Canon Continuous Mode - different exposures

Started 8 months ago | Questions thread
John Sheehy Forum Pro • Posts: 20,663
Re: Canon Continuous Mode - different exposures

JumboJunior wrote:

I am using Canon 80D and a Canon 5Dmk2 cameras and I notice the same behaviours with both cameras.

I am using AV mode with the aperture set to F5.6 (happens with other apertures as well).

I select AIServo and high speed drive (in other words continuous shooting).

I am using back button focussing, with the button held down.

When I take pictures of a subject in this mode, I get different shutter speeds between the shots in a kind of a pattern. The first may be 1/1250, the second 1/1000, the third 1/1250 and the fourth 1/1000.

Any ideas why this is happening, I would have expected all of the shots to be the same as the composition and light has not changed between shots (as far as I can tell).

I get the same kind of result if I use single shot mode, and depress the shutter multiple times in quick succession.

You're on the meter's cusp. It's affect is not analog; just shifting the camera a little or a tiny change in the lighting of 1/10000000 stop can toggle between the two of the camera's coarse 1/3-stop shutter speed values. Of course, this can make some images almost 1/3 stop apart in apparent brightness. You avoid this by using manual mode, or exposure lock (which I haven't tried recently, so I don't know how that works).

Why doesn't the camera have nearly virtual analog Tv values (or auto-ISO values)? That's a good question. There is some benefit to the quantized Tv/Av product, though, as slight changes in composition may fail to change needed exposure, which may be ideal. Either approach has its problems, and different circumstances benefit from different methods.

If the light source is not steady and is strobing quickly, then it is difficult to meter for unless the camera has anti-flicker intelligence and can see the pattern and always expose at the same light phase, but that can't help with every situation, especially with fast shutter speeds which may cause the exposure to be different and the light color different, up and down the frame, even if consistent between frames. If you are using a two-curtain focal plane shutter, any "shutter speed" above your normal flash sync speed on that camera is basically fiction. The shutter is never fully open for as little as 1/1250 second; a slit of open curtain a fraction of the height of the frame passes over the sensor vertically, over a period several times the 1/1250s.

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