Do lenses need to be made with more precision for Hasselblad's 60MPix camera?

Question

One of Hasselblad's cameras has a Double-Full-Frame sensor.

Does that mean that the lenses needed for it are incredibly more complex to build (to have a sharp, high quality, abberation free image on the borders ?)

Asked by Skippy Fastol

Answer

Back to film camera time, studio enthusiast where used to 6x6 (6cmx6cm) (or even 6x8) camera which have surface about 4 times the 24x36 (24mmx36mm).

If you look (as exemple) at the Zeiss lense for such camera (as exemple for Hasselblad body) you can see that the same optical formula are also used for 24x36 lenses (Zeiss optical formula is indicated by the name Planar, Distagon, Tessar,...)

Theses lenses are bigger and usually build for high-end users but designs are not very different

Answered by floqui

Check more discussion of this question.

How do I design a conceptual lens schematic?

Question

I would like to attempt to create a schematic for a simple or small compound lens, but the process seems very daunting. I have thoroughly read 'Science for the Curious Photographer: An Introduction to the Science of Photography', and fully understand most of the lens concepts and the basis of optics like index of refraction, Snell's Law, etc.

Knowing this, what is the process of designing a lens, and where do I start? If you could provide any resources that would be very helpful.

Asked by Dylansq

Answer

I'd expect that virtually all lens design and analysis these days is done with optical design software like Zemax or Code V. Unfortunately, these products are priced for people and companies that make a (substantial) living working in the field. However, I have come across an optical design tool for Windows called OSLO that offers a free version with reasonable limitations. I've just downloaded it, and it comes with some relatively simple lenses in its library, like a Petzval 50mm f/1.8. It doesn't look like the easiest software to get started with, but there are some users' guides on the download page.

Answered by coneslayer

Check more discussion of this question.

How does the quantity of elements and blades in a lens affect the optics?

Question

I am trying to get a greater understanding of how lenses work. Basically I bought this lens, the Sigma 10-20 f4-5.6:

The construction has really fascinated me. 14 Elements in 10 Groups?! What exactly does that mean? Are the elements the blades?

What does this do the optics?

If I think about a pair of glasses there is effectively just one element.

Asked by Rob

Answer

The "elements" and the "blades" are two completely different things.

An "element" is a single piece of glass in the lens. Most of what's shown on the diagram are the lens elements. Some elements are colored pink, to indicate that their shape is aspherical. Other elements are colored blue, to indicate that they are made of a special type of glass. Others are uncolored, because they're just ordinary "spherical" elements made of typical kinds of optical glass. Your eyeglasses have a single element for each eye. Generally, using more elements, and using exotic types of glass or aspherical elements, allows the designer to better correct aberrations, or optical faults, in a lens.

When two or more elements have their surfaces in contact with each other (instead of having a gap between them), they form a group.

The number of blades refers to the number of diaphragm blades, that form the aperture stop. The diaphragm is an iris that you can make bigger or smaller, to control the amount of light that can pass through the lens. The location of the diaphragm is noted in the drawing by the vertical lines near the middle of the lens, above and below the central axis.

The number of blades is not shown in the diagram. The diaphragm looks like this:

This diaphragm has 6 blades; you can see that the aperture (opening) is a hexagon (6 sides). The number of blades is one of the factors that affects the bokeh of the lens. Your eyeglasses do not have any blades, because there's no adjustable diaphragm in eyeglasses.

Answered by coneslayer

Check more discussion of this question.

What is the benefit of an internal focus lens?

Question

The "IF" in Pentax DA★ 200mm f/2.8 ED (IF) SDM stands for "Internal Focus". I know what this means: the lens doesn't change in size as I focus. (And it's true; it doesn't.) What's the point of this, and why is it important enough to rate a few letters in the product name alphabet-soup?

I know that a non-rotating filter ring makes it more pleasant to work with orientation-sensitive filters (like polarized and graduated ND filters), but as I saw in this (unrelated) lens review, lenses can have non-rotating filter threads without being IF.

