monument-Valley-May-1945Correct exposure of the image is critical to preserving as much detail as possible and also getting the effect you want for a particular image.  If you are doing a silhouette shot of a mountain in a warm sunset you will be underexposing an image to accomplish this.  If you were to expose this scene with a normal exposure you may get some detail in the mountain, but the sun would be very washed out and drab looking.


On the other hand, if you were taking a scene of a black dog sitting in a field of white snow, you would slightly overexpose the image to render the snow white.  A “normal” exposure of this scene would leave you with rather dark looking snow and probably an almost silhouette dog.  The next section will briefly explain how your camera measures this and gives you the correct result.

The Meter

The light that enters your camera must be measured somehow to give you the correct exposure.  Modern camera’s have several different methods of metering like average, average center weighted and spot metering.  Each has a specific use for creative purpose depending on the scene or subject to be metered.  We will only deal with average center weighted metering in this section to demonstrate how your camera decides what the correct exposure should be.  To do this it is easiest to think in black and white.

light meter

The camera’s meter only understands one “color” or shade, and that is midtone gray.  If you were to have a scale of shades from pure white to complete black, you get the idea approximately where this would fall on that scale.  So no matter what scene you point your camera at, it will try to average that scene out to become midtone gray.   Most beginners use their cameras in Automatic (also known as idiot mode ) and get reasonably well exposed pictures. This works well for the average scene where there are dark areas and light areas, and the camera is assuming your main subject is in the middle of the picture. It will make some allowances to try to expose your main subject properly (the “center weighted “part of the metering process).  However if the scene is not average, the camera throws its proverbial hands in the air and says  “I’m going to make you midtone gray”. Imagine again a scene of a black dog in a field of white snow. The majority of the scene is white with a small black area.  The camera left to think this out for itself will decide that the snow really should be midtone gray and that’s how your picture will turn out if you shoot without exposure compensation.  This means you have to add light to the scene to make the snow white again.

black wall_white_cats

Conversely if you have a white cat against a large black wall the camera again would try to turn this into midtone gray.  This would mean you would wind up with a muddy gray looking wall. In this case you would have to remove light from the scene to render the wall black.  So how is this done?


F Stop and Shutter Speed 

The two things that control how much light gets to your sensor are shutter speed and apeture setting or more commonly referred to as F stop.  The best way to wrap your brain around this is to get out of automatic mode, and turn your camera dial to “M” for manual mode. This allows you to have control of both these parameters independently, and you will gain much better understanding of how you’re camera works.


We will deal with shutter speed first and leave the F stop at a given value.

Shutter speed is controlled by the camera body.  You can try this out by again removing your lens and setting the shutter speed dial to 1 or more seconds.  When you push the exposure button you can see the mirror flip up and 2 shutter curtains travel back and forth exposing the sensor for whatever time you have set for the shutter speed.  (Just remember that the whole time you are doing this dust is collecting on your sensor which you will have to clean at some later date). Now turn the shutter speed dial to a quicker time, say for example 1/125 of a second and push the exposure button. You can see how much quicker the mirror flips up and down and the curtains slide back and forth.  Once you set the shutter speed even higher at 1/1000 of a second the whole process becomes a veritable blur. So what is the conclusion of this whole exercise?  Each time you double the shutter speed you are letting in half the amount of light.  So for example if you go from 1/125 (slower) of a second down to 1/250 (faster) of a second, you have half the light hitting the sensor and exposing your scene.  This would make your scene what is known as 1 stop underexposed.  Remember the white cat dark wall example.  Underexposing would make the wall dark again instead of the muddy gray that the camera wants to do automatically.  Conversely if you half shutter speed for example from 1/60 (faster) of a second to 1/30 (slower) of a second you are letting in 2X the light or adding one stop of light.  This would turn the gray snow in the previous dog example back to white again as it is overexposing the scene to compensate for all the white. So now you can see how shutter speed is one component of how much light enters the camera to expose your image.  Now on to aperture setting or F-stop.

