I have a Minolta Auto Bellows 100mm lens (no focussing functionality, just aperture), that I am trying to use to duplicate slides with a A7R III and Minolta bellows I, focussing rail and slide duplicator attachment. My goal is to take a 1:1 photo of a 35mm slide, but I'm having trouble getting the lens to focus within the range of the bellows and focussing rail. I can get the lens to focus on a slide that's about 400mm from the film place, but that's well beyond the range of the bellows.

From what I understand, the farther the lens is from the focus plane of the camera, the shorter the focal distance. I thought I should be able to extend the bellows a bit, then use the focussing rail to get the slide in focus, but no combination of lengths seems to work to get the slide in focus. Can any give me a good explanation of how this is supposed to work, or if it can work at all with a 100mm lens?

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Can any give me a good explanation of how this is supposed to work, or if it can work at all with a 100mm lens?

If you're set on using a 100mm lens, you will need to add some more distance between the camera and bellows, using extension tubes.

For 1:1 slide copy work, that bellows was intended to be used with a 50mm lens (or similar focal length). According to Rokkorfiles.com, the Auto Bellows 1 can achieve 0.7–3.0 magnification with a 50mm lens.

I use a Nikon PB-4 bellows (which is very similar to the Minolta Auto Bellows 1) with my full frame body with a 55mm ƒ/3.5 Micro to do my slide copying. The lens is quite sharp and has almost no field curvature (important for slide reproduction), and importantly, can be had easily for under $150.

1:1 (life-size) is achieved when the object is 4 times the focal length from the image plane (film plane or sensor plane). If a 100mm lens is mounted, the lens-to-image-plane distance will be approximately 400mm. The lens position will approximately split this distance i.e. 200mm from object and 200mm from image plane. These are approximate points because your camera lens consists of multiple lens elements, each with different powers. In other words, the center of the lens barrel will likely not be the optical center of the lens system. The above measurements are true for the rear nodal location of your specific lens. This location is a variable based on the optical formula (figure) of your lens. Meaning the rear nodal might fall forward or rearward of the physical center of the lens barrel.

  • How is 4x focal length determined? How can it be calculated for different reproduction ratios? – xiota Dec 6 at 0:34
  • L = distance object to image p = object distance q = image distance L = p + q ---- m = magnification L = p(1 + m) L = q(1+m) / m these are some of the many optical formulas. – Alan Marcus Dec 6 at 2:20
  • @xiota It comes from the thin lens equation, 1/ƒ = 1/d_o + 1/d_i. At 1:1 magnification, d_o = d_i, so 1/ƒ = 2/d_o ---> d_o = 2ƒ, d_i = 2ƒ. Total object-to-image-plane distance is d_o + d_i = 4ƒ. (Of course, this all assumes a literal thin lens, where there is no physical distance between the 1st principal plane and 2nd principal plane. For real lenses, the PPs are not in the same place. That is, the actual lens body has substantial distance, so that roughly needs to be added in to the 4ƒ distance). – scottbb Dec 6 at 4:03

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