For a macro lens, I see how IF might be important, since you might be at actual risk of bumping your subjects. But this lens has a close-focusing distance of about four feet, so that can't be a concern.

So what's the big deal? Is there an advantage I can't see? Wouldn't a non-IF lens be more compact for storage (when set to its minimum extension)? Are there any optical benefits? Are there any drawbacks — compromises in other areas which must be made to enable this feature?

Asked by mattdm

Answer

In my experience, IF lenses frequently autofocus faster, because there is less mass to drive back and forth. For non-zoom lenses, internal focusing probably means that the bellows effect (in which air is sucked into the lens) is minimised since the outside of the lens probably won't mocve during focusing. That means the interior of your camera doesn't get humid or dusty (and so less crud adheres to the sensor).

According to The Manual of Photography (ISBN 0240515749; page 147), internal focusing mechanisms make it easier to have elements of the lens move nonlinearly with focus distance. This means that some kinds of aberration can be better corrected with such systems, or adequately corrected over a wider range of focus distances (this reminds me of Nikon's "Close Range Correction" feature; it looks to me like all their CRC lenses are also IF).

Answered by James Youngman

Check more discussion of this question.

What about lens construction influences bokeh?

Question

I can understand how certain elements of a lens can correct an aberrations or make it sharper - but specifically, how do lens manufacturers influence the bokeh - the quality of the blur? There's a huge difference between the bokeh of say a 85mm f/1.4 and the standard 18-55mm kit lens.

Asked by rfusca

Answer

As has already been pointed out, out of focus highlights will (mostly) reflect the shape of the aperture. That may be the most obvious element of bokeh.

There is quite a bit more to bokeh than just that though. Spherical aberration correction is another important element. If a lens corrects spherical aberration perfectly, an out of focus highlight will simply increase in size compared to one that's perfectly focused, but the light will be distributed perfectly evenly throughout that disc. In reality, however, that isn't how all lenses work, and (in particular) it doesn't lead to what many think of as the nicest bokeh.

In reality, the illumination across that disc is often at least somewhat uneven. What most think of as pleasant bokeh results when the center of the disc is bright, and the brightness falls off toward the edges of the disc.

The opposite is also possible though: most of the light falls toward the edges of the disc, and the center is relatively dark. In severe cases, this leads to "double line" bokeh -- the center part of a disc is enough darker than the edges that the center seems to disappear, and what was one object looks almost like two instead. Even in less extreme cases, this tends to lead to a "nervous" looking bokeh where features and textures tend to stand out rather than blending together nicely.

The third possibility is that even though the same overall brightness is maintained across the disc, it's not the same color throughout. One typical manifestation of this is discs that appear to have a green outline around the outside.

One more point to bear in mind is that exactly the same lens will often exhibit more than one of these. In fact, nearly all lenses exhibit all three, to some degree or other. The first two (bright center vs. bright edges) will depend on the position of the out of focus highlight relative to the focus point. If the background shows bright edges, then the foreground will typically show bright centers (and vice versa). Nearly all show at least some minimal degree of color change across the disc, though it's certainly much more visible in some cases than others.

Answered by Jerry Coffin

Check more discussion of this question.

How do ultrasonic motors allow continuous manual focus?

Question

I understand the basic pros and cons of ultrasonic lens motors — called USM by Canon, SDM by Pentax, or SWM by Nikon. One of the chief benefits is that they allow "always on" manual focusing — you don't have to disengage the autofocus motor in order to make manual tweaks.

Many Pentax lenses using the traditional non-SDM have drives have a quick-shift clutch mechanism which accomplishes largely the same effect, but this is apparently different. Is the difference here something to do with the motor itself (and something to do with its "ultrasonic" nature), or is it that the gearing used by this type of motor is special?

