Should I be warming grey card calibrated shots?

Question

In a recent blog post at this site, Tim Grey recommended the following:

You could then use the white balance tool in Lightroom or Adobe Camera Raw (or similar tools in other software) to make the gray card appear perfectly neutral, and then simply adjust the Temperature slider to shift between blue and yellow. For the “typical” photograph that would involve increasing the Temperature value to add a little warmth to the image.

After reading other questions about using a grey card on this site, I do not see anyone else recommending "adding a little warmth" back in after using the white balance tool off of a grey card. Is this a typical practice? Do grey cards produce cooler colors then most scenes require?

Asked by dpollitt

Answer

I usually find, especially when dealing with artificial light, that an image that is white balanced that accurately is often cool and I tend to find it a little harsh as a result, so I've been known to warm up the image a little either with the white balance tool or a filter in Photoshop afterwards.

I would note that it wasn't all that rare with film, which was white balanced for appropriate light, to use warming filters to make it a little more pleasing, especially with people in the image. Cooling filters also existed, so it is entirely dependent on the look you want to achieve as to how you would filter.

At any rate, skin tones tend to look better if the blue is minimized...

Answered by John Cavan

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What is the 18% gray tone, and how do I make a 18% gray card in Photoshop?

Question

I have heard about 18% gray tone - what it real is, why 18% ? (and not 20%) and how can I make it in photoshop ?

Thank you in advanced.

To summarize an answer.

From white to black, the eyes see a range of greys. Because the eyes see logarithmic (and the ears hear logarithmic), what the eyes looks like the middle - this is actually not contains 50% black + 50% white, but 18%.

The middle point for the eye contain 18% Black On White.

To make this on Photoshop you fill a white background with 18% black pattern. So in photoshop if you fill with half black a white background, you do not get the middle gray that eye can see.

Many years ago I was made a page base on that 18% rule to calibrate the monitor. The different with other calibrations was that, I use 18% to fill the background with black and not 50%

I still have this gamma calibration page online. Blur your eyes and try to make the circle inside to disappear.

Asked by Aristos

Answer

Warning: this is a long, somewhat technical post that includes some math (but when you get past the superscripts and such, it's ultimately pretty simple math).

Some time ago, Thom Hogan published an article:

http://www.bythom.com/graycards.htm

that claims meters in Nikon digital cameras are calibrated for a mid-level grey that corresponds to 12% reflectance rather than the 18% grey of most standard grey cards.

Unfortunately, while the title and opening paragraph of the article are quite emphatic about 18% being a “myth”, the remainder of the article fails to provide much factual basis for this claim. Here’s what Thom gives as the basis for his statements:

ANSI standards (which, unfortunately, are not publically published--you have to pay big bucks to have access to them), calibrate meters using luminance, not reflection. 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%.

In the end, he seems to have no real basis for his claims, merely a statement that “12% seems to be correct,” with no real evidence, or even information about why he considers this correct. Despite this, however, this article is now widely cited on various photographically oriented web sites (among other places) as if it were absolute and indisputable fact.

Since this issue seems to be of interest to a fair number of photographers, I decided to see if I could find some real facts with evidence to support them. The first step in this journey was to find the standard in question. Doing some searching, I found the relevant standard. Contrary to Thom’s implication above, this is really published by the ISO rather than ANSI. This may be trivial to most, but when I was looking for the standard it was somewhat important – I put in a fair amount of work trying to find an ANSI standard that apparently does not exist. In the end, however, I found the relevant ISO standard: ISO 2720-1974, “Photography - General purpose photographic exposure meters (photoelectric type) - Guide to product specification (First edition - 1974-08-15)”.

I also found that Thom was (at least from my viewpoint) quite mistaken about prices as well – a copy of this standard costs only $65 US. This didn't strike me as "big bucks" -- in fact, it seemed like a fair price to pay for some real enlightenment (pun noted by not really intended) on the subject.

The standard confirmed part of what Thom had to say, such as calibrating meters directly from sources that emit light rather than from reflected light. Unfortunately, other parts of what Thom had to say are not quite so closely aligned with the content of the standard. For example, at the conclusion of his article, he includes a comment from “lance” that mentioned a "'K' factor", without specifying its exact meaning or purpose. Thom replied by saying: “No manufacturer I've talked to knows anything about a K factor, though, and they all speak specifically about the ANSI standard as their criteria for building and testing meters.”

As stated, this may not be exactly wrong – but it’s certainly misleading at best. In reality, a large part of the ISO standard is devoted to the K factor. Much of the rest is devoted to the C factor, which corresponds to the K factor, but is used for incident light meters instead (the K factor applies only to reflected light meters). It would be utterly impossible to follow the standard (at least with respect to a reflected light meter) without knowing (quite a lot) about the K factor.

