The Exposure Triangle Part 1 – ISO


Originally I wanted to write a single blog post that covered the exposure triangle in its entirety. Now that it’s finished, I’ve decided that it needs to be broken into multiple posts so fewer people hit the tldr wall. I’ll be releasing new posts on Wednesdays and Saturdays for the next two weeks.

The exposure triangle is one of the major foundations of the art of photography. Each component affects the final exposure of your image and achieving your desired outcome is about finding the best balance between each element. The exposure triangle is made up of ISO, aperture, and shutter speed each of which I will be covering in this blog series. I will do my best to explain each of these aspects without going into too much technical detail, however it will likely be a staple of my blog to have technical asides when I feel the topic deserves one.

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ISO

ISO is the current measurement for photographic film’s sensitivity to light.  Personally, I think explaining how ISO works makes the most sense when you consider how film worked, so we will be approaching the topic from that angle even though I’m sure most of you use digital cameras.

With film, photographic light sensitivity is based on the size of the granules that make up the film.  Slow film, with a low ISO number around 100 or 200, is made with small granules and requires more light for a proper exposure. Quick film with a high ISO number, such as 800+, has larger granules, and require less light for a proper exposure. The downside to quick film is that the large granules can cause the final photo to appear grainy, while the downside to slow film is that the film needs a more light to expose the same image. This is why people would purchase different speed films depending on if they would be shooting inside, outside, or some mixture of the two.

With digital cameras, the photographic speed is based on the gain setting for the image sensor. Gain is basically the amount that you are amplifying the signal coming from the sensor.  You won’t be setting the gain directly, as that is handled by the camera after you set the ISO number. What’s interesting is that this leads to the same pros and con’s as the film world.  As the gain for the sensor is increased any noise that was in the signal is also increased causing a similar grainy affect to occur in the image. The big plus for digital is that you can adjust ISO on the fly and don’t need to pick one setting to use with an entire role of film. On digital cameras the ISO range usually starts around 100 or 200. The high end will vary from camera to camera, on mine the high is 6400, but some newer high end cameras can go as high or higher than 400,000. ISO is a linear scale so a change from 100 to 200 will double your exposure speed. With the advancements of noise reduction on digital cameras the maximum ISO range where noise is not noticeable is much higher than film can produce. The range where ISO noise stops being noticeable will vary from camera to camera, on mine it is around 1600. Grain affects for my camera are show in the photos below, from left to right the ISO setting was 200, 3200, and 6400.  The affects are most noticeable around the bottom lens in the picture, but can be difficult to see unless viewed at 100% magnification.

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If you want to set ISO manually, most cameras have a quick method of changing ISO, on mine I have to hold down a button on the back that says ISO.  When the button is held down, ISO becomes the only element shown on the control panel display, then I can spin the rear selection dial to change ISO in either direction.  If you can reach all of these buttons by feel, the heads up display in the viewfinder will usually show any setting you’re changing while looking through the viewfinder.

If all of this sounds complicated, many cameras have an auto ISO setting, some of which let you to set a maximum ISO. This allows you to worry about other aspects of the exposure triangle and allow the camera to handle the ISO that best matches your other settings.  Some cameras will allow you to also set a maximum shutter speed for auto ISO so that you don’t cause motion blur.  This is usually found in the shooting menu, but check your manual for exact instructions and capabilities.

For those of you looking for advice on choosing the ISO setting on your own. I normally I like to keep my ISO set around 200 for everyday use and if I require a higher ISO setting I generally do not like to increase it above 1600.

My next blog post in this series will be covering shutter speed.

Peter

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5 thoughts on “The Exposure Triangle Part 1 – ISO

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