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I have some 35mm slides that I would like to digitalize. I am thinking about building something like this (modifying a slide projector to feed the slides and mounting a dslr to photograph the slide currently shown by the projector). I am wondering what type of lens would be best for the job. The projects I have seen uses a macro lens but to my question.

For this kind of flat to flat photography where both the sensor and subject is 35mm across what are the advantages and disadvantages of different lens types like macro lenses, projector lenses, and enlarger lenses?

What I have figured out of my own, might contain misconceptions.

  • Macro lenses: no fussing around with adapters and extension tubes
  • Projector lenses: tend to come with used projectors so it will be cheap
  • Enlarger lenses: optimized for flat to flat projection

I obviously realize that I will have to adapt any non-native lenses and add extension tubes until I can focus at macro distances.

So my question: Which kinds of lenses ate suitable for this and what are their respective advantages and disadvantages?

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Enlarger lenses and projector lenses are figured to work flat-to-flat. In other words, project an image of a flat negative or transparency onto flat paper or screen. The camera lens is figured to work a 3 dimensional world (objects at different distances) and project their images on flat film or flat digital image sensor. To accomplish, enlarger and projector lenses are generally symmetrical in design. The camera lens is generally un-symmetrical; thus when tasked to project flat-to-flat they display considerable curvature of field. For this reason most camera lenses with large apertures tend to yield substandard results as projector lenses.

Let me add, when stopped down to f/8 or f/11 satisfactory results are likely. The bottom line is, projector lenses with symmetrical figures are preferable. Another point, the visual focus of the enlarging lens and the chemical focus of the paper are different. Enlarger lenses are color-corrected to avoid such inaccuracy.

The macro lens is also figured to operate flat-to-flat, because when working close focus, depth-of-field is very shallow. When doing close-up work with a standard camera lens, higher sharpness is obtained by reversing the lens. This is because the camera lens projects on flat film or digital sensor -- thus the obverse will serve better when the subject is a flat surface.

An enlarger and a projection lens are generally tasked to project an enlarged image. High projection magnifications require a more complex design, especially when projecting color images. In other words, color correction planes for projection tasks are very different as compared to the camera lens task.

  • So projector lenses create less distortion than a regular camera lens? – MicroMachine Jan 28 at 20:14
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    @ MicroMachine -- When imaging flat-to-flat, a projector/enlarging lens is preferred. When imaging a 3D world on a flat film or imaging chip, a camera les is preferred. – Alan Marcus Jan 28 at 20:53
  • I'd love to see examples of both results - I feel visuals would speak more than words – MicroMachine Jan 28 at 21:06

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