The standard specifies that: “The constants K and C shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a number of observers, of a number of Photographs, for which the exposure was known, obtained under various conditions of subject matter and over a range of Iuminances.”

The standard also specifies a range within which the K factor must fall. The numbers for the range depend on the method used for measuring/rating film speed (or its equivalent with a digital sensor). For the moment, I’m going to ignore the DIN-style speeds, and look only at the ASA-style speed ratings. For this system, the allowable range for the K factor is 10.6 to 13.4. These numbers do not correspond directly to reflectance values (e.g. 10.6 doesn't imply a 10.6% grey card as mid-level grey), but they do correspond to different levels of illumination that will be metered as mid-level grey. In other words, there is not one specific level of reflectance that is required to be metered as mid-level grey – rather, any value within the specified range is allowable.

The K factor is related to a measured exposure by the following formula:

K = LtS / A²

Where:

K = K factor
L = Luminance in cd/m²
A = f-number
t = effective shutter speed
S = film speed

Using this formula and a calibrated monitor, we can find the K factor for a specific camera. For example, I have a Sony Alpha 700 camera and a monitor that’s calibrated for a brightness of 100 cd/m². Doing a quick check, my camera meters the screen (displaying its idea of pure white) with no other visible light sources, at an exposure of 1/200th of a second at f/2. Running this through the formula, gives a K factor of 12.5 – just above the middle of the range allowed by the standard.

The next step is to figure up what level of “grey” on a card that corresponds to. Let’s do that based on the sunny f/16 rule, which says a proper exposure under bright sunlight is f/16 with a shutter speed that’s the reciprocal of the film speed. We can mathematically transform the formula above to:

L = A²K/tS

Let’s work things out for ISO 100 film:

L = 16x16xK/.01x100

The .01 and 100 cancel (and they will always cancel since the rule is that the exposure time is the reciprocal of the film speed), so this simplifies to: L = 256K.

Working the numbers for the lowest and highest allowable values for the K factor gives 2714 and 3430 respectively.

Now, we run into the reason the ISO standard specifies light levels rather than reflectance of a surface – even though we’ve all seen and heard the sunny f/16 rule, the reality is that clear sunlight varies over a considerable range, depending on season, latitude, etc. Clear sunlight has brightness anywhere from about 32000 to 100000 lux. The average of that range is about 66000 lux, so we’ll work the numbers on that basis. This has to be multiplied by the reflectance to give a luminance – but the result from that comes out in units of “apostilbs” rather than cd/m². To convert from apostilbs to cd/m², we multiply by 0.318:

L = I x R x 0.318.

Where:

R = reflectance
I = Illuminance (in Lux)
L = luminance (in cd/m²)

We already have the values for L that we care about, so we’ll rearrange this to give the values of R:

R = L / 0.318 I

Plugging in our minimum and maximum values for I, we get:

R₁ = L / 10176
R₂ = L / 31800

Then we plug in the two values for L to define our allowable range for R:

R_1,1 = 2714 / 10176
R_1,2 = 2714 / 31800
R_2,1 = 3430 / 10176
R_2,2 = 3430 / 31800

R_1,1 = .27
R_1,2 = .085
R_2,1 = .34
R_2,2 = .11

In other words, between the range of brightness of the sun and the range of K factors allowed by the ISO standard, a reflectance anywhere from about 8.5% to about 34% can fall within the requirements of the standard. This is obviously a very wide range of values – and one that clearly includes both the 12% Thom advocates and the 18% of a typical grey card.

To narrow the range a bit, let’s consider just the arithmetic and geometric mean of the range of brightness from the sun: 66000 and 56569 lux respectively. Plugging these into the formula for the range of possible reflectance values gives:

R_1,1 = 2714 / 20988
R_1,2 = 2714 / 17989
R_2,1 = 3430 / 20988
R_2,2 = 3430 / 17989

The results from those are:

R_1,1 = .13
R_1,2 = .15
R_2,1 = .16
R_2,2 = .19

An 18% grey card is close to one end of this range, but still falls within the range. A 12% grey card falls outside the range; we have to assume an above-average light level for it to work out. If we average the four numbers above together, we get a value of about 16% grey as being the "ideal" – one that should work out reasonably well under almost any condition.

To summarize:

1. The ISO standard allows a range of calibrations, not just one level
2. Normal daylight brightness covers a fairly wide range as well
3. 18% grey is justifiable based on average light levels
4. 12% grey is not justifiable based on average light levels
5. Based on average light levels, the ideal value for a grey card would be about 16%
6. You meter might be calibrated to 18%, but probably isn't (and shouldn't be) calibrated to 12%.

