What Is ISO And How It Effects Your Images


The ISO (International Standard Organisation) or the ASA (American Standard Association) as it was formly called, is the standard for expressing the sensitivity of the sensor or the film to light. For a point and shoot camera this is normally 50 – 100. Digital cameras are more sensitive to light then film or fixed lens cameras, so normally the slowest ISO is often 200 or 100.

The ISO is expressed as film speed. The higher the number, the more sensitive or faster the film. Film with lower sensitivity to light, (a lower ISO) requires a longer exposure and is called a slow film. A film with hight sensitivity (a higher ISO) can shoot the same scene with a shorter exposure, or shutter speed and is called a fast film. Now, of course if you are shooting with a digital camera, you have that advantage that you can change the ISO for each image. With film photography, the whole film is the same ISO!

When shooting with shutter priority, the camera chooses the shutter speed for proper exposure and this will depend on the lighting, the film speed (ISO) and the aperture. A higher ISO, allows it to be exposed faster, so the shutter does not have to stay open as long for the same size aperture.

With a digital camera, when the ISO is increased, it amplifies the signal from the sensor. Each time the ISO is doubled, the camera can use a shutter speed twice as fast for proper exposure in the same lighting, at the same aperture. This I think is amazing! It was all so confusing to me at the start, but when I look at it now, it somewhat makes sense! I control the amount of light entering the sensor and I have 4 things at my disposal to use to alter the lighting.

Anyways, getting back to ISO….. the only down side of using a faster ISO is that this amplification can cause noise, or film grain so the photo will turn out bad quality. Again, this is an effect that the photographer may be wanting to create on purpose!

But for good quality photographs that can be enlarged and still clear and sharp, it is better to keep the ISO speeds set lower for a cleaner image. My camera ISO starts at 200 and goes to 1600.

You may not see the need to alter your ISO, but it is good to know the effect it will have on your images. A perfect example of needing my ISO altered, happened to me a few weeks ago. I was shooting a wedding. It was a beautiful day, the photos outside i could shoot away at an ISO of 200. But inside the church, was a different story. It was dark, and dimly lit! I wanted clear sharp images. I had a tripod, but didnt want to be using my flash during the ceremony. I needed to increase my ISO to 1600 to get clear bright images in this dark environment. Increasing your ISO increases your cameras sensitivity to light. So you can get the image you want, even if you only have a little available light.

I hope that has explained ISO to you clearly! Now get out and experiment!!

Good luck - Rebecca!

Photo credits: Quentin Bargate, Linqong, Tracy Hebden.

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December 07, 2009


Mine is locked on 100 on my Canon 5D. The only time I change it is in the middle of summer during the day, I drop it to 50.

November 24, 2009


Hi Rebecca:)
A useful articles.
I am very pleased to see my image here.

November 15, 2009


very informative Rebecca, well done.
i like to add too, that with some digital SLR it seems that even their AUTO mode can give you the best noise free image. this is especially useful for those who are just starting out using their new digital camera.
i noticed this by chance with my old and first digital SLR Olympus e300.
i guess you can say that some "AUTO mode computer chip" have artificial intelligence literally, as it know which ISO will give you the best results. as i said, some cameras, this may not be so with all digital cameras.

November 12, 2009


Well written article Rebecca! And nice use of DT images in it.


November 11, 2009


thanks for sharing those informations,useful

November 10, 2009


Good article and everything included in it is very true.

November 10, 2009


Great information! You have learned well since you wrote in one of your early blogs,"So.... I am stepping out and going back to college in September to do a diploma in photography, so that I can learn more and be better at what I love." Congratulations!

November 10, 2009


Thanks so much. Great article. Shooting in Manual mode, I try and keep the ISO as low as possible, but always consider dialing it up if necessary to achieve the right artistic mix of Aperture and Shutter Speed. Today's digital cameras are actually quite impressive with their high ISO performance.

November 10, 2009


Thanks for the article!

November 10, 2009


Great article...Thank you Rebecca!

November 10, 2009


hey Elianehaykal, i had not heard that.. but you could be right!!

To everyone else, thanks for the comments, glad the blogs are of a help to you all!! xx xx

November 10, 2009


Thanks for this beautiful series of blogs, was waiting for your blog, Thanks Rebecca for all the time and effort to educate fellow contributors.

November 10, 2009


Useful blog:)

November 10, 2009


Very useful informations, Rebecca. Thanks.

November 09, 2009


Good advice, thanks.

November 09, 2009


Thank you! Cheers :)

November 09, 2009


Another great blog! Cheers :0)

November 09, 2009


Nice blog :) just one thing: I've read elsewhere that ISO is not an acronym, it's Greek & means equal.

November 09, 2009


thanks,Nice blog Rebecca!!

November 09, 2009


Another reason to bump up ISO is to use a smaller aperture for increased depth of field, and the same shutter speed.

November 09, 2009


thanks for this :) always great to refresh!

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