BetterPhoto Q&A
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more about fast lens

Short to the point, strength & weakness of fast lens with smaller aperture sizes of 1, 1.4, 2, 2.8.

Thanks in advance,
Dr Phyo San Win
Myanmar)

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March 23, 2006

As light rays pass through, the lens manages each by bending and redirecting (refraction). Stated another way, the lens aims each ray in such a way that each is targeted to arrive at the film plane at precisely the right place to reconstruct an image of the scene. Rays passing through the lens near its center are bent the least while rays striking the lens at its periphery, are bent the most.

Now think of a line of shooters aiming at the bull’s-eye of a target. The shooters at the end of the line are disadvantaged. They must shoot at a target that is further away and at a steeper angle. The end of the line shooters are more likely to miss the bull’s-eye.

So too is the fate of the fast lens. To be fast, the lens must have a large diameter. Light rays originating from the lens peripheral are more likely to be misdirected. To make a good fast lens required complex design which increases the price.

Most lenses perform best when set to about their middle diameter. Generally the best sharpness is obtained about 2 or 3 f-stops down from max. The smaller apertures give more depth-of-field but sharpness is reduced due to diffraction. The larger setting suffers the most from aberrations.

When it comes to photographing under low light conditions, nothing beats a fast lens.
Simply stated, a fast lens allows you to work when light levels are severely low. While every element in the image making process is important, most name brand lenses exceed the imaging requirements of the average photographer.

Other solutions to low-light imaging:
1. Higher film/chip speeds (ISO) however, often the image quality suffers.
2. Slow shutter speeds allow the film/chip to accumulate more light over time. However, subject/camera movement limits just how slow you can go.
3. Supplemental artificial light is a good solution but not always practical.

You get what you pay for! Most moderately priced name brand lenses are quite good. Good fast lenses cost more. A good fast lens is a treasure and a joy for ever.

Alan Marcus

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March 24, 2006

Now, there is also the issue of optical quality. It takes a lot more optical engineering to produce the faster lenses, but this in no way translates to overall better optical quality. To the contrary, and in general, I have found that slower lenses perform better stopped down, while fast lenses perform better at wider apertures.
Most lenses perform their best 2-3 stops down from wide open. Hence, if you own a 1.4, you can expect your lens' image quality to peak at about f2.8-4. If you shoot in conditions where these wide apertures (read very limited DOF) are needed (low light, portraiture with a blurred background, etc), then you would be better off with the bigger, heavier, costlier fast lenses.
However, if you require lots of DOF (read landscapes, close-ups, etc) then you are much better off buying slower lenses. A 3.5 - f4 lens will perform its best at f8-11. On 35mm cameras, or DSLR's, these smaller apertures would be much more useful to the landscape photographer than a 1.4 lens that peaks out @ f4.
Defining your subject matter and work flow (tripod or handheld) can also lend a clue as to which you should purchase.
My pleasure shooting is landscapes, nature, flowers, etc. For that I need smaller f-stops for DOF, and thus, I choose the slowest lens in a given focal length when purchasing. Added benefit is the cheaper price, and lighter weight.
For my professional shooting in the studio, the same applies, although I typically use macros lenses, and they are usually only available in one speed.
In short, a fast lens will perform its best at its wider apertures, and a slow lens will perform its best at its smaller apertures. Your 1.4 lens will probably perform better @ f4 than my 3.5 lens, but my 3.5 lens will outperform the 1.4 @ f11 and f16 (generally speaking).
Michael H. Cothran

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March 24, 2006