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Why is it that on compact digital cameras the aperture never seems to go any smaller than about F8 ? Even on high-end compacts such as the Cannon G10 or Panasonic LX5. Is there some practical or physical limitation due to the size of the camera or the CCD which prevents this from being possible?

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4 Answers 4

up vote 19 down vote accepted

Although the relative aperture numbers — the ƒ stops — are the same regardless of format, the actual focal lengths of the lenses on small cameras are quite low: 5mm or 6mm at the wide end. That in turn means that the real aperture is small, which is why the diffraction limit kicks in sooner.

The smaller format also means that depth of field is hugely increased — even wide open at f/2.8, a camera like the Canon G10 has an infinite depth of field if you're focusing farther away than a few feet. So, there's not much difference in that aspect of changing aperture, so from that point of view there's no point even bothering. And that's presumably why there's usually not many choices besides wide open and one closed-down stop like f/8. (Because everything is small, and competitive price pressure significant, adding the mechanics for more intermediate stops is easily deemed not worth it.)

The other aspect of a smaller aperture is, of course, controlling exposure in bright light, without artificially dropping ISO beyond the sensor base or using very high shutter speeds. Some compact cameras actually use a dark neutral-density filter instead of closing down the aperture, specifically to avoid issues with diffraction.

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1  
Thanks for your very comprehensive answer - I never actually knew about the depth of field effect (or lack of effect above a certain distance) on these cameras. I have often wondered why I didn't seem to get the expected shallow depth of field with my LX5 despite often shooting with the aperture wide-open at F2. –  Matthew Dresser Feb 25 '11 at 13:54
    
I had the same doubts about my RX100 with f/1.8 (: –  Igoru Jun 2 '13 at 15:53
    
Sorry if I recover this old post: does the aperture with a smaller sensor mean the same in terms of shutter speed (with same ISO)? –  clabacchio Jul 11 '13 at 22:14

Yes, it's a physical limitation due to the size of the sensor (and thus the size of the individual photosites) that means that diffraction becomes a limiting factor sooner than it does with DSLRs, hence the aperture doesn't go smaller than f/8.

Older compacts do have a larger range, my old Fuji Finepix s602 went down to f/11

see also:

What is a "diffraction limit"?

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The biggest reason for smaller apertures is to increase the depth of field at a given focal length. Point and shoot cameras, with their small sensors, already have huge depth of field when compared to their larger cousins at any given focal length, so the smallest aperture is going to be selected to maximize depth of field and retain sharpness in the image (too tight an aperture leads to diffraction and loss of sharpness). So, it's a balancing act.

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How does sensor size influence depth of field? Isn't it really that small sensor size ==> small focal length ==> small depth of field? –  whuber Feb 25 '11 at 5:42
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@whuber it's small sensor size ==> small focal length [to get normal coverage] ==> large depth of field –  gerikson Feb 25 '11 at 6:04
    
@geriksen Sorry, and thank you; I meant to write "large depth of field," but the lure of repeating all the "smalls" (and the late hour) distracted me! –  whuber Feb 25 '11 at 6:12

And don't forget that many of those cameras have no adjustable aperture at all. They're always shooting wide open, any adjustment that might happen being done through changing the sensitivity of the sensor, no moving parts required.

These cameras aren't designed for people with an interest in photography, so there's no need for things that can be adjusted by the operator. In fact your average user doesn't want to adjust anything, just point and click, and the cameras are designed with that in mind.

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