What is effective focal length after cropping? SAT-like question.

[Daniel Browning]

Back to the post - why is the f-stop jumped to 16.4?

Same reason the focal length "jumped" to 1170mm. Which is to say, because it gives the "equivalent" for just about everything that matters.

the f-stop doesn't jump when we take our FF lens and stick on a crop sensor? What am I missing?

Well, the focal length doesn't jump when take our FF lens and stick it on a crop sensor either. But the *equivalent* focal length does.

In the same way, the f-number doesn't change with sensor size, but the *equivalent* f-number does.

That's why I say that 200mm, f/2.8, 1/60, ISO 100 on a digicam is equivalent to 1170mm, f/16.4, 1/60, ISO 5800 on FF35. But those are obviously different exposures, so how can they be equivalent? Let me answer that question with another question: why does exposure matter? If you think of all the ways that exposure matters, you'll find that in almost every case it *does* come out equivalent. Brightness? The ISO setting takes care of that. Noise? No problem. Given equal sensor performance, ISO 5800 on FF35 is the exact same as ISO 100 on a digicam.

Ahhhhh I get it now.... I can see the point as it relates to the pin hole effect (diffraction?) and DOF items would be in the same process.

Precisely.

Would light gathering be a separate process. i.e. it is still a f-x.x from a light gather standpoint same number of photons striking the same surface area - just a tiny surface area? I know that each pixel would be getting fewer as the density is higher.

Basically, the total number of photons falling on sensor area is the same between f/16.4 on FF35 and f/2.8 on the digicam. The only difference is that one has a strong intensity of light on a small area and the other is a low intensity of light over a large area.

It might help you to think of both sensors (large and small) as having the same pixel count (e.g. 1 MP), so that the large sensor has large pixels and the small sensor has small pixels. (Such circumstances are not actually required, but it helps to make things easier to think about.)

Hope that helps.