Why is there not a protective glass element that is between the lens and shutter in a SLR?

Question

With the danger of dust on all SLR cameras, it only seems logical that adding a simple glass element that went between the lens and shutter (mounted on the camera) would both protect the camera's interior and sensor from any intrusive particulate matter.

Logically thinking, I don't think it should effect the picture quality at all, seeing how we have glass sky filters already.

There are a couple of questions that I have:

Are there any cameras that currently have this feature?

Would implementing this feature detract from the normal function of a camera?

Is this in any way impractical?

To clarify, the blue region is where the question refers to.

Asked by Dylansq

Answer

Image quality and possibly expense

Introducing yet another element in front of the sensor will degrade quality of the pictures, and for various reasons.

a) being outside of protection of the shutter will mean that it is constantly in contact with the air and dust. This will mean it will require more constant cleaning than a sensor would. How often do you clean the mirror inside your SLR? Hardly ever because you know it won't show up on photos. How often are you going to clean this extra piece of glass?

b) The placement of being between the lens and the sensor will mean that any dust on the glass would be spread over a larger area of the sensor, degrading quality. We probably all know what an example of dust on the 'sensor' looks like. That dust is on the protective glass right in front of the sensor, so only really covers the few pixels directly under. Being on glass in front of this would cause a larger smudge to be affected by the dust.

c) CA, diffraction, and all of those other nasty things. You have just had the light of your subject travelling through an expensive lens filled with precision glass. That nice white L lens you bought has scored amazingly well on sharpness and contrast tests. Now, however all that precision engineered light is passing at a steep angle through a cheap piece of glass placed in front of your sensor. Unless they spend a lot of money developing some very good quality coated glass, it is unlikely that it will not effect the quality of your photo. And depending on what lens you have, and the angles of incidence that the light from it is hitting this glass, will likely change how the glass effects it. To make none of this happen, you would need multi-element glass, or effectively another lens inside the camera, which would increase cost and give you even more to need to maintain and clean.

Also, in the placement where you put the glass, it might not fit with all lenses. I know some rear lens elements protrude more into the body with others, and I don't have any measurements, but at a guess some have been designed to only just narrowly miss the mirror as it flips up. Putting this glass in the way would mean these lenses no longer fit those cameras.

To summarise, it's not practical, and would degrade image quality. The sensor already has filtered glass on it that can be cleaned, and that is protected by a shutter that opens when it needs to to minimise contact with the air. Introducing an extra glass element removes this simplicity and makes the whole process of keeping things clean more difficult.

Answered by Dreamager

Check more discussion of this question.

Why has no one invented a 4:5 aspect ratio APS-C/35mm-sized DSLR?

Question

People keep inventing different form factors and formats for cameras, but none seems to have hit on what seems (to me) like an obvious one. Why doesn't somebody build a camera with a sensor that has a 4x5 aspect ratio, but the diagonal size of the sensor the same as an APS-C and/or full-frame (35mm) sensor (and a mount compatible with their existing lens line)?

Maintaining the same diagonal size means this would remain compatible with (most?) existing lenses for the same mount. Since typical use would no longer involve cropping, a sensor with exactly the same number of sensels (and therefore, the same pitch/density/well size) as a current one would still have higher usable resolution.

Just FWIW, the sensor sizes seem to work out to around 18x23mm to work with APS-C lenses, and 27x34mm to work with full-frame lenses. Does anybody see a major downside to this idea?

Answer

The mirror has to be a lot bigger for 4x5, particularly at 90 degrees to the axis the mirror flips about. The shutter also gets bigger in the direction it has to move (there's a reason the shutter moves up and down not left to right). This means a slower moving mirror and shutter.

Another consequence of the bigger mirror is the lens mount has to be further away (greater registration distance), which gives you less space for the rear element and so you have to use a retrofocus design for normal lenses. You would also break compatibility with current lenses - despite the fact that the image circle is big enough, the mirror would hit the rear element!

Squarer formats are more common in medium and large format photography where more time is taken over a shot so a slow mirror/shutter can be tolerated (many use leaf shutters anyway) but anywhere where speed dominates, such as sports smaller and more elongated formats allow a little extra speed.

Another area where you tend to find 4:3 sensors is in compacts where you have no mirror to move. The only mass market 4:3 DSLR I can think of were the Olympus 43rds, and they're moving away from having mirrors...

Look at the full frame Canon 5D, it has a slower max shooting rate and a slower sync speed (a measure of how fast the shutter curtains move) than the equivalent APS-c model. Although Nikon have a high speed FF camera, it's extremely expensive, and I bet it would go faster with a smaller mirror.

Check more discussion of this question.

What are all the options or features a DSLR camera body can have?

Question

I was curious to know if there was a place that lists all the options that are possible on a DSLR camera or even which cameras have them?

for example: Video recording, Night vision, The speed in retaking a picture.... etc...

Answer

Snapsort is a great site that I think does what you're after. It's a really easy way to browse camera features and compare models side by side to decide what to buy.

Check more discussion of this question.

What are the consequences of making camera bodies black?

Question

Considering that silver and other camera colors are outsiders, I wonder if there is a specific reason to make black camera bodies.

I can think of several "marketing" reasons (discretion, first, or aesthetics - black is posh), but I wonder if more technical aspects play a role here, or on the contrary if the makers don't take some effects of the color into account to privilege design.

For example, black is probably pretty bad for the thermal protection (on hot days, the temperature of the body can jump due to light incidence), but good to avoid reflections. These are random thoughts...

Then what is the influence of the body color on the optical and mechanical system, or on the imaging process generally speaking? If there are sources somewhere explaining these aspects, that would be nice.

Answer

I think black is primarily chosen to be discrete, and because that's what people expect. Whilst some people like to be different, most want their camera to look like a camera. On the first point, a shiny camera would be a liability for nature photography, as light reflecting off it could scare away animals. People tend to use camo coverings over Canon's white lenses for this reason.

It's not a manufacturing limitation: there are plenty of silver plastic bodies, in the good ol' days bare metal was popular, and then there's Pentax ;)

The other argument comes down to temperature issues. Black absorbs the most solar energy, but also radiates the most heat energy back into the atmosphere so the problem isn't as bad as you might think.

Canon's supertelephoto lenses have traditionally been white (cream) and the reason given was to prevent heat expansion affecting the optics. Other manufacturers predominantly make back superteles (actually Nikon once made a 300 f/2.0 that was white), as Tzarium points out in the comments this may be due to their not using flourite elements and thus have fewer issues with heat expansion.

A white Nikon lens!

In the end then it comes down to marketing. White is Canon's trademark — when you see a sea of white lenses at a sporting event you know the majority of shooters are using Canon and that's a fantastic advert for them!

Check more discussion of this question.