f/stop and the aperture ratio

[wfooshee]

Aperture diameter has nothing to do with lens diameter. The diameter of the aperture actually is irrelevant as a measured number. Its importance is how it relates to the focal length of the lens. 200mm lens at f:4, the iris is 1/4 of 200, or 50mm. A 50mm at f:4, the iris is 1/4 of 50, or 12.5mm. But the actuall= diameter is irrelevant; what matters is that it's 1/4 of the lens's focal length. f:4 mean focal/4. f:16 means focal over 16. The ratio to focal length is how it's measured, not the diameter of anything.

Aperture diameter drives the lens diameter.
In your example, the objective lens of the 200mm f/4 lens has to be a minimum of 50mm in diameter.
Any smaller in diameter, and the aperture would be smaller than f/4.
Larger in diameter at the option of the lens designer.

The lens barrel, zoom and focusing rings would be larger in diameter than the objective lens.

I would describe it the other direction... Lens diameter dictates the available aperture. This is an economic decision by the manufacturer. You can have your 70-200 f:2.8 if you've got the bucks, but most will settle for f:4 or even f:5.6 for affordability.

My statement that the aperture diameter has nothing to do with the lens diameter was in response to the OP's sentence: "For example, common wisdom would tell me that an f/stop number of 8 or 13 will be the same diameter hole regardless of LENS DIAMETER. But why isn't that what actually happens in life?" I may have misunderstood his statement, but it appears that the OP was thinking that the f-number specified a physical size aperture, same size for any lens, when in fact it specifies that size as a ratio to the focal length, larger physical iris openings for longer lenses.

 

[Soocom1]

Aperture diameter has nothing to do with lens diameter. The diameter of the aperture actually is irrelevant as a measured number. Its importance is how it relates to the focal length of the lens. 200mm lens at f:4, the iris is 1/4 of 200, or 50mm. A 50mm at f:4, the iris is 1/4 of 50, or 12.5mm. But the actuall= diameter is irrelevant; what matters is that it's 1/4 of the lens's focal length. f:4 mean focal/4. f:16 means focal over 16. The ratio to focal length is how it's measured, not the diameter of anything.

Aperture diameter drives the lens diameter.
In your example, the objective lens of the 200mm f/4 lens has to be a minimum of 50mm in diameter.
Any smaller in diameter, and the aperture would be smaller than f/4.
Larger in diameter at the option of the lens designer.

The lens barrel, zoom and focusing rings would be larger in diameter than the objective lens.

I would describe it the other direction... Lens diameter dictates the available aperture. This is an economic decision by the manufacturer. You can have your 70-200 f:2.8 if you've got the bucks, but most will settle for f:4 or even f:5.6 for affordability.

My statement that the aperture diameter has nothing to do with the lens diameter was in response to the OP's sentence: "For example, common wisdom would tell me that an f/stop number of 8 or 13 will be the same diameter hole regardless of LENS DIAMETER. But why isn't that what actually happens in life?" I may have misunderstood his statement, but it appears that the OP was thinking that the f-number specified a physical size aperture, same size for any lens, when in fact it specifies that size as a ratio to the focal length, larger physical iris openings for longer lenses.

I am of that opinion as well.

But the real crux of this is the aspect of RATIO involved with the f number. Again I reference the function aspect. The 'f" is the final product of a load of numbers put into a mathematical equation.

 

[Sharpshooterr]

I saw this debate once before elsewhere and one of the things that came up and stuck in my mind is that with some lens makers /lenses is that the f stop is not a whole number or a single digit fraction eg f8/ 5.6 but the makers round up down to the nearest
I can’t cite the article so this just a memory recall so maybe wrong
Thoughts on this

