What does the F value mean for a lens

**Lowell Goudge** · 10-30-2012, 01:00 PM

Originally posted by desertscape

This subject of focal ratios can get complicated, especially when you have a lens that doesn't fit the normal definition. In a reversed telephoto design (wide angle) one can have as an example, a 45mm wide lens (medium format) with an entrance pupil of 60mm. The focal ratio would appear to be f/.75. However, the lens is given an f/4 by Pentax. The reason for this slower designation is because of the design. Most of the front elements are negative and therefore cause the light rays to diverge. This causes much of the light to be lost into the lens baffling and not reach the film /sensor.

The point is, that there is a diameter restriction somewhere in the lens, that determines the maximum opening. I agree when going to wide angle practical lens designs have a reverse telephoto effect, but that does not change the fundamental principle. Focal length over diameter is F number. It is only a question of knowing the true optical diameter.

**TomB_tx** · 10-31-2012, 05:52 AM

Originally posted by Anvh

We should actually call it the F ratio instead of number, maybe idea to start that as of today?

I learned photography in the early 1960s, and never heard the term "f-number" - it was always "f-stop" or "aperture." The f-stop fits with the instructions to reduce exposure by closing the lens "1 or 2 stops" - back when lenses had aperture rings. Most lenses had "half-stops" or clicks between the marked f-stops. The Zeiss lenses I have for my Leica now have 1/3 stops between full stops.

The term "f number" appears to have come in with more automatic cameras. The Pentax LX manual refered to f-stops, while the ME Super manual mentions f number.

Now that most people use cameras without aperture rings, I guess the f-stop terminology doesn't relate to a tangible control the way it used to.

Anvh · 10-31-2012, 04:58 PM

Originally posted by Lowell Goudge

The point is, that there is a diameter restriction somewhere in the lens, that determines the maximum opening. I agree when going to wide angle practical lens designs have a reverse telephoto effect, but that does not change the fundamental principle. Focal length over diameter is F number. It is only a question of knowing the true optical diameter.

effective aperture is better then pupil entrance, that takes into account the difference in lens design.

Anvh · 10-31-2012, 05:06 PM

Originally posted by TomB_tx

I learned photography in the early 1960s, and never heard the term "f-number" - it was always "f-stop" or "aperture."

Aperture is something else then f-stop.
Aperture is really just the opening, where as the f-stop is a value.

Aperture off 4 does not exist but f/4 does for example and the aperture can also be 2 centimeter or an inch.

stops, numbers, ratio are all about values so not a real differences there.
I understand your story and i believe it's quite plausible

**Lowell Goudge** · 10-31-2012, 05:33 PM

Originally posted by Anvh

effective aperture is better then pupil entrance, that takes into account the difference in lens design.

Agreed for retrofocus lenses, i.e. those who's focal length is roughly less than the regestry distance,. but pupil diameter works just fine above 50mm

**Just1MoreDave** · 10-31-2012, 08:53 PM

Originally posted by cyclone3d

What lens/camera was used for those newspaper crops? How far away was the lense from the newspaper? How big an area was cropped? Even at f2.8, a good lens should be a whole lot crisper than that at the edges.

It's a K-7, 28mm, the corner (not the edge), about 9 feet away, and a 100% crop, so something like 125 pixels wide for each aperture. I am not 100% sure which lens that was, I just picked one with a nice progression from fuzzy to sharper. I definitely have lenses that are worse than that.

Anvh · 11-01-2012, 10:52 AM

Originally posted by Lowell Goudge

Agreed for retrofocus lenses, i.e. those who's focal length is roughly less than the regestry distance,. but pupil diameter works just fine above 50mm

Not too fast, just the pupil is not correct, we are talking about the entrance pupil. I'm not sure but with retro focus is then the exit pupil used?

**Lowell Goudge** · 11-01-2012, 11:56 AM

Originally posted by Anvh

Not too fast, just the pupil is not correct, we are talking about the entrance pupil. I'm not sure but with retro focus is then the exit pupil used?

you are misreading my comment. I explicitly stated that above 50mm (i.e. non retro focus lenses) the entrance pupil works fine.

I agree it may not apply in retrofocus lenses, but, when dealing with telephotos, in all 30 years of photography, I have NEVER seen a telephoto that had a bigger front element than was dictated by what was needed to get the specified maximum aperture..

note that there is simply no need to have a retrofocusing lens when the focal length is above the system regestry distance, and why would you place extra elements, all of which have transmission losses and contrast reducing flare (regardless of how good the coatings are) when you don't need it.

desertscape · 11-01-2012, 01:44 PM

Originally posted by Anvh

Not too fast, just the pupil is not correct, we are talking about the entrance pupil. I'm not sure but with retro focus is then the exit pupil used?