It's my understanding that some ultrasonic motors are ring-type, but that others, including the one in the DA★ 200mm lens I'm testing, are "micro-motors". Both seem to have this same beneficial property of allowing manual focus without a switch, which leads me to wonder: would it be possible to use an ultrasonic in-body motor and get basically the same benefit — or does it not work that way?

Asked by mattdm

Answer

The "gearing" is special, in that it is essentially non-existent, at least between the motor and the main mechanical drive for the focusing mechanism. In order for a traditional (low-frequency or DC-driven) syncromotor to have enough control fineness (or even control finesse), it needs to be geared down significantly. The mechanical forces involved in gearing down are slight, but working backwards against the system (manually focusing while the motor is trying to drive the system) would mean applying enough torque to damage gears, bushings/bearings, and probably strip the mechanical coupling between the gears and shafts, etc. (Not to mention that if there's a worm drive in the system anywhere, it won't work backwards at all.)

High-frequency systems (and systems with enough poles, even if they operate at lower freqs) can be directly controlled with a lot more fineness because the phase angles involved can be larger for shorter physical intervals. Because they don't need to be geared down, there's no extra mechanical stress on any of the components when you physically oppose the motor.

I don't think it would be viable to have direct drive from the in-body focus motor. The screw drive would need to run at a very different speed for a lens meant to be driven directly at or around 1:1 gearing than it would for an older, geared-down lens. (I don't have a Pentax to play with, but my Nikon screw-drive lenses seem to be at about a 8-12:1 drive rate.) If the motor has enough torque to drive a 1:1-geared lens effectively, then it's probably got enough torque to strip the drive screw on an older lens (or if the driver and receiver are hard enough not to strip, then to destroy some even more expensive parts, like the attachment points on lens barrels or body castings).

Answered by Stan Rogers

Check more discussion of this question.

Why are there no fast APS-C zooms beyond 55mm?

Question

I am looking to add a longer lens to my current zoom (a Tamron 17-50) for my Nikon D300 (but I believe this question is relevant across aps-c lens manufacturers).

There is just one currently in production — the Pentax 50-135 f/2.8 (which is Pentax mount only).

What is the reason behind this? Is there a physical reason why making a smaller DX alternative to a 70-200 is not be possible? I have seen it mentioned that "beyond 100mm the advantage in making dx lenses is gone", but not sure if this is folklore or a genuine restriction.

Answer

Actually the same 50-135 f/2.8 lens was sold by Tokina for other mounts, but it was discontinued in 2009.

I'd say the primary reason is that there's no significant savings between making a telephoto lens just for APS-C or one that's also good for full frame. In fast telephoto lenses, the majority of glass goes towards achieving the big aperture needed, the smaller image circle won't introduce very significant differences.

Another reason is that since a fast telephoto zoom is an expensive lens, many of the buyers are already considering getting a full-frame body in the future, and would dismiss an APS-C lens. This leaves a smaller pool of potential buyers, meaning less potential profit. Pentax doesn't have to worry about it, since they don't have a full frame body, nor have they announced any plan of making one.

Check more discussion of this question.

How to read a lens spot diagram?

Question

How to read such a lens spot diagram?

Answer

A lens spot diagram is an excellent means of evaluating lens quality "at a glance". but needs to be used in conjunction with other measures.

A lens spot diagram shown how a circular "spot" of light which should appear as a "spot" actually appears when viewed via various parts of a lens. It provides an immediate visual indication of quality at that point on the lens. Aspects such as astigmatism, coma, spherical aberration, chromatic aberration and more can be viewed instantly. This is valuable compared with being provided with numerical results which may contain more data but which are harder for the brain to interpret. The diagram you supplied is low enough resolution that it is hard to see some of the important details.

The black central circle is the Airy disk - the best possible image that a perfect lens with perfect focus could achieve - it is diffraction limited. See Wikipedia - Airy Disk

Outside the Airy disk you see light which has been diverted due to various aberrations.

