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I have a Canon PowerShot G7 camera. In Manual mode it supports 1/2500s shutter speed at f/8.0, whereas at f/2.5 it supports only upto 1/1600.

But, should it not support higher shutter speed at higher aperture size and maybe relatively lower shutter speed at lower aperture size?

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It should support the same shutter speeds, regardless. Have you taken a photo at both these settings (f/8 @ 1/2500 & f/2.8 @ 1/1600)? Post the results. Are they the same exposure? Could be playing with the ISO settings. –  BBking Dec 7 '13 at 10:36
    
@BBking The two exposure values are not equal. f/8 is three stops slower than f/2.8. 1/2500 allows a little over 1/2 stop less exposure than 1/1600. So f/8 @ 1/2500 is 3 1/2 stops darker than f/2.8 @ 1/1600. –  Michael Clark Dec 30 '13 at 7:51
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4 Answers

Since the limitation appears to be with regard to specific positions of the aperture diaphragm it is most likely a mechanical limitation of the camera's design. The limitations you have described are clearly outlined on page 49 of the Canon PowerShot G7 Advanced Camera User Guide, so this is expected normal behavior for this camera. As the focal length increases the effective aperture will change for the same position of the diaphragm mechanism so it appears the specific shutter speed limitations apply to a specific position of the diaphragm.

This would lead one to assume, without any confirmation available from Canon that we could find on this point, that the camera has a leaf shutter. The wider the selected aperture is (lower the f-number) then the amount of time it takes the mechanical parts to move from being closed to open all the way becomes more critical to the amount of exposure on the edges of the frame and depth of field of the entire frame, thus the longer/slower the fastest possible shutter speed is for that selected aperture value.

Don't forget that aperture numbers are the denominator of a fraction. At f/2.8 the effective aperture is 1/2.8th the focal length of the lens. At f/8 the effective aperture is 1/8th the focal length of the lens. The aperture settings that limit the shortest shutter speed to 1/1600 second are the widest settings (f/2.8 at wide angle), those that limit the shutter speed to 1/2000 second are the mid-range apertures(f/3.2-3.5 at wide angle), and those that allow a fastest shutter speed of 1/2500 second are the smallest apertures (f/4 and up at wide angle). All shutter speeds below the shortest value for a given aperture are also available with that aperture.

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Actually, i felt it counter-intuitive for a camera to support higher shutter speed(1/2500) when amount of light striking the sensor is low(f/8.0) and lower shutter speed(1/1600) when amount of light striking will be high(f/2.8). –  Pranav Dec 6 '13 at 10:29
    
@pranav If you increase the aperture by one stop, and decrease the shutter speed by one stop, then the amount of light striking the sensor is the same –  laurencemadill Dec 6 '13 at 13:59
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@Pranav I see what you're saying. That might make sense if the limit were based on some maximum total exposure the camera could handle, but apparently that's not it — instead it is some physical thing. That makes sense this way around, because the faster shutter speed corresponds to a smaller opening. –  mattdm Dec 6 '13 at 14:13
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It's not necessary for the shutter to function as the aperture iris in order to see limits like these (in fact, that would complicate things mechanically to an unnecessary degree). The shutter typically opens centre-out at a finite speed (and closes periphery-in). Even if the shutter can fully open and close in something less than 1/2500s, the outermost part of the opening will be covered for much of that time (it's the last part uncovered and the first part re-covered), so the effective transmission (T-stop) is reduced at wider apertures when the shutter speed is high enough... –  user2719 Jan 1 at 0:09
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(cont'd) ...and the depth of field will increase because of the reduced contribution to the exposure from the outer edges of the lens. Ansel Adams covers this in The Camera, since knowing the differences in exposure at wide apertures and (especially) high shutter speeds is part of calibrating for the full-blown, graphs-and-calculations Zone System. –  user2719 Jan 1 at 0:14
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It should support the same shutter, regardless of aperture. If designed with separate shutter and iris like DSLRs.

This is not the case with shutters where the aperture and the shutter is in fact the same part. So the wider it is, the longer it has to travel to open up and close. Usually referred to as diaphragm- or leaf shutters. These are typical in top compacts and medium-large frame cameras, e.g. accordion style view-cameras, where the shutter (e.g. CoPal) was built into the lenses and controlled by spring systems like oldschool clocks.

I have searched for info about which type of mechanical shutter the G7 has, but only found confirmation that G11 and G15 has leaf shutter, and that is the reason for the limitation of shutter speed at larger aperture opening, because it has to close and open a farther distance. Since G7 has the same limitation I will assume that it has the same leaf shutter.

And as M.C. point out the aperture number is relative to focal length, so it is in fact the same physical aperture size that sets a certain limit, even though at first glance it looks arbitrary.

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Have you checked any of the G7 documentation? –  Michael Clark Dec 6 '13 at 9:41
    
The Sony RX1 has a leaf shutter which means the max shutter speed is reduced when shooting wide open, I wouldn't have thought a G7 would have one, but it's not impossible. –  Matt Grum Dec 6 '13 at 11:35
    
it is the only reason to have a dependency between F and s, though. –  Michael Nielsen Dec 6 '13 at 11:37
    
@MattGrum I think G7 will not have leaf shutters because its maximum shutter speed is 1/2500 which is 5 times more than maximum achieved for leaf shutters as per leaf shutters(See Cons) –  Pranav Dec 6 '13 at 13:07
    
@Pranav that article is talking about interchangeable lenses for medium format cameras. The limiting factor is the size of the iris. The G7 has such a small iris (on account of it's small sensor and shoft focal length lens) that a much faster leaf shutter is possible. However I would have thought such a camera is more likely to have an electronic shutter. –  Matt Grum Dec 6 '13 at 13:37
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It is quite normal for a leaf shutter to support a faster shutter speed when the lens aperture is physically smaller, i.e. at f/8 instead of f/2.8. So the question becomes 'does the Canon G7 have a leaf shutter?'

There's a simple test for that.

Set the camera's shutter speed to 1/60s and aim it at a blank wall. Force the flash to fire and take a photo. The result should be evenly illuminated, more or less. Now set the shutter speed to 1/1000 or so, and take another flash photo. If the result is still evenly illuminated, more or less, then it's a leaf shutter. If part of the photo is black, especially if it's only one side, top, or bottom, then it's a focal plane shutter.

(There is one other option – a shutter that's partially or entirely electronic. But as the camera does have a mechanical sound when firing this is unlikely, and electronic implimentations are less far less common than mechanical shutters.)

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Aperture blades emit a mechanical sound as well, unless shot wide open. –  Esa Paulasto Dec 30 '13 at 0:20
    
They may, so I checked the G16 that I have handy, and confirmed that the shutter sound is consistent at all apertures, but not at all shutter speeds. –  mpr Dec 31 '13 at 17:40
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In theory it should support the same shutter speed for any other settings, although my Canon's max shutter speed is reduced to 1/250 when the flash is enabled. This is by design because any faster than that the flash can't sync with the shutter properly, and part of the image is obscured by the closing shutter.

I suspect then that it's probably by design, could be any number of reasons, protecting the sensor from excess light (although I don't really see logic in that), it could even be a limitation with the way Canon have designed the shutter and aperture mechanism in the camera.

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