Katami, you are not wrong, I’m surprised it hasn’t come up before this in a discussion of light levels being inconsistent across what should be similar if not the same lens specs. The PROBLEM is more prevalent in the less expensive lenses.
Manufacturers will flat out LIE about their lens spec since there is virtually no way to call them out on it.
For example I believe it was either the Tamron or Sigma 150-500 which was notoriously well known for being 20-30 mm short(or more)! A lens is made short to make it easier and cheaper to make the lens, for example an f8 at the long end. It might need another pound of glass and an added inch to make it a true 500 but that might cost another $500!
The same is true of the f stop. A company can cheat by a 1/2 or full stop because again a true f stop would cost considerably more. O a combination of both!!
I’m sure you’ve read this more than once. An OP will buy say a canon 7Dmkll which is supposed to AF to f8. So they get a 150-500 f5.6-f8 lens and them complain that the camera won’t actually AF. Well the camera doesn’t have the capability of handling that lens because though the lens says f8, it’s in reality an f9 or even an f10 because the company is cheating!
It’s not a matter of rounding down a 1/10th(no one rounds up!) it’s a matter of cheating quite a bit, enough that a lens just won’t AF at the requested spec.
Testers can only compare the field of view from one lens that‘s known to be pretty accurate to the other lens.
Like all standards, the ISO/DIN headquarters, probably in Geneva keep, in vaults, standard lenses to use as precise comparisons. I can just assume that all the lens companies have fabricated their own sets. Designing them mathematically can be I assume very close but maybe not exact.
You can’t use the length of a lens for the ratio. Physically measure a 500mm lens and it’s only probably 400mm long. That’s because by design, the reason they’re called “tele”photo lenses is because they have a telephoto lens group in them that allows the len’s actual physical length to be shorter than the specified length. A good case in point is a D.O. which has a completely different set of elements allowing them to be even shorter than the prescribed attributed length. Try using a ratio to opening on that one.
All of the above combined with different types of better light transmitting glass and figuring out f ratios would take a lens engineer.
No modern today lens is designed using a simple long lens design without tele optics.
So yes, f4 is f4 except when a company is cheating to create a cheaper easier to build lens. If they where rounding down just a 1/10th stop we’d never see it but there is definitely a big discrepancy when we can see a huge light level difference from lens to another lens that are supposed to have the same specs!
SS

 

[petrochemist]

I saw this debate once before elsewhere and one of the things that came up and stuck in my mind is that with some lens makers /lenses is that the f stop is not a whole number or a single digit fraction eg f8/ 5.6 but the makers round up down to the nearest
I can’t cite the article so this just a memory recall so maybe wrong
Thoughts on this

Katami, you are not wrong, I’m surprised it hasn’t come up before this in a discussion of light levels being inconsistent across what should be similar if not the same lens specs. The PROBLEM is more prevalent in the less expensive lenses.
Manufacturers will flat out LIE about their lens spec since there is virtually no way to call them out on it.
For example I believe it was either the Tamron or Sigma 150-500 which was notoriously well known for being 20-30 mm short(or more)! A lens is made short to make it easier and cheaper to make the lens, for example an f8 at the long end. It might need another pound of glass and an added inch to make it a true 500 but that might cost another $500!
The same is true of the f stop. A company can cheat by a 1/2 or full stop because again a true f stop would cost considerably more. O a combination of both!!
I’m sure you’ve read this more than once. An OP will buy say a canon 7Dmkll which is supposed to AF to f8. So they get a 150-500 f5.6-f8 lens and them complain that the camera won’t actually AF. Well the camera doesn’t have the capability of handling that lens because though the lens says f8, it’s in reality an f9 or even an f10 because the company is cheating!
It’s not a matter of rounding down a 1/10th(no one rounds up!) it’s a matter of cheating quite a bit, enough that a lens just won’t AF at the requested spec.
Testers can only compare the field of view from one lens that‘s known to be pretty accurate to the other lens.
Like all standards, the ISO/DIN headquarters, probably in Geneva keep, in vaults, standard lenses to use as precise comparisons. I can just assume that all the lens companies have fabricated their own sets. Designing them mathematically can be I assume very close but maybe not exact.
You can’t use the length of a lens for the ratio. Physically measure a 500mm lens and it’s only probably 400mm long. That’s because by design, the reason they’re called “tele”photo lenses is because they have a telephoto lens group in them that allows the len’s actual physical length to be shorter than the specified length. A good case in point is a D.O. which has a completely different set of elements allowing them to be even shorter than the prescribed attributed length. Try using a ratio to opening on that one.
All of the above combined with different types of better light transmitting glass and figuring out f ratios would take a lens engineer.
No modern today lens is designed using a simple long lens design without tele optics.
So yes, f4 is f4 except when a company is cheating to create a cheaper easier to build lens. If they where rounding down just a 1/10th stop we’d never see it but there is definitely a big discrepancy when we can see a huge light level difference from lens to another lens that are supposed to have the same specs!
SS

