Posted on Saturday, 14th April 2012 by Cee
Why have a grainy speckled photograph when you can have smooth and despeckled one. The secret lies in your camera ISO. Which setting should you use to obtain nice, despeckled photos?
Camera ISO was mentioned briefly in Part 2 of this course to you. I used the analogy of employees working to take a picture. The number of employees you have to take that picture represents your ISO setting. So if you have 50 employees, your ISO setting is ISO50. A 100 employees represents a setting of ISO100. The analogy helps you understand that the higher the ISO setting, the quicker you’ll be able to take the picture. That is, you can use a faster shutter speed. Faster shutter speeds at higher ISO settings means that your ISO sensitivity is increasing. Your digital imaging sensor is becoming more receptive to the light that is landing on it.
A high ISO setting or sensitivity is also sometimes referred to as ISO speed. In film photography, the film ISO is its speed. An ISO50 is a slow film whereas a film that has an ISO3200 is referred to as a fast film.
There is one problem with using high ISO speeds especially in digital photography and that is the problem of digital noise. In fact, noise can arise when your images are underexposed from long exposure times and can vary from camera-to-camera. A lot of images taken at night can exhibit noise.
What Is Digital Noise?
Digital noise in photography is pollution inside your images in the form of speckles. It arises because photons (which are little wave particles of light) cannot reach your imaging sensor. Most of the time however, noise isn’t normally that evident and much of a problem. But… sometimes it is a problem in night photography and is clearly evident when you view your images having zoomed in at 100%. Here’s an example.
Canon 5D Mark II, EF 50mm f/1.2 L Lens : 1/80 | f/4 |ISO320.
The image that you can see without hovering is the full digitally-unmodified image at a size of 750 x 500 pixels. It looks acceptable in terms of noise – you can’t really see many speckles in the sky or anywhere else for that matter. However, if you hover over the image… you’ll see a cropped version near the lamp and the sky in the background zoomed in at 100%. This version provides clear evidence that there is an undesirable amount of noise.
Now take a look at the following example which has also been shot at ISO320 indoors with plenty of light.
Canon 5D Mark II, EF 50mm f/1.2 L Lens : 1/50 | f/1.2 | ISO320.
If you hover over this image, you’ll notice that it’s virtually noise-free even in the dark flower petals. There is more light in this image than the amount coming from the blue sky in the image above… and so, there is less noise.
So Which ISO Speed Should You Choose For Your Exposures?
The answer is, the lowest possible. The thing to do here under a given light source is to take a sample shot at an ISO speed that gives you an acceptable shutter speed if you’re hand-holding your camera. Then zoom in on the image to 100% on your LCD to check how much noise you have. If it’s too much, you’ll have to fill your subject with another light source. I shoot a lot of my images at ISO50 especially landscapes. The choice of your ISO speed will depend on how well your camera performs in the lighting conditions that are available to you. The more light you have in your scene, the lower the ISO speed that you’ll be able to set.
The other point to take away from this is that when you modify your images using software, you should not try to completely remove noise but reduce it. Noise gives definition to your image by bringing in detail. By removing it completely, you’ll soften up your digital images and they’ll look false – they’ll look like paintings not photographs taken by a camera.
This should provide you with a solid foundation on what ISO speed to select.
Homework – Take photographs at ISO50, 100, 200, 400, 800, 1600 and 3200 of the same scene at a fixed aperture and examine the results. The amount of noise should increase at higher ISO settings.