The speed of a retro focus/reverse telephoto is determined by its entrance pupil. When looking in the front of the lens, the aperture appears smaller than its physical size due to the demagnifying effect of the negative lens group in front of the diaphragm. This apparent size (entrance pupil) is used to divide into the focal length to determine the speed or focal ratio. This reduction in aperture size relates to what I said above, in that, light is lost due to the negative front group causing light to diverge.

Anvh · 11-02-2012, 10:27 AM

Originally posted by Lowell Goudge

you are misreading my comment. I explicitly stated that above 50mm (i.e. non retro focus lenses) the entrance pupil works fine.

I agree it may not apply in retrofocus lenses, but, when dealing with telephotos, in all 30 years of photography, I have NEVER seen a telephoto that had a bigger front element than was dictated by what was needed to get the specified maximum aperture..

note that there is simply no need to have a retrofocusing lens when the focal length is above the system regestry distance, and why would you place extra elements, all of which have transmission losses and contrast reducing flare (regardless of how good the coatings are) when you don't need it.

Nope you're misreading mine.

You're talking about the pupil but that's not fully correct, it must be the entrance pupil, slight but important difference.

Anvh · 11-02-2012, 10:28 AM

Originally posted by desertscape

The speed of a retro focus/reverse telephoto is determined by its entrance pupil. When looking in the front of the lens, the aperture appears smaller than its physical size due to the demagnifying effect of the negative lens group in front of the diaphragm. This apparent size (entrance pupil) is used to divide into the focal length to determine the speed or focal ratio. This reduction in aperture size relates to what I said above, in that, light is lost due to the negative front group causing light to diverge.

Thanks.

I was thinking that since it is a reverse design that the exit pupil might be used, seemed logical

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10-30-2012, 01:00 PM	#16
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,892	Originally posted by desertscape This subject of focal ratios can get complicated, especially when you have a lens that doesn't fit the normal definition. In a reversed telephoto design (wide angle) one can have as an example, a 45mm wide lens (medium format) with an entrance pupil of 60mm. The focal ratio would appear to be f/.75. However, the lens is given an f/4 by Pentax. The reason for this slower designation is because of the design. Most of the front elements are negative and therefore cause the light rays to diverge. This causes much of the light to be lost into the lens baffling and not reach the film /sensor. The point is, that there is a diameter restriction somewhere in the lens, that determines the maximum opening. I agree when going to wide angle practical lens designs have a reverse telephoto effect, but that does not change the fundamental principle. Focal length over diameter is F number. It is only a question of knowing the true optical diameter.

10-31-2012, 05:52 AM	#17
TomB_tx Loyal Site Supporter Join Date: Jun 2011 Location: Texas Photos: Gallery Posts: 1,446	Originally posted by Anvh We should actually call it the F ratio instead of number, maybe idea to start that as of today? I learned photography in the early 1960s, and never heard the term "f-number" - it was always "f-stop" or "aperture." The f-stop fits with the instructions to reduce exposure by closing the lens "1 or 2 stops" - back when lenses had aperture rings. Most lenses had "half-stops" or clicks between the marked f-stops. The Zeiss lenses I have for my Leica now have 1/3 stops between full stops. The term "f number" appears to have come in with more automatic cameras. The Pentax LX manual refered to f-stops, while the ME Super manual mentions f number. Now that most people use cameras without aperture rings, I guess the f-stop terminology doesn't relate to a tangible control the way it used to.

10-31-2012, 04:58 PM	#18
Anvh Veteran Member Join Date: Sep 2011 Posts: 4,616	Originally posted by Lowell Goudge The point is, that there is a diameter restriction somewhere in the lens, that determines the maximum opening. I agree when going to wide angle practical lens designs have a reverse telephoto effect, but that does not change the fundamental principle. Focal length over diameter is F number. It is only a question of knowing the true optical diameter. effective aperture is better then pupil entrance, that takes into account the difference in lens design.

