A common problem with mirrorless designs is that long, especially adapted or output-telecentric designs, still stick out far more than they would eg on a small film SLR or rangefinder. One obvious solution would be making the section of the camera body with the lens mount as thin as possible, and figuratively glueing the bare-die sensor to the back wall of the enclosure (preferrably MAKING the sensor circuit board the back wall. Probably actually feasible with metal-cored FR5 substrates :) ).

Are there less-widespread designs that do that, if not, what technical reasons (unless you want IBIS etc.) preclude such a design?

  • \$\begingroup\$ Just as a data point, my Olympus E-PL7, a camera with IBIS, has its sensor plane mark only ~9mm forward of the outside of the rear body wall under the flup-up LCD... so, even scrapping IBIS and going all-out towards this objective would only gain a few mm... \$\endgroup\$ Mar 13, 2019 at 19:47
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    \$\begingroup\$ "A common problem with..." why is it a problem? \$\endgroup\$
    – null
    Mar 13, 2019 at 19:51
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    \$\begingroup\$ vtc b/c How is this relevant to taking and producing photographs? \$\endgroup\$
    – xiota
    Mar 14, 2019 at 8:12
  • \$\begingroup\$ What photographic problem are you trying to solve? Why do you think designing cameras in the way you describe would make taking certain photographs possible/easier than current designs? \$\endgroup\$
    – Michael C
    Mar 14, 2019 at 20:29
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    \$\begingroup\$ Say you want a full frame camera with an (interchangeable) 50mm lens... almost any option will be significantly larger and less pocketable if you compare it to a small full frame film SLR(!!!!) like an X700 or Pentax ME.... \$\endgroup\$ Mar 15, 2019 at 14:08

2 Answers 2


I think this is a somewhat confused question: as xiota has answered the important thing is whichever of the flange-to-focal-plane distance (FFD) & the rear-element-to-focal-plane distance is smallest. This is the reason why, for instance, adapted SLR lenses stick out a long way on mirrorless cameras (the rear elements of SLR lenses must clear the mirror, which is not the case for dedicated mirrorless lens designs).

It's possible to see this by simply comparing FFDs for different mounts. There is a convenient table here.

  • Fuji X mount is 17.7mm
  • Sony E mount is 18mm
  • M4/3 is 19.25mm
  • M mount is 27.8mm
  • Pentax K is 45.46mm
  • Nikon F is 46.5mm
  • Canon EF is 44mm

It's immediately apparent from this that, whatever you do, you can't mount both a lens from the short end of this list and a lens from the long end of this list without one of them sticking out quite a long way from the body, unless you do something absurd like recess the mount flange into the body by many mm.

Even if this were not a problem, simply moving the sensor backwards in the body of the camera is also extremely nontrivial. Sensors are thicker than film was, and have support electronics which has to go somewhere. Almost all cameras also need to fit, behind the sensor, the screen which goes on the back of the camera, which also needs support and electronics. You can see the differences by comparing camera designs which have the same mount between film & digital bodies. A good example is M-mount cameras:

  • the M3 is 33.5mm thick;
  • the M6 is 33.5mm thick;
  • the Leica CL is 32mm thick, as is the Minolta CLE;
  • the Zeiss ZM is 32mm thick.

It looks like the thinnest a film M-mount camera can be is about 32mm: I suspect the Leicas are slightly thicker because they're more robustly made. Comparing this with the FFD for M mount above there is not much leeway here.

Now look at the digital Ms:

  • the M8 is 39mm thick (not full-frame)
  • the M9 is 37mm thick
  • the M10 is 39mm thick

The mount flange on a film M-mount camera is essentially flush with the front of the camera, so the digital Ms are not thicker because the mount is being moved back further into the body: they're thicker because the sensor, the support arrangements behind it and the support arrangements for the screen take up more space than film did. Looking at the above dimensions we can guess that that 'more space' is somewhere around 5-6mm (and we can also deduce from the M mount FFD that the film, pressure plate & back of the camera on, say, an M6, is a little under 6mm all in: it's fairly easy to confirm this is about right for most film cameras by opening the back of course, & measuring how thick film is).

Now of course, we don't know that it's not possible to make things thinner than that. But I think it's pretty clear that if Leica could, they would – there have been many negative comments about the size of digital Ms. An we know that they are getting sensors which are customised to their requirements (since they make cameras with monochrome sensors), so they are certainly talking to sensor manufacturers. So making the thing thinner is at least hard.

Putting this together we can see a couple of things.

  • It's not possible to mount lenses with widely-varying FFD distances on the same body without either one of them having a mount(-adaptor) which sticks rather far forward from the body (or recessing one of the mounts far into the body, which would I think be absurd). In the case of, say, E-mount, there is a difference of 26mm – more than an inch - between its FFD and the Nikon FFD.
  • Sensors & their support arrangements (including the back of the camera, screen &c in this) seem to be about 10mm or so at least.
  • A camera which can mount a Nikon SLR lens with the mount flange flush to the body would therefore need to be about 54mm thick.

It's then slightly hard to find dimensions for cameras since they often include various grips &c. However the Fuji X-Pro 2 is claimed to be about 35mm thick at its thinnest, which looks like slightly thicker than it needs to be (compare the FFD for its lenses, 17.7mm with the FFD for M mount lenses of 27.8mm and remember the M9 is 37mm thick and you can see how much work Leica must have done to make things thin). But to mount a Nikon SLR lens on an X Pro 2 with the mount flush to the front of the camera the camera would need to be nearly 20mm thicker. That's probably not a tradeoff anyone wants to make.


I would surmise that The sensors were placed at The same focal plane that film cameras had because the relationship the distance between the lens and the sensor needs to be the same as film cameras in order to use existing lenses. If the sensor was moved to the back of the camera a whole new set of lenses would have to be manufactured to accommodate that distance.

When professional photographers were switching over from film to digital they could buy the new digital cameras and still use their old lenses on that new camera. Had the sensor position been changed they would’ve had to buy a whole new line of lenses.

  • \$\begingroup\$ How would this apply to systems like Fujifilm X or Micro Four Thirds, which do have whole new systems of lenses? \$\endgroup\$
    – mattdm
    Mar 14, 2019 at 18:12
  • \$\begingroup\$ This answer applies to the transition from film SLRs to digital SLRs. It doesn't apply to mirrorless, for which all but one (that I'm aware of) abandoned the old lenses for shorter FFD designs. \$\endgroup\$
    – xiota
    Mar 14, 2019 at 18:45
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    \$\begingroup\$ I don't read the question as suggesting a change to the FFD, but rather "Why not push the sensor further back into the body, and make the mounting flange project less by a corresponding amount". \$\endgroup\$ Mar 14, 2019 at 20:58
  • \$\begingroup\$ Perhaps moving it back would increase the engineering and production costs of the new lenses or perhaps a safe space behind the sensor is needed to keep it protected better or perhaps engineers were not thinking outside the box but inside the existing box of where the sensor should be. ? \$\endgroup\$
    – Alaska Man
    Mar 14, 2019 at 21:12

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