F-stop is controlled by the aperture in your lens.  Most modern lenses are wide open at their maximum aperture.  When you push the exposure button the lens has little leaves that close down to your desired F-stop to limit the amount of light entering the camera.  You can liken this to going in your bathroom, shutting off the lights and standing in front of the mirror singing “Oh Canada” to yourself in French in the dark.  Now turn on the light.  Once that blinding feeling goes away you will notice that you’re pupils go from being very large and dilated from being in the dark to

being very tiny. apeture This may well be caused because your French is so bad, but more than likely it’s just your eye adjusting from being in the dark and needing all the light it can get (large pupil) to being blinded and trying to shut off the light supply as much as possible (small pupil).  This is exactly how the aperture setting works on your lens.  The only difference is that you can control it in conjunction with your shutter speed to let the desired amount of light on to your sensor.  If you have 5 dollars to spare take a trip down to your local pawn shop and buy an old manual lens that no one wants, and  you can stop down to see the lens opening open and close with different aperture settings.  Make sure you get one with a depth of field scale on it as well, as it will come in handy in a later lesson when we cover that area.


One more indirect way that light is controlled in the camera body is by setting the ISO or what was once film speed.  ISO rating is how quickly the sensor will absorb the light hitting it (the lower the number the slower the speed).  Lets say you are shooting a scene at ISO 100 and the sun is starting to go down.  You reach a point where you can no longer adjust the shutter speed low enough to compensate and get a proper exposure and still get a sharp image because you are not able to hold the camera still enough.  There are two things you could do.  First you could open the aperture of the lens up, say for example from F16 to F11 gaining one stop of light, or even to F8 to gain 2 stops.  But lets say you are shooting a landscape scene and you want to keep the F-stop at small (the larger the number the smaller the hole letting light into the lens) opening so you can get better depth of field (we will take on DOF in a future tutorial).  Your only other option is to increase the rate at which the sensor absorbs the light.  This is done by increasing the ISO.  The relationship in all of these options is two to one.  If you increase the ISO by doubling it, say from 100 to 200 you are doubling the light hitting the sensor assuming you leave the shutter speed and aperture as they are.   The inverse is true of the aperture and shutter speed.  If you double the shutter speed you are letting in half the light, or conversely if you half the shutter speed you are letting in 2X the light.   If you go to a bigger number in F-stop (say F4 to F5.6) you are letting in half the light and conversely going from F16 to F11 you would be doubling the amount of light getting to the sensor.  Each doubling or halving of the light is known as gaining or losing a stop of light.

Increasing the ISO does however have one unwanted side effect.  With an increase in ISO comes an increase in noise or that grainy effect you see in some images.   Most cameras can tolerate out to about 400 ISO and stay fairly clean but beyond that many cameras begin to display an increase in noise.  Some professional cameras stay very noise free out to ISO 1600 and beyond.  This becomes a great advantage in low light conditions where you need all the help you can get to get enough light to properly expose an image.  If you go from 400 to 1600 ISO you have gained 2 stops of light.  Imagine trying to hold your camera steady at 1/30 of a second ( pretty slow for most people to do and still get a consistantly non blurry usable picture) and suddenly being able to bump the shutter speed up by 2 stops (1/30 to 1/120th of a second).

Summary and Conclusion

The sensor needs X amount of light to properly expose the image as indicated by the light meter.  This is controlled by the the photographer  in any combination of three ways;  Changing F-stop, shutter speed, or ISO.  Probably the best way to understand this is with this excellent program that demonstrates this better than any words possibly can.   Click on this   camera simulator   to see the relationship between these three parameters and how they affect your meter and final image.

In conclusion this is the sequence of events that happens when you push the exposure button on your shiny new digital SLR.

Modern lenses have the aperture wide open to allow maximum light into the mirror, so when you look through the camera the viewfinder stays nice and bright and you can properly assess your scene.  When the exposure button is pushed the lens stops down to the aperture setting (F-stop) that you have set on the camera body.  The mirror then flips up out of the way and the shutter curtains behind the mirror slide aside to expose the sensor to the light coming through the lens.   The shutter stays open for the amount of time you have chosen for your shutter speed.   (These two settings were selected by looking at the meter  in the camera to determine proper exposure)  The shutter curtains then slide back blocking the light from hitting the sensor, the mirror flips back down, and the lens aperture once again goes back to being wide open.  The camera body then does its thing transferring the light you gathered through its little magic computer  and saving it as an image on your compact flash card to be later transferred to your home computer for final processing.

In the next section we will have a look at the  histogram  which is a visual display of your image in graph form.