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#1
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Aperture calculation - one more time
Okay, again, what's the formula for calculating maximum aperture?
Specifically, do I have to do a serial calculation - diameter of entrance pupil(lens) then consider the smallest aperture in the system, and exit aperture? My lens has a 112mm diameter front lens, 30mm center (where the shutter goes) and 106mm diameter rear lens. Then how does one measure the T-stop? |
#2
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Aperture calculation - one more time
"jjs" wrote in message ... Okay, again, what's the formula for calculating maximum aperture? The formula is aperture in stops = F / D where F is the focal length of the lens when focused at infinity and D is the diameter of the entrance pupil. But you knew that, I am pretty sure. Specifically, do I have to do a serial calculation - diameter of entrance pupil(lens) then consider the smallest aperture in the system, and exit aperture? I don't think so. My lens has a 112mm diameter front lens, 30mm center (where the shutter goes) and 106mm diameter rear lens. If 30mm is the maximum physical opening ("D") of the lens/shutter assembly, a quick aproximation to the maximum apertue would be given by F / 30 stops. To find the actual f/stop, you would have to measure the physical opening of the lens/shutter as seen from the front of the lens (search this NG for ways to measure that), in other words, the diameter you should use in the formula is not the real physical opening but the virtual opening (physical opening as seen thru the lens), after all, this is how big an opening the photons rushing into the lens "see and think" they have in front. Once the photons crosss the virtual opening and they realise the actual opening is in fact smaller (for instance), it is too late for them, they have been tricked :-) Then how does one measure the T-stop? With a photometer. |
#3
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"jjs" wrote in message ... Okay, again, what's the formula for calculating maximum aperture? The formula is aperture in stops = F / D where F is the focal length of the lens when focused at infinity and D is the diameter of the entrance pupil. But you knew that, I am pretty sure. Specifically, do I have to do a serial calculation - diameter of entrance pupil(lens) then consider the smallest aperture in the system, and exit aperture? I don't think so. My lens has a 112mm diameter front lens, 30mm center (where the shutter goes) and 106mm diameter rear lens. If 30mm is the maximum physical opening ("D") of the lens/shutter assembly, a quick aproximation to the maximum apertue would be given by F / 30 stops. To find the actual f/stop, you would have to measure the physical opening of the lens/shutter as seen from the front of the lens (search this NG for ways to measure that), in other words, the diameter you should use in the formula is not the real physical opening but the virtual opening (physical opening as seen thru the lens), after all, this is how big an opening the photons rushing into the lens "see and think" they have in front. Once the photons crosss the virtual opening and they realise the actual opening is in fact smaller (for instance), it is too late for them, they have been tricked :-) Then how does one measure the T-stop? With a photometer. |
#4
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"jjs" wrote in message ... Okay, again, what's the formula for calculating maximum aperture? The formula is aperture in stops = F / D where F is the focal length of the lens when focused at infinity and D is the diameter of the entrance pupil. But you knew that, I am pretty sure. Specifically, do I have to do a serial calculation - diameter of entrance pupil(lens) then consider the smallest aperture in the system, and exit aperture? I don't think so. My lens has a 112mm diameter front lens, 30mm center (where the shutter goes) and 106mm diameter rear lens. If 30mm is the maximum physical opening ("D") of the lens/shutter assembly, a quick aproximation to the maximum apertue would be given by F / 30 stops. To find the actual f/stop, you would have to measure the physical opening of the lens/shutter as seen from the front of the lens (search this NG for ways to measure that), in other words, the diameter you should use in the formula is not the real physical opening but the virtual opening (physical opening as seen thru the lens), after all, this is how big an opening the photons rushing into the lens "see and think" they have in front. Once the photons crosss the virtual opening and they realise the actual opening is in fact smaller (for instance), it is too late for them, they have been tricked :-) Then how does one measure the T-stop? With a photometer. |
#5
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Aperture calculation - one more time
"f/256" wrote in message
t.cable.rogers.com... "jjs" wrote in message ... The formula is aperture in stops = F / D where F is the focal length of the lens when focused at infinity and D is the diameter of the entrance pupil. But you knew that, I am pretty sure. My lens has a 112mm diameter front lens, 30mm center (where the shutter goes) and 106mm diameter rear lens. If 30mm is the maximum physical opening ("D") of the lens/shutter assembly, a quick aproximation to the maximum apertue would be given by F / 30 stops. To find the actual f/stop, you would have to measure the physical opening of the lens/shutter as seen from the front of the lens (search this NG for ways to measure that), in other words, the diameter you should use in the formula is not the real physical opening but the virtual opening (physical opening as seen thru the lens), after all, this is how big an opening the photons rushing into the lens "see and think" they have in front. Once the photons crosss the virtual opening and they realise the actual opening is in fact smaller (for instance), it is too late for them, they have been tricked :-) Ignoring the 'virtual opening' because I haven't found what that means yet, that makes my lens F 2.5. I'll try to find out what that virtual opening is all about. I just measured a 90mm F8 Super Angulon. Middle (shutter area) diameter is 14mm front glass is 46mm. By the (incomplete) metrics of F/D that's 6.4. Wow. That unknown virtual opening is a huge factor. |
#6
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Aperture calculation - one more time
"jjs" wrote in message ... "f/256" wrote in message Ignoring the 'virtual opening' because I haven't found what that means yet, that makes my lens F 2.5. I'll try to find out what that virtual opening is all about. The real opening is the actual physical size of the opening, when looking at that opening thru the front side, the lens itself will have an effect on the opening you see, that opening as seen thru the lens is what I am calling "virtual opening", but it is actually called entrance pupil. I just measured a 90mm F8 Super Angulon. Middle (shutter area) diameter is 14mm front glass is 46mm. By the (incomplete) metrics of F/D that's 6.4. Wow. That unknown virtual opening is a huge factor. I just did the same measurements on my 90mm Grandagon F/6.8, the glass is about 13.5mm at the rear of the front element, that would make it F/6.67 or so, not bad approximation if you ask me. The "virtual opening" or -better call it- entrance pupil, would have to be would have to be 13.2mm in order to match the F/6.8. I very very roughly measured it and I got 13mm (don't have a point source of light handy at this time), which gives me F/6.9, not bad either. I believe Richard.K method to measure the entrance pupil goes something like this: position a point source of light at the infinity focal plane of the lens, place a piece of translucent paper on the front of the lens, you should get a circle of light that is the image of the aperture, measure that aperture and you have the diameter of the entrance pupil. Guillermo |
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