Demystifying the numb3rs in photography – by Mark Pedlar

As a person with little experience in photography you may choose to join a camera club to meet people with similar interests and to learn more about the subject. You may well be looking to improve the standard of the images you produce. Then, when you attend your first meeting you find you’ve landed on Mars. People are asking you questions about your photos that you can’t answer and using abbreviations and acronyms you’ve never heard. It could just as well be another language. The object of this short article is to de-mystify some of those terms and to show how some of the numbers these people quote all come together.

Let’s start with your camera. It’s just a box with a hole in one side to let in the light. Inside the box is a flat panel that collects the light. You can easily make a camera with a shoe box. Cut the base out of the box and cover that side with grease-proof paper. Then poke a hole in the opposite side with a knitting needle. Stand in a darkened room and point the hole at the light. You’ll see an inverted image of the scene ahead of you on the grease-proof. Behold, a camera! However, the image you see is not clear, it’s fuzzy. To overcome that in a real camera we put a chunk of glass (lens) in the hole to make the image ‘sharp’. We’ll see how it works later. The other thing is that when you move the shoe box the picture goes away. So we need some way of storing what was on that screen. In your real camera the grease-proof is replaced by a sensor which captures the data electronically. All we need now is to control how much light gets onto the sensor; too much and it overloads and your image is all white, too little and it’s black.

Focal lengthWhatever lens your camera has it will be described by its focal length. We don’t need to go into this in great detail but the lens’s job is to bend rays of parallel light so that they come together to meet at a point. When they meet your image looks ‘sharp’ not fuzzy and it’s ‘in focus’. So a 28mm lens brings the rays together at 28mm and a 200mm lens does it at 200mm. This also means that the 28mm lens harvests light over a wider area (wide angle) and a 200mm lens gets light from a narrower angle (telephoto).


f8vsf16From here we are now concerned with getting just the right amount of light into the camera through the lens. We can consider the lens as a tap allowing the flow of light rather than water. So the factors affecting how much light gets through are the size of the hole and the time it is open. The term ‘f stop’ refers to the size of the hole. It is the diameter of the hole as a fraction of the lens focal length.

So, at F8 the diameter of hole on your 200mm lens is 25mm (1/8 focal length). The next thing is that the amount of light going through the hole is related to the area of the hole not its diameter. So if we go from F8 to F16 the hole has gone from 1/8 focal length to 1/16 focal length and the light is not half but a quarter of the original. This is why we have those funny F numbers like F11 because a hole with a diameter 1/11th of the focal length has twice the area of a hole with a diameter 1/16th  the focal length. This has all been about Aperture. The next issue is shutter speed.

The smaller the F no. the larger the hole, the bigger the precision piece of glass and the greater the cost.
Shutter speed denotes how long the shutter (tap) is open; how long the light is allowed to flow. Again the numbers are fractions. So 125 on your camera 1/125th second. Being a fraction, the smaller the number at the bottom, the larger the quantity that is being described. If the shutter is open for 1/60th second it lets in twice as much light as if it is open for 1/125th second. Now we have a beautiful and simple relationship between aperture and shutter speed. A lens set to F8 for 1/250th second lets in the same amount of light as one at F11 for 1/125th and as one at F16 for 1/60th. So if you want the shutter open for the shortest possible time, to minimise the risk of moving the camera, you’ll need to have a lens that opens up to a very wide aperture, maybe F2-2.8.

The implications of shutter speed are relatively easy to grasp. The longer the shutter is open the more chance there is that the subject may move, or that the photographer may move. Either will result in a blurred image. This may be the desired result, of course, but it’s a real bummer when it happens by accident. As a rule of thumb the shutter speed number should be no less than the focal length of the lens for hand held shots.

Images courtesy of Time Life Library of Photography – The Camera pp101-11

The implications of F stop selection may not be so immediately apparent. We need to have a look at why we have a lens in the first place and then how F stop affects a factor known as ‘depth of field’. When we talked about basic pinhole cameras at the start of this article I said that we needed a lens to overcome the blurry image otherwise created. In the illustrations above we can see that without a lens the image in the camera is made up of many small circles rather than sharp points of focused light. What’s more as the hole in the camera gets larger those circles get larger and the image becomes more confused.


Image courtesy of Time Life Library of Photography – The Camera pp101-11

Therefore, we put a lens in the hole so that we can focus the light accurately on the plane of the sensor giving us a sharp image.  Some or all of the image will now appear in focus. However, in reality, only objects on the same plane as those we focused on will be in true focus. Just how much else appears to  be sharp is what we call ‘depth of field’. Everything not on the exact plane where we focused is producing those circles of confusion we can see in the images relating to a ‘no lens ‘ situation.

In these cases the further the objects are in front of or behind the true plane of focus the larger the circles and the more ‘out of focus the objects will appear. The other thing affecting the size of those circles of confusion is the physical size of the camera aperture. The smaller the F stop number, the bigger the hole the less is the ‘depth if field’ (the area that appears to be in focus). Not only is the F stop important  but also the focal length of the lens. F8 on a 50mm lens is a hole 6.25mm across. F8 on a 1000mm lens is a hole 125mm across. So telephoto lenses have much lower depth of field at a given aperture than wide angle ones.

Image courtesy of Time Life Library of Photography – The Camera pp101-11 Top image 28mm @ f8, bottom image 50mm @ f8

Both of the two images above are shot on a full frame sensor (35mm) camera. The upper one is shot at F8 using a 28mm lens and the lower one At F8 with a 50mm lens.

Just one other set of numbers – ISO ( used to be ASA). There is one other factor that affects getting the right result from the amount of light getting into your camera. In the days of film this was the sensitivity of the film. Different emulsions were available which reacted to different intensities of light. Those that needed the least light and therefore worked in darker conditions tended to have larger chemical particles in them. When these photos were enlarged the ‘bits’ showed up. We call them ‘grain’. The relative sensitivities of the various films were described on a scale called ISO. This scale has been perpetuated to describe the alterations your camera allows you to make to the sensitivity of  its sensor. To some degree the effect of grain being more apparent at the ‘faster’ settings, those that work in less light, is still the case.

Basically, the image requiring 1/100th second exposure when the ISO is set to 100 will only need a 1/200th second exposure if the ISO is lifted to 200. Doubling the ISO enables you to use half the light.

The photographic language really isn’t all that complex once you have a key to the commonly used words, terms and acronyms . Don’t be put off. Jump right in. The water’s fine.