ISO – In Beginner Speak “Fast or Slow Film”

Continuing the series of posts where I try to explain some of the basic concepts of photography in “beginner speak”. If you like the concept, have anything to add, any futher questions or want to suggest another topic then let me know. Either add a comment below or email me.

ISO is often referred to when discussing the settings used to obtain a correct exposure but what is it, what affect does it have on your photos and when do you need to change it?

ISO actually stands for International Standards Organisation who are a group that define univeral standards in industry so that everybody is working from the same hymn sheet. What does this have to do with photography? Not a lot. The reason we use the term ISO is because the organisation defined the standard for the sensitivity of 35mm film called the ASA. Anyway enough of of pointless facts, what does it affect in my photographs.

Light Sensitivity

The ISO setting on your camera defines how sensitive it is to light. Normally ISO 100 is the least sensitive setting on your camera and as the ISO numbers double so does the light sensitivity setting of the sensor. 200 is twice as sensitive as 100, 400 half that of 800. “So what?” I here you ask. “What affect will this have on my photos?” Well as long as you maintain the same settings for aperture and shutter speed increasing the ISO setting will increase the exposure (brightness) of the photo you take and this can be very useful in certain situations.

Why don’t I just use the highest setting all of the time?

Good question, surely most sensitive is best. Well there is one big reason not to use max ISO (often 1600) all of the time. Noise – Noise is evil. It is a problem that occurs in digital cameras where some pixels in an image do not get recorded correctly and appear as speckles in a photo. You can noise in the images below taken at ISO 1600. The first image is a 100% crop of the smaller image and you can clearly see the noise speckles causing the photo to have a grainy effect.

100 % Crop

100 % Crop

Full Image

Full Image

Noise is worse at high ISO. To get all techy for a second this is because at high ISO settings the signal from the sensor is amplified more than at low ISO, this not only amplifies the signal but also the noise present in all electrical circuits, this noise then appears in the final image. Anyway the why is not that important, what is important is Noise = Bad, High ISO = More noise. Therefore we should always try to use the lowest ISO possible.

One of tha advantages of a DSLR over a compact is that they have less noise at high ISOs, but the still have noise as the image above, taken with my Olympus DSLR shows.

OK, so why don’t I just use the lowest setting all of the time?

Good idea, unfortunately some times it just not possible given all of the other constraints. Suppose we want to use a small aperture (large f number) to maximise the depth of field. The small aperture will decrease the amount of light let into the camera, if we don’t use a longer shutter speed the picture will be too dark. But suppose we want to hand hold the shot and the shutter speed is now too long. In this situation we can increase the ISO, useful isn’t it! Before I said noise = bad but is is better than not getting the shot.

Should I use Auto ISO?

If all the stuff above makes no sense to you then leave the ISO on Auto by all means, you will get the shot. However I like to leave it on the lowest setting in my camera, 100, and only increase it if it means I won’t get the shot.

Now go test it out. Fix the shutter speed and aperture and take a shoy each of your ISO settings, you should see them get progressively brighter.

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2 Responses to “ISO – In Beginner Speak “Fast or Slow Film””


  1. 1 The Bogtrotter 16 February 2009 at 1:40 pm

    One of the biggest uses of the ISO settings for me is when I use my zoom for wildlife shots.

    Going to a higher ISO allows you to use a faster shutter speed and therefore reduce camera shake. When using a 300mm zoom lense I normally set the ISO to 400 in reasonable light (or even 800 in some cases) to keep camera shake to a minimum.


  1. 1 The Exposure Tiangle - In Beginner Speak “The End of Auto Mode” « Fletch’s Photo Blog Trackback on 20 February 2009 at 5:32 pm

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Welcome to my blog

I am an amatuer photographer living in Sheffield, England. My blog will detail my photography journey, from fumbling beginer to, hopefully one day, competant and knowledgeable photographer

By sharing my experince hopefully I can help every one else with the same goals and maybe, just maybe impart some of the knowledge I have leaned

If you have any questions or comments about the blog then why not email me.
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