I am using a Nikon coolpix L830, which is basically a point and shot camera. Although I can not manually adjust the aperture, it does let me adjust the exposure. I understand the aperture controls the amount on light that is let in through the lens, but I would like to know how changing the exposure affects the auto aperture when taking photos? I have experimented with shots using different exposure levels and have seen a difference. I just don't know if changing the exposure automatically changes the auto aperture with my PAS.
Exposure-compensation is simply an offset to metering. A camera decides how to expose a scene, taking into account that offset. It must usually select a combination of aperture, shutter-speed and ISO in order to expose accordingly. You may fix the ISO, in which case it must vary the remaining two parameters which ends up being mostly shutter-speed.
The Nikon L830 however does not have a variable aperture. Instead, the opening is determined by the position of the zoom with F/3 at the widest and F/5.9 at the longest. If there is two much light to achieve the metered exposure, it slides in a 2-stop ND filter in the optical path. This simulates a smaller aperture in terms of light pass-through and is marked in the EXIF as if a smaller aperture was used. So, if you were at the widest, the effective aperture possible is F/3 or F/6. At the long end, it is F/5.9 or F/11.8 but given the opening is not really any smaller, it does not cause diffraction.
Basically, it just tunes how light or dark the camera's auto-exposure will aim to make the picture. It is the equivalent of applying an adjustment to your camera's "light meter".
Note: In reality your digital compact camera won't have a light meter but will judge the lightness of a scene by measuring the lightness from the sensor, then adjusting, then measuring again, and so on.
When aiming for a correct exposure, your camera will adjust all of the following settings to try and reach the exposure it deems to be correct:
- ISO - or the sensitivity of the sensor
- Shutter speed
It will do this according to its own algorithm, and each manufacturer will be different, but in basic terms they will all try to achieve the following:
In auto mode, shutter speeds slower than 1/30s or 1/60s are avoided as the camera cannot be safely hand-held.
Very small apertures are avoided if possible, and obviously you can't open wider than the widest aperture of the lens.
ISO is kept as low as possible to reduce grain/noise. Attempts will be made to adjust the shutter speed or aperture first, before increasing ISO. Auto exposure usually regards increasing ISO as a last resort (or second-to-last, before venturing into really slow shutter speeds).
Once the ISO reaches an absolute maximum it wants to do in auto mode, the camera may then start choosing shutter speeds even slower than the slowest reasonable speed for hand-held use. This will often be accompanied by a "shaky hand" warning icon.
Anything other than that, where it isn't hitting its limits for shutter speed or aperture, and doesn't need to increase its ISO, the camera will use its own judgement to decide on the best compromise between adjusting aperture and adjusting shutter speed.
The typical approach is to adjust them in a gradient, favouring an equal approach to making the aperture smaller and making the shutter speed faster, so that for a middle-of-the-range exposure, the aperture and shutter speed are both about half way along what the camera regards as its "preferred range" for these.
Note: A "program chart" is a chart in some camera manuals showing how the auto-exposure chooses between aperture and shutter speed adjustments. Here is the program chart for a Nikon F5. Some cameras may adjust their algorithm (represented by different charts) for different attached lenses.
Adding exposure compensation simply adjusts the target exposure the camera calculated from the start. So it'll target a lighter or darker exposure than its light readings would have suggested. It won't specifically alter the aperture, or shutter speed, or ISO - these will still all be adjusted according to the camera's algorithm, it'll just be targetting a different final exposure.
So, for example, if you increase exposure compensation (eg by +1.0 stop), the following might happen:
As long as the aperture can open wider and/or the shutter can be slowed without hitting the widest aperture or the slowest reasonably shutter speed for hand-held use, it'll probably adjust both a little.
If it's at the widest aperture already it'll slow the shutter a bit unless it's already too slow for hand-held use, in which case it'll boost the ISO.
If it's at the slowest reasonable shutter speed for hand-held use already, it'll widen the aperture unless it's already at its widest, in which case it'll boost the ISO.