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After seeing the Pentax Q which uses a sensor with 5.6X crop factor that is miniature compared to its mount and now the Nikon 1 system with its 2.7X crop factor also looks like its sensor is small compared to the mount, I am curious to know why these mounts are not much smaller?

At the other extreme, compare this to a full-frame model (technically we see the mirror but the sensor is almost the same size) whose mount is not much bigger than the circle it takes to fit the rectangular sensor.

So:

  • Why the mounts were not shrunk in proportion to sensor size?
  • Is there an advantage in having a bigger mount-to-sensor ratio?
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  • \$\begingroup\$ Maybe they just wanted to keep the mount large to accommodate large/heavy lenses? \$\endgroup\$
    – dpollitt
    Commented Nov 2, 2011 at 15:47
  • \$\begingroup\$ It may be nothing more than ergonomics -- I remember the Pentax Auto 110 being very fiddly (the "flatness" of the lens/body contact wasn't nearly as positive-feeling as my 35mms), and I didn't have to worry about going through the mount side to clean/maintain the sensor (you got a new one free with every film cartridge). \$\endgroup\$
    – user2719
    Commented Nov 2, 2011 at 15:58
  • \$\begingroup\$ Nothing more than a gut feeling, but I would imagine that the autofocus motors and other electronic connections and actuators would put a limit on how small the lens barrel would be, and then they didn't want to have a small mount on larger barrel lenses. The counter example is a Lecia rangefinder, where the lenses are often quite small even though it is a full frame 35mm, partially (I imagine) because they have no aperture actuator or AF motors. \$\endgroup\$
    – David Rouse
    Commented Nov 4, 2011 at 15:37

4 Answers 4

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I'm going to go out on a speculative limb here and suggest a few possible answers:

  1. Ergonomics for ILC. If the lens is too small, it's probably not as easy to handle or may be perceived that way.

  2. Optics. For handling wider apertures at longer focal lengths, you probably can only make the lens diameter so small and still make it practical for production costs and retail price.

  3. Adaptations. So, if there is an adapter (in the case of the Q, from K to Q), then the larger opening may reduce or eliminate the need for optical correction in the adapter or even make it possible.

It may be some combination of all those as well, or none at all. I'm speculating. :)

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    \$\begingroup\$ don't forget backwards compatibility. The DX size sensors were the originals, and placed in bodies designed for 35mm film, using mounts for that size. Those mounts were kept so buyers could benefit from decades of available lens hardware. \$\endgroup\$
    – jwenting
    Commented Nov 3, 2011 at 7:45
  • \$\begingroup\$ Those are pretty good guesses. #2 seems most probable, followed by #3. Ergonomics gets unfortunately very little attention from most brands and they seem to like to boast how small things are despite being impractical. That is why most small ILCs do not have a grip. \$\endgroup\$
    – Itai
    Commented Nov 10, 2011 at 14:15
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I have the suspicion that — particularly with the tiny Q — they wanted the ability to increase the sensor size in the future without abandoning the system. However, this is simply based on the same observation you've made, and not backed up by any knowledge of the actual image circle size.

I know this is a speculative answer, but I doubt anyone from Pentax or Nikon will show up with an authoritative one.

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    \$\begingroup\$ Sorry but I do not think this makes much sense because the lenses are designed for the smaller imaging circle, so the only thing they can keep of the system is the size the the mount if they change the sensor size. \$\endgroup\$
    – Itai
    Commented Nov 2, 2011 at 16:03
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    \$\begingroup\$ Has anyone checked the imaging circle of the lenses? \$\endgroup\$
    – mattdm
    Commented Nov 2, 2011 at 16:47
  • \$\begingroup\$ Checked not so much but it looks awfully small to me. The rear elements of the two Nikon 1 lenses I have here (10-30mm and 30-110mm) is there small. On the 30-110mm it measures 1cm diagonally. \$\endgroup\$
    – Itai
    Commented Nov 2, 2011 at 16:58
  • \$\begingroup\$ I suspect that it can still be the case. They can introduce a new model with a larger sensor which will still accept the older lenses with a crop (or vignetting — half of them are labeled "toy" lenses anyway), and then gradually change-out the lineup towards larger-circle lenses. \$\endgroup\$
    – mattdm
    Commented Nov 2, 2011 at 19:19
  • \$\begingroup\$ +1 for prescience :-) \$\endgroup\$
    – Philip Kendall ♦
    Commented Aug 27, 2014 at 11:15
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Don't forget that Pentax has committed to an in-body shake reduction system and thus there will be mechanical limits to how much can fit inside the mount.

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Let me add another speculation: brand positioning. These cameras are positioned for professionals or advanced enthusiasts who need good quality combined with small size and weight. The large mount gives a "professional" feeling. The small mount might make the camera feel "cheep".

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