---

Below are some somewhat stylised impressions of some typical aberrations.
These are copied from this superb page Notes for "A Gentle Introduction to Optical Design" By Bruce Irving, Optical Research Associates. He comments -

• Spot diagrams are graphs that show where rays from a point object will fall on the image surface (they must fall close together if the lens is to form a good image).

  The graph is usually highly magnified (as if you looked at the image spot through a microscope), and its shape can indicate the type and amount of aberration in the lens. Perhap most distinctive is the aberration coma, whose name is fairly descriptive.

He provides the following diagram

---

Also useful is this page from OSLO analysis.

OSLO is a lens designing program with a focus [ :-) ] on telescope lenses - details here.

He says:

• This is the best overall evaluation for a quick check of image quality, but it may not be enough by itself. For example, it is possible to have a good spot diagram even if there is more warfront error than desired. And sometimes you may make the spot diagram look better without really improving the images. So, it can be useful to check other analysis windows to double check the spot diagram. Another good tool is the Strehl ratio, which I will explain later on this page.

  The above image has 15 spot diagrams. The top row shows the spot diagrams for an off axis image which is 2 degrees away from the center. The bottom row shows the spot diagram for images on the axis.

  The columns show a focus shift. This means that the column on the left is 0.1mm inside of focus. The column on the right is 0.1mm outside of focus. The center column is at the current focus. You can examine the spot diagram at any focal position you want. For visual use a lens is normally focused for the primary wavelength because our eyes are very sensitive to green light, and less sensitive to blue and red light. But, you might want to examine a lens which is focused for polychromatic light, which would be a balance of all three wavelengths.

---

Note:

The diagram supplied is of low visual quality - some of the information needed to understand what is being seen is very hard to see. Also, the context is not absolutely certain. A link or reference to the original would be useful.

Check more discussion of this question.

Is overall light gathering of a lens only dependant on aperture?

Question

My impression is that the aperture value of a lens determines its light gathering ability, but I'm not sure I understand how it works...

When considering light gathering in telescopes, it is dependant on the diameter of the objective lens (or mirror). This makes perfect sense to me, since light is radiated in all directions, so a larger area means you gather more light. It seems to me it should be the same in camera lenses also - a larger lens would pick up more of the cone of light from the subject, and focus it onto the sensor.

What got me thinking about it was I've seen an F/0.95 lens, but it doesn't look hugely larger than F/2.8 lenses, so I don't understand the physics of how that would work.

Answer

Essentially yes, light gathering ability of a lens is determined by its maximum aperture. Transmission rates of the materials used also has an effect but it is very small.

You intuition is correct in that you would expect a large aperture lens to have a large barrel, however the aperture is specified as a ratio of the apparent* size of lens opening divided by the focal length. So a 200mm f/2.0 lens must have a front element large enough to see a 200/2.0 = 100mm aperture, so the barrel must be at least 10cm. However a 20mm f/2.0 only appears to have a 10mm aperture, which is small is comparison to most lens sizes.

To complicate matters wide angle lenses need larger front elements than dictated by their aperture to prevent vignetting across the frame. For focal lengths shorter than about 50mm lens sizes increase as focal length decreases despite apertures, and thus light gathering ability, also decreasing.

Here's nice example, this Nikon lens is only f/2.8:

but is absolutely huge, due to its extreme wide angle nature.

* note that 100mm f/2.0 doesn't mean the physical opening in the middle of the lens is actually 50mm diameter, only that the image of said opening when viewed through the front of the lens appears to be 50mm in diameter. The actually opening is often smaller, but the lens front element has to be large enough to accommodate its theoretical size.

Check more discussion of this question.

When you zoom in with a lens on an SLR why does the lens go in then out?

Question

I guess this is more a question of optics than photography but I just got an SLR with a basic 18-55 lens. I noticed that when going from 18 to 55 or 55 to 18 the lens physically comes back in and then physically goes back out?

