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#1




Formula for prefocusing
Is there a formula to calculate the required distance between the lens board
and the film plane for an object at a given distance from the camera? Thanks in advance. Steve (new to 4x5) 
#2




Formula for prefocusing
Steve Yeatts wrote:
: Is there a formula to calculate the required distance between the lens board : and the film plane for an object at a given distance from the camera? : Thanks in advance. I don't have an actual formula that I use. I've found that after time I know about how far the lens needs to be from the film for the different lenses. Also after focusing the first time with a lens the next shot will be close.  Keep working millions on welfare depend on you  
#3




Formula for prefocusing
Steve Yeatts wrote:
Is there a formula to calculate the required distance between the lens board and the film plane for an object at a given distance from the camera? Thanks in advance. If u is the distance from the lens to the subject, v the distance of the lens from the film plane, and f the focal length, then 1/u + 1/v = 1/f. So v = fu/(u  f) In words, multiply the subject distance by the focal length, and then divide that by the difference between the subject distance and the focal length. But this requires some additional qualification since the lens has an extent of its own. The subject distance is measured from a point called the front principal point and the film distance is measured from a point called the rear principal point. For most lenses, these points are both close to the center of the lens which in turn is close to the lens board. But for lenses of telephoto design, both these points may be far in front of the lens. For many wide angle lenses, the rear principal point is a short distance in back of the lens board. Usually lens specifications tell you what the rear flange focal length is. If you know that and the focal length, you can determine the position of the rear principal point. Steve (new to 4x5) 
#4




Formula for prefocusing
by the time you figure out the formula, you could have set up, focused and
exposed the film...;^) Steve, the best "formula" to use for focusing is to get under the dark cloth with a focusing loupe and get the scene sharp. Check the near and far, all the corners, etc. Keep in mind that you probably won't be able to get the entire scene in focus with the lens wide open, you'll want to stop the lens down and let your eyes get accustomed to the light, with a bit of practice you'll be able to focus. The math that Leonard uses is undoubtedly correct, but I think you'll find that in practice it's much better to actually see the image get sharp on the ground glass. John Emmons "Leonard Evens" wrote in message news Steve Yeatts wrote: Is there a formula to calculate the required distance between the lens board and the film plane for an object at a given distance from the camera? Thanks in advance. If u is the distance from the lens to the subject, v the distance of the lens from the film plane, and f the focal length, then 1/u + 1/v = 1/f. So v = fu/(u  f) In words, multiply the subject distance by the focal length, and then divide that by the difference between the subject distance and the focal length. But this requires some additional qualification since the lens has an extent of its own. The subject distance is measured from a point called the front principal point and the film distance is measured from a point called the rear principal point. For most lenses, these points are both close to the center of the lens which in turn is close to the lens board. But for lenses of telephoto design, both these points may be far in front of the lens. For many wide angle lenses, the rear principal point is a short distance in back of the lens board. Usually lens specifications tell you what the rear flange focal length is. If you know that and the focal length, you can determine the position of the rear principal point. Steve (new to 4x5) 
#5




Formula for prefocusing
John Emmons wrote:
by the time you figure out the formula, you could have set up, focused and exposed the film...;^) Steve, the best "formula" to use for focusing is to get under the dark cloth with a focusing loupe and get the scene sharp. Check the near and far, all the corners, etc. Keep in mind that you probably won't be able to get the entire scene in focus with the lens wide open, you'll want to stop the lens down and let your eyes get accustomed to the light, with a bit of practice you'll be able to focus. The math that Leonard uses is undoubtedly correct, but I think you'll find that in practice it's much better to actually see the image get sharp on the ground glass. This is the second response expressing a similar sentiment. I would agree that in practice one wouldn't focus by setting the film to lens distance based on the subject to lens distance and calculations. That seems rather obvious. But there are certainly other reasons for wanting to know the formula. The most common is when you want to know if you can focus at a certain distance with the maximum bellows extension your camera allows, but there are others. Steve's original request was quite simple and didn't suggest he intended to use that as a primary method of focusing, although perhaps his subject title could be interpreted that way. Generally, when someone asks a question like this, I assume he has a reason for wanting to know the answer, and, if I know the answer, I respond. Let's give him the benefit of the doubt instead of lecturing him about what he should be doing. By the way, my answer is correct. John Emmons "Leonard Evens" wrote in message news Steve Yeatts wrote: Is there a formula to calculate the required distance between the lens board and the film plane for an object at a given distance from the camera? Thanks in advance. If u is the distance from the lens to the subject, v the distance of the lens from the film plane, and f the focal length, then 1/u + 1/v = 1/f. So v = fu/(u  f) In words, multiply the subject distance by the focal length, and then divide that by the difference between the subject distance and the focal length. But this requires some additional qualification since the lens has an extent of its own. The subject distance is measured from a point called the front principal point and the film distance is measured from a point called the rear principal point. For most lenses, these points are both close to the center of the lens which in turn is close to the lens board. But for lenses of telephoto design, both these points may be far in front of the lens. For many wide angle lenses, the rear principal point is a short distance in back of the lens board. Usually lens specifications tell you what the rear flange focal length is. If you know that and the focal length, you can determine the position of the rear principal point. Steve (new to 4x5) 
#6




