Can I increase the back focus distance of a DSLR lens for a custom sensor setup?
Asked 6/16/2016
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I’m using a Canon 50mm f/1.8 in a custom imaging setup and need much more clearance between the rear of the lens and the image sensor—ideally 100mm or more. A typical DSLR lens is designed around a much shorter flange/back focus distance. Is there a practical way to increase that distance while still forming a usable image on the sensor? Would front-mounted negative optics, extension tubes, or lens modification help, or is a different type of lens the better solution?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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You cannot alter the back focal length of a lens without changing its optics. Specialized adapters exist to do this, but there is usually a large loss of image quality.
Your best bet is to not use a DSLR lens, but to use a large format lens designed for use with bellows. They will have considerably longer back focal lengths and allow you the requisite clearance.
Use the right tool for the job, don't contort a tool to do something it isn't intended to do.
Originally by user40937. Source · Licensed CC BY-SA 4.0
user40937
10y ago
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Generated from our catalog & community — verify before relying on it.
A DSLR lens is designed to project a sharp image at a specific flange distance, so you generally can’t increase its back focus a lot without changing the optics or the mount. In practice:
- Extension tubes increase lens-to-sensor spacing, but they mainly shift focus toward close-up/macro use rather than giving you a clean long-back-focus solution.
- Optical adapters or added negative optics can alter the path, but usually at a noticeable image-quality cost.
- Mechanical modification is possible in some cases, but it’s specialized work and not a general solution.
For a requirement like 100mm+ clearance, the most practical answer is usually to use a lens designed for longer back focus or bellows use, such as a large-format or other lens intended for that geometry. That’s typically a better fit than trying to force a standard DSLR 50mm lens into a setup it wasn’t designed for.
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