StewartR
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- Name
- Stewart
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I just had a call from somebody who was looking for some help/advice with his photography problem. It turned out to be way beyond my area of competence. I was able to discuss some general issues with him but I felt I was really floundering in terms of positive, practical help. Can anyone else out there help?
His situation is that he's got lots of 8mm film to digitise. A frame of 8mm film measures 4.5mm x 3.3mm. He has a custom built optical rig with an industrial CCD and a microscope lens on a precision focusing rail. The CCD is a 2/3" type, which is approximately 8.5mm x 7mm in size. So he's effectively got a magnification ratio of a bit more than 2, and that puts it into pretty extreme macro territory by the standards of the photography that most of us are used to. He estimates the distance from the film to the sensor as about 25cm.
His problem is that some of this film is old and in poor quality, and it's slightly bowed. His depth of field is so shallow that he can't get the whole frame in focus when it is bowed. Obviously with the film only 4.5 x 3.3mm across the amount by which it's bowed is only a fraction of a mm, but that's enough to put it out of focus. The film already runs through pressure plates either side of the imaging apparatus, so there's no obvious mechanical solution available to make it more flat.
The lens is a Russian microscope lens of unknown specification. It was chosen by a process of trial and error and delivers excellent results in most circumstances. It doesn't have an aperture control. My "correspondant" has the idea of cutting a precisely circular hole into a thin metal disc, and inserting that into the optical path to create an artificial aperture. A smaller aperture should create a larger depth of field, all other things being equal. Obviously that will require adjustments to the exposure time and/or sensor gain, or more light behind the film, but that's something he can manage. However, increasing the exposure time means slowing down the frame rate and therefore extending the time taken to digitise a whole film. Increasing the sensor gain risks degrading the image. So he wants to use the largest possible aperture which will deliver the required depth of field.
The questions are:
His situation is that he's got lots of 8mm film to digitise. A frame of 8mm film measures 4.5mm x 3.3mm. He has a custom built optical rig with an industrial CCD and a microscope lens on a precision focusing rail. The CCD is a 2/3" type, which is approximately 8.5mm x 7mm in size. So he's effectively got a magnification ratio of a bit more than 2, and that puts it into pretty extreme macro territory by the standards of the photography that most of us are used to. He estimates the distance from the film to the sensor as about 25cm.
His problem is that some of this film is old and in poor quality, and it's slightly bowed. His depth of field is so shallow that he can't get the whole frame in focus when it is bowed. Obviously with the film only 4.5 x 3.3mm across the amount by which it's bowed is only a fraction of a mm, but that's enough to put it out of focus. The film already runs through pressure plates either side of the imaging apparatus, so there's no obvious mechanical solution available to make it more flat.
The lens is a Russian microscope lens of unknown specification. It was chosen by a process of trial and error and delivers excellent results in most circumstances. It doesn't have an aperture control. My "correspondant" has the idea of cutting a precisely circular hole into a thin metal disc, and inserting that into the optical path to create an artificial aperture. A smaller aperture should create a larger depth of field, all other things being equal. Obviously that will require adjustments to the exposure time and/or sensor gain, or more light behind the film, but that's something he can manage. However, increasing the exposure time means slowing down the frame rate and therefore extending the time taken to digitise a whole film. Increasing the sensor gain risks degrading the image. So he wants to use the largest possible aperture which will deliver the required depth of field.
The questions are:
- Will that work?
- Does it make a difference whereabouts in the optical path the aperture disk is inserted?
- Is there any way of estimating how small the hole ("aperture") would need to be, to increase his depth of field to (say) 1mm?
