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#1
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Apertures and focal length
Maybe this is a silly question, but I'll ask anyways.
Suppose I have a shutter from a 210mm Trashagon with accurate aperture markings. If I instead screw in the elements of a 210mm Expensivegon will the same markings apply? In other words, is the physical aperture opening the same for all lenses of the same focal length? I _think_ it should be... TIA! Steve |
#2
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Apertures and focal length
Suppose I have a shutter from a 210mm Trashagon with accurate
aperture markings. If I instead screw in the elements of a 210mm Expensivegon will the same markings apply? In other words, is the physical aperture opening the same for all lenses of the same focal length? I _think_ it should be... -------------------------------------------------------------------------- ------------------------------------------------ Not so. If you had two one-element lenses made out of the same glass and of the same focal length, it would be true. But modern camera lenses are composed of many different elements of different glass types, each with its own light-transmission peculiarities. The physical size of the iris at any particular f-stop value would likely be different from one lens type to another, even if their focal lengths were identical. I'm sure there are many other considerations apart from glass type that enter into the equation. I'm no optical expert but I know from experience that the apertures will vary. Bob G |
#3
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Apertures and focal length
The physical size of the iris at any particular f-stop value would likely be
different from one lens type to another, even if their focal lengths were identical. The above statement is NOT true of f/stops. The old T/stop was an effort to factor in the light transmission of a given lens design, but the f/stop is defined as the simple ratio of the 'effective aperture' size:focal length. The 'effective aperture' measures the diameter of the beam of light entering the aperture and must be measured. But the effective aperture has nothing to do with the light transmission of the lens. The variability of light transmission in different lenses was minimized via coatings, but critical exposure differences are not eliminated entirely with the f/stop system. --Wilt |
#4
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Apertures and focal length
"Stephan Goldstein" wrote in message ... Maybe this is a silly question, but I'll ask anyways. Suppose I have a shutter from a 210mm Trashagon with accurate aperture markings. If I instead screw in the elements of a 210mm Expensivegon will the same markings apply? In other words, is the physical aperture opening the same for all lenses of the same focal length? I _think_ it should be... TIA! Steve The _effective_ size of the stop is affected by the magnification of the lenses in front of it. The calibrations are for this effective size, not the physical size of the hole. The image of the physical stop as seen from the front of the lens is called the Entrance Pupil. In general, its size and position will be different from the physical size and position of the stop. An example is the speed of a single element of a convertible lens like a Dagor. When the cell is located at the rear of the stop (its optimum position) the speed is about f/13 and the entrance pupil is at the stop. The _effective_ size of the stop is its physical size and position its physical position because there is no glass in front of it to either magnify or reduce it. When the half-Dagor is placed in front of the stop the speed increases to about f/12, not a big difference, but different. The reason is that the stop is now magnified by the lens. The location of the stop will also change. Since the half lens is positive in power it will make the stop seem closer to the front than the physical position. The effect on the stop will be opposite where the front cell is negative in power, as it is in a Tessar type lens. There, the entrance pupil will be smaller than the stop and seem to be behind it. The size of the entrance pupil can be measured by placing a small light source exactly at the focal plane of the lens. The focal plane is where it focuses light from an infinit source. The exact infinity focus position can be found by autocollimating. To do this you need a card with a small hole in it and a small light, like a penlight. You also need a flat mirror to place over the lens. You can do this in a view camera. First, place the mirror over the lens, a shaving mirror will do but make sure the flat side is against the lens. Place the card behind the lens and focus the reflected image of the lighted hole on the card. You will have to adjust it so that the image does not fall exactly on top of the light of course. The lens is now focused exactly at infinty. Now, place a translucent screen over the lens, thin paper will do. The circle of light projected onto the paper is the outline of the entrance pupil. If you measure its diameter you will have the diameter of the _effective_ stop. If the front cell doesn't have a lot of power this diameter will not be much different than the physical size of the stop, but, for some lenses it can be quite different. The above method works well when a new stop scale must be made for a lens. Its not even necessary to remove the cells for this. If you want to measure the location of the entrance pupil use a camera that can focus close, the closer the better. First, focus the camera on a reference surface, the rim of the cell is a good place. Now, move the whole camera until the stop, as seen through the lens, is in focus. The distance the camera moved is the distance from the rim of the cell (or whatever reference surface you used) to the entrance pupil. The location of the entrance pupil is sometimes useful, for instance, it is the correct point to rotate a camera to take panoramic pictures. Do you still think the question is silly? :-) -- --- Richard Knoppow Los Angeles, CA, USA |
