sensor - Why has no one invented a 4:5 aspect ratio APS-C/35mm-sized DSLR?

People keep inventing different form factors and formats for cameras, but none seems to have hit on what seems (to me) like an obvious one. Why doesn't somebody build a camera with a sensor that has a 4x5 aspect ratio, but the diagonal size of the sensor the same as an APS-C and/or full-frame (35mm) sensor (and a mount compatible with their existing lens line)?

Maintaining the same diagonal size means this would remain compatible with (most?) existing lenses for the same mount. Since typical use would no longer involve cropping, a sensor with exactly the same number of sensels (and therefore, the same pitch/density/well size) as a current one would still have higher usable resolution.

Just FWIW, the sensor sizes seem to work out to around 18x23mm to work with APS-C lenses, and 27x34mm to work with full-frame lenses. Does anybody see a major downside to this idea?

Answer

The mirror has to be a lot bigger for 4x5, particularly at 90 degrees to the axis the mirror flips about. The shutter also gets bigger in the direction it has to move (there's a reason the shutter moves up and down not left to right). This means a slower moving mirror and shutter.

Another consequence of the bigger mirror is the lens mount has to be further away (greater registration distance), which gives you less space for the rear element and so you have to use a retrofocus design for normal lenses. You would also break compatibility with current lenses - despite the fact that the image circle is big enough, the mirror would hit the rear element!

Squarer formats are more common in medium and large format photography where more time is taken over a shot so a slow mirror/shutter can be tolerated (many use leaf shutters anyway) but anywhere where speed dominates, such as sports smaller and more elongated formats allow a little extra speed.

Another area where you tend to find 4:3 sensors is in compacts where you have no mirror to move. The only mass market 4:3 DSLR I can think of were the Olympus 43rds, and they're moving away from having mirrors...

Look at the full frame Canon 5D, it has a slower max shooting rate and a slower sync speed (a measure of how fast the shutter curtains move) than the equivalent APS-c model. Although Nikon have a high speed FF camera, it's extremely expensive, and I bet it would go faster with a smaller mirror.