Forming an understanding of exposure calculation.

I am currently blessed again to help a friend in forming an understanding of the principles of photography. It would seem the best way to do this is to demystify all the menus and buttons.

The problem and how to solve it

Now you could leave your camera on one of the Auto settings and simply keep snapping but at some stage you will want more. You might even be brave enough to attempt one of the semi-automatic settings. In doing this you will start using the camera's light meter. Big mistake!

This article is the wrapping up of a one-on-one session where we did a bit of shooting and reviewed results as we went.

In the next related article I will start by explaining aperture settings and by then I will show some example images of the good and the bad. For now please bear with me and let's work through some theory to get to our goal.

The camera's built-in light meter is probably...no, DEFINITELY your greatest enemy until you understand how it reads a scene and controls the camera.

Calculating the settings to get the exposure you want should happen instinctively. The problem is that D-SLR cameras are more like computers in the shape of cameras and navigating all the buttons and menus within menus scares most people.

What we need to find is that idea that makes sense of it all.

All cameras work on the same basic principle of exposure. They have a shutter which mostly sits in the camera body. This is nothing but a mechanical device that opens and closes for a predictable period of time. The shutter therefore allows light into the camera for a controlled period of time. If you like, click the above or below orange text to see a short video of a camera shutter in action.

When we talk shutter or shutter speed we talk about a period of exposure regardless of the quantity of light that may or may not be entering the camera. You could see this as the opening and closing of curtains to let sunlight in for a period of time you deem appropriate.

The camera has a lens on the front and the lens is the eye of the camera. Now the lens has a few qualities beyond merely seeing and focusing the image onto the sensor or film. For now we will only deal with the lens' ability to regulate the quantity of light entering the camera over a specified amount of time. This is commonly referred to as the lens aperture.

When we talk lens aperture we talk about the physical size of the interior opening inside the lens. Consider filling your pool from empty. Now you could do it with a regular garden hose and it would take a while. Alternatively you could call the Fire Brigade and they would fill your pool double-quick with their huge hoses. Even simpler yet, watch how your own eyes react when you walk into a brightly lit environment from darkness. The eye opens wide in darkness and closes when entering a brightly lit environment. This is because the eye regulates the quantity of light entering the eyeball.

The camera lens mimics the human eye in this way.

The third aspect of exposure calculation is the matter of sensitivity. Back in the day we bought film which was rated by a set of numbers, ISO or ASA or DIN. These numbers expressed the sensitivity of the specific film to light and that was a fixed quality of the film.

The lower the number the more light was required for normal shooting. Summer family pictures at the beach in bright daylight would be done with, say ISO 100 or 200. Evening campfire pictures would typically require higher ISO numbers such as 800, 1600 or 3200.

In digital terms only the ISO numbers really survived. The camera's ISO sensitivity is set to the required number and can be changed from one image to the next.

OK, that is quite a mouthful. What do we do with all of that?

Simple really, let's summarise quickly. Remember at the end of my previous post I showed you the old classic camera lurking behind the D-SLR?

From the images and videos I shared above I trust you will understand why one of these old dinosaurs are my favourite props when I do one-on-one coaching.

Once we understand the three-way relationship that regulates exposure calculation any camera can be understood within minutes.

So, to start, let's decide on the sensitivity we wish to use. This is governed by the light we are shooting in. In bright midday light a low number such as 100 is fine. Late afternoon and sunset will see us typically using a number around 400 - 800.

Post-sunset and dark environments will see us using a number such as 1600 or higher.

Now, please remember, we are now dealing with the specific aspect of adjusting the camera to get a good exposure. Some time down the road we will look at secondary effects of these three aspects.

A few thoughts for ideas to come...

A very fast shutter will freeze motion and a very long shutter will show movement and blur moving elements in the images. There is the further aspect of using a tripod or not. This will be decided by the subject matter you are shooting and the image you wish to produce.

A very wide open lens will yield a very shallow depth of field and a very small lens aperture will yield great depth of field. This becomes much more relevant as we move away from wide-angle lenses to telephoto lengths.

A low ISO sensitivity number such as 100 or 200 will almost always yield better image quality than high sensitivity numbers such as 3200 and especially 6400 and higher. Noise in a digital image is first spotted in shaded or darker parts of the image when a very high ISO setting such as 3200 and higher is used. Sometime in future we will look at noise and how to work with it.

