You will need to measure the actual focal lengths and find the principle points. This is not difficult. You can use a view camera as an optical bench. To measure the focal length focus the lens on infinity and mark the position of the lens. The exact point doesn't matter, just that you need a reference that you can come back to. After setting the lens exactly at infinity focus it for exact 1:1 image to object reproduction. The extension of the lens from its infinity focus point is the exact focal length. A double check is that the distance of object to image at 1:1 is exactly four focal lengths. If there is no convient target far enough away to approximate infinity you can autocollimate the lens. You need a flat mirror which will cover the front of the lens. Put the mirror over the lens and set up a small light behind the lens. If you use a view camera you can just put the end of a pencil flashlight against the ground glass near, but exactly at, the center. The mirror will project the image of the light back to the ground glass. Focus it for best sharpness. the lens is then focused exactly at infinity. Once you have the focal length you can find the rear or second principle point by measuring one focal length from the image back toward the lens with the lens focused in infinity. This is by definition the location of this point. To find the other principle point turn the lens around and focus it for infinity again. Its focal length doesn't change with direction. Measure one focal length from image toward lens again and you have the front or first principle point. You need to know the location of the principle points to calculate the focal length of the combinations (of course you can just measure them the same way). The D or distance between lenses given in the formula is the distance between the rear principle point of the front lens and the front principle point of the rear lens, not the physical distance of the cells. For thick convex meniscus lenses one principle point will lie about in line with the front apex of the lens and the other a bit in front of it, just outside the lens. The exact positions will depend on the bending of the lens. For a single lens mounted behind the stop the entrance pupil is the stop itself so the f/stop is simply the lens focal length devided by the physical diameter of the aperture. When the lens is in front of the stop, or where combined lenses are used the stop is changed by the magnification of the stop by the lens. This changes the effective size of the stop and the position of the entrance pupil. To measure the _effective_ size of the stop arrange a small light source behind the lens at the exact infinity focal point (which you have already found). Place a translucent screen over the lens (writing paper will do) and measure the diameter of the circle of light projected onto it. This is the effective size of the stop. You do not need to know the position of the entrance pupil unless you are going to do panoramic work. The entrance pupil is the correct point about which to rotate the camera for panorams. Its poisition can be calculated but its easier to measure it by setting up a camera to focus on some convient part of the lens, say the rim of the cell, at al close distance. Now look at the aperture though the camera and move it until the aperture is in focus. The distance the camera moved is the distance from the point of reference to the entrance pupil. The entrance pupil can be in front or in back of the aperture depending on the sign of the power of the lens in front of it. Again, where a lens is used behind the stop the stop becomes the entrance pupil. These measurements can be tedious but are simple and accurate enough to determine the effective focal length and stop calibration. Note, BTW, that the speed of a single cell depends on its position relative to the stop. Since these are all positive lenses (they must be to form a real image) they will be a little faster when in front of the stop than when behind it. However, in general, the correction of aberrations is better when the lens is used behind the stop.
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