Westfield Photographic Club

EXPOSURE 1 - APERTURE

Nothing confuses a beginner in Photography more than the whole subject of exposure. The most important thing to realise is, and it’s so important I’ll devote a couple of lines and underline it;-

There is only one level of light on your sensor or film that will give you a perfect exposure.

There are three ways your camera controls the amount of light your sensor sees.

Aperture, Shutter Speed & ISO

1. Aperture. This is a multi leafed iris which forms a hole within the lens that can be opened or closed, the wider the hole in the iris the more light can get through, the smaller the hole less light (told you this was basic). OK that’s the easy bit.

The size of the hole has an f number which is what really confuses people. The f number is not arbitrary it has a precise size and takes into account the losses within the glass lenses that make up the lens, but more importantly how much area the lens can see, a wide angle lens can see the light reflected off a large area so gathers lots of light, a telephoto lens on the other hand has a narrower field of view and so would get a dimmer image everything else being equal.

This field of view the lens sees is determined by it’s Focal Length. So our f number needs to take this into account which it does by dividing the Focal Length by the hole diameter in mm (all optics use the metric system). As an example a lens with a Focal Length of  50mm set at f8 would have a hole diameter of 50 divided by 8 which equals 6.25mm, actually it would be a tad larger as the manufacturer has built in the losses in the glass used in the lenses that make up the lens and the fact that it‘s the area of the hole not just it‘s diameter that matters.

You may wonder why do the smaller apertures have the larger f number? It’s because the f number is part of a fraction. Everybody knows that ½ a cake is larger than ¼ of a cake. In the same way f/8 is larger than f/16. We meet fractions again with shutter speeds. The lower case f is the international symbol for Aperture and stands for Focal Length so f/8 is the Focal Length divided by 8, f/16 is the Focal Length divided by 16 and so on, as we saw above.

The f number enables us to have a standard amount of light flowing through any lens irrespective of its focal length, manufacturer and all other variables. f8 on a wide angle lens would allow exactly the same amount of light onto our sensor as f8 on a telephoto lens.

We have one final hurdle to jump to fully understand Apertures and that is the f numbers relationships with each other. You might think that f8 let’s through twice as much light as f16, Unfortunately that’s not so. The apertures are based on area, not just diameter, area has height as well as width so f8 allows through four times more light than f16. To get around this intermediate values are added, so now our scale looks like this with the widest aperture on the left.                                                                                         

Reading from the left every one of these stops from the widest aperture (f1) to the smallest (f128) allows half the light through the lens of the one immediately before it, so the light passing through an f8 aperture onto our sensor is only half as bright as it is with an aperture of f5.6.

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Aperture and shutter speed acting together in balance is what is termed Exposure.

A perfect exposure is one that records the most data, there is little latitude with digital, ½ a stop can make a noticeable difference, you can think of your camera as an Image Recording Device. 80% of what is important to the image the Composition, the Subject, the Lighting - it’s a long list, happens external to the camera, the cameras job is to record it with high fidelity and without adding any false data such as as artefacts and noise.