F stop technical limits?

Started 2 months ago | Discussions thread
Leonard Migliore
Leonard Migliore Forum Pro • Posts: 17,682
Re: F stop technical limits?

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?

The faster you make a lens, the harder it is. Anyone can do an f/8 lens. Making a sharp f/2 prime requires a large number of elements because the light must be focused on the sensor over a wide range of angles. Making a sharp f/1.4 prime is drastically harder and, in fact, I don't know of any lenses that are sharp across the field at f/1.4.

Now if you want to make a zoom, you have to have good correction over a wide range of angles while the glass is moving around to change the focal length. This is very hard at f/2.8 and gets much worse as you get faster. The lens gets very bulky and expensive because it needs a huge number of elements for it to work. Eventually you'll design a lens that nobody wants to pay for or use.

The only hard physical limit is that you can't make a lens faster than f/0.5. But the practical problems of making a sharp fast lens are so severe that you can't do a whole lot better than f/1 (NASA got Zeiss to make some f/0.7 lenses but I'm sure they've got NASA prices).

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Leonard Migliore

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