#5
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Apertures and focal length
Not so. If you had two one-element lenses made out of the same glass and
of the same focal length, it would be true. But modern camera lenses are composed of many different elements of different glass types, each with its own light-transmission peculiarities. The physical size of the iris at any particular f-stop value would likely be different from one lens type to another, even if their focal lengths were identical. I don't think this is correct. Lens apertures represent the diameter of the lens opening expressed as a fraction of the focal length. So I would think that at any given f stop the physical aperture size in two lenses of the same focal length should be the same regardless of the number of elements or the type of glass used. I think it's simply a mathematical calculation made by measuring the diameter of the aperture and focal length of the lens and then expressing the result as a fraction. But I could be wrong, I'm not an optical expert and some of the people who participate here are so they can correct me if necessary. In any event, lenses marked as a given focal length often, maybe always, aren't acutally that exact focal length. One lens sold as a 150mm lens might actually by 148mm and another might actually be 153mm, and in that case the physical size of the apertures at the same f stop presumably would be slightly different. "Bob G" wrote in message ... Suppose I have a shutter from a 210mm Trashagon with accurate aperture markings. If I instead screw in the elements of a 210mm Expensivegon will the same markings apply? In other words, is the physical aperture opening the same for all lenses of the same focal length? I _think_ it should be... -------------------------------------------------------------------------- ------------------------------------------------ Not so. If you had two one-element lenses made out of the same glass and of the same focal length, it would be true. But modern camera lenses are composed of many different elements of different glass types, each with its own light-transmission peculiarities. The physical size of the iris at any particular f-stop value would likely be different from one lens type to another, even if their focal lengths were identical. I'm sure there are many other considerations apart from glass type that enter into the equation. I'm no optical expert but I know from experience that the apertures will vary. Bob G |
#6
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Apertures and focal length
Perhaps I misunderstood the question but I thought he was simply asking
whether at F2.8, for example, the physical size of the aperture in lenses of the same focal length will be the same. I thought yes, you seem to be saying no but I'm not sure. In other words, have I been corrected? : - ) "Richard Knoppow" wrote in message link.net... "Stephan Goldstein" wrote in message ... Maybe this is a silly question, but I'll ask anyways. Suppose I have a shutter from a 210mm Trashagon with accurate aperture markings. If I instead screw in the elements of a 210mm Expensivegon will the same markings apply? In other words, is the physical aperture opening the same for all lenses of the same focal length? I _think_ it should be... TIA! Steve The _effective_ size of the stop is affected by the magnification of the lenses in front of it. The calibrations are for this effective size, not the physical size of the hole. The image of the physical stop as seen from the front of the lens is called the Entrance Pupil. In general, its size and position will be different from the physical size and position of the stop. An example is the speed of a single element of a convertible lens like a Dagor. When the cell is located at the rear of the stop (its optimum position) the speed is about f/13 and the entrance pupil is at the stop. The _effective_ size of the stop is its physical size and position its physical position because there is no glass in front of it to either magnify or reduce it. When the half-Dagor is placed in front of the stop the speed increases to about f/12, not a big difference, but different. The reason is that the stop is now magnified by the lens. The location of the stop will also change. Since the half lens is positive in power it will make the stop seem closer to the front than the physical position. The effect on the stop will be opposite where the front cell is negative in power, as it is in a Tessar type lens. There, the entrance pupil will be smaller than the stop and seem to be behind it. The size of the entrance pupil can be measured by placing a small light source exactly at the focal plane of the lens. The focal plane is where it focuses light from an infinit source. The exact infinity focus position can be found by autocollimating. To do this you need a card with a small hole in it and a small light, like a penlight. You also need a flat mirror to place over the lens. You can do this in a view camera. First, place the mirror over the lens, a shaving mirror will do but make sure the flat side is against the lens. Place the card behind the lens and focus the reflected image of the lighted hole on the card. You will have to adjust it so that the image does not fall exactly on top of the light of course. The lens is now focused exactly at infinty. Now, place a translucent screen over the lens, thin paper will do. The circle of light projected onto the paper is the outline of the entrance