Exposure Compensation

Exposure Compensation – that’s something else that cropped up once or twice for the chaps on my recent Norwegian Eagle workshop!

We had something like 420 or more dives from eagles during the trip, and very few if any were shot with flat metering, or 0Ev compensation.

What is Exposure Compensation, and why do we need to use it?

It all begins with this little button:

Pushing this button and rotating your main command dial will select a certain exposure compensation value.

Why do we need to use Exposure Compensation though?

Cameras, for all their complexity and “intelligent whotsits” are basically STUPID!  They don’t know WHAT you are trying to photograph, or HOW you are trying to photograph it.

They make a lot of very basic assumptions about what you are trying to do – 99.99% of which are WRONG!

The camera does NOT know if you are trying to photograph:

• A white cat in a coal shed
• A black cat in a snow storm
• A white cat in a snow storm
• A black cat in a coal shed

All it sees is a frame full of various amounts of light and shade, and depending on your metering mode (which should always be Matrix/Evaluative – see post here) it gives you an “average mean exposure value”.

Take a general scene of fairly low contrast under flat overcast light:

As discussed in the previous metering article mentioned earlier, only MATRIX/EVALUATIVE takes the entire frame area into account.

Okay, so that scene was fairly bland on the old tonal front, so let’s have a look at something a little more relevant:

Why would the image be so dark and under exposed?

Well here’s an approximation of the cameras average tone “thought process”:

But if we look at some averages WITHIN the overall image:

We can see that the tonal values for the subject are generally darker than the average scene value, therefore the camera records those values as “under exposed”.

This is further compounded by the cameras brain making the decision that the commonest tonal value MUST represent “mid grey” – which it DOESN’T; it’s lighter than that – and so under exposing the image even further!

Now I’m not going to get into the argument about “what is mid grey” and do Nikon et al calibrate to 12%, 18%, 20% or whatever – to be honest it’s “neither here nor there” from our standpoint.

What is CRITICAL though is that we understand the old adage:

“Light Subject Dark Background = Under, or negative exposure compensation. And that Dark Subject Light Background = Over, or positive exposure compensation”.

Okay, but what are we actually doing?

In any exposure mode other than Manual mode, we are allowing the camera to meter the scene AND make the decision over which shutter speed or aperture to use depending on whether we have the camera in Av or Tv mode – that’s Canon-speak for A or S on Nikon.

If we are in shutter priority/S/Tv mode then the camera sets the aperture to give us its metered exposure – that thing that’s usually WRONG! – at the shutter speed we’ve selected.

If, as in the case above, we ADD +1.3Ev – one and one third stops of POSITIVE exposure compensation, the camera uses the shutter speed we’ve selected but then opens up the aperture WIDER than it’s “brain” wants it to.

How wide? 1.3 stops wider, thus allowing 1.3 stops more light into the the sensor during the exposure time.

If we were in Av/A or aperture priority mode then it’s the shutter speed that would take up the slack and become 1.3 stops SLOWER than the cameras “brain” wanted it to be.

Here’s an example of negative exposure compensation:

In this particular shot we’re pointing towards the sun – a “dark subject, light background” positive exposure compensation scenario, or so you’d think.

But I want to “protect” those orange highlights in the water and the brightest tones in the eagle, so if I “peg those highlights” just over a stop below the top end of the cameras’  tonal response curve then there is no way on earth they are going to “blow” in the final RAW file.

Manual Exposure mode can still furnish us with exposure compensation based on metering if we engage AUTO-ISO.  If we decide we want to shoot continuously with a high shutter speed and a set aperture at a fixed ISO then our exposures are going to be all over the place.  But if we engage AUTO-ISO and let the camera choose the ISO speed via the meter reading, we can use the exposure compensation adjustments just the same as we do in Av or Tv modes.

This get’s us away from the problem of fixed ISO Tv mode running out of aperture in low light or when very high shutter speeds are needed; or conversely, stopping the aperture down too far when the sun comes out! – I’ll do a breakdown on this method of shooting later in the year – it’s not without it’s problems.

Next time you get the chance to stand by a large lake or other body of water, just take a moment to notice that the water is dark in some places and light in others. ambient light falling on a moving subject can easily be very uniform and so the subject basically has the same exposure value all the time.  But it’s the changing brightness of the background as the subject moves across it that causes us to need exposure compensation.

People seem to think there’s some sort of “magic” at play when they come out with me and I’m throwing exposure compensation values at them.  But there’s no magic here folks, just an ability to see beyond “the subject, framing etc” and to actually “see the light” and understand it.

After all, when we click our shutters we are imaging light – the subject is, for the most part, purely incidental!

And there’s only one way you can learn to see light and grasp its implications for camera exposure, and that’s to practice.

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