What is going on there? I would think that if I am zooming in the lens should be going out 100% of the time but the lens actually goes out and then comes back in.

Answer

The lens is retrofocal at the wide end and telephoto at the long end. A retrofocus lens is referred to as "inverted telephoto" because it is constructed similarly to a telephoto lens with the elements reversed. The effect decreases as you zoom in, until you reach about 35mm, at which the lens begins to extend and eventually becomes a telephoto configuration, where the size of the lens, front element to rear element, is less than the focal length. The lens is neither retrofocal nor telephoto between these positions. This results in the lens being longer at the extremes of the zoom range than at intermediate positions.

For more information on this design, see the Wikipedia articles on Angénieux retrofocus, which discusses the origin of the design for the wide end, and telephoto lens for what happens at the long end. According to the telephoto lens article:

Zoom lenses that are telephotos at one extreme of the zoom range and retrofocus at the other are now common.

This is essentially what is happening with your 18-55mm lens. As far as I am aware, Canon, Nikon, Pentax, and Sony (A-mount, not E-mount) 18-55mm lenses all share this design aspect.

Check more discussion of this question.

How can I get bokeh with an aperture of 3.5 as my max?

Question

I am trying to get DOF portraits of my daughter with my Nikon d7000, as per my previous post.

I have zoomed in to 100mm or so and tried to get a large aperture but as I zoom, the camera decreases f# to about 6.

I have also tried in aperture priority mode and still have the same problem lowering the f#.

What am I doing wrong?

Answer

You can't zoom to f/3.5 at 100mm on your lens, your lens will do at best f/5.6 at 105mm.

You can however get a shallow DOF effect quite easily, for example by shooting at f/5.6 at 105mm, with your subject quite close (say 6ft) and you background say 10ft behind the subject.

See this set of photos I took with a Nikon D70 (much older predecessor to the D7000!)

You can see the shot at f/2.8 has a shallower depth of field than at f/5.6, but at f/5.6 you can clearly see the subject in focus and the background blurred, you can even see that at f/8. (They aren't all sharp, I was on a tripod, long exposures, and caused the camera to move when I released the shutter, i used a 5s timer after that..!)

At f/5.6 at 200mm the effect is even more pronounced.

I'm sure at f/5.6 at 105mm you could do even better than I did, getting closer to the subject, and having the background further away.

f/2.8, 105mm

f/5.6, 105mm

f/8, 105mm

f/5.6, 200mm

Check more discussion of this question.

Why do zoom lenses and compact cameras have varied maximum aperture across the zoom range?

Question

Why does a camera's maximum (allowed) aperture get smaller when you increase its zoom?

Answer

The short answer is because it is cheaper to manufacture such lenses. The longer the lens and the wider the aperture, the larger the optical elements in the lens - thus larger the expense to produce them.

A lens like 70-200/2.8 must have a front optical element of 200mm/2.8=72mm, which is quite a chunk of glass. On the other hand, the 70-300/4-5.6 needs to be 300mm/5.6=54mm wide. If it were f/4 through its full range, the optical element would need to be 75mm wide - even larger than the much more expensive 70-200/2.8.

In your question, you say "the camera's maximum aperture". The camera does not have an aperture - the lens does. Minor but important difference, especially for SLRs - once you remove the lens you see that the camera is just a light bucket with a big hole in the front.

DETAILS:

The aperture is the ratio of the focal length of the lens to the size of the front optical element. Essentially

aperture = focal length / optical element size

For example, a 50mm f/1.8 lens has a 28 mm (50/1.8) element size.

If you're wondering why the f-stop numbers don't seem to be linear (they're not), it is because the amount of light collected by the lens is proportional to the focal length divided by the aperture. Because of this power of 2, f/4 collects twice as much light as f/5.6, since 5.6/4=sqrt(2).

Check more discussion of this question.

Why do wide angle prime lenses have relatively small apertures?