Answered by Jerry Coffin

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Why are greycards used for white balance?

Question

I know 18% grey cards are supposed to provide a neutral color to set custom white balance. But why are grey cards used instead of pure white ones?

My assumptions: Is a specific greytone easier to produce than pure white? Does white stain so much easier? (or Does grey just doesn't show stains that much?)

Answer

When you have already set your exposure parameters, white could be clipped in some single color channel (but not all, so your camera won't show it as blown), therefore not being very good basis for color balance adjustment.

Also, paper will turn yellow during time. And just looking at the sheets currently on my desk, there's three different tones of white papers already.

That said, I've used white paper many times and it will get you close enough in all but very critical studio shoots. Very often you'll have multiple different light sources, reflections from colored surfaces and/or shadows vs. lit areas, all having much more impact on colors than slight imperfection in metering the white balance. Just make sure you don't clip any color channels (e.g. by spot metering without compensation from the paper, or verifying exposure parameters using color channel histograms).

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How to get meter reading for a DSLR if not from a gray card?

Question

I read an article called Expose (to the) Right, which explained why you should try to get the graph of the histogram as much to the right of the scale as possible. The reasoning is that DSLRs record much more detail in bright areas of the subject than in darker areas.

You could then stop back in your photo editing tool to a "normal" correct exposed picture but with reduced noise and more range than with doing the shot with the "normal" correct exposure upfront.

That makes all sense to me, but in the end the author quotes someone else stating:

For film based photography, the highlight end of the scale is compressed by the shoulder portion of the D/log E curve. So as brighter and brighter objects are photographed, the highlight detail gets gradually compressed more and more until eventually the film saturates. But up until that point, the highlight compression progresses in a gradual fashion.

Solid state sensors in digital cameras behave very differently. As light falls on a sensor, a charge either accumulates or dissipates (depending on the sensor technology). Its response is well behaved right up until the point of saturation, at which time it abruptly stops. There is no forgiveness by gradually backing off, as was the case with film.

Because of this difference, setting up the exposure using an 18% gray card (as is typically done with film) does not work so well with a digital camera. You will get better results if you set your exposure such that the whitest white in the scene comes close to, but not quite reaching, the full digital scale (255 for 8-bit capture, 65535 for 16-bit capture). Base the exposure on the highlight for a digital camera, and a mid-tone (e.g. 18% gray card) for a film camera.

Source: http://www.luminous-landscape.com/tutorials/expose-right.shtml

So … how do I do meter reading for a DSLR if not from a gray card? My DSLR metering will always try to make my image gray, right? How can I avoid this, to make images which are exposed "to the right"?

Answer

I think the article is referring to using the histogram to judge exposure, after a test shot has been taken. Using the histogram as a guide you can increase exposure until the top of the histogram hits the right edge, indicating clipping may start happening.

If you have to rely on the in camera metering (which will meter assuming 18% reflectance as you suggest) then you can simply use exposure compensation to correct the camera exposure up a stop or two in order to expose to the right. Using the histogram is much more accurate however!

Whilst we're on the subject the luminous landscape article is a little simplistic and wrong in a few areas. Expose to the right doesn't increase detail it increases signal to noise ratio. The more light you let in, the more signal you get and hence better SNR. This even applies to increasing ISO in order to expose to the right, you increase the analogue signal above the noise floor by amplifying it.

Exposing to the right is not always desirable. Increasing SNR can come at the expense of limiting colour fidelity. The higher you go up the brightness scale the fewer colours can be represented.

Finally on every DSLR I've seen, the histogram you get is based on a jpeg image the camera creates - even if you shoot raw. I know of no camera that gives you a histogram of the raw values. This means you have to be careful of your jpeg settings (saturation and contrast) in particular when setting exposures based on the histogram.

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What's a good ad-hoc replacement for a gray card?

Question

I know that a real gray card is the best thing to use to determine exposure. But I'd like to know if there are other naturally available features that can be used in a quick and dirty way. Are fields of grass or road tarmac appropriate?

The reason for asking is that I suspect that my new used (manual focus) lens is exposing incorrectly using my camera body. I don't have access to a gray card, but I do have access to lawns and pavements, nicely and evenly lit by a cloudy sky.

Answer

Grass is generally considered to be 18% grey. I would guess if you shoot in grey scale mode and your exposure is said to be correct, it should come up 18% grey.

There's also the sunny 16 rule, which is on a sunny day, shoot at 1/ISO shutter speed, f16. This should produce a correctly exposed picture. You can check to see if this results in a correct picture.

Both of these were tricks I was taught back in film days.

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