It doesn’t matter whether you are shooting on a digital camera or using film, or whether you are using a manual setting or on auto – understanding the principles of ISO will definitely improve your photography.

We are here to demystify ISO, also known as ASA and Film Speed to help you get the look you want for your photos.

ISO in Film Cameras

The ISO is a measure of film’s sensitivity to light - some film is less sensitive (low ISO) and some film is more sensitive (high ISO). This can also be referred to as film speed, with low ISO being slow film and high ISO being fast. Slow film is used for bright conditions or capturing movement, fast film is used in low light conditions or for blur-free images of fast moving subjects.

ISO in Digital Cameras

In a digital camera the ISO refers to the sensitivity of the sensor. A point and shoot will have a smaller ISO range than a DSLR with the same pixel count because the sensor is much smaller. Basically, because the sensor on a DSLR is larger, the photosites are bigger and therefore can collect more light, compared to the photosites that are squeezed onto a small point and shoot sensor.

When to use a low ISO

  • Bright conditions

  • Still subject

  • Shooting on a tripod

  • When producing large images

A low ISO means that you can have the shutter open for longer because the film is less sensitive to light, allowing you to capture more detail or movement. Remember that because the shutter needs to be open for longer for the image to expose, you will need a steady hand or to use a tripod. The reason for using a high ISO for large images is because the graininess associated with high ISO is more noticeable on larger areas.

When to use a high ISO

  • Low light levels

  • Fast moving subject

  • Hand held shooting

  • When you can’t use flash

A high ISO means that you can have a fast shutter speed and still capture an image in low light. If this is the case – you may be wondering why we don’t just use a high ISO all the time – well, it is because there is a trade off, and that is picture quality. The high sensitivity not only captures images in low light, but also highlights the imperfections in the film, or the camera sensor. The effect is further exaggerated in shadows and not highlights, therefore more noticeable in darker scenes.

What ISO is best?

The general rule of thumb is that the lower the ISO the better – this is because with a high ISO you will get ‘noise’ or graininess in the image. However, you may want an element of graininess in a picture, especially if shooting in black and white.

Choosing Your ISO

The beauty of a digital camera is that you can change the ISO from shot to shot, whereas with a film camera you will often have to adjust the shutter speed and aperture to get the right exposure for the speed of the film you are using. Below is a guide to using the ideal ISO for varying light conditions.

50 to 200 ISO - bright sunny days
400 ISO - overcast days or indoors with natural light from a window
800 ISO - indoors without a flash
1600+ ISO – low light conditions, parties, concerts, plays etc

Do you have any ISO experiences or tips you can share with the community? I'd love to hear your comments.

July 12th, 2012
Also: ""The ASA scale was arithmetic, that is, a film denoted as having a film speed of 200 ASA was twice as fast as a film with 100 ASA.""

the International Organization for Standardization (ISO).
the American Standards Association(ASA) (now named ANSI)
July 12th, 2012
Very helpful, thanks!
July 13th, 2012
@byrdlip thanks, I never knew that, you learn something new every day!
July 13th, 2012
Great article and thank you for sharing!
July 13th, 2012
Great info, thanks!
July 13th, 2012
Very interesting and helpful. Thank you.
July 14th, 2012
Yes, in regards to taking pictures and not getting every reflection in the room. It's so hard, took the glass out, that helped, but never know how much indirect light to use in the room, if any? I just have a P & S camera. I'm trying to finish the thumbnail-challenge this week and my subject was pictures of grandchildren and it has really been hard. Any advice is appreciated.

July 15th, 2012
Thanks for information. I may try experimenting on the fireworks shots that I can never seem to accomplish.
July 18th, 2012
All this information is golden. Thanks for breakin' it down!
July 20th, 2012
thank you for the great info!
September 4th, 2012
What a great article but this sentence confused me - "The reason for using a high ISO for large images is because the graininess associated with high ISO is more noticeable on larger areas." Should it be 'using a low ISO'?? or am I a bit slow and not reading it properly?
October 5th, 2012
@joeymc I think it's supposed to read like this: "The reason for not using a high ISO for large images is because the graininess associated with high ISO is more noticeable on larger areas."

One typo in this isn't too bad...but yes, a little confusing. ;)

Great article though - nicely laid out...
January 2nd, 2013
Very informative! I was actually just wondering how the ISO's worked. I knew that the higher ISO was for darker settings but i didnt know why. Thank you! :)
February 25th, 2013
I still would wish the recommendation would be to "Start Low" in all light conditions, and only increase ISO as the last alternative to get a proper exposure. I don't like tables that start with "recommended" ISO settings that include 400, 800 or even 1600. Yes, you may have to resort to these, fast action sports shots under poor lights is a classic case, but don't just start high. Start low!

With many lenses now providing some form of Optical Stabilization, and many P&S and phone cameras having it too, Where it might have originally required ISO 800 to handhold in poor light, You can now do so with such lenses comfortably at ISO 100 and three or four stops slower on the shutter.

Yes, many editors have "noise reduction," but (a) many folks don't have or can't be bothered using editors, and (b) some editors are "more better" than others. But even in the best ones, the more noise you have and want to remove, the more your sharpness and clarity is compromised. That's just the scientific facts.

Except for that special "grunge" effect, ISO noise (and its companion long exposure noise) should be the last thing you introduce into your digital images. So keep ISO as low as you can, and always try to meter the current light conditions starting at say 100 or at most 200 and see what you have. Remember when your lens or camera has VR, and take into account whether that is 2, 3 or 4 stop protection. And except for action that needs "stopping" there is always a tripod, right?

Good shooting.
April 27th, 2013
May I also add another aspect to this discussion and that is dynamic range. Digital cameras permit a good level of capture of light between dark and light but not as good as the human eye. Using lower ISO settings will enable the digital camera to be able to be set to capture a higher dynamic range (i.e. the range between dark and light). So - this is yet another reason to agree with @frankhymus that you should select a lower ISO that you can manage for the shutter speed you need
September 7th, 2013
Experimented with different ISO and shutter speed when shooting Mesquite Rodeo, but could never get exactly what I was hoping for. Need more practice! My best ISO choice seemed to be 1600 in order to capture action.
May 31st, 2014
I have a new DSLR Sony A 3000, it's taking some time to get used to but I'm getting the gist. Not the highest quality camera but was in my price range.

My daughter is getting married in June and at dusk, any suggestions for ISO settings? I can't imagine much movement during the wedding but the party and before pics might require a higher ISO. Also, should I use a wide angle lens?
This is a great article, you made ISO clearer in my mind. Thanks in advance for the advice.
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