Is it difficult to focus on 1.4F prime lens, and if it is how to counter that?

Question

Does difficulty in focusing have anything to do with 1.4F (Nikon G)?

Will it be still difficult if I use manual focus?
Is auto focus too slow?

Is there a way to counter it?

This example photo (from a photozone review) was taken taken at f/1.4. Isn't the whole trunk in focus here? If it's so hard to get everything in focus at f/1.4, why aren't some parts of trunk out of focus?

Asked by Anisha Kaul

Answer

No matter what the maximum aperture of the lens, the plane of absolute focus is the same. That is, if you shoot with a 50mm f1.4 or a 50mm f2.8 at, say, 10 feet out, it is focused on the exact same specific point of your subject. The difference is that at f1.4 a thin slice appears to be in focus and at f2.8 a thicker slice appears to be in focus.

So, let's say you are using the 50mm f1.4 lens set to f8. The resultant picture shows a thick slice of area in focus, just as a normal zoom lens set to f8 would. When you are focusing to take that photo, regardless of which lens and what focusing mode you use, you aim to focus on the exact same plane. In that regard, focusing with either lens is neither more or less difficult.

Of course, experience says that's not quite true: most will agree that focusing an f1.4 lens is more difficult than focusing an f5.6 zoom lens. The reason it's considered more difficult is because of that narrower plane of focus which shows less in focus and therefore makes it a little difficult to tell what precisely has been focused on. That's sort of a misnomer, however, because if your lens is set to f8 placing the plane of focus in exactly the correct location is irrelevant: the resulting photo will have a larger depth of field and hide any inaccuracy you had when focusing.

But, people don't buy an f1.4 lens to shoot at f8; they buy them to shoot at f1.4! If you misfocus slightly when shooting at f1.4, it won't be hidden like when shooting at f8. You do need to worry about being more precise in placing focus where you want it when shooting with such a large aperture. Is it more difficult? I think that depends upon your perspective and understanding:

• Perspective: a thin plane of focus means it's easy to see exactly what is in focus. You can tell precisely where you have focused and it's no surprise what the resulting photo looks like.
• Understanding: using a wide-area AF mode is almost asking for trouble, for example, because it's trying to focus on much of your scene, which can't be rendered entirely in-focus at f1.4. Use a single-point AF mode to place focus precisely. If using manual focus you need to have a feel for how much throw the focus ring has to recognize how much to turn it to pull your subject into focus -- just racking back and forth trying to find it will get you nowhere.

Working with an f1.4 lens isn't anything to be concerned with, IMO, because after using it for a while you'll have a good feel for it and know best how to use it.

Regarding your example photo:

Isn't the whole trunk in focus here? Why aren't some parts of trunk out of focus then?

Look close -- the whole trunk is not in focus. near the edges of it you can see it's a little soft. I bet that they could have stopped down to f1.6 or 1.8 and the edges would be a little more in focus, and the rest of the scene would basically be unchanged.

Answered by Dan Wolfgang

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Are the Canon 15-85mm + 50mm f/1.8 adequate substitutes for the 17-55mm f/2.8?

Question

The EF-S 15-85mm lens is a relatively recent lens, and I haven't seen too many recommendations on it thus far. However, most of the reviews have been very positive and the lens is said to be optically on par with the 17-55mm. It sacrifices the constant wider aperture for better zoom range that could translate to fewer lens changes. As for the 50mm f/1.8, it is one of the most recommended lenses, dirt cheap and suited to low light & portrait photography.

The two lens combo is also cheaper than the single one (around INR 53,000 vs INR 64,000). The usage scenarios I have in mind are the following, along with my thoughts:

• General purpose\travel photography - the 15-85mm seems to be more versatile than the 17-55mm for this, especially when light is not a constraint
• Portraits - the 17-55mm would be quite versatile in this regard (however, 55mm feels a bit short for getting tight shots, and getting close can lead to distortion)
• Event photography (weddings, parties etc) - the 17-55mm has the low light advantage over the 15-85mm, and 50mm can get a bit long in closed environments
• Low light shooting - the 17-55mm is more versatile, but the 50mm has a wider aperture

So, which would be a better upgrade for my current setup comprising of the Canon 550D with the 18-55mm IS & 55-250mm IS kit lenses? Are there any alternatives worth considering in this budget?

