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miketnyc
Joined: 18 Apr 2018 Posts: 39 Location: NYC

Posted: Fri Apr 27, 2018 4:21 pm Post subject: 


William Hallett wrote:  Sorry  I should have defined the variables in the equation more carefully. The object distance is the distance from the object that you're photographing to the nodal point of the lens, while the image distance is the distance from the nodal point to the image (i.e. groundglass). To generate a scale, you calculate the image distance at a number of different object distances. Start at infinity (for which the image distance is equal to the focal length, since 1/infinity = 0), then do (say) 50 ft, 25 ft, 15 ft, 10 ft, 6 ft (these are usually the numbers on Graflex's scales). For each of these, calculate the difference between the image distance and that at infinity  this is then the distance from infinity to lay off on the scale for that particular point. Note when you are using the equation that your units must be consistent  i.e. all measurements must be in the same units, whether mm or ft or inches.
The only objection I can think of to this method is that the actual focal length of a given lens may be a little different from that marked on it. However, if you calculate the effects of an error of a couple of millimeters in focal length on the scale, you will find that it is almost negligible. The final test, of course, is to check your scale against groundglass focussing. 
Your last paragraph suggests what I was talking about. Measuring the distance between the lens and the subject is logical, but instead of trying to measure the distance between the lens and the groundglass (which would be prone to errors because it varies only a tiny bit for each distance) why not just focus the lens on that subject? That should move the lens forward by the correct amount, AND it would correct for tiny differences from the marked focal length. 

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William Hallett
Joined: 07 Jan 2012 Posts: 69

Posted: Fri Apr 27, 2018 6:02 pm Post subject: 


What you are describing is marking the scale experimentally, and certainly you can do that. The advantage of calculating the scale instead is that it is much faster, and that it avoids the errors that can occur in trying to mark off a location from the camera itself. If I were going to determine offsets for a scale experimentally, I think I would fasten a depth micrometer to the camera bed and use it to measure the front standard position as the camera was focussed to different distances. _________________ WilliamH 

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45PSS
Joined: 28 Sep 2001 Posts: 3894 Location: Mid Peninsula, Ca.


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