All this talk about "equivalence" is silly

Started Jun 10, 2011 | Discussions thread
Fabian628 Senior Member • Posts: 2,447
some parts are true.

Howard wrote:

Someone wrote a web page about "equivalence" some time ago. Let me first say that I perfectly understand the concept (it is really about perspective, DOF, framing, etc.), but I think all this talk about "equivalence" is silly, confusing and mostly bogus.

For example, the "equivalence" adherent will say that a 300 f/4 lens is "equivalent" to a 480 f/5.6 lens on a crop camera, or some such. But this has severely limited applicability -- that is, only if you are narrowly concerned about perspective, DOF and want to duplicate exactly the same image as on a FF camera. But this is not the only, or most important concern for most people. Actually, I want to argue that for most people, this is not a concern at all.

Some cases in point:

Case 1: if you want to stop action, in the same lighting condition, with the same ISO, a f/2.8 lens gives you the same shutter speed on a crop sensor or a FF sensor, so to same something like a 300 f/4 lens is "equivalent" to a 420 f/5.6 lens is completely misleading.

yes this is true

Case 2: if your primary concern is reach, then a 300 f/4 lens is decidedly similar to a 480 f/4 lens on a 1.6 crop camera, not 480 f/5.6.

But you forget to mention that the depth of field is not actually that shallow, therefore not appearing to be an f/4 480mm lens.

Anyway, I can go on. But I think this "equivalence" talk is very confusing to beginners and causes more harm (confusion) than good (clarification).


I agee there is alot of confusion amoung people. The worst confusion is when people think the apparent greater DoF menas that thier lens is not gathering as much light.

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cameras: 5D, 50D, D60, R2K
lenses: 17-40 f/4, 24-105 f/4, 100-400 f/4.5-5.6
24 f/3.5 TSE, 35 f/2, 50 f/1.4, Tamron 90 f/2.8 Macro

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