Making Auto ISO your friend

Talking exposure

ISO is one of three elements of the exposure triangle, the other two being, shutter speed and aperture. To correctly expose a scene you need to do three things

• Meter the scene to establish the overall brightness of the scene.
• Select a combination of aperture and shutter speed to correctly expose the scene.
• Select a different ISO if you cannot achieve your desired aperture / shutter speed combination.

So where does ISO come in?

ISO relates to how sensitive your sensor is in relation to the brightness of the scene. Bright scenes are usually associated with lower ISO settings and dark scenes with higher ISO settings.

Changing the sensitivity or gain of your sensor by adjusting the ISO, allows you to use your camera in all kinds of lighting conditions, helping you exploit the use of aperture and shutter speed combinations to meet your needs. Let’s look at an example scenario.

• A Wedding photographer
  • Shooting outside (bright light) might shoot at F8 @ 1/250s at ISO 200
  • Moving indoors (dark interior) they might change to F5.6 @ 1/60s and use an of ISO 800

The dark interior meant that in order for the photographer to use F5.6 @ 1/60s they needed to increase their ISO to 800 as when using ISO 200 they were not able to set the desired aperture and shutter speed combination as the lighting levels were too low.

If the wedding photographer could not adjust the ISO, then the only choice would have been to introduce more light to the scene, probably by using a flashgun. And in the days of analogue film that is what we used to do as we could not change ISO so easily.

Wedding photography is very demanding, you need to work fast, be competent in the use of your equipment and you always have to be keeping an eye on your exposure and camera settings. Todays digital cameras and their auto functions can help us when shooting under demanding situations.

Auto ISO

Auto ISO is one of those functions that can make shooting less demanding, especially when shooting in rapidly changing light conditions, for example, in our previous example of the wedding photographer, moving quickly from an outdoor location to an indoor location and back again. If you set your ISO to auto ISO then it is one variable you do not need to worry about. But what camera modes and what shooting scenarios merit using Auto ISO?

Best camera modes for Auto ISO

In general, we always want to have control of at least one of the elements of the exposure triangle, aperture, shutter speed or ISO. There are times when we can use full auto but I recommend controlling at least one of the three elements of the exposure triangle.

My go to set up would be to use the following;

• Camera mode – Aperture priority
• ISO – Auto
• Metering – Matrix / Evaluative
• White Balance – Auto

This set up allows you to control your aperture, remember that the aperture controls how much light your camera sensor receives, and all other settings are on Auto. Working like this means that if you shoot at your widest aperture, you will be maximising the amount of light entering your camera, keeping your shutter speed up and the ISO down. Your camera’s algorithm will probably look to

• Try and keep your shutter speed high enough to handhold your camera (1/30s~1/60s or higher )
• Keep your ISO as low as practical

But what if you don’t want your shutter speed to drop blow a certain speed or you want to shoot at a constant shutter speed? Then you are probably best switching to shutter priority mode.

Camera mode – Shutter priority
ISO – Auto
Metering – Matrix / Evaluative
White Balance – Auto

Now your camera is going to

• Maintain a constant shutter speed using whatever shutter you decide on, 1/500s for example.
• Balance out the aperture and ISO

Final thought

As photographers we are normally concerned with

• portraying our subject to its best.
• exposing our subject correctly.
• Putting our own photographic interpretation on a scene or subject, our creative intent if you like.

Digital cameras offer us a lot of flexibility and functionality when taking pictures, and we must learn to exploit this functionality and use our cameras intelligently.

Learn to exploit your cameras technology to your advantage, experiment and use what works best for you.