4:41 pmOff
Pushing Exposures
It's been suggested that raising the exposure in the raw processor would generate a cleaner file than raising the ISO by the same amount, since the camera doesn't have internal software as sophisticated as the raw processors you run on your computer.
Nonsense.
For one thing, when shooting raw files, the software in the camera has absolutely nothing to do with the resulting image. That's what shooting raw means: the raw data from the sensor, once digitized by the A/D converter, is not processed further and is dumped as is on the memory card. Understand that when the ISO is changed, it doesn't simply mean that the camera will "push process" the file with its internal software—that would be a completely pointless enterprise which would render even the concept of selecting an ISO meaningless. Why would you bother changing the ISO at capture time if you could simply push a slider farther right in your raw processing software in post?
Raising the ISO is not a matter of software, just as raising the gain on any signal processor isn't:
In digital camera systems, an arbitrary relationship between exposure and sensor data values can be achieved by setting the signal gain of the sensor.
This cannot be relegated to post production, as by then it is too late. If you have underexposed an image, no amount of pushing will recover lost information—just like it was with negative film. Having raised the signal gain prior to digitization (by setting a higher ISO) gives you additional information (especially noticeably in the shadow area), albeit at a cost of higher noise.
Back in the film days, it was a well known fact that a "push" processing was detrimental to the quality of the image (unless, of course, that particular "look" was what you wanted, for aesthetic reasons):
Push processing allows relatively insensitive films to be used under lighting conditions that would ordinarily be too low for adequate exposure at the required shutter speed and aperture combination. This technique alters the visual characteristics of the film, such as higher contrast, increased grain and lower resolution. Saturated and distorted colours are often visible on film that has been push processed.
If you were shooting slides (positive film, which is closer to the behavior of digital exposure), you could push an exposure by a larger margin than if you were shooting negative film, and the opposite was true of "pulling" (which is the opposite of pushing).
This was, of course, the film days, and there might be various reasons for doing so (such as the sudden need for a different ISO when the film had already been loaded). I am not a avid film shooter, so I cannot discuss in detail about the many motivations behind the practice, but the fact remains that pushing definitely didn't have the same effect as using a higher ISO (i.e.: not just an equivalent increase in grain).
In digital photography, pushing a file not only increases the noise (which is okay—raising the ISO does so as well), but more importantly, it also compresses the dynamic range of the file in a way that raising the ISO doesn't. That, you want to avoid.
Same aperture and shutter speed, ISO 800, 400 and 200.
Here is the boring picture of a camera shot with various ISO speeds. As you can see, the histogram reveals that tonalities are not just generally farther to the left, they are also more compressed to the left—bringing the exposures on a par in post will unavoidably be harder on the underexposed frames.
But those are raw files, and we're working with very powerful, sophisticated raw processors, aren't we? Let's see what happens when we bring the three exposures even (moving the white point and black point):
Pushing the exposures
Well, well! It seems the file with a higher ISO turned out cleaner than the files pushed in post! The more you push, not only do you get more noise everywhere (especially in the shadows), but the files get muddier, and tonal gradations are less smooth. Why is that? Because there is more information in the highlights than in the shadows (hence "expose to the right").
The morale of the story is that you are better off raising the ISO to get a proper exposure than compensating for underexposure in post.
3:09 amOff
Resolution Shmesolution
As I've said before (here, here and here), setting the resolution doesn't mean anything until actualization—that is, until we're talking about printing the image, having it become a measurable object you can hold in your hands. In other words, both detail (quality) and file size are unaffected by the value you set for the resolution until actualization.
Of course, here I am not talking about resolution in the sense of number of pixels, as we often hear people say when talking, for example, about the resolving power of digital camera sensors ("My camera has a resolution of 12 megapixels!") This unfortunate choice of words perpetuates an age old confusion about what we mean by "resolution", "size", "dimensions", "number of pixels", etc. Here, I am referring to the proper sense of the word, which means pixel density, not number of pixels.
Resolution = Number of pixels / Physical space they occupy
Note that even the "Guideline for Noting Digital Camera Specifications in Catalogs" of the Camera & Imaging Products Association (CIPA) explicitely advises that "The term 'Resolution' shall not be used for the number of recorded pixels", but I am not merely arguing that we should stick to using the word "resolution" when we mean "pixel density". That is a valuable point to defend, but is not so problematic as the fact that people actually confuse "ppi" (by sheer definition a measure of density) and "number of pixels" or "file size". (There is also a related confusion between ppi [pixels per inch] and dpi [dots per inch], the latter being a matter of print heads, but that is an issue I won't be discussing here.)
