A few words on Equivalence and comparing systems

Started Apr 2, 2013 | Discussions thread
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slimandy Forum Pro • Posts: 16,736
Re: I'm just glad I picked a system with a 2x crop factor...

slimandy wrote:

The actual aperture does not change but neither does the actual focal length. If you double the focal length to get a equivalent focal length you need to double the aperture to get the equivalent aperture. I think that should be simple too. Obviously it isn't.

Oops! Having said it should be simple I have over simplified it. If you doubled the aperture from f2.8 you'd get f4 but as we're are talking area rather than length we have to square it, in other words we have to double it again, hence f5.6.

The calculation for 'relative' aperture (which is what we really mean) stays the same.

Focal length / physical aperture = relative aperture.

100mm/36mm = f2.8

200mm/36mm =f5.6.

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