f/22 & f/32?

[Souljer]

Hi,

These might be simple (dumb) questions. Please help cure my ignorance.
I got an Elmarit 135mm f/2.8 a few weeks ago and I noticed that it has f-stops smaller than I've ever seen on a lens.

• Is there any special reason for these f-stops? A special purpose like landscapes, etc. for expanding the depth-of-field?
• Are these super small stops to balance the relatively slow 2.8? As in, I might be using faster film to compensate, but in daylight I need to stop down.
• Are these small f-stops somehow connected with the longer focal length? Due to the physics or design of longer lenses? I've never had a lens this long or longer before.
• Why don't other lenses (35mm, 50mm, etc.) have these smaller f-stops?

No doubt this will be a short thread.
Thanks in advance for any insight.

 

[Richard G]

Yes, in part it's the focal length, and in part the practicalities of fitting an aperture system with only so many positions. So far as diffraction is concerned, f22 is a wider diameter hole in your lens than f22 in a shorter focal length lens, and so not as prone to diffraction problems. Having said that, my widest lens, 18mm, still has f22. It may be that diffraction problems are greater in the intermediate focal lengths for f22, as none of my three 50s has smaller than f16, and all my lenses from 28 and wider do have f22.

 

[peterm1]

I take it you mean its maximum f stop (ie the smallest sized aperture) goes own to f32. An f stop basically measures how much light is hitting the film plane / sensor. So the same f stop in a longer focal length will have a physically larger hole size than it would in a shorter focal length lens. If you compare say f5.6 on your 135mm lens it will be physically much larger than f5.6 would be on a 35mm lens. (In a longer focal length lens it is further away from the film plane so it has to be physically larger in order for the same amount of light to be hitting the sensor / film at this f stop.)

As f stops get smaller in size (i.e. the numbers get bigger) diffraction kicks in and image quality is degraded. Usually in lenses of 35mm or 50mm focal length for example you begin to see this effect at f11 or f16.

But this is mainly related to the physical size of the aperture through which the light is passing.
Because on the same reasoning as above f32 is physically much larger on a 135mm lens than it would be on a shorter focal length lens, then its less affected by diffraction.

As a result of all of the above, lenses which have a shorter focal length tend not to get built with f stops like f32 - if they did have that f stop the chances are the result would be awful when that f stop was used as diffraction would degrade the image quality too much.

I can only presume that the Leica engineers calculated that f32 on this lens would be physically big enough so it would not suffer from image degradation from this source so they could afford to include it in this lens (but not others of shorter focal length).

Ansell Adams who used large format cameras routinely used f stops like f 64 (or even smaller I think) which would be impossible to do on a smaller format camera due to the degradation the image would suffer when this f stop was used on a lens designed for a smaller camera.

 

[Richard G]

No doubt this will be a short thread.

I bet you're wrong.

 

[EdwardKaraa]

Yes, in part it's the focal length, and in part the practicalities of fitting an aperture system with only so many positions. So far as diffraction is concerned, f22 is a wider diameter hole in your lens than f22 in a shorter focal length lens, and so not as prone to diffraction problems. Having said that, my widest lens, 18mm, still has f22. It may be that diffraction problems are greater in the intermediate focal lengths for f22, as none of my three 50s has smaller than f16, and all my lenses from 28 and wider do have f22.

I actually agree with what you and Peter said. Just to add, the reason why some lenses like the 50s don't have the smallest apertures is because of their widest apertures, as they're usually f/1.4 or f/2. There is physically a limited number of apertures that you can include, so having an extra stop on the wide side usually means one stop less on the small side. Of course this doesn't apply to lenses with electronic diaphragm control.

 

[Richard G]

I then did a little reading and it is a whole lot more complicated. Ed, my little f2.8 Elmar also only goes to f16. It turns out according to one thing I read that the actual length of diverging rays in the OP's longer lens and therefore to a wider circle on the film/snesor cancels out the advantages of the increased size of the f22 aperture in a longer lens. I found an online calculator for different formats, and basically f22 is the limit of diffraction acceptable apertures for 35mm full frame with the calculator taking no account of the focal length as it is deemed irrelevant.

 

[hlockwood]

While it's obvious to many of us, designation of numerical aperture as, say, f16 is more properly written as f/16. This is not a quibble; it's a formula. It means that the diameter of the aperture is given by the focal length, f, divided by 16. So, a 50 mm lens at f/16 has an aperture diameter of 3.125 mm, while the same "aperture" on a 135 mm lens is 135/16=8.44 mm.

Diffraction effects limit the resolving power of a lens (in any optical instrument) in a complex manner and (ignoring aberrations) can contribute to fuzzy edges in an image. The resolving power of the lens (at a given aperture) is inversely proportional to the diameter. Thus, edge fringing or fuzzyness will be more apparent with a shorter rather than a longer focal length lens at a given numerical aperture.

For the brave of heart, see Raleigh Criterion.

HFL

 

[Rodchenko]

I have seen a lens with f/64.

 

[teleparallel]

The F number, as said before is the relative aperture to for a given lens

F = f/d

Where "f" is the focal length of the given lens, and "d" is the diameter of the iris of the aperture.

Aperture can be made as small as like, but usually don't go higher than F/22, since diffraction effect's affect to much the final image quality.

Diffraction is an effect that depends on the "d" value and not the F by itself. So longer focal lengths are more prone to diffraction effect's.

So, longer focal length lenses more commonly to go up to higher F numbers, since it's easier to be made, and diffractions is less problematic.

I had an F/45 sigma lens once.

 

[Roger Hicks]

The smaller the format, the more it will be enlarged, and the more important diffraction limitations will be. For a VERY rough guide, divide the aperture into 1500 to get the maximum useful resolution in lp/mm on the film or sensor. At f/128 (not too rare on lenses for 10x8 inch) you get about 12 lp/mm -- more than enough for a contact print to look very sharp indeed. Enlarge the same resolution from 35mm, however...