Formula for prefocusing
John said:
Steve, the best "formula" to use for focusing is to get under the dark cloth with a focusing loupe and get the scene sharp. Check the near and far, all the corners, etc. . . . With all due respect, I disagree with this advice. It's often difficult to stop down to the taking aperture and tell whether everything is "sharp" or not. Many of us photograph early in the morning and late in the evening or in fog , rain, cloudy conditions, or other times when the light isn't very bright. Then we're stopping down to at least F22, maybe F32, F45 or whatever. At those apertures and that kind of light I'm sometimes lucky if I can even compose an image on the ground glass much less stop down to the taking aperture and tell whether everything in the scene is sharp or not. And even in bright sunlight there often are shadow areas where the light is dim. So many of us to resort to something other than just looking at the ground glass to tell whether everything looks sharp or not. I use the method discussed in Tuan's article on focusing the view camera found at www.largeformatphotography.info but there are other methods too that IMHO are more practical than trying to figure it out from the ground glass. "John Emmons" wrote in message ... by the time you figure out the formula, you could have set up, focused and exposed the film...;^) Steve, the best "formula" to use for focusing is to get under the dark cloth with a focusing loupe and get the scene sharp. Check the near and far, all the corners, etc. Keep in mind that you probably won't be able to get the entire scene in focus with the lens wide open, you'll want to stop the lens down and let your eyes get accustomed to the light, with a bit of practice you'll be able to focus. The math that Leonard uses is undoubtedly correct, but I think you'll find that in practice it's much better to actually see the image get sharp on the ground glass. John Emmons "Leonard Evens" wrote in message news Steve Yeatts wrote: Is there a formula to calculate the required distance between the lens board and the film plane for an object at a given distance from the camera? Thanks in advance. If u is the distance from the lens to the subject, v the distance of the lens from the film plane, and f the focal length, then 1/u + 1/v = 1/f. So v = fu/(u  f) In words, multiply the subject distance by the focal length, and then divide that by the difference between the subject distance and the focal length. But this requires some additional qualification since the lens has an extent of its own. The subject distance is measured from a point called the front principal point and the film distance is measured from a point called the rear principal point. For most lenses, these points are both close to the center of the lens which in turn is close to the lens board. But for lenses of telephoto design, both these points may be far in front of the lens. For many wide angle lenses, the rear principal point is a short distance in back of the lens board. Usually lens specifications tell you what the rear flange focal length is. If you know that and the focal length, you can determine the position of the rear principal point. Steve (new to 4x5) 
#7