Question

I've noticed that many of the wide angle prime lenses (at least for Canon) have somewhat smaller apertures than their normal or telephoto counterparts. E.g. the regular Canon 24mm prime is f/2.8 while the 50mm prime is f/1.8.

Theoretically, it should be possible to make large aperture wide angle lenses, as their opening will be much smaller than primes with longer focal lengths. So, why are there no wide angle lenses with larger apertures? Does a larger aperture place limits on the smallest aperture a lens can have, as this could have an impact on the depth of field for landscape photography.

Answer

Broadly speaking wide aperture lenses are easier to design the longer the focal length. The reason that you don't see any 400mm f/1.4 lenses is due to manufacturing difficulties, e.g. keeping dispersion low while producing elements of the size required for such apertures. It's worth restating that the designation f/1.4 means that the size of the aperture stop is the focal length divided by 1.4, which for a 400 f/1.4 is a whopping 285mm. Technically it's the image of the aperture stop that must be that size, which means the front element has to be at least that big.

If you look at the widest of Canon's superteles you see a pattern that 150mm seems to be about the limit of what is economical:

• 400/2.8 = 142mm

• 600/4.0 = 150mm

• 800/5.6 = 142mm

Lenses with focal length less that the registration distance (about 46mm for most DSLRs) have to incorporate what's known as a retrofocal design, which is essentially a reverse telephoto group (or "wide converter") at the back of the lens. The wider the lens the more corrections have to be performed due to the retrofocal design, and these corrections are more difficult for wide apertures lenses.

You can see this if you look at the design of the Canon 24mm f/2.8 and 50mm f/1.8:

Canon 24mm f/2.8

50mm f/1.8

The reason 50mm offer such good price/performance ratio when it comes to aperture is that for 35mm cameras that 50mm sits at the sweet spot where the focal length is long enough to allow a simpler non retrofocal design, but not too long that large pieces of glass have to be used to give a good f/number.

Check more discussion of this question.

Does focus breathing make a lens slower when close focusing?

Question

I've heard that focal length of some lenses will become noticeably longer when focusing to a close distance, an effect called "focus breathing". Since f-number is focal length divided by diameter of physical aperture and aperture size does not change, it seems logical to conclude that such lens should become slower when focusing close.

Is that really so, or is there something I'm overlooking?

Answer

That is true, and very noticeable in macro lenses. For example a Nikon 105mm f/2.8 VR (at infinity) is f/4.8 at it's closest focus distance of 30cm or so.

Check more discussion of this question.

How can a lens like a 18-55mm have more than one focal length?

Question

How can a lens have a range of focal lengths? Shouldn't it be a single number?

Isn't focal length supposed to be fixed for a lens of a particular shape and geometry?

What am I missing?

How many lenses are actually there in a "lens"?

Answer

A range of focal lengths indicates a zoom lens. There are two major classes of lenses. Primes, or primary lenses, have a single focal length. They tend to be higher quality, as there can be fewer lens elements, and fewer moving element groups. One exception to this rule is super telephoto prime lenses, particularly faster lenses (f/2.8), which are some of the most advanced optics on earth, and contain numerous element groups with special types of lens elements (i.e. ultra low dispersion glass, fluorite lenses, aspherical elements, etc.)

The other class are zoom lenses, and they have a range of focal lengths. They generally have more element groups overall, and usually have several moving groups. The length of a zoom lens can be increased or shortened, thus changing the focal length. The quality of a zoom lens depends on its construction. Some have great quality at one end of the range, and lesser quality at the other end. Some have great quality at the ends of the focal range, but lesser quality in the center of the range. Super telephoto zooms also often have special types of lens elements.