Cheers,

R.

 

[Robert Lai]

The Tominon 105mm lens on my Polaroid 180 goes down to f/90. I've never used that setting, but it is there.

For 35mm, f/32 is about the smallest that I've seen. It's there on my Micro Nikkor 70-180 and on the 300mm f/4.

 

[Dwig]

The Tominon 105mm lens on my Polaroid 180 goes down to f/90. I've never used that setting, but it is there. ....

Lenses for Polaroid equipment often have such small f/stops. This is solely because Polaroid's common B&W film was ASA 3000. The small aperture was needed when shooting outdoors in full sunlight without an ND filter.

As others have correctly stated, f/stops are written the way they are because it is a mathmatical formula. They are most often written in fraction notation as f/n where "n" is some number and "f" stands for "focal length" and should be written in lower case. At times it is written as the "focal ratio", a technical term see often when discussing telescopes, using ratio notation. With camera lenses this is usually seen on the barrel engravings along with the focal length (e.g. "50mm 1:1.4 on a 50mm f/1.4 lens).

One as yet unmentioned reason that many lenses began loosing their smaller apertures back in the '60s was the advent of auto-stopdown iris mechinisms in SLRs and meter coupling. The mechanical and electrical mechanisms needed for coupled metering present a greater limitation to the possible range than iris construction does. This is seen in Nikon's Ai coupling where older 105 f/2.5s which originally had a minimum aperture of f/32 would only stop to f/22 after conversion.

 

[Spavinaw]

I know I brought this up several years ago, but since no one had an answer and we are on the general subject of small f-stops I'm going to bring it up again. I have a Vivitar 3X Tele-Converter. It has a built-in calculator. You set it to the same f-stop that your lens is set to, and it shows you the actual f-stop you are shooting at (it calculates a three stop smaller aperture). Anyway, the last of the scale for the actual aperture runs like this: 22, 32, 45, 65. WTF---65? Surely Vivitar knows better. A typo? I've seen it on other converters besides mine. Not meaning to hijack the thread (of course I'm hijacking the thread, it's just what other people say when they won't admit it. Sorry 'bout that). Well, (a deep subject) maybe if someone can explain f/65 or has a Vivitar that says 64, they will post a teeny, tiny reply and we can get back on the original topic Souljer asked about. Anyone?

 

[Souljer]

Hi,

Looking at Wikipedia's page about f-stops shows this scale among many (many used at different times or for different applications).

I see a '64'.
Why do you think that is incorrect or a typo on your scale or lens?


Click image to enlarge.

By the way thanks everyone.
I'm getting a lot out of this. Hope this is helping others too.

 

[Spavinaw]

Souljer--when I click on the scale you posted I see 64 right between 45 and 90. I don't see 65 anywhere on the scale. Anyway, the f-stop scale is based on the number two and the square root of two. The half of the scale based on two is 1, 2, 4, 8, 16, 32, 64, 128, etc. The other half of the scale is 1.4, 2.8, 5.6, 11, 22, 45, 90, etc. This latter scale involves some rounding and shifting of numbers; however, the former scale, based on two, is a prefect progression and 64 is part of that progression. Peterm1 in his post above referred to f 64 and that's what it's supposed to be.

 

[Souljer]

Hi,

Oh, I see what you mean now. Apologies.

So you are saying that the number should be 64 and not 65 and all these scales and lenses are incorrect?

That's interesting.
I wonder what's going on.

 

[Dwig]

... Anyway, the f-stop scale is based on the number two and the square root of two. ...

That's a bit of a simplification.

The light instensity, NOT QUANTITY as many have said, is based on the area of the aperture. Doubling or halving the area results in a one EV change, equivalent of doubling or halving the exposure time. Since the area of a circle, the typical shape of the aperture, is related to the diameter throught the formula area = pi * radius^2 and since diameter is merely radius *2 it all boils down to a progression of 1 EV spaced f/stops where each smaller aperture is equal to the previous f/stop multiplied by the square root of 2, or approximately 1.41421356237. Hence, f/1.0 f/1.4, f/2.0 f/2.8, ... f/64.0 ... .

A two stop increment is thus a sequence where each is exactly double the previous. I you start at f/1 then f/64.00000000 is the value in question. f/65 would be insignificant lysmaller, photographically. Given that accuracy (read: inaccuracy!) of the 3x extenders actual magnification any attempt at great precision in calculating the 3-stop shift is futile.

 

[Spavinaw]

Of course if you wanted to get down to nitty, gritty accuracy, you would want your lens to be calibrated in T-stops like the Bell & Howell Foton. Anybody want to tackle that ball of wax?

 

[tunalegs]

Yes, in part it's the focal length, and in part the practicalities of fitting an aperture system with only so many positions. So far as diffraction is concerned, f22 is a wider diameter hole in your lens than f22 in a shorter focal length lens, and so not as prone to diffraction problems. Having said that, my widest lens, 18mm, still has f22. It may be that diffraction problems are greater in the intermediate focal lengths for f22, as none of my three 50s has smaller than f16, and all my lenses from 28 and wider do have f22.

Diffraction will be practically the same regardless of the focal length. Even though the aperture at f22 is much larger on say a 400mm lens than it is on a 50mm lens, because of the angle of view the diffraction remains the same.

Super tiny f-stops are acceptable in large format because the negative is enlarged less for a given print size, therefore the effect of diffraction is not magnified to the degree it would be for a 35mm negative.

 

[Landshark]

The calculator on the tele converter is giving you values for the amount of light lost by the converter. Not by actual aperture isn't it? The converter itself doesn't have a variable aperture.