Light meters, grey cards and the ultimate answer (its 12, not 18 and 42 is totally off)

Ok, 12 is not the number that explains everything. But its a 12% grey that the light meter in our camera is calibrated to, not the commonly published 18% grey.

Yes, a Kodak grey card is 18% grey. But the light meters in our cameras are calibrated to the ANSI standard. Thom Hogan went into great lengths to explain the difference in his article Meters Don’t See 18% Gray. Lazy as I am, I just quote him:

“Light meters are calibrated at the factory using ANSI standards. The standard has always been for a luminance value that is roughly equivalent to the reflectance of 12% gray.”

12% grey is a 1/2 stop lighter than 18% grey.
So, if you are a serious photographer who uses a grey card with the light meter of your camera to nail the optimum exposure, you are 1/2 stop off. So far, so bad.

If you think you are smart (like I thought of me) and use a hand-held light meter to measure the incoming light (opposite to the reflected light, like the camera meter or a hand-held spot meter), you are walking in to an other obstacle. Different light meter manufacturers use different luminance values:

Thom Hogan again:
“…it may be that it’s unclear what the companies producing meters are really doing. It doesn’t help that the “technical” information from many of the companies involved in meter production contains contradictory information. For example, Sekonic’s web page mentions 14% and claims Minolta uses a higher setting, while Minolta’s English pages claim yet a different value.”
“For an ANSI calibrated meter, the most commonly published information I’ve seen is that the luminance value used translates into a reflectance of 12%. I’ve also seen 12.5% and 13% (so where the heck does Sekonic’s 14% come from?), but 12% seems to be correct–one half stop lighter than 18%, by the way. I haven’t seen anyone claim that ANSI calibration translates into a reflectance of 18%.”

Now I know why my former Seconic flash meter gave a little different metering than my current Minolta meter. Gossen light meters did seem metering equivalent to Seconic’s when I tested them years ago in the shop.

Is this 12% to 18% difference really important?
Maybe. 🙂
If you use film or shoot JPG in your digital camera and you want to nail the exposure it can make a difference.
While that is mostly irrelevant for negative film (because of its wide exposure latitude) a 1/2 stop wrong exposure can ruin the exposure of your slide film (remember those?) or blow the highlights in your JPG’s. A 1/2 stop too dark exposure will increase the noise in shadows regardless if you use RAW or JPG.
If you use a hand-held light meter and not the internal light meter of the camera you have to calculate the light loss of your lens in to the equation as well.

Your lens is slower than you think
Because the F-stop that is written on your lens like in 50mm/1.4 is a geometrical value, it describes the diameter of the entrance pupil in proportion to the focal length. Or easier said, the F-stop is the diameter of your aperture in proportion to the focal length of your lens.
The F-stop tells you nothing about the internal light loss of a given lens, caused by light reflection on every lens element surface. The “real life” aperture (geometrical F-stop + light loss of lens elements) is described by the T-stop.
Because lenses with more lens elements have more internal light loss, the ugly truth is that zoom lenses are normally a bit slower than prime lenses. For example your big, heavy and expensive high-speed 70-200mm/2.8 zoom lens with its 12-15 lens elements is in reality “only” a T/3.3 or T/3.5 lens (1/2 to 2/3 stop slower than expected), while a 180mm/2.8 is about a T/3.2 lens (1/3 stop slower in real life).

Are you still with me?
I hope you recovered from the shock of finding out that your precious f/2.8 lens is not a 2.8 lens in terms of real world light transmission. You can take a soothing reassurance that an ultra fast 50mm/1.2 lens ist only a T/1.4 lens as well… I suffer a bit from the fact that my fast f/1.4 prime lenses are only t/1.6 lenses, but not too much.
Unfortunately a more pedestrian f/3.5-5.6 zoom lens, like a Nikkor 18-200mm/3.5-5.6 is wide open at 200mm only a t/7.1 lens. If you know that the autofocus sensor of your camera is specified to work more or less precise up to f/5.6 and AF performance declines with slower lenses, you may understand that the lens is exceeding the specifications of the AF sensor, resulting in slow and/or unprecise auto focus operation. But that is a whole different story…

The metering system in our DSLR’s are constructed slightly wrong anyway.
It gets worse. I mentioned the increase of noise in the shadows.

With every new camera model the manufacturers increase the noise handling of digital cameras. There are noise reduction algorithms in our camera software, noise reduction functions in our computer software, and the like. While they decrease the noise, this algorithms also rob some image detail. Why should I buy a sharper lens and a new camera with higher mega pixel count, if the noise reduction destroys some of the image detail advancements I just paid for? While I’ve got quite a high tolerance against noise, I rather have as little noise as possible to start with (one of the reasons I switched from the D300 to the D700, but the main reason was actually the bigger view finder).

In 2003 Michael Reichmann wrote an article in Luminous Landscape describing how and why a photographer should slightly overexposure his RAW file (you shoot RAW, do you?), without clipping the highlights, and correct the “too bright” photo in his RAW-converter to increase the measly tonal values in the shadows and decreasing the shadow noise as well.
He called it “expose right”. With right, he ment to the right side of the histogram. (for newbies: The right side of the histogram show the white, while the left side of the histogram show the black parts of the picture). Since then “expose to the right” (ETTR) is a quite common therm for experienced RAW shooters.

The ultimate answer
Some days ago Michael Reichmann posted a great follow-up: Optimizing exposure
Apart from the slightly disturbing fact mentioned in the beginning that our “digital” sensors are analog devices, while old film is essentially binary (“digital”) in nature, that post is a great tutorial why someone should expose against common wisdom to get better exposed files with more tonal values and less noise in shadows in the first place, rather than destroying image details by using noise reduction software after the fact.

He also described how camera manufacturers should reengineer their metering systems to expose the sensor stronger to a slight extend, while darkening the review to the same extend, thus normalizing the picture. That would give us a normalized review, but more tonal values in the shadows and less shadow noise.

If you haven’t read the article yet, jump over and take a look of the distribution of the tonal values of a 12bit RAW.
A bit further down, you will stumble over this:
“My 5D Mark II has a noise level of ~70 units at its maximum highlight level of 16,383 (on a 14-bit scale), and a noise level of ~30 units at a much darker signal level of 16 (i.e., 10 stops darker). The highlights appear clean because the SNR is good (16,383 vs 70). The shadows appear gross because the SNR is dismal (16 vs 30) – in fact, the signal is buried in the noise.”


Whats the bottom line?
The bottom line is, the camera manufacturers should transform the old film style metering system in our cameras to the real life demand of digital cameras, like Michael Reichmann described so eloquent.

What has this 12% grey of our light meters and the 18% of a grey card to do with it?
Not much… I started this post with the 12% fact as opposed to the widely published 18% common wisdom, and got a bit carried away. 😉
Also the F-stop vis T-stop difference in lenses is not too essential if you never use an external light meter, but good to know non the less.

The noise bit?
In my book, the possibility to enhance tonal values by using the “expose to the right” scheme is something every RAW shooter interested in getting the most out of his digital camera should try out. It will not give better technical results every single time, but you will not needlessly waste some of the quality your camera is capable of.

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1 Response to Light meters, grey cards and the ultimate answer (its 12, not 18 and 42 is totally off)

  1. Pingback: Off-setting a light meter

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