No lens will ever be exactly the aperture/focal length quoted, there's always some error & usually significant rounding going on too. Yes marketing departments for lenses will usually round specs to make the lens look better, but never by a whole stop. Some manufacturers are no doubt worse than others for this.
I rather suspect the AF issues & larger errors you quote are more down different T-stops rather than f-stops. Light transmission through a lens is not only affected by the aperture, the quality of anti reflection coatings, number of elements, & the glass used will all have an effect. The light transmission will vary depending on the light source used for measuring it, as reflection & absorptions are all affected by wavelength, but this is very rarely specified on photographic lenses anyway (It is used in cine/video).

ISO does not keep standard lenses, for reference, the required properties are referenced to physical dimensions i.e. the meter - which is now defined using multiples of the wavelength of a specific lightsource, the physical reference used 100 years ago is nowhere near accurate enough for some of todays uses.
All SI units are tied to three primary standards/definitions, length (in meters), time (seconds) & mass (kilograms)
Mathematical determinations of these is much more exact than just comparing to a physical reference, every time a mass reference is touched it can have atoms worn off & get contaminated via other materials. Even without touching it some change can occur from atoms getting knocked off by light...

Lens manufacturers will no doubt use carefully calculated instruments to measure parameters like focal length, but a fairly accurate determination can be done for single elements on a home built optical bench, even a bit of card & a ruler can get quite close (using clouds in the sky as a subject). Field of view can also be determined fairly accurately at home with a tripod & tape measure & this can be used to determine the focal length of more complex rectilinear lenses (fish-eye focal length are much more awkward). Once the focal length is known a simple Vernier caliper is sufficient for measuring the apparent aperture to high enough precision for practical use.

Yes the physical length of a lens has little relationship to it's focal length. My 40mm or 50mm SLR lenses are considerably shorter than most of my 10mm SLR ones (a factor of 2 to 3) several of my 500mm lenses are only about 1.5x the length of the 10mm ones. A few prime examples:

Samyang 10mm/2.8 (mid way in length between my 10+ zooms) is 77mm long,
Pentax M50/1.7 (roughly mid way among my many 50's) is 31mm long,
Panagor 500mm/8 is 120mm long.

 

[zombiesniper]

That's It!

I'm designing a new lens that just rates the max aperture in mm.
Of course I can't call it aperture since I don't want it to be confused with those OTHER lenses.
So I'll have to call it the lens orifice. Now you'll be forces to ask someone "How large is your orifice?"

 

[Soocom1]

The Illuminati made the specifications for camera lenses because they wanted to fool everyone with f stop numbers and make really bad photos while members of the Illuminati got lenses made in secret that have actual and real f stop numbers that are exactly the same from one camera and lens to the next.

And no you don't have the money to buy one because the Illuminati controls the distribution and market to only their rich elite photogenic members.

 

[petrochemist]

That's It!

I'm designing a new lens that just rates the max aperture in mm.
Of course I can't call it aperture since I don't want it to be confused with those OTHER lenses.
So I'll have to call it the lens orifice. Now you'll be forces to ask someone "How large is your orifice?"

Sounds a bit like one of my pin holes...

 

[Original katomi]

Re post 20
100 iso, s speed 250 O value.......
so therefore when the the image is over/under exposed you can honestly say
O, poo

 

[TWX]

The Illuminati made the specifications for camera lenses because they wanted to fool everyone with f stop numbers and make really bad photos while members of the Illuminati got lenses made in secret that have actual and real f stop numbers that are exactly the same from one camera and lens to the next.

And no you don't have the money to buy one because the Illuminati controls the distribution and market to only their rich elite photogenic members.

So the Illuminati use T-stops?

 

[star camera company]

Bell and Howel Foton, using Cooke lenses, uses the T stop system.