Originally Posted by
Merijn
[*]The length of the bellows (I actually understand how moving the lens back and forth focusses the image, I'm unclear on how bellows length affects shutter time)
Lenses are described by their maximum aperture and focal length at infinity. For example, a lens might be described as an f/4 200mm. What that means is (more or less) that when you focus a 200mm lens on a far object (more than 60 meters or so) the lens will be 200mm from the film plane (the focal length is 200mm).
Aperture is defined as focal length divided by the diameter of the hole in the lens. So, in our example, that would be 200/x = 4, so x (the diameter of the hole) = 50mm.
To focus on something closer than infinity, you need to move the lens farther away from the film plane. That clearly changes the focal length. Say, for example, we focused on something close, and then measured the distance between lens and film plane and it is now 400mm. At infinity focus the lens is an f/4, but if we have a focal length of 400mm, and a hole of 50mm, then our aperture for this case is f/8 (400/50 = 8). Keeping the same hole size, you will have to adjust shutter speed one stop less than shooting a subject at infinity distance to get the same exposure. There are tables available to tell you how much addition exposure you need to provide for a given bellows extension (called bellows factor table), but they are based on the change in aperture described by the simple equation above.
[*]The plane of the ground glass and the plane of the film need to be in exactly the same place. I've seen so many different setups ranging from combined fresnel lenses + hand sanded glass to people just using baking soda paper for a test. If you're using a 2mm plate of glass, does the film need to be flush with the front plane of the glass or the rear plane?
Normally the frosted face of the ground glass will be against the frame, so thickness of glass does not matter. When you put the film holder in, the face of the film should be in the same plane that was occupied by the frosted face of the glass.
[*]The pitch and yaw of the lens assembly affect depth of field
Not really. They change the plane of focus. Normally the plane is parallel with the film plane, but when you apply tilts you can modify the plane of focus so that it is tilted as well. In the most common application, this allows you to get a foreground and distant background in focus. A true plane has no thickness. Depth of field is the zone of acceptable sharpness on either side of the plane of focus, and you can think of it as adding thickness to the plane. When you tilt the plane of focus that thickness goes with it, but remains perpendicular to the plane.[/LIST]
Another thing I'm wondering about is how does the film back ensure a light tight seal with the camera itself? In videos it usually just looks like a drawer that get's pushed into the camera. An overlapping edge covering the seams between the film slide and the camera. Is that really sufficient for a light tight seal?
Yes, its just a lip and a rebate. Light will not go around corners unless you have a reflective surface.
Anyway I'm at the information gathering stage right now so any pointers you have are welcome. I'm also really interested in sourcing a suitable lense / shutter assembly, bellows and ground glass plate since I doubt I'll be making those myself.
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