To answer the question about "how many lenses are in a lens", the question depends. Some lenses have few lens elements (individual glass or other material lenses within a camera lens body), and others have many. The number of lens elements in a given lens is usually an indication of a few things. Lower quality lenses tend to have fewer elements, and the glass used in those elements tends to be of a lower quality (less dense, greater chance of splitting light and causing things like chromatic aberration.) Lower-end lenses may have 3-5 lens elements total. Higher quality lenses tend to have more elements, and often larger elements. Many high-end lenses tend to have more lens elements, around 5-10. Higher end telephoto lenses may have 14-20 lens elements or more. More glass is generally required to produce wider apertures, and having a very wide front lens element often requires additional lens elements to focus that light down to a size that can pass through the lens mount into your camera. As focal length increases, maintaining the quality of the generated image often requires additional elements that serve different purposes. High end telephoto lenses often contain aspherical elements, ultra-low dispersion elements, fluorite elements, movable element groups, etc. These additional lens elements increase the control the lens has over light, but also increase the overall weight of the lens, and its manufacturing complexity.

Check more discussion of this question.

How do constant aperture lenses work?

Question

Cheaper zoom lenses usually are faster at the wide end and slower at the long end (for example, the $150 Canon EF-S 18-55mm f/3.5-5.6). More expensive constant-aperture zoom lenses have the same aperture regardless (for example, $800 Canon EF 17-40mm f/4.0 L).

My question is: are these good lenses sandbagging at the wider settings, or do they have a different optic system that allows them to maintain the same aperture throughout the zoom range?

Answer

They have different optics and are usually substantially bigger lenses for the same focal range (compare a 70-200mm f/2.8 to a 70-300mm f/4.5-5.6 and see that the latter is small in comparison). To get the constant aperture at the long end, you need to have a bigger barrel because the aperture is a ratio versus the focal length. However, if you do the math for your examples:

18mm f/3.5 means a 5.14mm opening 55mm f/5.6 means a 9.82mm opening

17mm f/4.0 means a 4.25mm opening 40mm f/4.0 means a 10mm opening

It's clear that the aperture's physical diameter can be bigger in both cases. So, in either case, you would theorize that at the widest end you should be able to be f/2.0 or thereabouts and your sandbagging scenario would then apply to both. On the other hand, for the latter, the optics may be simplified and thus approaching prime quality in result. So... Tradeoffs.

In any case, zooms have pretty complex construction involved, much more so than a prime lens ever would, and so there are a lot of considerations around optical correction at various focal lengths, the effect of the aperture on that correction, and so on. It may be, given the lens design and costs associated, that attempting get wider on the short end would result in a hugely unacceptable softness in the image or some other forms of abberation.

Finally, for certain there are different optical constructions between the two. Heck, there's different optical constructions between lenses of the same configuration but different manufacturers. It all comes down to cost versus benefit and, in the end, what price the market will bear for a lens of a given construction.

Check more discussion of this question.

What are the differences between two generations of Tokina AT-X 124 AF PRO DX AF 12-24mm f/4?

Question

I have been looking for a wide angle lens for a long time and have finally decided on the Tokina 12-24mm. When I went looking on eBay, I found there are two different types:

• The Tokina AT-X 124 AF PRO DX AF 12-24mm f/4
• The Tokina AT-X 124 AF PRO DX AF 12-24mm f/4 II

The only difference I can find is they've put some different coating on the glass – but there has to be more differences than that, right?

Also, does anyone have this lens? If so, what do you think of it?

Cheers.

Answer

From looking on DXOMark it appears the new one has its own focussing motor and it's a bit lighter as well as it has a new coating.

Check more discussion of this question.

What are the differences between two generations of Tokina AT-X 124 AF PRO DX AF 12-24mm f/4?

Question

I have been looking for a wide angle lens for a long time and have finally decided on the Tokina 12-24mm. When I went looking on eBay, I found there are two different types:

• The Tokina AT-X 124 AF PRO DX AF 12-24mm f/4
• The Tokina AT-X 124 AF PRO DX AF 12-24mm f/4 II

The only difference I can find is they've put some different coating on the glass – but there has to be more differences than that, right?

Also, does anyone have this lens? If so, what do you think of it?

