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Travel / walk around lens for my EOS 600D

Started Oct 1, 2019 | Discussions thread
Steve Balcombe Forum Pro • Posts: 15,582
Re: F4.0 is too slow for indoor
1

Rexgig0 wrote:

Wave forest wrote:

The point is that F4.0 at 600D is too slow. It is equivalent to F5.6 at a full-frame camera.

OK, I pruned it down to the above two sentences. This is the part I would like to ask about, please. How do you know this to be true? I am questioning the math/science.

A full explanation of equivalence would be a very long post, and not necessary because there are numerous online resources. Closest to hand, and from a trusted source, is https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care

But the conclusion is very simple to state - in terms of both total light collected (which is fundamental to image quality) and depth of field control, a 1.6x crop sensor requires a lens which is approximately 1 1/3 stops faster to get the same result as full frame. That's in conjunction with the better known and generally better understood focal length equivalence.

So an f/4 lens on a 600D gives the same performance (in those respects) as an f/6.3 lens on full frame. People often round the difference to one stop, which is where the f/5.6 figure comes from, but it's actually slightly worse than that.

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