Sunday, 10 June 2012

Exposure, the Metering Pattern and the Histogram

What is correct exposure?

What criteria do you use to assess correct exposure? Is it reading the histogram? Is it setting the camera to P A S and flipping to M to “match needle”? Is it buying this pro looking external exposure meter, then transferring the settings to the camera on M? Is it the way the photo “comes out?

There is no correct exposure. There is contextually suitable or relevant exposure. That means if you are wanting the face just right, it is. If you want the sky, it is. If you want that bright sky and that darker ground just right, well that’s being just…. greedy.

  1. The exposure histogram describes the brightness levels of the scene in numbers.
    1. It’s not a visual assessment. I’ll say it again, its not a visual assessment.
    2. It does not show whether the exposure is correct (see point 1 – there is no correct exposure). you can have what looks like a “good” histogram, whatever that is, but the face you want is contextually too dark or too light.. ditto with the sky and the earth.
    3. It can show that you are clipping highlights (255) or the shadows are going too dark ( 0 ). that does not mean you photo is “wrong” – it just shows that technically in numeric terms, you have lost digital data – that does not mean you have lost visual aesthetic.
    4. The averaged histogram may be misleading in technical terms. because your separate r g b channel histograms are different and one of them may already be clipping whilst the averaged histogram may not indicate that.
    5. There is more than one histogram for the same scene. Yes. There are:
      1. the Liveview histogram which is quickly computed so that the camera’s Liveview does not slow down. It is the stats BEFORE the shot is captured.
      2. the exposed photo JPEG histogram – this is the stats AFTER the shot is captured.
      3. the invisible and yet-to-be-deciphered raw file histogram. The reason why it is not visible can can’t be displayed is that the raw file has not been processed against your preferred set of contrast, saturation gamma curves. And if we did apply our preferences, it would no longer be the raw histogram, it would be the processed photo histogram.
    6. You might consider the highlight / shadow blinkies as an alternative to the histogram. But, the highlight and shadow blinkies give you even less information than the histogram – it can distract you by corrupting your perception of whether your exposure is contextually correct by painting your LCD with big blobs of red.
  2. You will often encounter naturally lit scenes that exceed the dynamic range of a single shot, even if you shoot raw. You will have to turn the tables (Star Trek’s Kobayashi Maru) by any trick – come back another day / season / time, stand with the sun behind your back, carry out exposure blending from multiple exposures, big a bigger sensor, shoot film).

Generally, we have:

  • Spot Metering
  • Centre Weighted Metering
  • Evaluative / Matrix Metering

Spot Metering

It tries to make anything you point at, 12% grey in tone. Point it at a white shirt and it will try to make it grey. Point it at a black shirt and it will try to make it grey.

That’s not a general use, point and shoot metering pattern. Some people figure they can use it like this, well and good for them.

Centre Weighted Metering

Centre Weighted covers a central area – yes, around the centre AF spot. The area may be hard edged. Or not. On optical viewfinders you may not be able to estimate how big the area is.

It has sufficient area that if you point it at something, it will average that bit of face, that bit of shirt and that bit of background to 18% grey. That might be just what you want – the face might turn out alright.

It has a small enough area to exclude the bright sky or that dark table shadow from dominating your camera’s exposure proposal.

There is also on some cameras, a spot biased centre weighted choice.

Matrix / Evaluative Metering

You could get the meter to measure the whole scene and take a dumb average. If you do that, some bright sky could “pull” the meter reading quite unfavourably towards a darker exposure.

So the camera makers came up with Matrix / Evaluative. Some cameras even have computationally enhanced evaluative (with elements of Artificial Intelligence, database statistics on image scene modes, face detection etc…)

This is the metering pattern (well it is not a pattern, it is a tremendous state of the art calculation) that the camera makers put a lot of their skill and knowledge into. If there is one thing they can create to help the millions of customers who just want to aim and press the trigger, this is the epitome of technological prowess.

So far on my cameras which are not leading edge nor are they the most expensive, this form of metering still can’t make it happen for the majority of my shots. Yet. And I can’t predict in which direction and by how much it will bias the camera’s setting since it is using some dynamic logic, not a standard pattern.

The Bottom Line

I have met a whole bunch of amateur photographers of various experience levels. And encountered even more on the web and in forums. Using my preference – Centre Weighted and the subjects I shoot, let me put forward some advice:

  1. Each country (e.g. Australia, Malaysia), each season (blue sky summer, gauzy overcast winter) and each time (Golden Hour, mid day, etc..) merits different consideration and care.
  2. There is no set-and-forget. And there is. Depends on your style and what you approve of and accept. You can accept that, for example, Street Photography of strangers cannot be controlled and is an unrepeatable point in time so any shot is good, even a technically imperfect shot. Or you can put on your grumpiest attitude and criticise the hell out of every landscape / panorama shot.
  3. These are modern cameras. Centre Weighted should be predictable. I didn’t say what you want (who knows what you want?). But it should be predictable. On a “standard” scene “what is standard?”, it should either be pretty close to good, or within 0.7 dark or light. If it is too dark or light, remember how it behaves and set the bias semi-permanently. It is semi permanent because there is no standard scene.
  4. Learn to identify several types of scenes. Why? Because that is what the camera makers are also doing for point and shoot people – that mechanism is called SCENE Modes. I’m NOT saying you should use SCENE mode. I’m saying you need to identify the scene you are pointing at, test it out beforehand how your camera’s Centre Weighted Pattern predicts and develop your own exposure biases so that they become instinctive and intuition.
  5. If you are using an EVF / LCD mirrorless camera, it’s even easier. Set your Liveview to exposure simulation mode (they’re already defaulting to that) and point, assess the darkness / brightness of the scene and flick your Exposure Compensation Dial. Just like that.

Further Reading

1 comment:

Mr S said...

Fantastic lunchtime reading! I've always used spot metering only because it seem to properly expose my subject (although it doesn't take into account the rest of the scene) but I have never ventured out of that metering method. I must give Center Weighted Average metering a try sometime.

Thanks for the handy tips and advice Ananda!