Which Lens Should I Get?
I am asked frequently "which lens should I get" or something along those lines. As with the sensor / mega-pixel discussion - I see a lot of people go for a top shelf camera and then skimp on the lens. Of course, it all depends on what you want to shoot, but in general, I am a big fan of good glass.
I don't think anyone will disagree with the statement that there is no right answer about what is the right lens or lenses for a particular person.
So, what do I look for in a lens? Maybe that is a good place to start.
For example, I did a comparison of chromatic aberration for Canon EF 24-70mm f/2.8 L II and a Canon 17-85mm 1:4-5.6 IS USM lens. At 24mm and f4.0 for both, you can see that at 2/3 out from the center, the f2.8 lens has virtually no aberration in the reds and about 0.03 aberration in the blues compared to about 0.02 of red and about 0.06 in the blues for the 17-85mm lens. See http://www.dpreview.com/reviews/lens-widget-fullscreen?compare=true&lensId=canon_24-70_2p8_ii&cameraId=canon_eos7d&fl=24&av=4&view=mtf-ca&lensId2=canon_17-85_4-5p6_is_usm&cameraId2=canon_eos7d&fl2=24&av2=4 for a total geeky way to look at this. You will also see sharpness drops off terribly for this lens.
So, what do I look for in a lens? Maybe that is a good place to start.
Aperture
I will always look for the biggest aperture I can find. If I can afford a lens in a f1.4 versus a f2.8, I will try to get it, if I can afford it! Why the desire for "fast glass"? (At small apertures, the lens lets in more light so you can shoot at a shorter exposure - thus the "fast" moniker.)
There are several reasons I like fast glass:
- I can capture the maximum amount of light in a given situation before needing external lights, or giving up.
- I can get a very shallow depth of field and nice bokeh for portraiture (blurry background).
- The sharpest aperture of the lens will be "better". In general, the sharpest aperture is never the maximum, but starts to drop off a few stops below. So, of I shoot at f4.5 on a f2.8 lens compares to a f4.5 lens (widest lens setting) - in almost every case, the f2.8 lens will be sharper - of course assuming other parameters are similar.
- The wider the aperture, the brighter the image for focusing.
Depth of Field
The wider open the lens (smaller f-stop), the shallower depth of field along with capturing more light. Sometimes, this has to be managed...
I was talking to someone who was complaining about his 50mm f1.4 lens. This was a professional series lens on a professional full sensor body! He was saying that the "lens" was not sharp. Could that be? What could be the issue?
Well, it turned out that he was using auto focus and not paying attention to where the camera was focused. At a 3 foot distance to his subject, the depth of field is less than 1/2 inch - that means anything 1/2 inch closer or farther from the focus point will be out of focus. What was happening was the camera was choosing an ear, shoulder or who knows what to focus and the face would be totally out of focus!
When used properly, like focused on the eyes for instance, a f1.4 lens can produce fantastic portaits - BUT - the photographer has to control the focus point.
Focal Length - Zoom vs. Prime
Choices really depend on what your prime objectives are. For a general purpose lens, anything in 30mm'ish range to 100mm'ish range should allow you to get a good lens that is affordable and will work in a wide range of situations.
Be careful in getting lulled into a big telescopic zoom lens unless you are willing to pay top dollar as the trade offs will be significant. Personally, I would chose the lens with a larger aperture (smaller f-stop) over the bigger zoom - especially if this is will be an only lens.
Also, for larger zooms, image stabilization becomes necessary for hand held use.
In my opinion, a 300mm lens that is not image stabilized and with a minimum f-stop of 5.6 is not useful in most circumstances. This of course is made worse if you are using a crop sensor camera so actual length is 480mm!
I find that in most situations, it will be too difficult to get a clear image because you are going to need to set your shutter speed to at least 1/1000 to stabilize the shot - and likely will need to push your ISO to point where it is just too grainy.
I am not saying "save your money" - what I am saying is get a shorter lens with image stabilization and a bigger aperture. My go to long lens is a 70mm to 200mm f2.8 image stabilized lens. On a crop sensor camera, this is even tough to use with moving subjects!
A prime lens has a fixed focal length. In general, a prime lens will be less expensive than a zoom in a particular focal range, and will allow for a better aperture selection. For instance, a 50mm f1.4 prime lens will cost significantly less than a zoom at even f2.8. The lack of moving parts, etc. makes these much less expensive. One still needs to pay attention to lens quality (see chromatic aberration, etc) though.
