Understanding exposure

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Lexy spent her formative years taking a lot of photos and dreaming in technicolour. Nothing much has changed now she's covering all things photography related for CNET.

Exposure modes

Once you've understood the basics of what makes up an exposure, you can start to familiarise yourself with the manual modes on your camera. Most digital SLRs will have the following options available on the mode dial.

Mode dial

Program mode (P) The camera will choose the optimum combination of aperture and shutter speed for you, but you can control variables such as white balance and ISO. On some cameras you can also toggle between different exposure combinations for a little more control.
Aperture priority mode (A) You select the aperture you want in order to achieve a desired effect (for example, opening up the aperture to blur the background) and the camera will select the optimum shutter speed required for obtaining a correct exposure. You also have control over ISO and white balance.
Shutter priority (or Tv) mode (S) Depending on what brand of camera you have, this mode will either be S or Tv (Shutter priority or Time value). Like in Aperture priority mode, you get to choose the shutter speed you want to achieve a desired effect, and the camera will automatically determine the aperture required to obtain a correct exposure. Again, you have control over ISO and white balance.
Manual mode (M) You have complete control over all aspects of the exposure and can adjust aperture and shutter speed freely. You also have control over ISO and white balance as before.
Bulb mode (B) All digital SLRs have this mode but only a few will have it on the mode dial. B stands for bulb mode, used for long exposures where the shutter needs to stay open for a significant length of time.

Light meter

So now you're probably wondering how your camera determines a correct exposure. It all comes from the light meter. A light meter measures the reflected light off a subject and uses that measure to determine the optimum exposure.

Light meters come in two flavours, internal and stand-alone. A stand-alone light meter looks like the image pictured below.

Light meter

(Credit: Sekonic)

All digital cameras have a light meter built into them so photographers no longer have to carry around a dedicated meter, or refer to techniques like the zone system and the sunny 16 rule.

If you have a digital SLR, look inside your viewfinder while you are in P, A, S or M mode. You should see a line or a bar in the middle. Depending on your brand it will look something like the following:

Nikon light meter Canon light meter

Nikon light meter

Canon light meter

Alternatively, on some digital cameras you can look at the light meter and exposure measures on the LCD screen.

Nikon LCD screen with light meter and exposure readings

(Credit: Nikon)

When the pointer is in the centre sitting on 0 it means that you have a correct exposure according to the camera's light meter. The light meter will continue to measure the incoming light as you adjust the combination of aperture and shutter speed (and ISO) while in manual mode. You can choose to underexpose, or overexpose, despite what the light meter tells you, to achieve different effects.

Adjusting the aperture and shutter speed is a different process depending on what brand of camera you have. Usually, there will be a rotating dial located just underneath the shutter button that you can turn to adjust shutter speed. Often, the same dial will be used to adjust aperture as well, and a specific button needs to be held while you turn the dial to toggle between shutter and aperture selection.

Nikon D90 and Canon 50D shutter and aperture dial

Nikon cameras, like the D90, have the aperture/shutter speed selector dial around the front of the camera. Canon cameras, like the 50D, have the dial on top of the camera body. (Credit: Nikon, Canon, CBSi)

Medium grey

As we've discovered, all cameras take a light reading using the internal light meter in order to determine a correct exposure. This reading comes from what's called reflected light — so the light bouncing off an object or subject. Light meters are calibrated to assume that all light is reflected off "medium grey".

As you've probably guessed, not everything you point your camera at will have the same light-reflecting properties of medium grey, therefore giving a less than optimal exposure. This is why you might see some photographers carrying around a specially designed medium grey card from which to take an ambient light reading and set their exposure accordingly.

In the next instalment we'll look at the role of exposure bracketing, as well as exposure compensation and the histogram. Click here to read the next part.

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Add Your Comment 8

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KevinH2 posted a comment   

very useful


CMc posted a comment   

informative and to the point. not too technical yet I feel like I reviewed / learned a lot


Ajith posted a comment   

Good writeup..easy to understand


Bec posted a comment   

Thanks for the write up. the explanations are very easy to understand. Many Thanks :-)


Aaron posted a comment   

Great article. Very helpful to understand some of the more technical terms that overwhelm new photographers


Nath posted a comment   

Great write up, thanks big help.


Mavis posted a comment   

Excellent write up and easy to understand. Thank you!


anysia posted a comment   

Good job on the write up. Many new to digital SLRs can be overwhelmed, and end up using the Auto settings only, because reading the manuals' same tech phrases over and over again can be mind numbing. This was written for the new users in mind.

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