inane prattle

Started Apr 8, 2013 | Discussions thread
Anders W Forum Pro • Posts: 21,468
Re: That statement is wrong for a different reason...

Paul De Bra wrote:

It is true that sensors have become better and IBIS is very good as well. But there is another reason for still wanting faster lenses:

- Typically a lens is at its best when stopped down at least 1 stop but in most cases 2 stops.

That's true as a purely descriptive empirical generalization, although if we are talking FF lenses rather than MFT lenses, you'd have to add another stop or two. With such lenses, at least if they are pretty fast, it is not unusual that you have to stop down three or even four stops before the optimum is reached.

But why is it that you have to stop down to reach optimal performance? Is it because it is impossible to design an f/4-lens that is just as good wide open at f/4 as an f/2-lens stopped down two stops? I think not. Rather, I think it is because it is easier to keep optical aberrations at bay at f/4 than at f/2.

Personally, I'd much rather have f/2-lenses that peak at f/4 than f/4-lenses that peak wide open even if the performance from f/4 on is just the same. But there is no doubt in my mind that the f/4-lenses we are talking about could be built.

- With the smaller m43 sensor (relative to 35mm) diffraction sets in earlier. The optimal f-stop for m43 seems to be f/4, with f/5.6 still quite usable but f/8 starting to show diffraction.

These two factors combined mean that unless a lens is already at its best wide open you need at least an f/2.8 lens (1 stop down means f/4 and 2 stops down means f/5.6 so that just does it) and that f/2 is actually a safer bet (with 2 stops down reaching the optimal f/4 stop).

So the only really valid argument that says we don't need faster lenses (except for shallow DoF) would say that we don't need lenses faster than f/2.

If you would translate this story to APS-C it would say that f/4 lenses are acceptable and f/2.8 is better, and for 35mm f/5.6 is acceptable and f/4 is better.

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