Formula for prefocusing
Vladamir30 wrote:
John said: Steve, the best "formula" to use for focusing is to get under the dark cloth with a focusing loupe and get the scene sharp. Check the near and far, all the corners, etc. . . . With all due respect, I disagree with this advice. It's often difficult to stop down to the taking aperture and tell whether everything is "sharp" or not. Many of us photograph early in the morning and late in the evening or in fog , rain, cloudy conditions, or other times when the light isn't very bright. Then we're stopping down to at least F22, maybe F32, F45 or whatever. At those apertures and that kind of light I'm sometimes lucky if I can even compose an image on the ground glass much less stop down to the taking aperture and tell whether everything in the scene is sharp or not. And even in bright sunlight there often are shadow areas where the light is dim. So many of us to resort to something other than just looking at the ground glass to tell whether everything looks sharp or not. I use the method discussed in Tuan's article on focusing the view camera found at www.largeformatphotography.info but there are other methods too that IMHO are more practical than trying to figure it out from the ground glass. I entirely agree with your comments. Even in bright light, I find it difficult to see much of anything if I stop down below f/16 or f/22. It is true that my retina is not that sensitive to dim light after 60 years of myopia, but I once posted a question about this and found that many people agreed with me that it is very difficult to evaluate depth of field by stopping down to the taking aperture. Steve Simmons has suggested the use of screen brighteners and a loupe. I have a Maxwell screen which is about as bright as anything you can find, and I still can't see much of anything when I stop down to the taking aperture. The use of a loupe is more involved than one might think. In effect, you are magnifying the picture. If you used a 10 X loupe to view a 4 x 5 transparency, you would expect less DOF than you would see with a 4 X loupe. So your choice of loupe involves implicit assumptions about how much the final image will be enlarged and how it will be viewed. As a result, it is going to be a individual thing. One person may find that a certain procedure, developed after years of experience, may work well for him/her in evaluating DOF, but it doesn't follow that the same procedure will work for someone else. The advantage of the focus spread methods, as described by Tuan at www.largeformatphotography.info, is that they are more objective and less dependent on the individual. Having said that, let me add that it doesn't hurt to stop down as far as your eyes will allow you to check that DOF is in the right ball park and extends equally from front to rear. Experience will help you estimate how much more you will get if you stop down further. In my case, I usually use f/11 to f/16 for that purpose. If you can get to f/32, all the more power to you. "John Emmons" wrote in message ... by the time you figure out the formula, you could have set up, focused and exposed the film...;^) Steve, the best "formula" to use for focusing is to get under the dark cloth with a focusing loupe and get the scene sharp. Check the near and far, all the corners, etc. Keep in mind that you probably won't be able to get the entire scene in focus with the lens wide open, you'll want to stop the lens down and let your eyes get accustomed to the light, with a bit of practice you'll be able to focus. The math that Leonard uses is undoubtedly correct, but I think you'll find that in practice it's much better to actually see the image get sharp on the ground glass. John Emmons 
#8




Formula for prefocusing
"Leonard Evens" wrote in message news Steve Yeatts wrote: Is there a formula to calculate the required distance between the lens board and the film plane for an object at a given distance from the camera? Thanks in advance. If u is the distance from the lens to the subject, v the distance of the lens from the film plane, and f the focal length, then 1/u + 1/v = 1/f. So v = fu/(u  f) In words, multiply the subject distance by the focal length, and then divide that by the difference between the subject distance and the focal length. But this requires some additional qualification since the lens has an extent of its own. The subject distance is measured from a point called the front principal point and the film distance is measured from a point called the rear principal point. For most lenses, these points are both close to the center of the lens which in turn is close to the lens board. But for lenses of telephoto design, both these points may be far in front of the lens. For many wide angle lenses, the rear principal point is a short distance in back of the lens board. Usually lens specifications tell you what the rear flange focal length is. If you know that and the focal length, you can determine the position of the rear principal point. Steve (new to 4x5) Its not hard to find the principal points of a lens. If you know the focal length its only necessary to focus the lens exactly at infinity and measure one focal length from the focal plane toward the lens. By definition that is the principle point. To measure the front or first principal point turn the lens around and again focus it for infinity. Again, measure from the focal plane toward the lens, that will be the first or front principal point and is the point to measure object to lens distance to. If there is no handy target far enough away to approximate infinity you can focus by autocollimation. You need a small mirror and a card with a small hole in it, and a flashlight to place behind the card. Place the mirror over the lens. Place the card in back of the lens, adjust the card (or lens) until an image of the illuminated hole is reflected back to the card. It should be near the hole. When this image is sharply focused the lens is exactly at infinity focus from the card. In a view or press camera you can usually get sufficiently good accuracy by placing a small light source over the ground glass near the center and focusing the lens on that. Since you know the focal length and infinity focus position you can calculate the amount the lens must be moved from infinity focus and use that to adjust for some closer distance. Its still necessary to know the location of the front principal point so you can measure the distance. For any lens the distance from the infinity focus point for any closer distance is: X = f^2 / u  f Whe X = extension from infinity focus f = focal length u = subject distance   Richard Knoppow Los Angeles, CA, USA 
#9