10-31-2012, 05:06 PM	#19
Anvh Veteran Member Join Date: Sep 2011 Posts: 4,616	Originally posted by TomB_tx I learned photography in the early 1960s, and never heard the term "f-number" - it was always "f-stop" or "aperture." Aperture is something else then f-stop. Aperture is really just the opening, where as the f-stop is a value. Aperture off 4 does not exist but f/4 does for example and the aperture can also be 2 centimeter or an inch. stops, numbers, ratio are all about values so not a real differences there. I understand your story and i believe it's quite plausible

10-31-2012, 05:33 PM	#20
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,892	Originally posted by Anvh effective aperture is better then pupil entrance, that takes into account the difference in lens design. Agreed for retrofocus lenses, i.e. those who's focal length is roughly less than the regestry distance,. but pupil diameter works just fine above 50mm

10-31-2012, 08:53 PM	#21
Just1MoreDave Site Supporter Join Date: Feb 2007 Location: Aurora, CO Photos: Gallery \| Albums Posts: 6,340	Originally posted by cyclone3d What lens/camera was used for those newspaper crops? How far away was the lense from the newspaper? How big an area was cropped? Even at f2.8, a good lens should be a whole lot crisper than that at the edges. It's a K-7, 28mm, the corner (not the edge), about 9 feet away, and a 100% crop, so something like 125 pixels wide for each aperture. I am not 100% sure which lens that was, I just picked one with a nice progression from fuzzy to sharper. I definitely have lenses that are worse than that.

11-01-2012, 10:52 AM	#22
Anvh Veteran Member Join Date: Sep 2011 Posts: 4,616	Originally posted by Lowell Goudge Agreed for retrofocus lenses, i.e. those who's focal length is roughly less than the regestry distance,. but pupil diameter works just fine above 50mm Not too fast, just the pupil is not correct, we are talking about the entrance pupil. I'm not sure but with retro focus is then the exit pupil used?

11-01-2012, 11:56 AM	#23
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,892	Originally posted by Anvh Not too fast, just the pupil is not correct, we are talking about the entrance pupil. I'm not sure but with retro focus is then the exit pupil used? you are misreading my comment. I explicitly stated that above 50mm (i.e. non retro focus lenses) the entrance pupil works fine. I agree it may not apply in retrofocus lenses, but, when dealing with telephotos, in all 30 years of photography, I have NEVER seen a telephoto that had a bigger front element than was dictated by what was needed to get the specified maximum aperture.. note that there is simply no need to have a retrofocusing lens when the focal length is above the system regestry distance, and why would you place extra elements, all of which have transmission losses and contrast reducing flare (regardless of how good the coatings are) when you don't need it.

11-01-2012, 01:44 PM	#24
desertscape Pentaxian Join Date: Feb 2009 Location: Arizona Posts: 1,272	Originally posted by Anvh Not too fast, just the pupil is not correct, we are talking about the entrance pupil. I'm not sure but with retro focus is then the exit pupil used? The speed of a retro focus/reverse telephoto is determined by its entrance pupil. When looking in the front of the lens, the aperture appears smaller than its physical size due to the demagnifying effect of the negative lens group in front of the diaphragm. This apparent size (entrance pupil) is used to divide into the focal length to determine the speed or focal ratio. This reduction in aperture size relates to what I said above, in that, light is lost due to the negative front group causing light to diverge.

11-02-2012, 10:27 AM	#25
Anvh Veteran Member Join Date: Sep 2011 Posts: 4,616	Originally posted by Lowell Goudge you are misreading my comment. I explicitly stated that above 50mm (i.e. non retro focus lenses) the entrance pupil works fine. I agree it may not apply in retrofocus lenses, but, when dealing with telephotos, in all 30 years of photography, I have NEVER seen a telephoto that had a bigger front element than was dictated by what was needed to get the specified maximum aperture.. note that there is simply no need to have a retrofocusing lens when the focal length is above the system regestry distance, and why would you place extra elements, all of which have transmission losses and contrast reducing flare (regardless of how good the coatings are) when you don't need it. Nope you're misreading mine. You're talking about the pupil but that's not fully correct, it must be the entrance pupil, slight but important difference.

11-02-2012, 10:28 AM	#26
Anvh Veteran Member Join Date: Sep 2011 Posts: 4,616	Originally posted by desertscape The speed of a retro focus/reverse telephoto is determined by its entrance pupil. When looking in the front of the lens, the aperture appears smaller than its physical size due to the demagnifying effect of the negative lens group in front of the diaphragm. This apparent size (entrance pupil) is used to divide into the focal length to determine the speed or focal ratio. This reduction in aperture size relates to what I said above, in that, light is lost due to the negative front group causing light to diverge. Thanks. I was thinking that since it is a reverse design that the exit pupil might be used, seemed logical