Thursday, May 04, 2006

Why Size Matters

C-Net has an article on Digital SRL Lens sizes.

There seems to be a lot of confusion about the switch from 35mm to Digital. The sensors used in high end Digital SLRs are much larger than the ones used in consumer models but they are all quite a bit smaller than a 35mm film negative.

As a result the field of view of a Digital SLR is somewhat narower than the field of view of a 35mm camera with the same lens. Put a 50mm lens on a Nikon DSLR and the field of view will be what you see with a 75mm lens on a film camera.

With film cameras the manufacturers all agreed to use a common sensor (i.e. film) size and so a 50mm lens on a Nikon gives the same field of view as a Cannon, a Minolta or a Pentax. Digital allows each manufacturer to make their own choice. For reasons best known to itself Cannon has made two different choices. Their professional line of cameras has a 1.3 conversion factor, their consumer DSLR has a 1.6 factor.

What this means is if you do portrait photography, sports, birdwatching or anything else that requires a long focal length a DSLR gives you a 50% bonus in focal length. If you are taking landscapes and want the widest possible field of view you have to get a lens that is 33% wider than you would need for film.

Contrary to a claim I have seen in a number of other reviews the focal length extension factor does not affect the perspective or the depth of field. All you are doing is cropping the negative. That does not change the depth of field and certainly not the perspective.

Although it is true that your existing wide angle lenses are no longer quite as wide there is a compensation. When you buy a new wide angle lens that is designed for the new digital format (Nikon call them DX series lenses) you will find that a 10mm lens for the digital format is considerably cheaper than an equivalent quality 10mm lens for the 35mm format. Nikon sell a 16mm lens for $550 that works with film or digital and a 10.5mm lens (i.e. 16mm equivalent) for $550 as well. What is going on?

The answer is quite simple. The new digital lenses don't have to cover such a wide field of view. The flaws in wide angle lenses generally show up at the edge of the picture. The focus is usually a little softer, on cheaper lenses there is often noticable distortion. Designing a lens that produces a flat picture right out to the edge of the frame is very hard. The new digital formats mean that the lens designers only need to get the picture right in the middle of the frame.

The result of all this is that when you switch to digital all your existing lenses become 50% longer. You now have a super-telephoto if you didn't have one earlier. If you did you now have a super-super telephoto. While you may decide to replace all your lenses over time the only lens you are likely to find you need in the short term is a new wide angle to replace your current widest lens.

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