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 Canon G10 or Panasonic LX5. Is there some practical or physical limitation due to the size of the camera or the sensor which prevents this from being possible?
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 means the diffraction limit kicks in sooner, reducing sharpness as one stops down.
The smaller format also means that depth of field is hugely increased even at the widest-available apertures — 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.
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
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.
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.
l can only be brief, this is how I have done it: I took an old 2 megapixel pocket-sized digital camera, carefully stripped the objective lens and just replaced the original aperture which has a 3mm hole with a ultra thin brass shim of 0.05mm thick with a precise hole of 0.33mm at the center.
As we know sensors do permit light fluctuations but will claim more exposure time. I am not an expert in such, but the results are good. All I can say is do not go below a 0.27mm hole because the entry of light will suffer and cause soft blur .
I have also done this with a cellphone camera and it works very well , It offers very good pictures with extraordinary depth of field - very useful for miniature landscape. For example 1 sq foot of miniature landscape will look as big as 1 acre of land .
Unfortunately manufacturers do not want to see it because it dose not suit their plans: they want customers to be always buying new cameras hoping that the new product may have these features .
Due to diffraction, and aperture size, F8 on a compact camera is equal to f22 on full frame, and F2 on compacts is equal to f5.6 full frame , and that's why f2 (F5.6) doesn't give the same background blur as F2 full frame. Therefore Sunny 16 rule for compact cameras is sunny 5.6 rule- so instead of 1/100th at F16 ISI 100, you would have 1/800th at F5.6
F8 = F22 F5.6 = F16 F4 = F11 F2.8 = F8 F2 =F5.6
It's a business decision, because you can have very small apertures on cameras - yes as small as 0.33 mm or f/80 with absolute sharpness from 12 mm to infinity.
I say so through experience. The only setback is that it needs 4x more exposure during the day time and is very difficult at night without a strong light and a steady hand. The diffraction of light begins at 0,27 mm aperture or f/92. Today technology allows such but manufactures are not interested in any improvements because it clashes with the set protocol which is not to satisfy the customers but their own ambition.