More about these ideas in the future. For now we are simply looking for the required calculation and how to find it.

So, back to exposure calculation...let's start with ISO

Let's say we are out in midday sunny conditions. There is hardly a cloud in the blue sky and all is beautiful.

For a start we will set the camera to ISO 100.

Please note that not all cameras are capable of ISO 100. The camera's ISO capabilities are listed in a "normal" range and then expanded possibilities exist at the high end and also at the low end. These expanded options are best avoided at first in either the low or high end as they are artificially induced and image quality can be compromised so please test them first.

For this article we will stick to basic classic numbers.

Right, so we have selected ISO 100 due to bright midday light. ISO 100 is a low sensitivity setting for the camera sensor and it requires good light.

The ISO numbers follow a logical sequence:

100 - 200 - 400 - 800 - 1600 - 3200 - 6400.

Every step is doubling or halving the neighbouring sensitivity value. Now your camera will show more options. Between 200 and 400 you might find 250 or 320. Again, like shutter speeds, these inbetween numbers simply give more precise adjustments. Let's stick with the basic numbers for now.

Do we consider movement or depth of field?

The next step would be decided by the subject matter we are photographing. If we photograph children playing or animals that move we would probably wish to freeze motion. Now shutter speed is expressed in fractions of a second and you will see numbers such as 1/60, 1/125, 1/30 or 1/500. Now if you access your camera's shutter speeds you will see many other numbers such as 1/90 or 1/50. These simply give more control for more precise exposures. Let's for now stick with the following sequence:

1 - 1/2 - 1/4 - 1/8 - 1/15 - 1/30 - 1/60 - 1/125 - 1/250 - 1/500 - 1/1000 - 1/2000.

Now, going down the sequence you will see that every step going up or down is either doubling or halving the exposure time.

Similarly, the ISO numbers follow a logical sequence:

100 - 200 - 400 - 800 - 1600 - 3200 - 6400.

Every step is doubling or halving the neighbouring value. Now your camera will show more options. Between 200 and 400 you might find 250 or 320. Again, like shutter speeds, these inbetween numbers simply give more precise adjustments. Let's stick with the basic numbers for now.

Are you spotting the pattern yet? Double or halve... From ISO 100 to ISO 200 the camera sensor's sensitivity to light is doubled. From ISO 800 to ISO 400 the camera sensor's sensitivity to light is halved.

Switching the shutter speed from 1/500 to 1/1000 will halve the exposure time. Switching the shutter speed from 1/500 to 1/250 will double the exposure time.

When we get to lens aperture values the plot thickens. No, it becomes downright crazy! The numbers make absolutely no sense. Please remember that the basic idea of doubling or halving the exposure is there. The doubling and halving is hidden in numbers which will make sense if you wish to delve into some mathematical concepts such as calculating the area of a circle. Any takers? No? Didn't think so and fair enough. There are a few things that can suck all the joy out of photography. Aperture setting numbers are one of them.

I suggest memorising the following classic numbers:

f/4 - f/5,6 - f/8 - f/11 - f/16 - f/22

There are smaller numbers at the low end and higher numbers beyond f/22. Both these options require further discussion best skipped for now.

Now, again, your camera will show a good few more options inbetween the above sequence of numbers exactly as it will with shutter speeds and ISO settings. Those ISO and shutter speed numbers made some sense. The above f/numbers make no sense AT ALL. For exactly that reason we will leave them for the next article. What is important is to know that a large f/number will represent a small lens aperture and a small f/number will represent a large aperture. Remember the double and halve. We will explore it in the next article.

If you made it this far down, well done and thanks for bearing with me.

Let's end this off with a tip on caring for your lenses. The best thing you could carry in your bag is a Lenspen. You will find them at any decent camera shop.

With a soft retractable brush you can get rid of loose dust and dry particles on your lens glass.

The applicator tip used to clean the lens glass is protected by a screw cap when stored. You really should not even consider anything else to take care of your lens glass.

See you soon for the next part and the third very confusing bit of exposure calculation...aperture settings.

Happy shooting and play often!

Anton