Why should we be interested at incident metering?

Question

Everyone says that incident metering is clearly better than reflective metering because the first one measures the real light on the subject, and not that reflected by its (which depends on its reflection coefficient, which the camera does not know and just supposes it is that of a grey 18% object).

Well, let's follow this reasoning. You have a subject and use reflective metering (through the camera light meter) with Spot Metering on it. Well, why should we be interested at the light falling on the subject?

What is important is the subject being well-exposed to the sensor. Not the light falling on it. Otherwise I wouldn't have chosen the Spot Metering pointed at the Subject. The light the subject sends to the sensor must fall at the center of the sensor dynamic range. Not the light falling on the subject. If I want all the scene properly expose, I'll use reflective metering with evaluative metering mode.

I've read that some sensors have a calibration constant of 250 lux seconds. This means that the light coming to the sensor (reflected by the subject) during the shutter time should be around this value to generate a well exposed image. Why should I consider the light falling on the subject?

Answer 1

You said:

What is important is the subject being well-exposed to the sensor.

Exactly – you've hit the nail on the head.

Imagine photographing a bride and groom. The groom has worn a black suit and the bride a white dress. If you spot-meter off the bride's dress, you'll get one reading. If you spot-meter off the groom's suit, you'll get a different reading. Which one is correct? Neither. That's because the camera will try to expose anything as a midtone, but these are not midtones. The suit should appear darker than a midtone, and the dress should appear lighter than a midtone. In order to get a correct exposure, you'll have to have this knowledge in your head, and start messing around with exposure compensation (or, meter compensation, as I personally like to call it).

If you instead just take an incident reading of the light falling on your subjects – which is the same intensity on both, and so by definition requires a uniform exposure – and use that reading instead, then the blackness or whiteness of the subjects themselves won't have any negative influence.

_{It's probably worth also clearly stating that to take an incident meter reading, you don't just point your camera at the light source. You use a dedicated incident light meter, which will have a diffusor fitted.}

Answer 2

Consider this…

Your subject is wearing a black jumper on a sunny day.
Incidence metering will expose correctly for the overall scene, because it 'knows' the light falling on the scene. Reflective is likely to come out too bright, because it's trying to compensate for the black jumper.
if you're really trying to force it to make a reasonable decision using spot metering for such as a portrait, then meter on the subject's face, then recompose.

You can also see the effect if the subject is not ethnically 'white'. 'White' skin is close enough to 18% grey that the camera's metering is a reasonable guess. Non-'white' skin does not fit this pre-determined average lighting reflection.
Incidence metering would not make that mistake.

The camera can't 'see' the scene. It has to guess an average - which is where Matrix/Evaluative or even centre-weighted metering can come in handy. one divides the frame into regions & tries to make a guess based on the difference between lightest & darkest, the other uses the centre of the frame as a bias to metering, based on the assumption that that's where your subject is most likely to be.

^{Don't shoot me for ethnicity descriptions, I'm doing my best for an old white guy, who doesn't care what ethnicity you are so long as I can get my exposure right for your picture.}

Answer 3

Light meters are just tools.

Sometimes incident metering produces better photographs. Sometimes reflected metering produces better photographs. Sometimes sunny 16 does, and the same is true for dumb luck.

Nobody cares what color pants the photographer wore and nobody cares how it was metered. All that matters is the pictures.

And the only proper exposure is what the photographer intended. Sometimes that may mean crushing the blacks or blowing the highlights or both or neither.

Answer 4

Prior to the invention of the photographic light meter by Edward Weston in 1932, cameras were set mainly by guesstimate. There were devices used to peer at the scene using filters with varying opacity. There was a thermometer method. Take the ambient temperature in sunlight and in shadow. With or without these gadgets, one consulted charts and tables. Setting right or wrong were revealed after the film was developed which was not immediate.

The pitfalls were immense. When using a reflective light meter, we learned in photo school: Sidelight increase ½ stop. Backlit increase 1 stop. Small, bright subject against dark background decrease 1 stop. Small dark subject against bright background increase 1 stop. Average subject with snow or sand in bright sun increase 1 stop.

Shortly after the Weston and GE meters were marketed, Kodak told us to place the Yellow Box in the scene and take a reading. The Yellow Box reflected 18% of the ambient light. Ansel Adams popularized the use of a battleship gray placard with a reflectivity of 18%.

Now consider Hollywood shooting an extravaganza; lots of film, high paid actors, horses, the works. Under or over exposure were not an option. Now comes the incident light meter. Incident is old French for about to happen. The incident meter is calibrated to give the same reading as a reflection meter reading the gray card.

The incident light meter worked for Hollywood; it saved them from countless retakes. Now today’s cameras sport a reflection metering system fortified with computer logic. Likely it will beat the handheld reflection or incident meter for accuracy. In other words, technology marches on. In other words, we have the best of the best built-in.

Answer 5

Especially in the days of film, portions of a subject that were near the middle of a film's dynamic range would have higher contrast than portions that were near the ends. If one were to take five photographs of the same subject, but with progressively decreasing amounts of light, and were to then print them while using correspondingly shorter exposure times in the lab, the overall gray balance of the photos might be similar, but some the amount of contrast in the lighter or darker portions of the subject would vary.

If one is interested in trying to produce the single best picture of a subject that can be produced, having a variety of photos which bring out different parts of the subject may be desirable. If, however, one is trying to produce a collection of photographs of various combinations of various subjects, some of which are lighter or darker than others, exposing each shot according to optimally match the subjects therein may result in the contrast levels of different subjects varying according to what other subjects are in the frame. If one instead uses incident metering, then subjects may not be exposed optimally, but they will be exposed consistently, regardless of what other subject share the frame with them.

Digital photography involves different trade-offs from film. Most sensors are rather less forgiving than films at the ends of their dynamic ranges, but they behave more uniformly than film nearer the middle of their dynamic ranges. If a camera doesn't have enough exposure latitude to do a perfect job of exposing light and dark subjects simultaneously, it may be better to photograph each shot using the maximum exposure that will avoid blown highlights than to try to expose every subject in context-independent fashion. If it's not possible to avoid having the some parts of a dark subject become murky while avoiding blown highlights on a light subject that shares the scene, having those parts be murky in those pictures, but clear in pictures without the light subject, may be preferable to having them murky everywhere. If the extra clarity that's present in some pictures but not others is distracting, the clearer pictures could be made murky in post to match the rest.

Answer 6

What is important is the subject being well-exposed to the sensor. Not the light falling on it.

The **correct way of using an incident meter is from the subject's position with the sensor/dome pointing towards the camera. In this way it is also metering the light that will be reflected back to the camera (assuming an irregular/diffused subject) and not really just the light falling on the subject.

The incident metering result is subject agnostic... just as a reflective reading is really. Neither meter reading is necessarily "correct." E.g. with the bride/groom scenario; if the light is strong enough to clip the whites of the dress, the incident metered exposure will probably cause the black suit to clip instead. Depending on your camera you might be better off letting the whites clip a little and recovering in post (recording raw files), or "underexposing" to save the highlights (exploiting ISO invariance).

The main advantage of incident metering is for studio use; where it allows you to set up the lighting to be reasonably close before your subject(s) ever arrive. It also allows setting up specific lighting ratios with multiple lights somewhat more easily if required/desired. And while working with a subject it basically allows "chimping" the exposure w/o putting the subject in front of the camera (as such); which can be a touch quicker, and more relaxing for the subject.

---

** there are other ways to use an incident meter for other specific purposes/reasons...