Formula for prefocusing
Richard Knoppow wrote:
What you say below is correct. But there is one minor problem for perfectionists. You have to know the actual focal length, which may be different from the nominal published focal length. Its not hard to find the principal points of a lens. If you know the focal length its only necessary to focus the lens exactly at infinity and measure one focal length from the focal plane toward the lens. By definition that is the principle point. To measure the front or first principal point turn the lens around and again focus it for infinity. Again, measure from the focal plane toward the lens, that will be the first or front principal point and is the point to measure object to lens distance to. If there is no handy target far enough away to approximate infinity you can focus by autocollimation. You need a small mirror and a card with a small hole in it, and a flashlight to place behind the card. Place the mirror over the lens. Place the card in back of the lens, adjust the card (or lens) until an image of the illuminated hole is reflected back to the card. It should be near the hole. When this image is sharply focused the lens is exactly at infinity focus from the card. In a view or press camera you can usually get sufficiently good accuracy by placing a small light source over the ground glass near the center and focusing the lens on that. Since you know the focal length and infinity focus position you can calculate the amount the lens must be moved from infinity focus and use that to adjust for some closer distance. Its still necessary to know the location of the front principal point so you can measure the distance. For any lens the distance from the infinity focus point for any closer distance is: X = f^2 / u  f Whe X = extension from infinity focus f = focal length u = subject distance 
#10




Formula for prefocusing
"Leonard Evens" wrote in message ... Richard Knoppow wrote: What you say below is correct. But there is one minor problem for perfectionists. You have to know the actual focal length, which may be different from the nominal published focal length. Its not hard to find the principal points of a lens. If you know the focal length its only necessary to focus the lens exactly at infinity and measure one focal length from the focal plane toward the lens. By definition that is the principle point. To measure the front or first principal point turn the lens around and again focus it for infinity. Again, measure from the focal plane toward the lens, that will be the first or front principal point and is the point to measure object to lens distance to. If there is no handy target far enough away to approximate infinity you can focus by autocollimation. You need a small mirror and a card with a small hole in it, and a flashlight to place behind the card. Place the mirror over the lens. Place the card in back of the lens, adjust the card (or lens) until an image of the illuminated hole is reflected back to the card. It should be near the hole. When this image is sharply focused the lens is exactly at infinity focus from the card. In a view or press camera you can usually get sufficiently good accuracy by placing a small light source over the ground glass near the center and focusing the lens on that. Since you know the focal length and infinity focus position you can calculate the amount the lens must be moved from infinity focus and use that to adjust for some closer distance. Its still necessary to know the location of the front principal point so you can measure the distance. For any lens the distance from the infinity focus point for any closer distance is: X = f^2 / u  f Whe X = extension from infinity focus f = focal length u = subject distance Its not difficult in principle to measure the exact focal length, but it does require some precision, especially for shorter lenses. With the above formula one can determine the focal length from the extension required for a known distance or from magnification. The easiest is the set the lens for unity magnification since the focus extension from infintity to 1:1 is exactly one focal length. It may be easier to measure the extension for a known distance than it is to set the image for exactly 1:1. However, knowing the exact distance requires knowing the location of the princpal points and that requires knowing the focal length unless one has access to an optical bench and a nodal slide. So, the unity magnification is perhaps the most practical method. The lens is focused for infinity with either a very distant target or by autocollimating as above. Then the lens is set for exactly 1:1 image to object size. The displacement of the lens from the infinity focus point is one focal length. A double check is the total distance from focal plane (ground glass) to object. At 1:1 it is exactly four times the focal length. The principal points are found by measuring one focal length back toward the lens from the focal plane at infinity focus. The first or front principal point is found by making the same measurement with the lens turned around. These points are somtimes useful to know for calculating focus, depth of field, exposure correction, and magnification.   Richard Knoppow Los Angeles, CA, USA 
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