pupil. If you measure its diameter you will have the diameter of the _effective_ stop. If the front cell doesn't have a lot of power this diameter will not be much different than the physical size of the stop, but, for some lenses it can be quite different. The above method works well when a new stop scale must be made for a lens. Its not even necessary to remove the cells for this. If you want to measure the location of the entrance pupil use a camera that can focus close, the closer the better. First, focus the camera on a reference surface, the rim of the cell is a good place. Now, move the whole camera until the stop, as seen through the lens, is in focus. The distance the camera moved is the distance from the rim of the cell (or whatever reference surface you used) to the entrance pupil. The location of the entrance pupil is sometimes useful, for instance, it is the correct point to rotate a camera to take panoramic pictures. Do you still think the question is silly? :-) -- --- Richard Knoppow Los Angeles, CA, USA |
#7
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Apertures and focal length
Reciprocity Failure wrote:
Perhaps I misunderstood the question but I thought he was simply asking whether at F2.8, for example, the physical size of the aperture in lenses of the same focal length will be the same. I thought yes, you seem to be saying no but I'm not sure. In other words, have I been corrected? : - ) Read Richard Knoppow's explanation again. As he remarks, it is the effective aperture which is used in calculating the f-stop, not the physical size of the opening in the diaphragm. "Richard Knoppow" wrote in message link.net... "Stephan Goldstein" wrote in message ... Maybe this is a silly question, but I'll ask anyways. Suppose I have a shutter from a 210mm Trashagon with accurate aperture markings. If I instead screw in the elements of a 210mm Expensivegon will the same markings apply? In other words, is the physical aperture opening the same for all lenses of the same focal length? I _think_ it should be... TIA! Steve The _effective_ size of the stop is affected by the magnification of the lenses in front of it. The calibrations are for this effective size, not the physical size of the hole. The image of the physical stop as seen from the front of the lens is called the Entrance Pupil. In general, its size and position will be different from the physical size and position of the stop. An example is the speed of a single element of a convertible lens like a Dagor. When the cell is located at the rear of the stop (its optimum position) the speed is about f/13 and the entrance pupil is at the stop. The _effective_ size of the stop is its physical size and position its physical position because there is no glass in front of it to either magnify or reduce it. When the half-Dagor is placed in front of the stop the speed increases to about f/12, not a big difference, but different. The reason is that the stop is now magnified by the lens. The location of the stop will also change. Since the half lens is positive in power it will make the stop seem closer to the front than the physical position. The effect on the stop will be opposite where the front cell is negative in power, as it is in a Tessar type lens. There, the entrance pupil will be smaller than the stop and seem to be behind it. The size of the entrance pupil can be measured by placing a small light source exactly at the focal plane of the lens. The focal plane is where it focuses light from an infinit source. The exact infinity focus position can be found by autocollimating. To do this you need a card with a small hole in it and a small light, like a penlight. You also need a flat mirror to place over the lens. You can do this in a view camera. First, place the mirror over the lens, a shaving mirror will do but make sure the flat side is against the lens. Place the card behind the lens and focus the reflected image of the lighted hole on the card. You will have to adjust it so that the image does not fall exactly on top of the light of course. The lens is now focused exactly at infinty. Now, place a translucent screen over the lens, thin paper will do. The circle of light projected onto the paper is the outline of the entrance pupil. If you measure its diameter you will have the diameter of the _effective_ stop. If the front cell doesn't have a lot of power this diameter will not be much different than the physical size of the stop, but, for some lenses it can be quite different. The above method works well when a new stop scale must be made for a lens. Its not even necessary to remove the cells for this. If you want to measure the location of the entrance pupil use a camera that can focus close, the closer the better. First, focus the camera on a reference surface, the rim of the cell is a good place. Now, move the whole camera until the stop, as seen through the lens, is in focus. The distance the camera moved is the distance from the rim of the cell (or whatever reference surface you used) to the entrance pupil. The location of the entrance pupil is sometimes useful, for instance, it is the correct point to rotate a camera to take panoramic pictures. Do you still think the question is silly? :-) -- --- Richard Knoppow Los Angeles, CA, USA |
#8
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Apertures and focal length
Thanks, it's nice to learn something new. I always thought it was the
