Tech Tuesday: Exposure

No, I am not talking about being seen on the latest social scene. I mean the exposure of a photo. I was asked recently about how I approach taking a photo? The first is to determine the exposure. Whether using a P&S on auto or the newest most sophisticated DSLR you still need to understand what exposure is.

Exposure is the sum of the three main elements of photography. Aperture, Shutter speed and ISO. I call this the triangle pf exposure They are interconnected and you can not change one without effecting at least one other. In the coming weeks we will look at all three more in depth, right now I want to give an overview of what makes an exposure.

ISO is the speed in which the film or sensor absorbs light. From the old film days you had ISO/ASA 100, 200, 400 etc. the larger the number the faster the film absorbs light.

Aperture is the diameter of the hole in the lens expressed in f stops, such as f2.8, f4, f5.6 etc. The smaller the number the larger the opening thus the more light allowed in. conversely the larger the number the smaller the opening and the less light allowed in.

Shutter speed is the time that the shutter is open, to allow in light and expose the film/sensor. It is expressed in fractions of a second such as 1/60, 1/125 etc.

As I said you can not change one without effecting at least one other side of the triangle. Let’s say either you or the camera set ISO 100, f16, 1/125 for an exposure, by the way this is the sunny 16 rule and more about that when we post about metering. now if the light changes or you decide to change one of the settings what will happen ?

I should explain that all three are measured in what is called stops. ISO 100 is one stop slower than ISO 200, f2.8 is one whole stop more open than f4 and 1/125 is one whole stop slower than 1/250.

From our basic settings lets say we change to f8 instead of f16. F8 is one whole stop more open than f16 so the shutter speed increases by 1 whole stop to 1/250. Same thing would happen if we increased the ISO to 200. To keep the same exposure the aperture would have to increase to f32 or the shutter speed up to 1/250. You see one stop of change must result in one stop of change in one of the other two. It is Newtons law for every action there is an equal and opposite reaction.

Why does it matter which one changes? We will delve into that when discussing choosing the correct exposure for a given situation.

Maybe by the end of the summer most of you will have gotten your camera off of the green square.

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