In short, ISO is the sensitivity of the film or digital sensor.
To give you the best example of what changes in ISO does to a photo, I made a chart below. Using a ribbon origami rose that I made, these photos are taken with a Canon 40D, so different cameras will have different results. The large picture at the top is taken at the lowest ISO of 100 to show what the entire picture is. Below that I have images of each ISO setting at 1/3rd increments. These are 100% of the pixel size right out of the camera. For each ISO change I had to also change the shutter speed. The aperture was the same in each at f/5.6.
At the beginning I have the two extremes of the highest and the lowest next to each other to show the dramatic difference. Then at the very bottom that full photo is taken with 1600 ISO. The reason I did this is to show, depending on the use (here being the web), a high ISO will still be a suitable option. At this small size on a website, the noise is barely noticeable. But blow it up to a 16×20 print and the difference between them will be quite intense. I very rarely would use 3200 ISO, and isn’t really native to this camera (thus the H). But it is nice to have available in a pinch.
So what is it all about then?
ISO is a standardization that was originally brought about to have a fair comparison of film between manufacturers. It measures the sensitivity to light. The sensitivity of the film was created through the use of the silver halides in the emulsion on the film plane itself. The smaller the halides, the slower the film was sensitive to light. The larger the halides, the faster or more sensitive it was. This sensitivity is equated as a ‘speed.’
With digital sensors, ISO speeds are still used, but they are actually ISO equivalents. Digital, instead of using a silver emulsion like film, actually increases signal gain on a sensor chip to increase its sensitivity. This is how a sensor can have multiple ISO speeds, whereas film is limited to one per film strip. These actually use an exposure index, but are translated to be the equivalent to a film’s ISO. One amazing thing that digital has accomplished over film (beyond the many other obvious advantages) is that the chips far surpass light sensitivity compared to film. Currently the highest ISO rating on a digital sensor is 102,400. My camera certainly doesn’t go that high, so I don’t know the quality of it, but that is still pretty incredible.
There is a ton more technical information regarding ISO and film/digital speeds, but I don’t think it is really that necessary to know.
So what do I really need to know?
The main concern with ISO is that it affects the exposure of your photograph. So the lower the ISO number (such as 100) the less light will be absorbed by the film/sensor in the same amount of time of a higher ISO number (such as 1600). In fact, with each doubling of the number, the light sensitivity is doubled. So ISO 200 is twice as sensitive as ISO 100. Or 800 has half the light sensitivity of 1600.
Where this matters is when setting your other manual exposure settings in your camera. If you change your ISO, you have to make an equivalent change in either your shutter speed or aperture setting. ISO on digital cameras (as well as aperture and shutter speed) generally have increments in 1/3rd stops. So a 1/3rd stop adjustment, say from 100 to 125, would require a 1/3rd stop adjustment in shutter speed, say from 1/60 to 1/80.
Generally ISO is set once for a scene and left there. Then shutter speed and aperture are adjusted accordingly between each other. You can gather your own assessment from the chart I made above, but I generally recommend, if possible, to stay in the 100-400 ISO range. Your photographs will result in the best quality, with much less noise. They will also turn out sharper, because the noise in higher ISO images can mess with the edges. Another issue with noise is when sharpening in post with Photoshop, the noise itself will get sharpened and become much more prominent than it already was.
Unfortunately, I am unable to make an equivalent chart for film. Noise (actually the silver halide grains) starts to show around ISO 800. I know of film that goes up to 3200 ISO, and thought I had heard of film going to 6400, but that about tops out its sensitivity. My recommendation on film speeds mirrors what I said about digital, 100-400 is ideal. 800 is fine to use also, and 50 is a great speed if you have fast enough lenses.
What if I want noise?
Then use film.
Seriously though, noise and grain are definitely different things. That is one reason why I and many other photographers still love shooting film. I don’t shoot it often, but sometimes the look of film is a very desired look. Noise in high ISOs is caused by high heat in the image sensor due to the increased gain. In film, grain is caused by the actual size of the silver halides. So noise is still on a pixel by pixel basis in a digital image. There are plugins and filters to add grain into digital photos, but they never capture the true essence of film grain. So, like I said, if you really want a grainy look to your images, my best recommendation is to shoot a high speed film like in the 1000-1600 range.
I can’t really think of anything else of importance with ISO, so if you have any other questions, feel free to ask in the comment section below. I love to help and teach.
For more on Shutter speed read this article from the archives:
Camera Basic – Shutter Speed
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