physical aperture, perhaps because of Ansel Adams' saying in "The Camera" that "the lens aperture is simply the diameter of the lens opening expresed as a fraction of its focal length." I guess he was wrong or I misunderstood what he was saying or he was oversimplifying or something else. But reading Richard's explanation again led me to Stroebel's explanation of the same thing and between the two I think I get it. "Leonard Evens" wrote in message ... Reciprocity Failure wrote: Perhaps I misunderstood the question but I thought he was simply asking whether at F2.8, for example, the physical size of the aperture in lenses of the same focal length will be the same. I thought yes, you seem to be saying no but I'm not sure. In other words, have I been corrected? : - ) Read Richard Knoppow's explanation again. As he remarks, it is the effective aperture which is used in calculating the f-stop, not the physical size of the opening in the diaphragm. "Richard Knoppow" wrote in message link.net... "Stephan Goldstein" wrote in message ... Maybe this is a silly question, but I'll ask anyways. Suppose I have a shutter from a 210mm Trashagon with accurate aperture markings. If I instead screw in the elements of a 210mm Expensivegon will the same markings apply? In other words, is the physical aperture opening the same for all lenses of the same focal length? I _think_ it should be... TIA! Steve The _effective_ size of the stop is affected by the magnification of the lenses in front of it. The calibrations are for this effective size, not the physical size of the hole. The image of the physical stop as seen from the front of the lens is called the Entrance Pupil. In general, its size and position will be different from the physical size and position of the stop. An example is the speed of a single element of a convertible lens like a Dagor. When the cell is located at the rear of the stop (its optimum position) the speed is about f/13 and the entrance pupil is at the stop. The _effective_ size of the stop is its physical size and position its physical position because there is no glass in front of it to either magnify or reduce it. When the half-Dagor is placed in front of the stop the speed increases to about f/12, not a big difference, but different. The reason is that the stop is now magnified by the lens. The location of the stop will also change. Since the half lens is positive in power it will make the stop seem closer to the front than the physical position. The effect on the stop will be opposite where the front cell is negative in power, as it is in a Tessar type lens. There, the entrance pupil will be smaller than the stop and seem to be behind it. The size of the entrance pupil can be measured by placing a small light source exactly at the focal plane of the lens. The focal plane is where it focuses light from an infinit source. The exact infinity focus position can be found by autocollimating. To do this you need a card with a small hole in it and a small light, like a penlight. You also need a flat mirror to place over the lens. You can do this in a view camera. First, place the mirror over the lens, a shaving mirror will do but make sure the flat side is against the lens. Place the card behind the lens and focus the reflected image of the lighted hole on the card. You will have to adjust it so that the image does not fall exactly on top of the light of course. The lens is now focused exactly at infinty. Now, place a translucent screen over the lens, thin paper will do. The circle of light projected onto the paper is the outline of the entrance pupil. If you measure its diameter you will have the diameter of the _effective_ stop. If the front cell doesn't have a lot of power this diameter will not be much different than the physical size of the stop, but, for some lenses it can be quite different. The above method works well when a new stop scale must be made for a lens. Its not even necessary to remove the cells for this. If you want to measure the location of the entrance pupil use a camera that can focus close, the closer the better. First, focus the camera on a reference surface, the rim of the cell is a good place. Now, move the whole camera until the stop, as seen through the lens, is in focus. The distance the camera moved is the distance from the rim of the cell (or whatever reference surface you used) to the entrance pupil. The location of the entrance pupil is sometimes useful, for instance, it is the correct point to rotate a camera to take panoramic pictures. Do you still think the question is silly? :-) -- --- Richard Knoppow Los Angeles, CA, USA |
#9
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Apertures and focal length
In article .net, "Richard Knoppow" wrote:
A substantial tome in answer to what I thought was a simple question (snipped for brevity). Do you still think the question is silly? :-) Indeed not! Thank you, Richard, for your clear and careful explanation. I understand now - the physical aperture corresponding to a given f-stop for a given lens is a function of the actual entrance pupil as magnified or reduced by the power of the front elements. So I can't just willy-nilly swap element sets between shutters and expect the aperture markings to be correct. A pity, but with your instructions I can make my own scales. They won't be as nice as those from the factory or from SK Grimes, but they'll be less expensive. Thanks again, Steve |
#10
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Apertures and focal length
"Stephan Goldstein" wrote
I understand now - the physical aperture corresponding to a given f-stop for a given lens is a function of the actual entrance pupil as magnified or reduced by the power of the front elements. Yup, though now it can be told: for 'government work' the old quick and dirty method works reasonably well: Hold the lens at arm's length and measure the entrance pupil with a ruler held against the front rim of the lens. Experimental results: Lens Measured Calculated Marked 150mm 'Gauss' 26mm f5.8 f5.6 16mm fisheye 5.5mm f2.9 f2.8 200mm tele 45mm f4.4 f4.0 -- Nicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. |
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