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#1
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Apertu geometric vs. real
Hello all,
As I know, and I hope am correct about it, a lens's aperture is the ratio of the front pupils' diameter to the focus length. And it is this number that is indicated as the maximum aperture and is also marked on the aperture ring, while the actual illuminance produced by a lens on its focal plane depends on many factors like (multi)coating, the number of optical elements e.t.c. Taking this into accout, the reading of an ideal external light-meter may differ from that of an ideal internal (say TTL) one. My question is whether this correction coefficient (or difference, if expressed in logarithmic terms) has a name for it in the photographic slang. Thanks in advance, Anton P.S.: Asking this because I recently got a tele- scopic lens with two mirrors in it, and the declared apperture of f/8 seems to be about one step too light... |
#2
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Apertu geometric vs. real
"Anton Shepelev" wrote in message ... Hello all, As I know, and I hope am correct about it, a lens's aperture is the ratio of the front pupils' diameter to the focus length. And it is this number that is indicated as the maximum aperture and is also marked on the aperture ring, while the actual illuminance produced by a lens on its focal plane depends on many factors like (multi)coating, the number of optical elements e.t.c. Taking this into accout, the reading of an ideal external light-meter may differ from that of an ideal internal (say TTL) one. My question is whether this correction coefficient (or difference, if expressed in logarithmic terms) has a name for it in the photographic slang. Thanks in advance, Anton P.S.: Asking this because I recently got a tele- scopic lens with two mirrors in it, and the declared apperture of f/8 seems to be about one step too light... You are correct, the definition of f/stop is the ratio of the diameter of the entrance pupil to the focal length. The entrance pupil is the _image_ of the stop as seen from the front of the lens. The actual transmission through the lens depends on various factors that can attenuate the light, you've named them pretty well. In the case of a reflecting objective there will also be some loss at the mirror or mirrors although that is generally low. It is unusual for the transmission to be full stop less than the f/stop but possible. Its easy to measure the diameter of the entrance pupil by autocollimating the lens to find the exact infinity focus and placing a point source light there, then place a translucent screen over the lens. The diameter of the circle of light projected on it is the diameter of the entrance pupil. It can be smaller or larger than the physical aperture depending on whether the lenses in front of it are magnifying or diminishing. I've posted instructions for autocollimating to this list several times in the past, its not difficult especially on a view camera which can be used as an elementary optical bench. If you can't find them write and I will repeat it. -- -- Richard Knoppow Los Angeles, CA, USA |
#3
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Apertu geometric vs. real
"Anton Shepelev" wrote in message
... Hello all, As I know, and I hope am correct about it, a lens's aperture is the ratio of the front pupils' diameter to the focus length. And it is this number that is indicated as the maximum aperture and is also marked on the aperture ring, while the actual illuminance produced by a lens on its focal plane depends on many factors like (multi)coating, the number of optical elements e.t.c. Taking this into accout, the reading of an ideal external light-meter may differ from that of an ideal internal (say TTL) one. My question is whether this correction coefficient (or difference, if expressed in logarithmic terms) has a name for it in the photographic slang. Thanks in advance, Anton P.S.: Asking this because I recently got a tele- scopic lens with two mirrors in it, and the declared apperture of f/8 seems to be about one step too light... Hello Anton. I've used several makes of mirror lens. I agree that their "f8" seems optimistic. I reckon, judging from the shutter speeds I use, that the lenses are no better than f11. Best wishes, Rog. --- news://freenews.netfront.net/ - complaints: --- |
#4
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Apertu geometric vs. real
"Anton Shepelev" wrote
I recently got a tele- scopic lens with two mirrors in it, and the declared apperture of f/8 seems to be about one step too light... To be expected. There is nothing subtle about this - you don't need to worry about the calibration of TTL Vs. handheld meters or the effects of flare. A mirror lens has part of the front entrance pupil blocked by the mirror that is glued to the back of the front element. Then part of the rear mirror is missing because of the hole cut in the center for the exit pupil. To arrive at the largest f number, the manufacture simply divides the diameter of the front lens by the focal length and doesn't correct for either of these light stops. -- Nicholas O. Lindan, Cleveland, Ohio Darkroom Automation: F-Stop Timers, Enlarging Meters http://www.darkroomautomation.com/da-main.htm n o lindan at ix dot netcom dot com |
#5