For studio photographers, primes tend to be standard since one can control the setting, etc. For general use however, it is important to remember that you need to "zoom with your feet"!
I heard complaints from a colleague who has an awesome Leica with a 35mm f2.8 prime lens. This person was used to a 28mm-300mm telephoto lens and could not get comfortable that with the 35mm lens, you need to walk to your action.
A prime lens will be much smaller than a zoom lens, so for travel, some people like to have one so as to not carry around a bazooka sized super telephoto. Of course, if you choose a 40mm pancake lens for your digital SLR, keep in mind that if you want to capture those long distance shots - have a telephoto in the bag too!
Focus
In general, your camera body does the focusing, but the lens is the thing that actually has to focus. Different lenses will focus at different speeds - and this is likely due to a number of things such as the motor technology as well as the software inside the lens. Software in a lens? Yes. Some newer lenses now have USB ports so you can update the firmware in the lens as well as fine tune lens parameters.
As mentioned above, when you focus an SLR, the lens is generally in it's widest aperture position - then when you take the picture - it closes down to your selected f-stop. What this means is that if you are focusing in a dark room, an f2.8 lens will allow more light for focusing than a f5.6 lens. This is a second reason I like to call these fast glass - since in general - they will focus faster too!
Unless you are shooting sports or your kids really never sit still, focus speed tends to be a low priority for me. The differences of technology are noticeable, but the least impacful on your final images. It is worth thinking about and another reason to consider faster glass if your common usage will be indoor non-flash shooting.
Image Stabilization
IS or no IS, that is the question. Image stabilization steadies the lens internals from small movements common to hand holding. Different manufacturers have different methods and I can't say that I have much experience with the differences between them.
What I can say is that when it works, it is worth its weight in sharp pixels. My 200mm lens has image stabilization and it claims it provides "two stops" of image stabilization. What that means is that at 1/250 of a second, it is as steady as if I was shooting at 1/1000 without stabilization. This is of no help in shooting a moving object, but the steadier your lens is, the better chance of getting a sharp shot.
There is no rule of when you should have an IS lens. Somewhere starting at 100mm is about where I think it makes sense - definitely on anything longer or if your effective focal length is longer because of a crop sensor lens.
Chromatic Aberration
Lens technology has come a long way over the last few decades, and I am continually amazed at the improvements in lens design. One area that I have seen improvements in is chromatic aberration. You can read about it in Wiki or elsewhere, but basically the problem is that all colors do not focus equally. The most common problem you tend to see is a haloing just around a high contrast image.
I was looking to purchase a compromise lens that was an older version of a lens that was selling for over $2K. The older version was "only" $600 or so and I figured how bad could it be? Well, after speaking with one of the guys at Adorama, I was told that this lens had some real problems with high contrast situations. So, as I tell everyone, I went over and asked to see that lens on my same camera body. I turned around, found a bright shiny chrome part and took a closeup of it. When I zoomed in on the back of the camera - the purple haze around the outside of the chrome made the decision easy. I gave it back and said not thanks. One shot. That was all it took. The chromatic aberration was clearly not something I wanted to live with. Of course, as with understanding limits of a sensor, if you know the limits of a lens, you can work with it.
When you see a $150 300mm lens, or other ridiculously low cost lenses, I would bet dollars to donuts that one of the limitations to this lens will be fairly poor chromatic aberration. I was just looking at a set of pictures from a friend who just got back from a trip. His 300mm telephoto lens was both cheap and cheap. As soon as I looked at a picture of an eagle he took flying in the sky, it looked like it had a ring of purple feathers around it!
Read the reviews. This is not a discussion about differential equations - the reason for the problem is - but the effect is very easy to see. Don't believe me? Put a pile of shiny paper clips on a dark surface in a well lit area - zoom in and take a picture. Are the edges of the paper clip clean and sharp when you view them in maximum zoom, or do you see a funny color at the edge. That is chromatic aberration.
Summary
I will spend more on a lens than on a camera. There is always a balance, but I would take a pro-lens and semi-pro body rather than the other way around. This is a personal choice, but that is how important good glass is to me.
Before selecting a new lens, do your homework...
Here is a much better and well written lens buying guide...
I also highly recommend looking at the lens reviews on dpreview too
If you have the time - look at some of the test results on this site to better understand why you pay more for a lens. You can change the focal length and aperture of a lens on the controllers to see the test results.
Another great resource is use reviews on B&H and Adorama. When you are looking at pricing, see what others have to say. (And don't read just one review - be open minded!)
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