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ISO – What is it and how do I use it?

Do you ever wish you could get sharper shots when the light starts to go down, without using your flash or using a tripod?

Most cameras these days have an ISO setting, either on a button or somewhere in the menu. This post will tell you how to use the ISO settings to improve your ability to get sharper photos in low light. Thanks to Tony for asking for a post on this subject 🙂

ISO – increasing sensitivity to light

Let’s start by thinking back to before digital cameras, when we used to put film in before we could take photos. I know, seems crazy doesn’t it, and you had to wait for the film to come back from the processing shop before you got to see your shots too. Anyway, with film, if you were going to be shooting in lower light conditions – inside for instance, at a concert or church, you would load up your camera with faster film – the film would be 400 or 800 ASA rather than the normal 100 or 200 setting. This faster film was specially formulated to be more sensitive to light than the normal film, so you could still shoot and get hand held shots without the shutter speeds decreasing to a point where blur from hand shake would spoil the shots.

Nowadays, digital cameras don’t have film, so how do we set the camera up so we can get fast shutter speeds in lower light? By using the ISO setting to make the camera sensor more sensitive to light.

Taking control of your shutter speeds

You need to understand that most people can’t get sharp shots at shutter speeds much less than 1/30th of a second – maybe even 1/60th if your hand is shaky. Your hand and body movements transmit to the camera and introduce blur to your shots. And as you use your zoom the problem intensifies – the general rule of thumb is that whatever the zoom length you are using, you need to have a shutter speed of that zoom length/th of a second to get a sharp shot. So when you are zoomed out to 200mm trying to get little Tommy on stage in his angel outfit, if your shutter speed is less than 1/200th of a second you are unlikely to get a sharp handheld shot.

Yes, this does mean that you need to become aware of your shutter speeds when taking your shots. But that is a fundamental for good photography, and if you want to move away from full auto on your camera and begin to take more creative control, then you need to begin to use this knowledge to make your creative decisions as you take each photo. On most digital SLR cameras, when you are looking through the viewfinder, there are 2 numbers in the bottom of the screen. On compacts this information is normally on the back screen. One of these is the shutter speed, shown as a number which should be read as a fraction of a second. The other is a number with an “f” in front of it – we can ignore that for this post. So if you see 125, that’s 1/125th of a second. If you see 30 that’s 1/30th of a second. But if you see 15, or 8, or 4, they mean 1/15th, 1/8th or 1/4 of a second – all these are slower than 1/30th and when you see these you need to do something or your shot will be blurred. Even if you see 60 (1/60th of a second) if you are zoomed out at 100 or 200mm then you are still going to get movement blur when handheld.

So what ISO setting should I use?

In normal light, a setting of 100 or 200 is fine, and will give you the best quality shots. As you move into indoor lighting, which looks OK to our eyes but is much darker to a camera, settings of 400 or even 800 will give you a chance of handheld shots still being sharp. As ISO doubles, shutter speed halves, so going from 200 to 400 ISO will change your shutter speed from say 1/30th to 1/60th. Moving the ISO from 400 to 800 will halve the shutter speed again – from 1/60th to 1/125th. So you check as you frame your shot for what your shutter speed is, then you can adjust the ISO until you get a speed within the range where you can hold it steady.

So why not just use the highest ISO possible?

Good question. As the ISO number increases, the camera is actually pouring extra power into the sensor. In lower end cameras, the sensor pixels have a tendency to fluoresce as the power input increases, and so higher ISO settings result in some purpley grain-like spotting appearing, especially in darker areas of your shot. This is why it is always best to shoot at the lowest ISO you can, but if it’s the difference between getting the shot and not, then bang up the ISO!

So you use ISO to get handheld shots in lower light by dialling up the setting – the higher the ISO, the faster shutter speeds you get, but the down side is a slightly lower quality of colour graininess.

What about Auto ISO?

Many cameras come with an Auto ISO setting. This will automatically increase the ISO as soon as your shutter speed falls below a given value – typically set at 1/30th or 1/60th of a second (although this can be changed on some cameras). This is great for novices, but one thing to check and adjust is the higher end setting. If the Auto ISO range is say 100 to 3200, then shots taken using 3200 ISO are going to be very grainy looking on compact and entry level SLR sensors. The thing to do is experiment with the settings at 800 and above to see when the results get too grainy for you to be happy, and then set the upper Auto ISO limit to that.

What ISO should I use for night photography?

All the above has assumed that you are handholding your camera. If you are out in town doing a night shoot to get lovely urban or river shots at night, you are on a tripod. In this case, you set the ISO back down to your camera’s lowest ISO setting – probably 100 or 200. And turn Auto ISO off when’re you are on a tripod shooting at night, as you are going to be having shutter speeds of several seconds, even up to 30 or more. Really high quality night shots need the blacks to be proper black with no colour graininess showing, and this needs low ISO.


I hope this has demystified ISO for you. It’s a tricky subject to make accessible, so please leave a comment if you have any questions and I’ll do my best to respond 🙂

Joe Houghton Photography runs small group photo shoots, individual 1 on 1 tuition and photography assignments. You can see some of Joe’s photography on ArtistRising or at The Canvas Works, or on his Flickr site.

If you enjoyed this post, please leave a comment, “Like” it on one of your social media sites or even better subscribe to the blog to get all the posts as they go up. Happy shooting 🙂



About Joe Houghton

As a self confessed gadget and app lover, here are my personal musings and thoughts on useful, interesting or maybe just odd technology which caught my eye...


4 thoughts on “ISO – What is it and how do I use it?

  1. ISO is tricky at first, but most get the hang of it after some try and error.

    Posted by knudsens | April 3, 2012, 2:54 pm


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