What does the ISO setting on my camera do?
Wednesday, December 3rd, 2008
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The question has come up a few times in the last few weeks about what exactly the ISO setting on a camera is and why it is important. To answer that correctly, let's go back to the days of film.
ISO and Film
Since the invention of photography in the early 19th century, images have been captured using chemical processes that react to light. Camera film is just an emulsion containing silver halide. The silver halide particles become opaque when exposed to enough light and, when you look at all of those particles collectively, you end up with a representation of the light from a subject in the form of a picture.
The larger the particles in the film, the quicker the overall image can be correctly exposed, but each dot of pigment is lager and therefore grainier. Conversely, the smaller the particles, the finer the quality of image, but the time to expose the shot increases.
fine grain, low-speed film
large grain, high-speed film
What do the numbers really mean?
If you go to the store and buy film, you'll see that it comes at different speeds typically measured in intervals of 100. ISO stands for International Organization for Standardization. This is number is essentially a calculation of optical density over time. The higher the ISO number, the higher the particle density for the same period of time.
This means that film at ISO 100 will need 2x the light exposure to to have as much pigment as ISO 200 film. Or, better put, ISO 200 film needs 1/2 the time to be exposed the same as ISO 100 film. That is why higher film speed numbers are called 'faster', because they correctly expose in less time.
Like we mentioned before, the speed of exposure is controlled by particle size and the larger the particle, the quicker the exposure. So ISO 400 has larger particles than ISO 100 film. So you can have either low grain or low exposure times, but adjusting one will impact the other.
For digital cameras, there are no particles to expose, instead there are photovoltaic detectors called pixels. The number of pixels determines the resolution of the picture being captured. ISO for digital censors is a measure of the signal-to-noise ratio of the picture captured. The effect on the final image is the same, the higher the ISO, the more grain (noise) present in the picture.
Conceptual Representation of a Pixel
How ISO impacts other camera settings
Proper exposure of a shot is a delicate balance between shutter speed, aperture and film speed (ISO). This table below shows how the three components interact. Things in red denote reduction in light sensitivity and green denotes an increase in light sensitivity;
| | Shutte Speed | Aperture | Film Speed | What happened | | Base Exposure | 1/125 | f/4 | ISO 200 | Typical normal exposure. | | Fast Subject (adjust aperture) | 1/500 | f/2 | ISO 200 | Shutter speed is faster but depth of focus is narrower. | | Fast Subject (adjust ISO) | 1/500 | f/4 | ISO 800 | Shutter speed is faster but picture is grainier. | | Huge Depth of Focus | 1/4 | f/16 | ISO 200 | Depth of focus is very deep, shutter speed is slower. | | Low Grain Needed | 1/60 | f/4 | ISO 100 | Picture has fine grain, shutter speed is slower. |
Camera adjustments are usually multipliers of 2 meaning that one "stop" lets increases or decreases the ability to expose the shot by double or half respectively. ISO ranges are 50,100,200,400, 800,1600, 3200, 6400, 12800, 25600. Some cameras allow adjustments in 1/2 stops or 1/3 stops for finer control.
When/Why to change your ISO settings?
Unless it meets your artistic objectives to have a high amount of grain, it typically best to shoot with the lowest ISO possible. Adjusting the ISO should be done only as a last resort. If your shutter speed is fast enough and your f-stop has a deep enough DOF then keep your ISO low.
Typically outdoors, in the good sunlight, you should be able to shoot at ISO 100. When you move indoors or it becomes dark outside, you may have to shoot at ISO 400 or even ISO 800.
However, that all depends on how good the sensors are. Some cameras ISO 3200 look like the ISO 400 on other cameras. The amount of light needed is still the same on both sensors, but there is less noise, so the picture looks better.
dark room ISO 100, low noise
dark room ISO 1600, high noise
Another way to look at ISO use a way to guarantee a minimum shutter speed is possible. No matter how low the grain is, if a subject is blurry it most likely won't matter. Finding the balance is the trick.
Summary
Most cameras have automatic settings that will set the ISO value correctly depending on the light conditions available so fully understanding ISO may not be needed. On the other hand, if you are willing to understand ISO, you can be much more flexible when shooting and are that much more likely to get the effect you are looking for.
To learn more about ISO on Wikipedia click here. |
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