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Apertu geometric vs. real
"Nicholas O. Lindan" wrote
A mirror lens has part of the front entrance pupil blocked by the mirror that is glued to the back of the front element. Then part of the rear mirror is missing because of the hole cut in the center for the exit pupil. To arrive at the largest f number, the manufacture simply divides the diameter of the front lens by the focal length and doesn't correct for either of these light stops. The hole in the rear (i.e., primary) mirror is irrelevant because it's already blocked by the secondary mirror, which is rather large. (If the hole through the primary is larger than the secondary, then the manufacturer's designer's got his/her head up the you know what.) |
#6
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Apertu geometric vs. real (correction)
The secondary *could* be smaller than the primary hole (plus the primary's
surrounding baffle!) that is sized according to the required diameter of the camera's lens mount.... The hole in the rear (i.e., primary) mirror is irrelevant because it's already blocked by the secondary mirror, which is rather large. (If the hole through the primary is larger than the secondary, then the manufacturer's designer's got his/her head up the you know what.) |
#7
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Apertu geometric vs. real
"Howard Lester" wrote
"Nicholas O. Lindan" wrote loss of light due to hole in primary mirror The hole in the rear (i.e., primary) mirror is irrelevant because it's already blocked by the secondary mirror Hmmm...yes, I should know that. If the hole in the primary was visible from the front of the lens it would be a bit of a bummer with 'non image forming light' getting to the film. -- Nicholas O. Lindan, Cleveland, Ohio Darkroom Automation: F-Stop Timers, Enlarging Meters http://www.darkroomautomation.com/da-main.htm n o lindan at ix dot netcom dot com |
#8
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Apertu geometric vs. real
Thanks to everybody for their replies.
Richard Knoppow: I've posted instructions for autocollimating to this list several times in the past, its not dif- ficult especially on a view camera which can be used as an elementary optical bench. Do I understand correctly that you refer to a device that produces a parallel beam of light? If so, this is really very interesting for camera/lens measur- ing/adjusting purposes. I'll try find your post(s) in the archieve and will get back to you if I fail. I currently don't do any optical measurements except for calibraing viewfinders in SLR cameras to get correct focusing. For that I use a darkened glass in the film plane, with some scratches in it. A bright light is placed behind the glass and the lens' focus is adjusted to get the sharpest possible image of the scrathes on the opposite wall. This is the "true" focus. Anton |
#9
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Apertu geometric vs. real
"Anton Shepelev" wrote in message ... Thanks to everybody for their replies. Richard Knoppow: I've posted instructions for autocollimating to this list several times in the past, its not dif- ficult especially on a view camera which can be used as an elementary optical bench. Do I understand correctly that you refer to a device that produces a parallel beam of light? If so, this is really very interesting for camera/lens measur- ing/adjusting purposes. I'll try find your post(s) in the archieve and will get back to you if I fail. I currently don't do any optical measurements except for calibraing viewfinders in SLR cameras to get correct focusing. For that I use a darkened glass in the film plane, with some scratches in it. A bright light is placed behind the glass and the lens' focus is adjusted to get the sharpest possible image of the scrathes on the opposite wall. This is the "true" focus. Anton Its pretty easy to set a lens for its exact infinity focus by autocollimating. This is done with the aid of a flat mirror that fits over the front of the lens. A first-surface mirror is ideal but a flat shaving mirror will work OK. The idea is that the light passes through the lens twice so that it comes to focus at the source. Since the path from source to image is folded by the mirror the lens is at its infinity focus and the light emerging from the front is collimated. The method is simple especially if done using a view camera as an optical bench. The source should be a small lamp. I penlight flashlight will do. For rough measurements such as finding the diameter of the entrance pupil of a view camera lens, its enough to put the end of the light against the ground glass, near, but exactly at, the center. The mirror is placed over the lens. The distance is not important but it is important that the mirror not be tilted. If its held against the rim of the lens cell it will be quite perpendicular to the optical axis. The image of the source is brought to focus, the lens is now exactly at infinity focus and several measurements can be made on it. If something approaching a point source is now placed at the center of the axis at the focal plane the light will be projected from the front of the lens as a parallel beam, that is, a collimated beam. This will have constant diameter regardless of distance (not counting scattering by the air for you purists). A translucent screen placed over the