F stop technical limits?

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HighlandApe New Member • Posts: 13
F stop technical limits?

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.

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Leonard Migliore
Leonard Migliore Forum Pro • Posts: 17,702
Re: F stop technical limits?
1

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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sybersitizen Forum Pro • Posts: 15,235
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.

So, what's the reason?

Basically, the reason is cost. For most manufacturers and most buyers, the cost makes that impractical and unappealing compared to the return on investment.

alcelc
alcelc Forum Pro • Posts: 12,444
Panasonic has one latest
2

10~25 f/1.7...

which has a very short focal length of 20~50 eq AoV of FF only, but weight 690g, 128mm in length and has a diameter of 88mm.

Look at the 35~200 f/2.8, it weights 357g, 100mm long and has a diameter of 67mm.

Comparing the 2, a wide to nearly standard focal length lens vs a mid telephoto lens, that f/1.7 making it like an adult vs a child... Not to mention the f/1.7 cost US$1,800 vs US$900 for the f/2.8...

Can't imagine what a f/1.4 10~25 would be. Then a 35~100 f/1.4 of M43... If there is one it must be very expansive, beat every single purpose of the size advantage of using M43 system, and will be for the niche of the niche market only. Not a commercially sound idea I think.

Shooting with a crop system, certain compromise has to be expected in return for the size and weight. There is no free lunch.

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Albert

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threw the lens
threw the lens Senior Member • Posts: 1,984
Re: F stop technical limits?

Leonard Migliore wrote:

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).

It would only make it a more expensive sparkler when they send it diving into Saturn's atmosphere at the end of a mission when the fuel runs out.

Great Bustard Forum Pro • Posts: 43,962
Lower f-numbers bend light more...
1

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.

Lee Jay Forum Pro • Posts: 55,262
Re: Lower f-numbers bend light more...

Great Bustard wrote:

...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.

And the reason for that is (as I understand it), the properties of glass (refractive index, dispersion, etc.) do not scale.

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Lee Jay

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OP HighlandApe New Member • Posts: 13
Re: Lower f-numbers bend light more...

Many thanks all, I have learned something 👍🏻

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Gerry Winterbourne Forum Pro • Posts: 16,644
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.

All simple lenses suffer aberrations of several types. They are corrected by using several different simple lenses ("elements") in combination so that their effects cancel out https://www.lensrentals.com/blog/2010/10/the-seven-deadly-aberrations/

Aberrations are caused by the angle of the glass surface, which is steeper as one moves away from the axis of the lens. Thus, the wider the aperture the worse the aberrations; and the worse the aberrations the harder they are to correct: this typically entails (a) more elements and (b) special - and therefore expensive - formulations of glass. The result is that fast lenses are bigger, heavier and more expensive than slower ones.

For primes of around 50mm on full frame these effects are relatively small; but move away from that and the effects can be huge. Canon has two 300mm primes - the f/4 model weighs 1190g and costs £1269; the f/2.8 model (just 1 stop faster) weighs 2400g and costs £5559.

And zooms are more difficult to correct than primes so fast zooms just aren't produced.

The latter shouldn't be any wider than the current pro zooms and, I assume, shorter if not lighter.

Look at the first diagram in the above link. The aperture diaphragm is towards the back of the lens and is pretty small; the size of the lens is controlled by the front element. That particular lens has a very big front element; they aren't all so big but it's the front element that gathers the light so you can't just look at the f-numbers.

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?

See above.

Interested to hear if anyone knows.

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Gerry
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xtominus
xtominus Junior Member • Posts: 36
Re: F stop technical limits?

F-stop is a physical measurement, so to reach certain F-stop. the lens has to be physically big. And that's not a business profitable product to make for camera/lens company.

If you are talking about light sensing ability or bokehlish, look at the google pixel phone, they are superb at artificial light processing and bokeh.

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BJL Veteran Member • Posts: 9,339
F stop technical limits? there are 30x constant f/1.7 zooms, at a price
2

There are commercial video "box lenses" from Canona and Panasonic that offer constant f/1.7 over a zoom range of about 30x (and then a bit more zoom at the long end with f-stop increasing rapidly); some reach 350mm (true focal length) at f/1.7. The latest are for the new 1.25" format 8K video cameras (about 2 micron pixel pitch) so resolution must be fairly good.

They weigh about 50Kg and cost about $200,000.

So the limits are mostly cost and weight.

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Smaller lenses, better in low light, more telephoto reach:
you can have any _two_ at one time.

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Dennis Forum Pro • Posts: 19,332
Re: F stop technical limits?
2

HighlandApe wrote:

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.

Even if you could do that theoretically, what's the market ? How many people are going to want to spend the money and carry the weight just to put equivalent lenses in front of a smaller sensor ?

The selling point of smaller sensors is smaller cameras. People who want both (big/capable and compact) are generally content to buy both. Instead of a full frame challenger (that isn't) you use full frame and when you want to shoot something compact, you shoot something compact. You're probably buying the same stuff anyway, because you'd want a compact body for the small lenses and a bigger body to handle the bigger lenses.

When the EM1X was announced, a lot of people expressed their opinions that Olympus was heading in the wrong direction and that focusing on smaller, lighter systems was the right path (and the reason m43 buyers bought into the system in the first place).

So whether or not there are technical limits, my guess is there's far too small a market to flesh out a lineup of big, fast FF equivalents. (Same applies to APS-C ... if you're going to bother with the weight and expense of f/2 zooms, just get f/2.8 zooms for FF).

- Dennis
--
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OP HighlandApe New Member • Posts: 13
Re: F stop technical limits?

Dennis wrote:

HighlandApe wrote:

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.

Even if you could do that theoretically, what's the market ? How many people are going to want to spend the money and carry the weight just to put equivalent lenses in front of a smaller sensor ?

The selling point of smaller sensors is smaller cameras. People who want both (big/capable and compact) are generally content to buy both. Instead of a full frame challenger (that isn't) you use full frame and when you want to shoot something compact, you shoot something compact. You're probably buying the same stuff anyway, because you'd want a compact body for the small lenses and a bigger body to handle the bigger lenses.

When the EM1X was announced, a lot of people expressed their opinions that Olympus was heading in the wrong direction and that focusing on smaller, lighter systems was the right path (and the reason m43 buyers bought into the system in the first place).

So whether or not there are technical limits, my guess is there's far too small a market to flesh out a lineup of big, fast FF equivalents. (Same applies to APS-C ... if you're going to bother with the weight and expense of f/2 zooms, just get f/2.8 zooms for FF).

- Dennis
--
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Versatility. 
One system or even body with add on grips does it all. Small lenses for travel, street and family events In daylight. Big beasts for safari or if you are a pro that wants scaleability. No need to memorise different control layouts or other peculiarities of different bodies. Or even buying different bodies if like most you can only justify/afford the one. Just add the accessories you need to deal with the task at hand

Were it possible to have that sort of flexibility and decouple the sensor size from low light abilities there’s a market. Especially for Olympus who clearly would gain by expanding their market. 
As nobody has tried it there had to be a good reason and the forum has given me several so thanks again. 
My original question wasn’t about commercial viability though, just to understand the art of the possible. Even if it’s five figure sums for small sensor video superzooms that probably weigh more that all of the photo kit I’ve ever owned!

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fferreres Veteran Member • Posts: 3,778
Re: F stop technical limits?
1

Leonard Migliore wrote:

The only hard physical limit is that you can't make a lens faster than f/0.5.

f0.5 in air, but could be faster if submerged in oil or the sensor was adhered to the rear element. However, when people say one can make it as fast as one wants, generally, there's the hard limit posed by refraction indexes.

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