This site and others have provided invaluable information on how to do quality VHS captures. I finished capturing my VHS tapes and have started restoring the video with AviSynth and VirtualDub. I will use these programs and Film9 on my captured films once I get there.
My question may be out of range for this forum, but I'm not sure where else to ask it. I am asking it here because I see that Film9 has an extensive thread here. I have followed VideoFred and other people who have done great work creating an extensive film restoration script and Film9. The filmshooting forum would be an appropriate place to ask my question, but it has been closed to new members since May.
I've narrowed my choices to two similar, megapixel-rated C-mount cine lens that should work well with my FLIR Blackfly S camera. Each of the lenses is 50mm and suitable for the 1/1.8 sensor in my camera. The only major difference between the choices is the back focus. One has a 20.1mm focus the other has a 34.5mm focus. How would the difference in back focus affect my setup? To me it seems the longer focus will give a higher magnification than the other. It should be like adding a 14.4mm extension to the 20.1mm focus lens--true? I will be using extensions as required to magnify the 8mm, super 8mm, and 16mm film for the camera.
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I had to Google "back focus" just to make sure I was understanding the term correctly. I think you are using the term incorrectly, and therefore I'm not quite sure what you're asking. Most of the definitions describe how an auto-focus system works, and that they all tend to focus slightly in front or slightly behind the subject. Since you obviously would never use autofocus when transferring film, this does not apply.
I have also seen a use of that term to describe the distance from the back element of the lens to the sensor. This can change the magnification, so that you get more magnification from the same length lens. This is actually an extremely complicated relationship. For instance, I still use the lenses from the first Nikon I ever bought, back in 1970. They still work with modern cameras. However, my first digital camera, the Nikon D70, has a sensor that is much smaller than 35mm film. As a result, it only grabs the image from the center of what is projected by the lens, resulting in a lot of magnification: a 50mm lens becomes the equivalent of about 75mm.
When choosing a camera and lens to capture images from a film chain, there are dozens of variables to consider. In general, you want something that will have a wide depth of field to allow slight curvature of the film, which is tough to completely eliminate, without any loss of focus at the edges. You also need to consider vignetting, something that is quite common around the edges. Chromatic aberration is also quite common.
I can go on, but you really need to test each lens, if you can borrow one. There is no hard and fast rule.
In transfers (assuming reasonable quality film, not warped), there is only one focal plane to worry about and that is the plane of the film. If your rig is set up with a rigid, fixed distance, just set it and forget it (assuming it doesn't get bumped or have such terrible focusing machinery to where it gradually loses focus).
Magnification should be whatever combination gets you to full frame of the sensor with no cropping and no wasteage of sensor area.
Use whichever lens gives you the least aberrations, as long as it's fast enough and fulfills those other above requirements.
I've studied this quite a bit over the last couple of days. Back focus is used in two different ways. Back focus can mean focusing behind the subject when taking a picture. Back focus for a lens is the distance from the back lens to the sensor. On a C-mount lens, the flange distance is about 17.5mm so a lens with a 20.1mm back focus means the back lens is recessed in the housing about 2.6mm from the flange mount point. Another lens has a 34.5mm back focus so its back lens is recessed inside the housing about 17mm from the flange. I believe this lens will have greater magnification because the lens is farther away from the sensor.