ISO: A *simple* guide for beginners. Locked

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ISO: A *simple* guide for beginners.

For this tutorial, I'm going to skip what ISO is all together and instead concentrate on what the ISO control on the camera does in terms of using the camera. This tutorial will skip the effect of the ISO setting when using external lighting, such as flash/strobe. To that end, I will simply say that higher ISO settings will result in less flash power whereas lower ISO settings will result in more flash power.

OK, we first need to consider the exposure mode that is being used. For this tutorial, I'm only going to consider P, A, S, and M modes (P, Av, Tv, and M modes for Canon). I recommend that those using Auto Mode simply set the ISO setting to Auto and let the camera choose the ISO setting in addition to the relative aperture (f-number) and exposure time (in fact, setting the ISO setting to Auto is useful in other AE modes, as well). In all cases, it is presumed we are photographing the same scene.

First up: P Mode. In this exposure mode, the camera will choose the relative aperture and exposure time for you. If you choose a higher ISO setting, the camera will use a more narrow aperture (higher f-number) and/or a shorter exposure time. The lightness of the resulting photo (how bright the photo appears to be) will not change, but the photo will be more noisy, since the more narrow aperture and/or shorter exposure time will result in less light on the sensor.

Next up: A Mode (Av Mode for Canon). In this mode, you choose the relative aperture, and the camera chooses the exposure time. A higher ISO setting will result in a shorter exposure time. which will mitigate motion blur. but also result in less light on the sensor, and thus a more noisy photo. The lightness of the photo, however, will remain unchanged.

For S Mode (Tv Mode for Canon), you choose the exposure time and the camera chooses the relative aperture. Higher ISO settings will result in more narrow apertures (higher f-numbers) which will give more DOF (and may also result in sharper photos), but, once again, project less light on the sensor and hence the photo will be more noisy. The lightness, however, remains unchanged as with P and A (Av) modes.

Lastly, we have M mode. In this mode, you choose both the relative aperture and the exposure time. For a given relative aperture and exposure time, higher ISO settings result in brighter photos (photos with greater lightness). There are reasons to use lower ISO settings that result in a photo that is "too dark" or higher ISO settings that result in a photo that is "too bright", but that's beyond the scope of this tutorial. Using Auto for the ISO setting in M mode is often very convenient, but there are definitely reasons to choose the ISO setting yourself.

Bottom line: what ISO setting should you use? In M mode, choose the ISO setting that gives the desired lightness for the photo. There are reasons to choose otherwise (if shooting RAW), but that is a more advanced topic. In P, A, or S mode (P, Av, or Tv modes for Canon), use the ISO setting that gives you the relative aperture and/or exposure time you need, so long as the photo is not "too noisy". If the photo is "too noisy" at the selected ISO setting, you can choose a lower ISO setting, but be aware that it will result in a wider aperture (resulting in a more shallow DOF and possibly less sharp photo) and/or longer exposure time (which may result in excessive motion blur). Alternatively, external lighting is another option to consider, but this, too, may not be possible, practical, and/or desirable.

Hope this helps!

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