Make That Digital Camera

Exposure - Again?

We've looked at the topic of exposure a couple of times now, but so far we have only really scratched the surface of this important topic. We've talked about f-stops and shutter speeds but never actually explained what is going on here. Just what is an f-stop anyway?

Well, let's look a little deeper and explain some of these concepts. It will make things a little easier if you are aware of just what you are actually doing when you are moving things around.

That Strange Creature the f-stop.

In the beginning...

To explain this, we need to go back all the way to the beginning of photography.

In those early days when the pioneers of our art were discovering and learning about how to make things work, the chemicals they were using to produce their images and the camera equipment they had were very crude and rudimentary compared to what we have available today. One big difference was the huge amount of light that their chemicals needed in order to produce the image.

Exposure times were quite literally measured in hours rather than the fractions of a second we are used to. This meant that the camera did not need to be much more than a wooden box with a lens to gather and focus the light. The action of the shutter was performed by the photographer simply removing the lens cap and re-fitting it when he judged enough time had passed.

As things progressed, the emulsions used to coat their metal or glass plates became more sophisticated and more effective. They became faster to react, meaning they needed less light.

Also the lenses they were using became much more effective at gathering the light and projecting it onto the plate.

Slow Down

It now became necessary to slow things down a little, particularly in bright conditions. In order to do this, a photographer would place a sheet of wood behind the lens to stop the light getting into the camera. Hence the term 'stop'. A small hole would be drilled in this stop to allow some light through, but the job of what became known as a stop was to stop much of the light in order to increase exposure times again.

Different sized holes would be drilled into various stops so that some degree of control over exposure could be maintained.

Shutter Up

Eventually even this became insuficient as chemicals and lenses continued to improve. Some kind of mechanical shutter mechanism was required in order to speed things up as well as bring some kind of accuracy and consistency that could not be achieved by simply removing and replacing the lens cap by hand.

All the Same.

As things progressed it became obvious that some kind of standardisation was needed, particularly with regard to the stops. If we were all to start drilling holes in wooden stops, clearly we would all get different results even when using identical film and developing systems.

There was also the small detail of using lenses of different focal lengths as this also affected the exposure.

The standard that eventually evolved related the stop size to the focal length of the lens, hence f-stop. The diameter of the hole when set at f/8 is, as f/8 suggests, the focal length of the lens divided by 8.

The numbers that were settled upon were also chosen with great care such that each larger stop allowed exactly twice as much light to pass as its smaller neighbour. This brought the advantage that shutter speeds became easy to arrange, you simply arrange the speed to double at each change, thus halving the amount of light passed.

It now became very easy to juggle f-stops and shutter speeds because a one stop increase on one could be compensated by a one stop reduction in the other.

Film Speed

You are probably aware that roll film is labelled with a number that looks like ISO100 or ISO200 etc. This relates to what is called the 'speed' of the film. Basically, the faster the film reacts the less light is needed to create the image, .

Once again, this is directly related to the aperture and shutter speed standards in that an ISO200 film needs half as much light as an ISO100 film. That is one stop difference.

This means that an ISO200 film will give you one extra stop or an ISO400 will give you 2 stops over an ISO100.

Digital ISO

Now, you don't use roll film in your digital camera, but many of them have an equivalent feature.

Why Do I Need it?

Fair question. What use is film speed adjustment when I am not using film?

There are two main occasions when you will be glad you learned how to make this adjustment. The first is when you are shooting action shots particularly at sporting events. If you wish to capture the moment a sprinter crosses the finish line you will need to freeze the action. This will provide a much more captivating image than if there is any motion blurring unless it is confined to the background.

You will be using a fairly large aperture in order to reduce the depth of field to make sure your hero stands out from his surroundings. If you are lucky enough to have fine weather, you will then need a high shutter speed to both freeze the action and to cope with the large aperture. This is when you wish you had spent a little more money on you camera, because you may not have a fast enough shutter speed.

You could compromise your image by reducing the aperture, but remember those extra stops available by changeing the ISO number? Change to a slower film speed you will rescue your shot.

The second set of circumstances arise when shooting in-doors or in low light conditions. Using ISO200 or more film will force you to use flash in these circumstances, but you may not want to risk things like red-eye or the rather harsh, contrasty image that often results.

Once again you have f-stops to spare if you utilise those ISO numbers.

Conclusion

Well, we covered a lot of ground in this one. If you are unfamiliar with these concepts you may want to read through this again ;>)

Remember that the best way to familiarise yourself with this stuff is to pick up your camera and take photographs. Take advantage of the speed and convenience of digital photography by taking lots of photos. Experiment with different settings and compare your results. This really is the very best way to learn; no amount of reading can compensate for this. Treat all of those books, magazines and articles like this one as an introduction and use them as the spur to grab your camera.

Most folks read the books then promise themselves they will try the new ideas next time they are out with the camera. We've all done that, and we've all forgotten everything we read by the time we decide to make the effort.

Don't fall into this trap. Worse still, don't say 'There's nothing to shoot, it's raining outside' and all of those similar excuses. We are not talking about creating great art here, just learning the principles and doing the ground work.

No one but you will see these shots so who cares what you take? Stick a bowl of fruit on the dining table. That coffee cup by your elbow, move it to the other end of the desk and shoot it.

Close the curtains to simulate low light and try changeing those ISO numbers.

If you get up and do it, you will probably be one of a very small number who will bother. This means you will be the one producing saleable images.

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