Note: I had asked a similar question regarding upgrades for the kit lens, but that was without a particular scenario or lens in mind.

Also, on a related note, I will need a bigger camera bag once I get the new lenses - any suggestions for the upgraded setup (2-3 lenses)? Update: I went and got the Lowepro Fastpack 350 Multi Use Backpack that contains a laptop storage compartment as well.

Answer

Before you get any other lens, get the 50mm f/1.8. Its a must-have lens for anyone even remotely interested in portrait photography. You just can't go wrong with it. Actually I never used the 18-55 after I got my fifty prime.

For your zoom needs, you would be better off with a third party lens like the the Tamron 17-50 f/2.8 (non-VC). At INR 21,000 ($440) it costs a third of the Canon 17-55mm f/2.8, and has great overall performance.

Here is my story:

I had a similar dilemma a while back. I had a Canon 500D with the 18-55 kit lens and was looking for a new lens. Since my budget was tight I purchased the 50mm f/1.8 and I was blown away by what a difference the wider aperture made. Amazing low light performance and sweet looking bokeh made it difficult to take a bad shot with it.

But with a crop factor of 1.6, I found it a little too tight for indoor use and while 'zooming with your feet' is the norm with a prime lens, I found myself running out of space indoors.

Also I used to shoot a lot wide open and found that shooting a portrait at f/1.8, even getting both eyes in focus becomes a challenge. Get a second person in the frame and there is no way you can get both people in focus. So I started stopping it down to about f/2.8 which gave me workable DOF while still giving great low light performance.

This made me realize that I needed a lens with the versatility of the kit lens and the wide aperture like the prime. I was on a tighter budget and hence purchased the Tamron 17-50 f/2.8 (non-VC). I have not been disappointed since. I still turn to the 50mm f/1.8 when I really need the extra 2/3rd stop or really shallow DOF, but the Tamron is what I use 95% of the times.

I've shot portraits, concerts, weddings and its always performed well. It would be better if it were a little sharper wide open and if the autofocus wasn't as loud, but its overall performance and versatility trump these minor issues.

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PS: I'm a huge fan of bokeh and shoot mostly in low light, so I thought it would be useful to show some examples of what f/2.8 lets you do.

This one shows how you can use the shallow DOF at f/2.8 to get sweet bokeh. The 50 prime would have also done wonders here, but you can't keep switching lenses at an event.

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This one was shot with just a candle to the left (out of the frame) and the lighter (in the frame).

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What is a “fast” lens?

Question

I've been reading lots of camera reviews lately and have ran across several references to "fast" lenses.

What exactly is a fast lens and what are its advantages compared to other lenses?

Answer

Ysap and Matt gave great answers. I understand things best with metaphors, so here goes.

Consider a container that you wish to fill with water. In this case, the water will represent light, and the container will represent your lens. The wider the opening in the container (the aperture), then the faster you'll be able to fill the container with water (light).

If you were to take two containers, one with a very large opening and another with a very small opening, and dump the same amount of water onto each of them (for the same amount of time), which container would be filled with more water? Clearly the one with the bigger opening. If you wanted both containers to end up with the same amount of water, then the one with the smaller opening would have to have water dumped on it for a longer period of time than the one with the larger hole.

It works the same way with lenses. A camera with a small F-stop is a lens that can open up very wide, thus having a bigger opening to swallow up light. What this means to a photographer is that he/she can use much faster shutter speeds and still get adequate light in his or her photo. In this case, consider a swimmer coming up very briefly for a breath of air. The swimmer would much rather open his mouth up wide to take in air than purse his lips and try to breath in. A fast shutter speed with a wide aperture means more light can get in during the short period of time that the shutter is open.

Consider shooting a ballet in a dark theater. It would be best to use a lens with a wide aperture, because of the dark environment. Additionally, you would be able to use a faster shutter speed to capture the fast-moving dancers.

I hope this makes sense!

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