lens will have the image of the entrance pupil focused on it. The diameter of this circle of light is the diameter of the entrance pupil, which determines the f/stop of the lens. This is a simple method of calibrating irises. Since you have the infinity focus point you can also determine the focal length of the lens. This is done by adjusting the lens to produce an exact equal size image of some object. At exatly 1:1 the focal length can be calculated from two things: One is the distance the lens moves from its infinity focus. That is by definition one focal length. It can also be determined by measuring the distance from the image to the object, that will be exactly four times the focal length. These two can be used to check each other. Now, knowing the exact focal length and the diameter of the entrance pupil makes is possible to determine the geometric f/stop. Also, when the lens is set to exactly infinity focus the rear principle point will be exactly one focal length from the image measured back toward the lens. The front principle point is found by turning the lens around, refocusing, and again measuring. You now know the exact focal length, the true f/stop, the location of both principle points. Now, its easy to find the location of the stops or pupils. The pupils are the images of the stop as seen from the outside of the lens. The front pupil is called the entrance pupil and teh rear is called the exit pupil. The pupils are the _images_ of the stop as seen from the outside of the lens. They can be displaced in space from the physical stop depending on the power of the lenses beween the stop and the outside in just the same way as the size of the pupils is chaned by the lens power. To find the location of the pupils one needs a camera capable of being focused at a close distance. The procedure is simple: First set the camera up close to the lens and facing it. This is done so that a small change in distance can be accurately determined. Focus the camera on a convenient reference point, for instance on the rim of the lens mount. Record the location of the camera. Then _move the entire camera_ until until the iris is in focus. The camera may move toward the lens or away from it. Now, record the location of the camera and measure the distance and direction its has moved. Now lay this distance against the lens from the reference point you used for the first measurement. That is the location of the pupil. Pupils can lie inside or outside the lens as can principle points. The entrance pupil is important both because it determines the light gathering power of the lens and because it is the correct point of rotation for panoramic pictures to prevent relative movement of forground and background parts of the image. It is often thought that the front principle point is the proper pivot point but it is not, rather the front entrance pupil is the correct point. The entrance pupil is also the correct point to use when calculating the magnification of the lens. These simple measurements allow you to amost completely characterize a lens. While precise measurements are difficult without a proper optical bench very close measurements can be made. Its interesting to experiment with the location of the principle points of various lenses becuse they may not be where you would think. For instance the single cells of meniscus convertible lenses like the Dagor or Protar have the principle points displaced so that the lens is somewhat retrofocus when in the usual position with the concave side facing the object and somewhat telephoto when placed with the convex side facing the object. While the best correction is gotten when the concave side facing out with the stop in front the shortening of the required bellows draw will often allow a long single objective to work on a camera with limited bellows capacity when reversed from ideal practice. Whew, that's a lot of typing... -- -- Richard Knoppow Los Angeles, CA, USA |
#10
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Apertu geometric vs. real
"Bob AZ" wrote in message ... On Jan 1, 4:39 pm, Anton Shepelev wrote: Hello all, Anton et al FWIW From the book "The Focal Encyclopedia of Photography" "the focal point of the lens is the point at which the lens can be revolved such that the focus plane remains stationery. The focus point is not always within the body of the glass/lens. Also the revolvement to try this is limited by the field of view of the lens. Bob AZ Optical benches are equipped with what is called a focal slide. This allows small fore and aft movement of the lens while wiggling it. The image of a very distant object, such as the target in a collimator, is viewed and the slide adjusted until there is no motion of the image as the lens is wiggled. The axis or rotation is now exactly at the principle point. I just posted another way of finding the principle points but this is the academically correct one. A view camera is actually a simple optical bench but without the calibrations for all movements and without a microscope (or telescope depending the distance of the virtual image) to examine the aerial image. A view camera can give you very useful data on longer lenses. For short focal length lenses one really needs the precision of a true optical bench. -- -- Richard Knoppow Los Angeles, CA, USA |
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