How can I increase the flange focal distance (back focal length) of DSLR lens?

by Zythyr   Last Updated June 16, 2016 08:07 AM

I am working on a custom setup where I am using a DSLR lens (Canon 50mm f/1.8) to project an image onto an image sensor (image plane). For my application, I require a very large distance between the last lens and my image plane. The issue I am running into is that a typical DSLR lens has a short (~42mm) flange focal distance (back focal length). I posted a link to the schematic of my setup in comment section.

Is there any way I can increase the flange focal distance of the DSLR lens? I prefer to have 100mm or greater flange focal distance.

I read about retrofocus and I think the solution is is to add a "negative" lens group in front of my DSLR lens. Is there a standard off-the-shelf set of negative lenses I can put in front of the 50mm DLSR lens to increase the back focal length? If so, how do I go about choosing the correct approach?

Answers 3

You could use extension tubes, that add directly their length to the focal distance or you can buy macro negative lenses and you can mount them on the front lens. Also inverting the lens could be a solution, but is less used because you lose AF and aperture.

Alberto Perro
Alberto Perro
June 21, 2016 08:24 AM

If you need to retain infinity focusing capability, one solution might be using lenses with longer focal length, like 135mm or longer. At least some will have the rear element far away from the sensor plane. It does not even have to be native Canon lens, Canon bodies can adapt lenses from other brands, like Nikon, Zeiss etc. - with appropriate adapter.

June 21, 2016 17:36 PM

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.

Brandon Dube
Brandon Dube
July 21, 2016 18:06 PM

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