Understanding Exposure

By Peter Ryan

Your camera manual is full of it. Web pages are full of it but many people coming to my workshops struggle to understand the relationship between Aperture, Shutter and ISO. There is much written about what each one does but not a lot on how they work n a reciprocal arrangement.

Let me try and explain.

The exposure trilogy of the Aperture, Shutter and ISO are all related.

The camera’s light meter gives you the correct exposure reading for a given image (albeit it sometimes needs some fine tuning but let’s leave that for now) and the Shutter (in the camera body) and Aperture (in each lens) manage the amount of light required to record the image according to the light meter reading.

The Shutter controls Time in photography (hence Tv = Time Value; some use S or Sv) and allows you to freeze the action (with fast shutter speeds) or show movement with blur (at slower shutter speeds). The shutter speed required will depend on how fast the subject is moving and also whether it is moving across your vision or towards/away from you.

The Aperture controls the Quantity of light allowed to enter the lens. The aperture is used to control Depth of Field (that part of the image that is sharp in a photo).

As I said the light meter tells you how much light is required to record an image correctly exposed and if you are using either of the semi automatic modes of Aperture Priority or Shutter Priority the camera adjusts these to allow you the correct exposure. Now it does not have unforeseen powers so it just starts off by selecting settings based on the last time you used the camera. So if you were in Aperture priority and using, say, f8 as your aperture then the camera would set a shutter speed to permit the correct exposure for the new subject based on f8.

There is a reciprocal relationship between Aperture and Shutter so if you change one then the other one will change to compensate and maintain correct exposure. For example, if you moved the aperture from f8 to say f11 (one stop less light) then the shutter would need to stay open longer to compensate and so the camera would automatically adjust the shutter to be one stop slower (to let in more light and maintain the correct exposure).

The ISO represents Quality control in photography and adjusts the sensitivity of the digital sensor (the recording medium) to light. At ISO 100 or 200 the image quality is good because it needs more light to record an image at these lower levels. As you increase the sensor to 400, 800 or 1600 the sensor does not needs as much light to record an image but the quality deteriorates.

So putting it all together: –

If you are using Aperture priority but cannot get the Shutter speed you need to freeze the subject, based on the Aperture size you want to use (i.e. to get a certain DOF); then,

• if you increase the ISO setting (making the sensor less sensitive to light),

• you do not need to leave the shutter open as long to maintain correct exposure, so you can obtain a faster shutter speed.

• the camera adjusts the shutter for you when you change the ISO setting so again you do not need to do anything special.

Of the trilogy Aperture, Shutter and ISO you have told the camera that the most important one for you at this time is Aperture by choosing Aperture Priority so: –

• if you change the ISO you do not want the Aperture to change

• so the camera adjust the shutter to maintain correct exposure.

If you were in Shutter Priority (telling the camera this is the most important on of the three for you) then any change in the ISO would adjust the Aperture setting.


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