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).

Here is the schematic of my setup:


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?

  • Schematic: i.stack.imgur.com/jvzLX.png Explanation of how wide angle lenses: photo.stackexchange.com/a/4492/53134
    – Zythyr
    Jun 16, 2016 at 6:02
  • 1
    It sounds like you're looking for extension tubes -- they'll let you increase the flange focal distance. But I'm confused why you think the solution of adding to the front of the lens will increase the space at the back. Jun 16, 2016 at 10:37
  • @DanWolfgang Possibly to retain infinity focus?
    – mattdm
    Jun 16, 2016 at 12:26
  • 2
    From your last comment, it sounds like the real question here is, "is there any way to mount Canon-mount lenses on Nikon bodies?"
    – scottbb
    Jun 16, 2016 at 18:32
  • 1
    @Zythyr: based on that description I think you want this: photo.stackexchange.com/questions/62000/…. Yes? Jun 16, 2016 at 19:10

4 Answers 4


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.


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.

  • You can use the extension tubes, but this not only extends the rear focal distance, it also increases magnification and decreases the amount of light hitting your sensor significantly. Light falls off the square of its distance, including that distance within your camera. Changing the distance changes the magnification.
    – bethanyP
    Jul 11, 2016 at 4:23

It can be done without adding optical adapters, but it requires some machining knowledge. An eBay vendor ("Retro Foto House" on YouTube, "kosbik84" on eBay) does it for vintage Helios lenses, adapting them to Nikon F-mount cameras. He made a YouTube video tutorial to show us the process, "Helios 44-2 on NIKON (proper infinity adjustment)."


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.

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