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Is it something like the sensor captures a circular image and then what we get is the cropped version?

Something like this:

rectangle inscribed inside a circle

Or did I get it completely wrong?

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2  
Cropped image should be labeled "camera sensor" or "film" and you'd be dead on. –  Jordan H. Jul 19 '10 at 16:26
7  
Is it just me that wishes they'd give us round sensors and let us do the cropping? All that lost image data. :( –  Peter Boughton Jul 20 '10 at 13:20
    
@Peter Boughton: I agree. –  Lazer Jul 21 '10 at 12:18
    
There was an April Fools joke in 2010 about a company named "Rokton" developing a circular DSLR sensor photographyhappenings.blogspot.com/2010/04/… A lot of real disappointment out there. –  Craig Walker Aug 12 '11 at 14:50
1  
@Peter a round sensor would be a very bad idea: you'd need a bigger shutter and mirror which would mean you'd have to move the lens further away from the sensor, compromising your ability to offer a good WA lens. The pentaprism would have to be huge also. Unless you used a small sensor, which has it's own drawbacks plus some lenses have baffles/hoods which project a rectangle anyway. And due to lack of options for displaying/printing circular images you'd end up wasting huge amounts of sensor space by cropping. So you'd need a new camera lenses and mount, for a tiny potential market! –  Matt Grum Aug 12 '11 at 16:26

5 Answers 5

up vote 16 down vote accepted

No, you are correct and the reason photos are the size and shape they are just has to do with the size and shape of the film or digital sensor used to capture the image from the lens. The rest of the image that falls to the top, bottom, and sides of the film or sensor are just not recorded.

...interesting thought, I never really realized the circular image/rectangular photo thing before!

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The lens will always produce a circular image - the sensor however is rectangular, and only captures the portion of the image falling on it.

With some "digital optimised" lenses (be they third party or something like Canon's EF-S mount), the circle is much smaller, as they tend to be optimised for an APS-C sized sensor. Conversely, you can get Tilt/Shift lenses which generate a much larger circle of an image to allow for the shifting.

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I was writing this up as well...I'm a slow typer :(. –  Marc Jul 18 '10 at 17:04

It's the way you've illustrated. The only exceptions are circular fisheye lenses, which have it other way around. (E.g. you have a full 180° circle in center of your photo, and everything else is dark. See pictures from Peleng 8mm Fisheye.)

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I would change it to "The only exceptions are some circular fisheye lenses" - Canon's lenses cover the whole frame, AFAIR. –  ysap Aug 12 '11 at 7:44
    
@ysap: agree, Samyang 8 mm is also circular fisheye and also produces stardard rectangular photo (at least on APS-C). –  Juhele Aug 12 '11 at 12:04
    
@ysap: There are two types of fisheye lenses. Circular and full-frame. If a lens produces standard rectangular photo, it's not circular fisheye. See en.wikipedia.org/wiki/Fisheye_lens#Types_of_fisheye_lenses –  che Aug 16 '11 at 11:12
    
@che - you are right - from some reason I overlooked the word "circular" (although I did mention it in my correction...) –  ysap Aug 16 '11 at 13:02

As seen from your illustration, some of image circle always gets wasted by missing the imaging area. You still have to pay for and carry the glass that creates the unseen parts; a lens with rectangular image would create an ugly blocky bokeh. At least those croppped parts are the lowest quality parts of image (same quality that you have in the recatngular image's corners).

The square image you get from 6x6 medium format cameras is probably the closest to maximum utilization of image circle you can get (not counting lenses with undersized image circles, such as circular fish-eyes or lenses designed for smaller sensors, and for those square is most efficient use of imaging area).

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Not only is the film/sensor in a camera rectangular, but so are our display mediums: LCDs (and their pixels) and photo paper (mostly because their printers are designed around rectangles). Not to mention photo albums. We live in a very orthogonal society.

So, somewhere in the photographic chain, you're going to have to turn that circle into a rectangle.

As it happens, it's cheapest to do that high up the chain: both film and digital sensors are more economical when produced as rectangles. Having circular versions of those would pretty much be a waste, and would be more expensive to boot.

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