Understanding ISO in Exposure: Lesson 6

Understanding ISO (eye-so) is a fundamental element to exposure as is aperture and shutter speed. In the last post, we covered shutter speed. Continuing on with the goal to shoot in manual mode, I will now explain the last component of the exposure triangle.

What is ISO?

Think of ISO as the sensitivity of your sensor to light. The lower the ISO number the less sensitive and the higher the ISO number the more sensitive.

If you recall in the days of film we had different ISO or speed film. For example ASA 200, 400, 1000. In those photos with higher ISO like 1000, grain could be seen in your photograph. These were tiny dots in the film that gave it a grainy appearance (noise).

In the digital era, we still have noise in our images at higher ISOs. Instead of dots, they are actually darker pixels that appear as grain like in the days of film.

As a general rule when it is bright outside an ISO of 100-200 would suffice. A cloudy day you might bump it to 400. Indoors without a flash 800 might work or even higher. As you can see your ISO is dependent on light and it is most useful in low light situations.

ISO Scale

From the scale above you can see that the numbers double when increased. These are stops. For example in a dimly lit room: Each stop increase makes the image brighter while decreasing the ISO will make the image darker.

ISO Stops

Reducing Noise

More often we don’t want a lot of noise in our images unless for artistic reasons. An example is this stairwell I shot in a dim environment. With my aperture and shutter speed set to where I wanted it, the image initially was too underexposed. I increased the ISO up until 2500 gave me a proper exposure.

If you look closely you can see what appears to be a grainy appearance to the photograph.

I imported the image above into Adobe Lightroom CC and used the slider in the develop module to reduce the noise. As you can see a lot of the grain has been removed.

The point I’m trying to make is that even if you do have to shoot at a higher ISO in low light, you can reduce your noise in post-processing. If you are interested in doing this see my post in Adobe Lightroom CC. It can be done in other software like PicMonkey and LightZone which is free.


We started with the Exposure Triangle. Now we have covered understanding aperture, shutter speed, and ISO. This is a framework for exposure. Now that you understand these principles you are ready to start learning to shoot in manual mode!

Assignment: For this assignment, I want you to set your aperture to around f/8 and a shutter speed of around 1/200. Take your camera into areas with shade or a dimly lit room and start with an ISO setting of 200. Increase your ISO a full stop (remember the number will double so the next ISO stop is 400). Take 5 photographs at each increased ISO stop.

Your photographs are not going to look perfect. What I do want you to see and understand is the change in brightness when you adjust your ISO. If you can enlarge your image on your LCD screen, look for noise in the higher ISOs.

Understanding ISO in Exposure: Lesson 6
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Welcome! My interest in photography spans 40 years and started with the purchase of my first SLR. Since then my passion has grown to landscapes, beach scenes, and travel photography primarily. Photography techniques I particularly enjoy are long exposures, and what I call splash art. You can see these on my gallery page if it interests you. Currently, I live on Amelia Island in Northeast Florida with my beautiful wife and our wonder dog, Fuji.