A few words on Equivalence and comparing systems

Started Apr 2, 2013 | Discussions thread
slimandy Forum Pro • Posts: 17,071
Re: A few words on Equivalence and comparing systems

Alumna Gorp wrote:

Macx wrote:

I understand the point of thinking about lenses in focal lengths and focal ratios, but for the purpose of comparing lenses for one system with lenses for another system, it would be much better to talk about what angle of view (or field of view) the lens is providing and the physical size of the aperture. It really cuts through to what is important with regards to framing, perspective, depth of field, diffraction and light, and the argument that "Angle of view = angle of view" and "Physical size of the aperture = physical size of the aperture" are easy to grasp.

Two systems using lenses that each provide the same angle of view and with the same physical size of the aperture are equivalent.

And I think that as we're finally moving towards a new paradigm of what the goal of exposure is in digital photography, the old idea about "f/2.8 is f/2.8" might also be slowly on the way out.

Why would or should the thoughts that a 2.8 is a 2.8 be on the way out, its always`s been this way, take medium format v 35mm there both very different but I can`t say I have ever come across anyone who has concerns over dof, its just something you live with or get over

For me a 2.8 is a 2.8, it always will be, dof may be different that`s for sure, you adapt, adjust and get on with it, its not a problem for most of us in the real world of things.

But calling a 2.8 a 5.6 or what ever is being a bit over the top and makes no senses at all.

It makes perfect sense. 100mm f2.8 has an opening (actual aperture) of 36mm. 100mm/36mm=f2.8

So if you want the equivalent of FF it would be 200mm/36mm=f5.6.

And as the same maths is borne out in terms of DOF it makes sense whichever way you look at it.

You can insist f2.8 is still f2.8, but only if you accept that 100mm is still 100mm.

100mm f2.8 is the actual focal length and relative aperture. The nearest equivalent to FF is 200mm f5.6. It makes no sense to mix the two.

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