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since all lenses are round, full frame cameras' sensors can be 36 × 36 millimeters. Correct? Is it possible, and if so, why it has not been done yet? Would that qualify it as medium format?

  • Are there any common uses for a square format? Most final applications I can think are one of 2,000 different rectangular formats. – JS. Jun 6 '16 at 21:52
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    @JS With square format you shoot first and crop later to your desired aspect ratio. Also no need to rotate the camera body to switch between portrait/landscape orientation, which can lead to using the camera in an unergonomic fashion. Why do you think the Hasselblad V System was so popular for so many decades? – osullic Jun 6 '16 at 23:00
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    I remember marking common aspect ratios on the focusing screen as a reminder to leave enough room in case your customer wanted to use a standard frame. Hasse is a fine camera, but a square format doesn't make aspect/cropping issues go away. – JS. Jun 7 '16 at 0:17
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    You use "roomy" framing for stock photography. Most commission photography sessions have a layout provided by the art director or designer to use for composing your miracle. – Stan Jun 8 '16 at 0:47
  • I just asked a similar question about making round sensors to accommodating any aspect ratio and allowing portrait orientation w/out rotating the camera if the sensor was round. I think this would be a great offering to the market, well worth a few extra bucks for extra sensor area. If most money is spent on lenses it would make sense to use the lenses to the fullest. – Michael Nov 29 '18 at 22:50
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That's not correct. Look at this picture:

36x36 vs 36x24

The green rectangle is a 36x24 sensor. The green circle, which has a diameter of 43.3mm, is the minimal light spot needed for that size. The blue square is 36x36 sensor. The blue circle, which has a diameter of 50.9mm, is the minimal light spot needed for that size. As you can see a lens suitable for 36x24 does not necessarily cover the whole 36x36 frame.

As for the second part of your question - it is possible to qualify such sensors (in my opinion) as medium format because the most of available "full-frame" lenses will not cover these bigger sensors and so you'll need other lenses.

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    Same comment as I wrote to Caleb: draw the diagonals :-) – Carl Witthoft Jun 6 '16 at 15:26
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    Even if one couldn't use a full corner-to-corner square, I would think that an image sensor captures a larger area than the light spot could give a photographer more options for cropping after the fact than one which is entirely within light spot, especially if the material would be masked to a non-rectangular shape after cropping anyway. – supercat Jun 6 '16 at 16:45
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    @kamuro No, i have no sources. But why would a manufacturer build a lens which covers more than needed? – Zenit Jun 6 '16 at 16:48
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    @Alex Manufacturers build lenses that cover more than needed for a very good reason: Lenses don't perform as well towards the edge of the image circle. So if you want good performance at the corners of your sensor, you need a lens that projects an image circle larger than your sensor. – osullic Jun 6 '16 at 23:23
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    Doubling the linear size of the sensor doesn't quadruple the cost. It increases it by a factor of about 10. – Michael C Jun 7 '16 at 19:57
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Is it possible and why it has not been done yet ?

Not necessarily. A 24x36mm sensor will easily fit in an image circle that's too small for a 36x36mm sensor. Specifically, a 24x36mm sensor requires a minimum diameter of about 44mm to cover the sensor. A 36x36mm sensor would require an image circle of about 51mm diameter. A square sensor is certainly possible, but square frames were possible with film as well.

image circle

The diameter of the image circle has to be at least as large as the sensor diagonal. You can see from the diagram that the green 24x36mm rectangle fits within the yellow image circle, but a 36mm square does not -- the corners fall outside the image circle and will be dark.

There's a variety of aesthetic and practical reasons that non-square images are preferred. Indeed, people often prefer images that are even wider than the 3:2 aspect ratio that we have in the typical full frame sensor.

If you prefer square images for any reason, you can set many cameras to record a square image, or you can crop after the fact.

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    It might be easier to understand this answer if you explain that the minimum image diameter is the length of the diagonal of the sensor region :-) . (and draw a picture) – Carl Witthoft Jun 6 '16 at 15:25
  • Sounds good @CarlWitthoft. – Caleb Jun 6 '16 at 16:00
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    Worth including: the square that does fit within the same image circle as 36x24mm is about 30.6mm. – hobbs Jun 6 '16 at 17:01
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The sensors could be made to a square format (though the current diameter would not accommodate 36x36mm, it would need to be about 30mm) if there was a demand for it. But by that logic the question we may actually need to be answering is why aren't sensors circular given that lenses present a circular image?

There were some attempts at circular sensors in the phone space when Nokia were working on the PureView cameras like the 808 before PureView just became a marketing thing for their best cameras. The camera then used the surface area it needed to make the chosen aspect.

Going back to squares - other square sensors are available in the 35mm size range outside of the SLR space (like the SBIG stx-16803 aimed at astronomers - thanks Dan.)

One of the things is that inside a DLSR you also need space for a mirror to move into and also for a shutter mechanism. While the shape of cameras could change to accommodate it, it's a lot of R&D for a small payoff.

It's also telling that even the medium format cameras use a non-square sensor with the Leica S, Pentax 645D, Leaf AFi, Hasselblad H5D and PhaseOne medium-format systems all adopt a rectangular aspect-ratio similar to the 6x4.5 systems than 6x6.

Any increase in sensor area also means a non-linear drop in wafer yield (the number of useful sensors you can get out of a silicon wafer) as demonstrated graphically in this answer - again it's extra cost.

Little payoff is where the idea really falls down - Canon, Nikon, Sony etc all put huge amounts of money into R&D, if any of them thought there was a viable market then they'd go for it. I'm not seeing one and the lack of such DSLR cameras on the market would back that up.

  • Some specialty cameras do use square sensors. eg the SBIG stx-16803 has a 36.8x36.8mm 16 megapixel sensor. Square sensors are relatively common in high end cameras designed for astrophotography (less expensive ones are more likely to use a commodity sensor in more traditional aspect ratios). – Dan Neely Jun 7 '16 at 11:21
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Square is not necessarily larger. 24 megapixels will hold:

3:2 6000x4000 pixels
4:3 5657x4243 pixels
1:1 4899x4899 pixels

And "pictures" have mostly always been rectangular, from most film, and paintings too (like Rembrandt). We did have square medium film, but the print paper was rectangular . It seems a matter of strong preference.

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    This is not 100% accurate: my parents' venerable Kodak Brownie camera was square format, and so were the prints. (now get offa my lawn you young punks, etc) – Carl Witthoft Jun 6 '16 at 15:22
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    @CarlWitthoft - And let's not forget good ol' Hasselblad's square 6cm x 6cm film format. :-) The actual dimensions of the negatives and slides were 56mm x 56mm, if I remember correctly. – Mico Jun 6 '16 at 19:48
  • I don't see how this answers the question. – David Richerby Jun 6 '16 at 22:26
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    @CarlWitthoft My first camera shot square using 126 cartridges: 28mm x 28mm. – user50888 Jun 7 '16 at 1:52
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    Come on Matt, you ought to know about vignetting. Vignetting reaches into diagonal corners, and affects regions of any shape sensor that far from center of lens (the diagonal corners). Sensor shape is NOT a factor, only extent of diagonal is (i.e., distance from center of lens). In the case of 6000x6000, the height will be greater and the width will be less, but the DIAGONAL will NECESSARILY be exactly the SAME (for the same lens to cover it). – WayneF Dec 22 '16 at 16:32

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