Camera exposure explained, forget Auto and be creative

Understanding the basics of photography so you can use those manual settings and get the images you want rather than the “correct” exposure that the cameras computer sets is something that confuses a lot of people.

So lets keep it simple and see if we can look at the principles that go to make up the exposure of an image when using a camera. First we have the sensitivity of the recording material, it used to be film but now it’s a light sensitive chip. With film you had to buy a whole roll of one sensitivity type, it might have been 100, 400 or 1600 ISO. Now the chip in a digital camera can have it’s sensitivity changed for each shot if required at the turn of a dial or even automatically on many instamatic type cameras.

Each doubling of the ISO rating is a doubling of the sensitivity of the chip, so 200 ISO is twice as sensitive as 100 ISO and so it goes on doubling through 400, 800, 1600, 3200, and 6400. Most cameras have reached the end of their range by 6400 and by this sensitivity one is asking the chip to record detail with very little light. Which is why the quality starts to drop as the chip is asked to read an image from less and less total light.

We have two other features that go to make up the exposure on a camera, the shutter speed and the aperture. Two very simple concepts that often have people totally confused.

The shutter speed is the simplest, it’s quite literally the amount of time the chip is exposed to the light. In very bright light situations the shutter speed would be very quick and in low light situations it would be slower. It’s having too slow a shutter speed that can cause blurred images from camera movement during the exposure.


2012 Olympic Flame
A fast shutter speed to freeze the flames motion.

In the days of film cameras because there was so little choice of fast film and it was so very grainy that many photographers would use slower ISO film and a tripod to stop any camera movement during an exposure. All works well until you get movement of the subject though, which can again add blur to an image. The difference being the blur will just be on the subject that is moving and not the whole image.

The shutter speeds on cameras started with a similar principle to the ISO rating of doubling every step, so hand held exposures could be 30th of a second, 60th, 125th, 250th, 500th, 1000th of a second. Now with digital cameras and the infinite power of the computer they normally have what we would call half stops between these 125th 160th 250th etc. But it’s worth noting a half stop makes very little difference to an exposure.

The last aspect of exposure is the aperture, basically how big the hole is that lets the light in to expose on the chip. Make the hole twice as big and you get double the amount of light in. The aperture is described in F-stops and this is where most people start to get confused and that’s because the F-stop numbers for a camera work in a reverse to the logic of numbering systems. One would think that F16 would be a bigger hole than F8, but no it’s the reverse, F8 is twice as big a hole as F16 when measured across it’s width. F numbers normally start at F1.8, F2, F2.8, F4, F5.6, F8, F11, F16, F22 and F32.

You’ll note that every number doubles every other step. This is because a diameter hole twice as big actually lets 4 times the amount of light in because it’s double it’s size in two directions – both north and south and east and west. So the intervening number is actually the equivalent of twice the amount of light.

So we have three elements that affect the exposure, the chip, the aperture or hole and the shutter speed or time the hole is open. All in scales that half or double depending on how you move along them. This gives us a very big advantage.

If you can change a setting on one scale by one position you can maintain the same exposure by doing the opposite on one of the other scales.

Taking an example average expose on a cloudy day which might be F8 at 125th of a second on a 400 ISO rated chip. If we needed a faster shutter speed to capture a fast moving object we might decide to use 250th of a second, to keep the same exposure you could either double the sensitivity of the chip to 800 iso or keep the same ISO and open the aperture to F5.6. So doubling the amount of light reaching the chip in half the time.

Understanding this basic principle of photography is the most fundamental aspect for taking back control of your camera. When you realise getting enough light to a chip is just like pouring water into a bucket. The bigger the buckets mouth (aperture) the less time it takes to fill or the faster you pour the water (shutter speed) the quicker it fills. This then makes the chip sensitivity equal to how big your bucket is!

So why would you want to adjust these variables yourself? Well, the first one we have already mentioned, for a fast moving object you might want to have a minimum shutter speed so you always freeze the action. In the same way if you are shooting a moving waterfall you might want a slow shutter speed to blur the flow of the water to a soft shape rather than the splashy detail.

But there is one further aspect that this control can assist with and that’s what we call depth of field. That is the amount of an image that is in focus. A shallow depth of field can look great on portrait photography to concentrate the viewers attention on the face and blur the background detail. Or for a landscape shot you might want everything in focus from the flowers close to the camera right to the distant mountains.


Westminster Abbey
A big depth of field to get everyting in focus

The depth of field is controlled by the aperture. Put simply, if the hole is big there is far less time for the image to focus detail than if the hole is small and a longer time of exposure is given. Therefore to get a great portrait use a big aperture like F2.8 and for a landscape a small aperture like F22 to suit your aims for the image.


Event Entertainment
A shallow depth of field caused by a big aperture focus’s the glass ball only.

That’s the basic principle. Adjusting the ISO sensitivity of the chip can considered to be the final decision or the first decision depending on your knowledge, the chip sensitivity is really the aspect that puts the other two elements within range of the lighting conditions. If you are out in bright sun conditions on the beach you know you can start with a slow chip sensitivity like 100 ISO if you are in a dark restaurant then you are going to need a fast ISO like 3200 ISO.

If you have started trying to take a shot in the restaurant with the camera set to a low ISO and you can’t get the shot without camera shake even with the aperture at it’s widest then you know you have to up the ISO rating.

Most cameras whether they be in phones, compact cameras or larger SLR cameras have the automatic setting to record what the camera thinks is best But with a little thought you might actually know better what you are trying to achieve and that is where photography starts to become exciting and not just a recording instrument.


By Brian Russell

Copyright October 2014