Cheers.

Answer

From looking on DXOMark it appears the new one has its own focussing motor and it's a bit lighter as well as it has a new coating.

Check more discussion of this question.

Why do filter sizes vary for lenses of the same focal length?

Question

I have a Nikkor 18-55 kit lens that has a 52mm filter, and recently I was looking that the 17-55 lens has a 77mm filter.

Is it because the latter is a better quality lens? I assume a bigger circle lets more light in.

Or because it has a greater maximum aperture (F/2.8 for the 17-55 vs F/3.5 for the kit lens)?

Or something else?

Answer

As Nick mentioned, there are two reasons why a lens might need a large filter diameter:

1. Your front element needs to be at least as large as your apparent aperture size.
2. If your lens has a wide field of view, you may need a large front element to avoid vignetting.

In the particular case of the 17-55, I think it's more of the latter than the former -- the aperture on a 55mm f/2.8 is 19.6mm; far smaller than the 77mm filter size of the lens. Even the old Nikkor 55mm f/1.2 AI had a 52mm filter ring.

For visual proof, here's the lens at 55mm, f/2.8:

As we can see, the apparent aperture is much smaller than the front element, even at f/2.8.

If we look at an angle at 55mm, f/2.8:

we see the edge of the image circle before the edge of the front element.

Considering 17mm,

once again, the aperture is much smaller than the front element.

However, this time, if we tilt the lens,

we can still see through the lens at an extreme enough angle that our aperture appears adjacent to the edge of the front element. I'm fairly certain that the wide angle, in combination with the lens's long physical length, is the reason this lens needs such a large filter size.

Check more discussion of this question.

What does the number of elements and groups in a lens mean?

Question

All lens specifications include a statement of how many elements the lens contains, and in how many groups, for example:

• Nikon AF-S VR Zoom-NIKKOR 70-300 mm 1:4,5-5,6G: 17 elements in 12 groups (two ED glass elements);
• Nikon AF DX Fisheye-NIKKOR 10,5 mm 1:2,8G ED: 10 elements in 7 groups
• Nikon AF-S DX Zoom-Nikkor 18-55mm F/3.5-5.6 G ED II (kit): 7 elements in 5 groups
• Nikon AF-S MICRO NIKKOR 60mm/2.8G ED: 12 elements in 9 groups
• etc.

What does this signify? How is this important for the lens? Does it make a difference in image quality? What is better: fewer or more elements/groups? Or does it not matter — in which case, why they are included in the specification?

Answer

I think manufacturers list the number of elements it just so you know how much effort they put into a lens!

There's no simple answer to whether more of fewer elements is preferable. More elements generally means greater correction for distortion, chromatic aberration etc. however this extra correction might be necessary due to the design or the performance characteristics of the lens, not a sign of better image quality. Elements are often paired up, so the number of groups gives you a better idea of the number of corrections.

However the more bits of glass the light travels through the more surfaces there are for reflections etc. so contrast and sharpness can be reduced. As an example, let's compare the Canon 50mm f/1.0L with the Canon 50mm f/1.8II

First the f/1.0 version:

11 elements in 9 groups

Now the f/1.8 version

6 elements in 5 groups

Now stop both down to f/8 and the II would almost certainly be sharper. But which is better? You can't really say, because the first version has an ultra wide max aperture. It's a high performance lens which necessitates a lot of optical correction.

Even comparing the degree of correction can be misleading. You'd think that a better corrected lens is preferable, but it can lead to other defects. Correcting for spherical aberration in particular often makes the bokeh worse (which is why some lenses leave it uncorrected). Lens design is all about compromise.

So in summary, the number of elements/groups can be informative, but it's very rarely an absolute measure of quality or a reason to prefer a specific lens. The more important factors are the inclusion of special types of glass, such as low dispersion, (extra low dispersion) or flourite elements, and aspherical elements which perform better but are harder to make.

Check more discussion of this question.