F stop technical limits?

Started 2 months ago | Discussions thread
Great Bustard Forum Pro • Posts: 43,932
Lower f-numbers bend light more...

HighlandApe wrote:

Having had cameras from compact to full frame (via 1", micro 4/3 and APS-C) I've always wondered why you don't see very fast zooms in any format.

For example, a 70-200 2.8 should have the same objective lens diameter as a hypothetical 35-100 1.4 for micro 4/3. The latter shouldn't be any wider than the current pro zooms and, I assume, shorter if not lighter.

If you could make something like that (and humour me please and not get into equivalence arguments or why one sensor size is better than others) one could theoretically produce a compact, small sensor system that allowed for very compact lenses for casual daylight use and full frame challengers at the other if you wanted to carry the weight.

Now, nobody has done this as far as I can see. Sigma has a couple of f1.8 zooms for APS-C but they have a shorter range than the full frame equivalents and there's a reason they did that. Olympus used to produce f2 zooms for 4/3 but seem to have given up and never went faster anyway. The reason can't be manufacturers wanting to up-sell to more expensive, bigger sensor cameras as Olympus never had any skin in that game but did and does produce expensive, quality glass and would logically want to make micro 4/3 as attractive as possible to as many as possible. Ditto Fuji, no interest in full frame but sell quality APS-C glass and cameras, don't sell f2 zooms - although they are working on a f1 prime I understand.

So, what's the reason? Is it physically very difficult/impossible to design zooms (or indeed primes) beyond a certain f number whatever the size of lens or sensor? Is that 35-100 f1.4 impossible to make due to some hard limit on the ratio of focal length to entrance pupil irrespective of size?

Interested to hear if anyone knows.

...and it becomes exponentially more difficult to control aberrations as the light is bent more and more as the physical limit of f/0.5 is approached, as more lens elements are added to apply the necessary corrections, resulting in a larger, heavier, and more expensive lens.

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