It actually means how fast the film itself reacts to light. A higher number meant you needed less light to record an image. If it was a dull day and you were photographing a motor race you’d need a fast shutter speed to freeze the motion, so you’d choose a high ISO film so you could have the shutter speeds you need in the light that was available. If on the other hand it was a very bright sunny day you could use the lower ISO film because you already had enough available light to get a fast enough shutter speed.
It’s exactly the same with your digital camera. High ISO means you don’t need so much light hitting the sensor to record an image, which means you can use smaller apertures or faster shutter speeds.
Question: Why bother with low ISOs at all?
As with all things photographic there are costs and payoffs. The cost of higher sensitivity comes at the price of grainier photos. The higher your ISO the more grain you’ll get. So for best image quality a low ISO is best, but if there’s not much available light you may have to increase the ISO so you can use the shutter speed and aperture combination you need to capture the image you want.Let’s take a closer look at a couple of images.
As you can see it’s a bit grainy but it’s still a great image because of the light and her ‘rapt’ expression. But as you can see from the image below, you couldn’t enlarge or crop into it very much before that grain starts to become a quality issue.
This means that with a high ISO you should always compose your shot ‘in camera’ when you take it, rather than cropping into it on the computer.
Question: Why not just use flash? Flash kills great light stone dead – look at the images below.
See how much nicer the second image is? It’s because of the beautiful light coming from a window to Lorna’s left which the flash totally killed.
Low ISOs
Low ISOs can safely be used in low light provided nothing’s moving and your camera is securely fixed onto a sturdy tripod. For this food shot I used 200 ISO for the best possible image quality and a middle aperture of f/8 to get the depth of field I wanted.
Because of the low ISO I had to go right down to 1/10th second shutter speed because I was shooting with available light and there wasn’t much of it. As nothing was moving and the camera was locked off on a tripod it worked perfectly. The image is beautifully fine grained – even when you crop right into it.
As I said in the video – get your camera set up in the garden and experiment with your ISO and its effect on shutter speed. It doesn’t matter if you’re not shooting an award winning image, but you do need to understand what happens and the best way to do that is to experience it for yourself.
These days image quality with high ISOs is becoming less of an issue and it’s one of the few times that spending more money on kit will help your images. High end cameras like Nikon’s D3X and Canon’s EOS 5D for example can shoot much better quality images at ISOs of over 6000 than my old D70 could at 400.
Here’s a little formula which might help you and it goes like this.
Increase ISO – camera’s more sensitive – you can use faster shutter speeds (for freezing movement) and smaller apertures (for greater depth of field) - but you get more grain.
Decrease ISO – camera’s less sensitive - best image quality – but if there’s not much available light you sacrifice fast shutter speeds and small apertures.
source: http://www.ephotozine.com
