What is "correct" exposure?

Question

Photography has been my hobby for far too many decades — first on film, and now digital (for about seven years). I recently had occasion to use exposure bracketing seriously for the first time, when I was shooting difficult subjects (high contrast, large light value range, intense lights in shot, etc). Much to my surprise, the one stop underexposed shot was virtually always the best shot. I have since been experimenting with this and find that it applies to virtually all the shots I take. It also applies if the is a lot of sky in shot, and if there is very little. The colour range is more realistic, the level of detail is greater, and the balance much better.

So what is going on? Am I just getting old and decrepit with my vision crumbling? Do digital sensors react better to low exposures? Is the manufacturers calibration suspect? Something else?

What is "correct" exposure for a modern digital system?

FWIW I use Nikon equipment; with all but one lens (a Tokina extreme wide-angle zoom) being Nikon. All the shots I refer to above were taken by natural light in the daytime, and a substantial portion of them were landscapes.

So, the skies look good in your photos. What about the landscapes themselves? Do they simply become silhouettes? Do they have any detail or color remaining? Are you using a D800/D600/D3200, in which case you have two additional stops of DR over any other camera, and can lift those shadows and still retain some detail? — jrista, Apr 27 '13 at 16:27
I think it is a similar question, since it relates to high contrast landscapes. Oh the other hand, the answer may be in the referenced question. — Paul Cezanne, Apr 27 '13 at 21:04
Which metering mode you use for high contrast scenes will greatly affect the results. — Michael C, Apr 27 '13 at 23:56
I rolled this question back to its first version. See Should edits which add the accepted answer to the question be reverted? for the reasoning for removing the answer added in. There was also another edit which seemed more like discussion rather than a clarification of the question. — mattdm, Jul 07 '15 at 15:00

U007D · Accepted Answer · 2013-04-27T23:14:21.620

I'm not sure if this is a "how is exposure defined" question or an "is my camera busted" question, so I'll try to address both. :)

Definition of proper exposure

ISO standard 1271 contains a definition for photographic exposure.

Bypassing the math, "correct" exposure averages a scene's luminance and renders that luminance at a particular (but arbitrary) level, measured in lux-seconds, at the image plane.

That level has been chosen based on an assumption that the the average scene's peak luminosity is <=7.8x its average luminosity (again, an arbitrary figure).

The standard provides manufacturers with a small amount of wiggle room (it specifies a constant K, the value of which the manufacturer can select, within a narrow, defined range) to compensate for transmission light losses in the optical pathway, as well as for a rendering a particular manufacturer feels is most pleasing.

In simpler language, "correct" exposure maps a particular shade of "average" grey in a scene to a specific RGB value in the image.

Anything in the scene brighter or darker than this "average" simply falls where where it falls in your image. Or put another way, depending on the average luminosity of the scene, dynamic range of the scene, dynamic range of your imager, etc., etc., it is entirely possible to experience clipping (in shadows and/or in highlights) in a "properly exposed" image.

Is my camera exposing correctly?

In practice, manufacturers have developed sophisticated metering systems to properly weight or discount areas of an image to achieve a higher rate of pleasing images. In effect, this adjusts the the shade of grey the camera considers to be "average" in the scene.

The fact that you prefer the image your camera delivers when its meter says the scene is one stop underexposed may indicate that your camera meter is out of calibration, or may simply be revealing your personal preferences.

Your histogram may offer some clues (be sure to evaluate a RGB histogram, not just a single-channel histogram), but it would be best to shoot a calibrated grey reflective target to see where your meter places the grey in your image file--should be at least 2.96 stops below saturation for 12.8% grey target or 3.46 stops below saturation for 18% grey target.

Hope that helps,

+1 for exposure definition and calibrated grey reflective target. It is closer to a "how is exposure defined?" question. I do appreciate that it is also a question about my personal preferences - none the less the consistent nature of the exposure modification came as a surprise. — Chris Walton, Apr 27 '13 at 23:07
Thx, Chris. You may know this already, but most SLR's allow you to bias your exposure meter to expose the way you prefer, rather than the way it was set from the factory. That way you don't have to any mental gymnastics over this every time you take a shot. — U007D, Apr 27 '13 at 23:18

score 4 · Answer 2 · answered May 01 '13 at 06:16

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I'd dare to say the correct exposure is whatever is needed for the artist to get the effect he or she desires. That might be technical perfection, but it might also be deliberate over- or underexposure used to get specific artistic effects.
I've used this myself to get seriously blown out highlights, causing a winter beach scene to look like a desert under a blazing sun. Or pushing the darker background of a photo to be completely black so that the light face and clothes of the baby resting on a dark blue blanket pop out without any distracting background elements. Technically both images are bad, but they achieve the desired effect so the exposure is correct.

answered May 01 '13 at 06:16

jwenting

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I wouldn't even say that technically both images are bad. (That's why this is impossible.) – mattdm May 01 '13 at 12:15
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@mattdm possibly. But there are certain exposure ranges that are considered "correct" by the "schools" of photography, that's what I consider "technically correct" in this context. And those you could measure nowadays by automatic comparison of histograms. The things asked in the question you linked are indeed impossible to quantify (except for noise, but that's influenced so much by the storage media/format that it becomes meaningless in the context of that question, which tries to measure a camera's performance based on an image stored with lossy compression). – jwenting May 02 '13 at 05:30

AJ Henderson · Answer 3 · 2013-04-27T17:04:40.857

The "correct" exposure may vary some from one image processing system to another, but the general goal is to make the darkest and brightest parts of the image both fall within the dynamic range of the camera with a good white balance and natural contrast.

