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I was puzzled recently when I did some test shots with my Canon A800 by shooting the same (landscape) scene with and without full optical zoom and with the camera program set to SCN - Sunset. (Sunset mode seems to produce really good DOF regardless of aperture and/or shutter speed.) The focus was equally sharp for both and the shutter speed for the zoomed shots was almost consistently double (or more) that of the non-zoomed shots. All of which seems somewhat inconsistent with generally accepted rules of thumb so I wonder if the camera is making some kind of 'intelligent' adjustments.

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Hi Gabe, welcome to photo stack exchange! I'm not quite clear on what your question is, are you wondering basically what the scene mode "sunset" does on your A800? – dpollitt Oct 31 '11 at 15:12
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I think it'd be helpful if you'd spell out the "generally accepted" rules you're wondering about — there's many such rules and our guess for what you're referring to could easily differ from your understanding. – mattdm Oct 31 '11 at 15:59
Thanks for your answers. I should probably have explained that I took 40-odd shots at the beach in the afternoon on a partly cloudy day. I was taking a series of shots in all the SCN modes so I could get an idea of what kind of effect each would have. The Sunset mode shots turned out to have the best exposure and depth of field focus; I was actually quite pleasantly surprised with the results of this 'starter' point and shoot. The thing that puzzles me is that almost consistently the Zoombrowser EXIF data showed that at full optical zoom shutter speeds were as high as 1/1000; without full opti – Gabe Oct 31 '11 at 17:10
I have done a Google search along the lines of increasing shutter speed for the A800, but the consensus seems to be that with such a low end pocket camera it's just not possible without introducing a lot of noise - that's why I thought I might take a different approach and try to duplicate the rather good results from the Sunset mode in P mode. By good DOF I meant sharpness from the foreground to the horizon - I found that most bird photographers prefer a shallow DOF so that the subject stands out, but for creatures in/on the water my preference is for the water to be in sharp focus as well as – Gabe Oct 31 '11 at 18:15
mattdm - I read posts here every day but this was the first time I tried posting and when I tried to figure out how to do as you suggest, I was stumped. For future reference, can you give me a quick how-to? – Gabe Nov 1 '11 at 14:39
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The Canon Powershot A800 IS features a variable maximum aperture from f/3.0(W) - f/5.8 (T). What this means is that as you zoom in the maximum aperture will be smaller, and less light will be allowed into the camera sensor. Exposure is combined of three variables, your sensor sensitivity, shutter speed, and aperture. If you zoom in, your maximum aperture will have to decrease if you are already in low light such as at sunset. Thus, either your film speed will have to increase, or your shutter speed will have to decrease to a longer speed.

You mentioned that your shutter speed doubled or more when you zoomed in. If this is the case, either the scene is becoming more bright from the sun, or the sensor sensitivity is being increased in the camera. The ISO(sensor sensitivity) is set to Auto ISO in the sunset mode of your camera, so this is possible as the culprit.

If you want to know what is going on for sure, take a look at the properties of one of your images and it will tell you the ISO, shutter speed, and aperture.

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Canon PowerShot A800 is a digital camera; the ISO number stands for sensor sensitivity, not film speed (there's no film). – Imre Oct 31 '11 at 15:39
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Yea, you are right. I call it film speed, because I consider the sensor to be my film, and speed to be sensitivity. I will keep my opinions secret from now on :) – dpollitt Oct 31 '11 at 15:42

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