So far we have considered changing exposure, or bracketing, using either aperture or shutter speed (or a combination of both). I’ve assumed that we all have manual mode but David asks what happens if you only shoot on automatic? Well, I’ll deal with that in a little while — but it is a good question.

What is this ISO?

This is another thing that harks back to the age of film, and this helps us understand things a little.

In the days of film we used to buy film of a certain speed. Most everyday film had a speed of around 100 or 200 ISO or ASA (these are both mean the same thing). On top of this there were ‘faster films’ of about 400 ASA for use in dimmer lighting conditions or for when you needed very high shutter speed (for example when ‘freezing’ fast action sports). There were also some even faster, expensive films, with ISO rates of 1000 or more.

Faster films sacrificed quality for speed. The 400 films could give good results, but the film did not have as much latitude (it was prone to bleaching) and was not of the same high resolution. With film we talked about ‘grain’. The faster the film the grainer the photo, i.e. you could see the individual ‘pixels’, or dots, of film.

Sports photographers, for example, often ‘pushed’ their film even faster by extending development time. This resulted in very grainy prints and no doubt you’ll have seen many of these.

Back to Digital

To some extent digital suffers from the same thing. The faster the ISO speed the lower the quality of the image. Basically, the sensor has less light to deal with when capturing detail and captures it at a lower resolution. Again you can see the ‘grain’ on very ‘fast’ shots although often this is not as attractive as it was with film.

The good news with digital though is that modern cameras are no where near as sensitive when it comes to ISO.

Film cameras often started their speed at 50 and went up to 1600 or 3200. Today’s digital cameras start at 100 or 200. The detail at 200 ISO is often just as good as 100. This is good for photographers as it gives us an extra ’stop’ of light to play with.

For many shots you may well find that you have to go beyond ISO 800 to see a real drop in quality. You can see the difference on big prints but for normal prints, or use on the web, you may well find you get very acceptable images up to ISO 800. When I first had my digital camera I took to the hills for two weeks. It was only when coming home I noticed I’d shot the whole trip at ISO 800! However, the images are still useable.

But, despite the tolerance of new cameras, shoot at the lowest ISO setting you can. The quality will be better and the latitude of the shot (the range it which it captures clear detail) will be better. Step up to ISO 400 only in poor light.

As ever these rules are there to be broken and a creative photographer can use higher ISO speeds to get all kinds of effects. But for most shots — and when starting out — shoot at ISO 200.

Bracketing with Automatic Cameras

And finally, to David’s question!

Yes you can bracket with an automatic camera — and ISO is the tool you need.

Let’s suppose we’re taking a shot of the land and the sky. It is a very bright day. It is possible that the camera will expose for the brightness of the sky and loose the detail of the darker, shadowy, earth. Or alternatively, you may have only a small section of brightness in your picture. The camera may expose for the majority dull bits but it will risk bleaching out detail in the small, but important, bright bit!

Got it?

So, we might decide to bracket our shots to capture the image, it being such a wonderful view!

We’re stuck with camera automatic. This will calculate its base exposure whatever you do. But we want to over-ride it.

Let’s suppose we think we might need a shot with more light, in other words were worried about over exposing the shot. We need to ‘fool the camera’. The first shot (with standard exposure) was shot at ISO 200. To get a bracketed shot with more exposure simply reset the ISO dial to ISO 100.

This might seem counter intuitive but it will tell the camera to expose more detail, By going from 200 to 100 we will have bracketed by one stop – i.e. over exposed by one stop.

On the other hand, if we think the camera might be bleaching out detail, bracket the other way. Set the ISO to 400 and the camera will think it needs less light. You will be bracketing by one stop in the other direction.

This is a little crude David but you’ll find it works. The next time you are out in the hills find a good shot with reasonably stable light. Take a range of shots at different ISO speeds (with the first being ‘normal’) and just see how the details differs when you get back home.

Thanks to Des Horan for pointing out I’d got the bracketing the wrong way round for this last bit — I’ve now changed it, mind you I’m still confused. Best stick to manual !!!