Not Your Everyday Banana

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Being a novice in photography and reading about preferable settings of my camera I'm a little bit confused about ISO settings. Sometimes I'm advised to set as little ISO as possible to have higher quality shots, and sometimes I come accross the contradictory advise to raise my ISO settings in order to make shutter speed as fast as possible to have higher quality shots as well.

I understand that here quantity of light available is involved, but are there any general rules on how to set ISO value, or everything should come through painful experience of trial and error?

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4 Answers 4

up vote 16 down vote accepted

Generally speaking, increasing ISO will not really improve the quality of your shots. Higher ISO means more noise in most cases, which can drown out details. The story is a little more complicated than that, however. To put it simply...if you can't get a shot at the lowest ISO, increase it. Getting a shot at all is certainly "higher quality" than missing a shot because you cant set a high enough shutter speed.

Let me give you a little bit more background on what ISO is, and hopefully that will help you make the appropriate decisions on your own as to when you might use a higher setting. ISO determines the sensitivity of your camera's sensor. Another way to put it is the ISO setting indicates how rapidly your sensor gathers light...the higher the number, the more rapid it gathers...the more sensitive it is.

Exposure is a triad, and is composed of three different settings working in concert to produce an image: aperture, shutter speed, and sensitivity (of film or a digital sensor.) ISO settings are rated in stops, just like aperture and shutter speeds, so adjusting it has the same proportional effect as adjusting one of the other two settings. If you are shooting with an aperture of f/16, a shutter speed of 1/100, and an ISO of 100, you can adjust any two settings and maintain proper exposure. Here is a table of various one-stop changes that produce the same exposure:

  A   |   S   |  I  
====================
 f/16 | 1/100 | 100
 f/11 | 1/200 | 100
 f/16 | 1/200 | 200
 f/22 | 1/50  | 100
 f/22 | 1/100 | 200

Generally speaking, you will want to keep your ISO setting as low as possible (usually 100, sometimes it may go as low as 50) to minimize the noise produced by your sensor. There are occasions, however, where using an ISO 100 setting is simply not possible. This usually occurs for one of two reasons (or possibly both in concert): you have your aperture wide open and there just isn't enough light to get a stable shot, or you are shooting action and need a very high shutter speed. In low-light conditions, increasing shutter speed will generally allow you to get a shot, but you are also most likely to get additional noise as well due to the low signal-to-noise ratio of darker pixels. In better lit action scenes, increasing the ISO setting will allow you to use a higher shutter speed than may normally be possible, improving your chances of stopping motion.

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Thank you, this helped me a lot. –  BBischof Sep 12 '10 at 23:32
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The three aspects of exposure (ISO, aperture, shutter) interact, so each will always depend to some extent on the others - in other words "as low ISO as possible" really means "low as possible in a given situation" rather than the minimum on the camera.

ISO determines the level of noise, which has a fairly minimal effect on the composition of most photographs, so I think it's best to work out the required ISO from the other two aspects of exposure, which have a much greater effect. In general, they work as follows (assuming a fixed level of light):

Aperture: a narrower aperture provides greater depth of field. This will in turn require a slower shutter speed or higher ISO. Sometimes you can use a very wide aperture (and may even want to), which allows shutter speed to raise or ISO to lower.

Shutter: A faster shutter speed stops motion more effectively, but will require a wider aperture or a higher ISO. Other times, you might be happy with the minimum shutter speed you can comfortably shoot hand-held, which would allow you to stop down the aperture, or lower the ISO.

If you can work out the general constraints on those two aspects, it will lead you directly to the ISO. This does take practice, but it's a bit better than random trial and error.

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As a quick example, this usually works approximately like this for me: "It's dark, so I need a wide aperture. People are in a group, so I can't go as wide as possible, to help keep them in all in focus. Say f/4 minimum. They'll be moving a little, but 1/60 or 1/125 should be sufficient." Then I set the camera to those settings, and adjust the ISO until the meter says "yes," hopefully - sometimes it just doesn't work out. –  ex-ms Jul 31 '10 at 7:55
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In general, as others have stated already, you should normally aim for the lowest ISO setting that will allow you to get the image that you want. However, this is not entirely true. The sensor in a digital camera has a sort of "base ISO" (this is often, but not always, the lowest ISO setting available). ISO settings above this "base ISO" means that the camera will amplify the signal (which is where the noise is introduced).

In some cameras, the highest ISO setting is identified not by its number, but by the letter "H" (as in "High"). This setting is not a normal amplification of the signal itself, but is roughly the same as taking an underexposed shot at the nearest lower ISO setting and then digitally making the image brighter (just like you adjust exposure in the computer). The same goes in the other end: there may be ISO settings indicated by "L"; these settings are the same as overexposing the image at the nearest higher ISO setting and then digitally adjusting the brightness.

The bottom line is that in the low ISO setting end, going into the L-settings will not really buy you any image quality. I may however be useful if you want the longer shutter speed.

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The ISO setting is how light sensetive the sensor is. This emulates how you could use films with different sensetivity.

You should set the ISO as low as possible, as long as there is sufficient light to take a good picture.

In low light situations you can raise the ISO to reduce the need for light. This will give you more noise in the image, but this is generally less of a problem than motion blur caused by too long exposure times.

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