You've heard of the Exposure Triangle?

People refer to the Exposure Triangle endlessly. Someone has finally made an interactive one that makes sense.

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

• Interpreting the Histogram (this blog)
• What is Correct Exposure (this blog)
• Nikon article on the histogram and getting a histogram of a portion of your scene
• DPReview forum search on histograms

IDHDR–the confessions of a former HDR reluctant

No, don’t look at the title, and leave. Just because every Google+ photographer seems to have a fetish for HDR doesn’t make this current vogue or passé.
I’ve previously recoiled a bit from HDR. Trey Ratliff has been the sifu and legendary evangelist of HDR and some of his works are exemplary, not just in HDR but in wide angle lens compositions. But as with anything if you see too much of that, you get jaded. And I am seeing too much of the dramatically emphasised tone mapped HDR the past month at G+.
I’ve tried HDR – the Photomatix way. The EasyHDR way. The Dynamic Photo HDR way. They’ve all been klutzy. Really, taking the time to shoot at least 3 shots. Coming back to the computer to blend them. No, I won’t use Photoshop super alignment whizzo. I’m about as keen to Photoshop anything as plucking my eyebrows.
In-Device HDR, on the other hand, I’m recently more than happy with. My Samsung Galaxy S (Android) phone-cam has an easy no brainer app HDR Camera+ – costs heaps, all of AUD 2.82 – yes, I haven’t made a mistake with the decimal point. You point, you hold steady for 3 clicks, and that’s about it. You can even post up to Google+ or Facebook. The small sensor phone-cam really benefits from extending the handling of dynamic range – skies improve instead of being dead white.

Rob and Rae's big Eucalypt at sunset – a tough shot in view of the shadows




Early morning at the Monash Staff Development Unit on Wellington Road with deep shadows and bright, bright happy but cold sun.

The Four Stages of Exposure Awareness

Stage 1: The first thing that newbies learn about is that there is an Exposure Triangle. Some Peterson guy is said to have wrote about it in a book. I haven’t read it. I’ve seen his videos. Maybe his intentions are good and he knows what he’s doing. But a heap of newbies don’t “get it”

Stage 2: Eventually it dawns on people that the Exposure Triangle has a Fourth Side.

Stage 3: After rummaging around, comparing effective techniques of whether to use P A S M or figuring out which metering pattern is better – Evaluative Matrix vs Centre Weighted vs Spot vs the classical Sunny 16 rule vs Interpreting the Histogram vs ETTR and asking themselves where the hell they put the white towel / Kodak 18% Neutral Gray Card or the XRite thingamajig, someone mentions that Adams chap who wrote about the Zone System. And bang! Smack on the head. There is no Correct Exposure. There is what the camera measures as an instrument and what the artist (you) choose to convey and interpret. The two are not and do not have to be the same thing

Stage 4: So far, so good. People are shooting decent shots. But they’re not spectacular. Like those gorgeous smooth skin tones and sharp, clear irises of the girls in the portraits. And so on. So we ask, how on earth does so and so get this shot with his iPhone but we can’t and we’ve almost spent as much as a Nikon D3s? And the penny drops. We can’t. If we REACT to the scene. Often times, the pros don’t react, they’re pro-active. They light up the scene the way they want. Or gain a vantage point if they can’t control the light. And having done their utmost to light the scene well, they touch up with Photoshop. Delicately and Emphatically. Not the other way around.

Interpreting The Histogram

Lots has been written about the Photographic Histogram. Many people still don’t understand what it is and what it does. Here are my notes:

What the histogram is

1. The histogram is a coarsely scaled and over small 2D representation of the tones. Some cameras have separate RGB histograms.
2. The histogram has two edges, the left and the right edge of the bounding box
3. If it looks like a peak has a slope extending past a box edge it means that there is some technical data that has been lost / will be lost regardless of RAW.
4. RAW has a higher latitude because the contrast curve is not "baked in" and the camera histogram may not show the RAW histogram, it may show the histogram of the embedded JPEG in the RAW. So, you infer that you have half a stop or more of latitude even though the in-camera histogram is showing that there is a truncation of the slope or tail of the peak.
5. With cameras with LiveView, the histogram is a predictive forecasting tool - it tells you before you press the trigger, what could be captured in tones. Shooting RAW or shooting a bracket of 3 or 5 shots is *NOT* a predictive tool - you have made the shots, you can retire without seeing the shots or you may take the time looking at the shots in camera - either way, you have lost oppurtunity or time. This lost time may be ok for a landscape shot but not useful for a quick assessment of a marching parade or anything in motion.
6. Some cameras don't have histograms
7. Some cameras don't have a good EVF or an LCD (e.g. my Kodak P880). This causes me to underestimate the visual quality of the image.
8. Some cameras have an over beautiful EVF or LCD (e.g. the newer Canon G models and reportedly the Panasonic LX-3, the Nikon D90 DSLRs set....). This can cause people to overestimate the visual quality of the image.
9. Some cameras don't have auto bracketing e.g. the Nikon D60.

What a histogram IS NOT

1. The histogram does not measure the visual quality of the shot. It is after all a crude 2D graph of tones.
2. The histogram does not tell you which subject or part of the subject constitutes the tail or slope of the peaks that is being cut off.
3. There is no "correct" histogram - the idea of "correct" does not apply because of points a. and b.
4. The histogram does not tell you how to compose the shot.
5. The histogram does not tell you what is wrong with the shot. It does not advise you that you are shooting into the sun, it does not tell you that you could enhance detail and micro contrast by changing your position and your lighting angle. It does not tell you that your metering pattern is inappropriate. It does not tell you that you are metering for a 12% grey target when you are facing white snow.
6. The histogram does not tell you clearly how much to compensate in EV - i.e. how much EV to dial in to move the peak and thus the tail of the hills. You can do some experiments and gain some understanding by dialing in EV and shooting a test subject. And watching the histogram move horizontally. You do that as homework, not on the day.
7. The histogram does not tell you how much tail to chop off or to force into the bounding box - that's your choice and you have to visualise using whatever method you understand (zone system, experience with the subject etc....). This is a human, visual assessment, certainly the computation and artificial intelligence is getting smarter but the histogram isn't - the histogram is a crude 2D graph.

If a person does not understand how to use the histogram, there is nothing stopping the person from doing a bracket of shots or interpreting visual quality by looking at the LCD screen or the EVF.
The histogram is an informative and useful tool above the:

1. It's sunny so I use f/16 1/100 @ ISO 100
2. Oh, the meter says it is EV 17 @ ISO 100
3. Heck, I'l just dial RAW and shoot a bracket of 5 shots at .3 intervals, one of them should work.

If a person does not want to use the histogram and has some magic recipe that works for them, just do it.

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