The bright and dark part is easy if the scene doesn't exceed the dynamic range of the camera since it can simply make sure the brightest white and darkest black are not clipped. If the scene uses less than the full dynamic range, it becomes a bit more subjective and the metering mode comes in to consideration. A variety of different metering modes are supported by many cameras. Some choices are things like "Evaluative", "Center Weighted Average", "Average" and "Spot".

Spot metering is probably the easiest to explain, you simply choose a spot in the scene to evaluate the exposure at. It looks at that spot and sets that point to make sure the exposure is good (if it's black, make it dark but detailed, if it's white, make it bright but detailed, if it's color, make it detailed around that color.)

Average is similarly fairly easy, take the average intensity of the scene and focus the ability to resolve detail there. Center Weighted Average is similar but it gives more value to the center of the shot than the outsides.

Finally Evaluative is the most difficult to describe. The camera makes an evaluation based on some internal criteria and what the scene looks like. Typically it will try to identify the subject of the shot and focus on getting a good exposure of them.

As far as why your camera would consistently under expose (in your opinion), this may be to try to avoid blowing highlights. In general blown highlights are more noticeable than lost detail to dark. Our eyes have a much wider dynamic range than a camera and our pupils adjust to avoid things getting too bright. We aren't used to washed out images in real life, but we are used to things being too dark to see.

There is also a school of thought that under-exposing (particularly when shooting raw) to give more headroom can be a better way to shoot and then bring it up after the fact. If you are shooting RAW, it could also be that the adjustments are being made for the RAW file but then the render of the RAW is based on what the camera thinks is good. Generally, what is displayed on the camera has very little to do with if a RAW was well exposed. You don't know what information is really there until you get it in a RAW processor and play with fine tuning the exposure.

Nitpick: I think he's saying that the camera is consistently over exposing relative to his preferences -- that is, the one that the camera brackets as a stop under nominal EV is preferred. — mattdm, Apr 27 '13 at 16:55
@Mattdm - now you have me curious, I keyed in on "Do digital sensors react better to low exposures?" but now looking back, I see where you see the EV adjustment bit as well. — AJ Henderson, Apr 27 '13 at 17:01
Ok, updated it to be a little more ambiguous to try to address either possibility and highlight that when shooting RAW, the on screen display isn't necessarily exactly what detail was captured. — AJ Henderson, Apr 27 '13 at 17:04
Another nitpick: With digital RAW files, details are more recoverable in areas just below saturation (which appear blown out in 8-bit previews) than in deep shadows, which tend to be noisy. Thus the preference of many to ETTR is more common than underexposing if the light curves will be edited before conversion to an 8-bit format. — Michael C, Apr 27 '13 at 23:50
But it's also really hard to keep to that just below saturation point when there's diverse lighting in the scene, and with modern miraculously-low-noise sensors, the shadow noise isn't that big of a deal, so.... — mattdm, Apr 30 '13 at 20:58

score 3 · Answer 4 · answered Apr 28 '13 at 01:42

Ansel Adams developed the Zone System to allow him to select the exposure levels of specific objects in his photos in relation to other specific objects with different luminosities rather than basing the exposure on a single meter reading of an object with the approximate average luminosity of the overall scene. We often forget that cameras have only had built in exposure meters for much less than one half the history of photography. The availability of a built in meter with more than one metering pattern is an even more recent development.

As others have covered the technical standards very well, we'll just say that your camera's meter should operate similarly to other cameras when metering the same scene. But realize the technical standards are applied to a scene with a standardized level of variation between the brightest parts of the scene and the average brightness of the entire scene.

Also realize that exposure compensation will vary from one metering mode to the next. I've got a photojournalist friend who shoots Nikon gear owned by his employer and Canon gear when shooting personally or for a side job. He hated Nikon's matrix metering mode in all the Nikon bodies he has used prior to the D4. He feels that even the D4's matrix mode, while more usable than prior Nikon bodies, considerably trails the evaluative metering mode in his Canon bodies that are several years older. In both cases these modes base the exposure calculation upon contextual clues in the scene. For instance, if the upper two thirds of a scene is much brighter than the lower one third, the calculation will be based on the assumption that the bright area is sky.

score 0 · Answer 5 · answered May 02 '13 at 12:50

The camera wants it to be Zone 5 if you smush all the tones together.

The histogram wants it to be solid black at it's darkest pixel and pure white at it brightest (though thats more of a contrast thing)

You can have it anyway you choose.

Kudo's to guy who mentioned the Ansel Adams system. The king of the correct exposure.

What is "correct" exposure?

5 Answers5

Definition of proper exposure

Is my camera exposing correctly?

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