Understanding Exposure

All Camera built in meters are calibrated to expose for medium tones with the exception of Matrix (Evaluative metering for Canon) metering which bases its computation on some logic depending on the contrast in the scene.

This article is based on spot metering which meters the light falling on a small percentage of the area (typically 3%-8%) of the focus spot selected in the Camera.All camera meters read the reflected light – and then provide a reading to make whatever you have pointed at, to a medium tone.

This means if you have your camera on A or S mode (Av or Tv for Canon) it will set the aperture and shutter speed all the time to make the meter set at Zero (you can see it inside the view finder).

Every time the meter is at Zero it means you are going to get a medium tone.Medium toned subjects reflect 18% of the light falling on them. If your subject is indeed a medium tone, then everything is fine, just shoot at the meter reading.

Some examples of medium tones are Green Grass, Blue Sky, and Faded Jeans etc etc.

This is exactly why people use a Grey Card because it's calibrated to reflect 18% of the light falling on it.Black Colour as in a Black Shirt or a Crow for example will be rendered Medium Grey by the meter because its calibrated that way. It thinks there is too little light being reflected from the Black and hence over compensates making the Black appear as Grey.

Similarly a White Shirt or a White subject like a butterfly for instance will make the camera think there is too much light coming back and it under compensates to make white appears as grey again.

The trick here is to tell the camera that its thinking is wrong and compensate for it – This is achieved by Exposure Compensation.This is done by several methods.

You will have to do this, when using A Mode. (similarly for S Mode)

* Point the spot meter at the subject you have in mind. Lets say your subject is a White Rose.

Make sure you point the meter at the brightest spot of the Rose – best option you have is to select your subject which is in even light and not differential lighting.

* Select an aperture, lets say F/8 is what you want to use.

* Camera will select a shutter speed which will make your meter reading Zero. If you were to shoot at this shutter speed, your rose will be rendered Medium Grey.

* Press the Exposure Lock button (AE-L for Nikon Cameras) and then dial in +1.5EV or 1.7EV using the Exposure Compensation Button located on your camera.

What you are essentially doing is asking the camera to let more light in (more by 1.5 to 1.7 stops) so that the Rose is now rendered White and not Grey.

* Please be careful because the exposure lock won’t remain there for more than a few seconds. In some Cameras like the F100 for example (and most digitals) one can use the Custom Function Menu to tell the Camera to hold this lock for something like 20 seconds.

Check your camera as to what settings it permits. If you take a longer time than this, then you don’t have any control and Camera will do what it will !!!.

* If your subject is dark black coal or a black shirt for example you dial in negative compensation, something like –1.5EV or –1.7EV. You are essentially telling the Camera to let less light in, so that Dark colours are rendered Dark and not medium Grey.

A much better way to handle this situation is to use the M or Manual Exposure Mode in your Camera. Many people confuse Metering with Autofocus and Manual Focus.

One can use Auto Focus in the M Mode as well – M Mode only gives you the control on setting the A and S values independently and thus obviating the need for the Exposure Lock button.

This is one reason why Henry always says, take the easiest way out which is manual. More steps means more opportunities for error.* Set your Camera on M Mode and lets assume you are shooting a White Rose again.

* Set your Aperture, lets say you want f/8.

* Now set the Shutter Speed in such a way that the dial or the electronic meter (green in most Cameras) shows the green sticks which represent +1.5 or 1.7EV. Nikon readings are set to read + on the left side and – on the right side of Zero.

So if you see Green Sticks on the left side of Zero then you are giving + compensation. Each Green Stick by default represents plus or minus 1/3 EV. Some Cameras have a custom function to make it +/- 0.5EV as well. * Once you have done this setting simply shoot. In the M mode you don’t have to worry about AE-L or Exposure Lock. No matter where you point the camera later the Camera still will get you the right exposure.

* This is why I always harp on using the M Mode, its that much simpler and less error prone to use.There are also some intuitive situations in life where one can simply use the Sunny f/16 rule.

If many of the subjects in the scene are medium tones, then you simply need to set your aperture to f/16 and shutter speed equaling 1/ISO you have set in your camera or the ISO of the film used you will have the right exposure.

This will also mean that at equivalent exposures will also provide the right final exposures.

For example if you used a ISO100 film then f/16 @ 1/100 seconds is the same as f/11 at 1/200 seconds, f/8 at 1/400 seconds or f/22 at 1/50 seconds………Again, please note that all you are doing is controlling the amount of light hitting the film or sensor.

Metering has nothing to do with Auto Focus or Manual Focus….Get this right again, I see a lot of confusion amongst the membership on this issue.

Long back I used to be confused about this aspect as well.

Everything you shoot wont be Black or White or Medium.

There are tones which fall in between. Pls use +/-0.7 EV, +/-1.0 EV etc accordingly.

If you use a grey card, then you need set the meter to read Zero. But the problem is that one cannot use a Grey Card in many situations.

Over a period of time, you will be able to calibrate your eye well to judge the tonalities.

Bracket when in doubt.

-Ram