Why do shutter speed and exposure change when zooming in on a Canon A800 in Sunset mode?
Asked 10/31/2011
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On my Canon PowerShot A800, I shot the same landscape scene in SCN "Sunset" mode at both wide angle and full optical zoom. The photos looked similarly sharp, but the zoomed shots often used a noticeably different shutter speed. I expected zooming in to reduce the amount of light because the lens is slower at the telephoto end, so the results seemed confusing. Does Sunset mode make automatic exposure changes such as ISO adjustments, and is that why shutter speed changes when zooming?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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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.
Originally by user4892. Source · Licensed CC BY-SA 4.0
user4892
14y ago
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Generated from our catalog & community — verify before relying on it.
Yes. On the A800, zooming changes the lens’s maximum aperture: it is brighter at wide angle and dimmer at full telephoto. That means less light reaches the sensor when zoomed in.
Exposure is controlled by aperture, shutter speed, and ISO. In a scene mode like Sunset, the camera is still making automatic choices. So when you zoom in and the lens becomes slower, the camera may compensate by changing shutter speed, raising ISO, or both.
That’s why your results can differ from simple “rules of thumb.” Scene modes are designed to favor a certain look rather than give you direct control. If the shutter speed became faster instead of slower, the camera was likely also increasing ISO, or the brightness in the framed scene changed slightly.
The depth of field impression is also normal on a small-sensor compact camera like the A800: these cameras tend to show a lot of depth of field across many settings, especially in landscape scenes.
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