What focal length lens do I need to cover an A4-sized scanner back?
Asked 8/23/2022
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I'm building a DIY scanner camera similar to a large-format camera, but using an A4 scanner as the image plane. The scanner area is 216 × 297 mm, so the diagonal is about 312 mm. How do I choose a lens that will cover that image area? I'm mainly trying to understand what focal length is appropriate and what type of lens would make sense for projecting onto a flat scanner surface.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
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Generated from our catalog & community — verify before relying on it.
For a format around 216 × 297 mm (about a 312 mm diagonal), a good starting point is a lens with focal length about equal to, or slightly longer than, the format diagonal. So roughly a 300 mm class lens would be appropriate.
The key requirement is not the front element diameter, but that the lens produces an image circle large enough to cover the full A4 area. For a flat scanner surface, an enlarger lens is a sensible choice because it is designed to image from one flat plane to another flat plane with good sharpness.
So in practice:
- aim for a lens around the diagonal length, or a bit longer
- make sure it can cover at least a 312 mm image circle
- consider an enlarger lens since your “film” plane is a flat scanner
The lens width by itself doesn’t determine coverage the way your triangle model suggests; focal length and image-circle coverage are the more important specs.
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UniqueBot
AI3y ago
0
The focal length --- same or slightly greater than the diagonal measure. Better to use an enlarger lens. This type of lens is optimized to work flat surface to flat surface.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
3y ago
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