So, in keeping with that definition of the word, for resolution to mean anything, we must know both the number of pixels and the number of inches we're dealing with. Until the image is printed, pixel density is either immaterial or is a value that fluctuates depending on the context. My main display, for example, projects 1920 × 1200 pixels on a 24" panel (~94 ppi). My laptop, on the other hand, projects 1680 × 1050 pixels on a 15" panel (~129 ppi). Using a projector? You could make 1024 × 768 pixels occupy 6 feet wide (~14 ppi)! Thus, if you look at, say, the same 900 × 600 pixels image in any of these contexts, the actual resolution of the image has changed considerably—and you didn't have any say in the matter. So the only time you actually have the control over the resolution is when you send an image for print, because then you know the ultimate size of the image, and you can finally lock down the "number of inches" part of the equation.
In their confusion, people will say something like: "I don't need a large image, just give it to me in, say, 100 ppi", as if that meant anything. Even more egregious, they'll say something like: "Send it to me in 72 ppi so that it can be attached in an email", or "For images displayed on the web, set the resolution to 72 otherwise they'll take longer to download" as if resolution had any relation to file size. Conversely, they'll make the opposite mistake when confronted with such file properties:
File properties in Bridge
"Well, gee, 72 ppi means I am obviously dealing with a small, low quality file here!" No you aren't—notice the dimensions! This is a rather huge image, of potentially pretty great quality, I would think.
Look at it this way: 5000 wide pixels at 72 ppi, or 5000 wide pixels at 600 ppi is the same file, the same data, it doesn't mean anything until we decide to print. In the same manner, an image of 100 measly pixels wide set at 8000 ppi is a ridiculously little file compared to a 5000 pixels wide image set at 8 ppi.
What matters for the quality and size of a file is the number of pixels. What matters for the quality of a print is the resolution—but you need the pixels to be able to afford it, so it still breaks down to the number of pixels.
So, okay, you could maintain that what they are actually trying to say, albeit technically improperly, is that if you take your original picture and you resample it down to, say, 72 ppi, then you would end up with a much smaller image. Apart from the fact that resampling and setting the resolution are two thoroughly different things (it just happens that you do both of these things in the same location in Adobe Photoshop), maybe. But the inescapable problem which then arises is that we'd have to know the initial resolution in order to reduce it down to 72. What is it? Let's see... The sensor in my camera is 36mm (~1.42") wide and produces images 5616 pixels wide. Does it mean that the original image has a ... 3962 ppi resolution? That can't be right!
Hey, no problem, let's just posit that the initial resolution is something like 240, or 300 ppi, for whatever reason. Well it's still a problem, because if you simply resample down from an arbitrary 300 ppi, everybody will end up with considerably different image sizes, since different cameras have different number of pixels (say, 21 MP versus 12 MP, a quite realistic difference these days).
Again, this all boils down to the fact that there is no such thing as "image resolution" until you actually decide how big you want it actualized. Let's go to the epicenter of confusion, the "Image Size" dialog box in Adobe Photoshop:
Changing image resolution in Photoshop's "Image Size" dialog box
As can be seen here, setting a different resolution has no effect whatsoever on the file size, and no effect whatsoever on the quality of the file either. (Of course, if you did print this, you'd go from a 18,72" wide high quality image to a 78" wide poor quality image—then, and only then, would it mean anything.)
The "Image Size" dialog box has all of the interrelated terms in the same place, and you can see that Adobe chose their vocabulary carefully:
- "Pixel Dimensions" is the relevant measure of image quality and file size in this dialog box. It is the number of pixels you're working with—the more pixels you have, the more [potential for] detail you have.
- In this example, 5616 × 3744 are the pixel dimensions of images coming out of a Canon EOS 5D Mark II (a 21 megapixels camera). Indeed, if you multiply these two numbers to get the number of pixels, you get 21.026304 million pixels.
- File Size (the value in parentheses) is a factor of the number of pixels, number of channels, and bit depth, but those last two aren't configurable from this dialog box.
- If you take the number of pixels, multiply this by 3 channels (red, green and blue in an RGB color image) and then by the bit depth (in this case 16 bits (2 bytes) per channel), you indeed end up with 120.31 MB. If you're working with an 8 bits per channel image, or if you're working with a grayscale image (only one channel), or if you're working with a CMYK image (four channels), etc., this value would change accordingly, so be careful when interpreting it. What's more, this value doesn't take anything else into consideration (such as layers/adjustment layers/layer styles, embedded Smart Objects, color profile, color lookup table, vectorial data/paths, text, masks, notes, file format, compression level, full size composite, preview, metadata, etc.), so it has little to say about the ultimate file size on disk.
- While we're on the subject... "File size" is another misleading term trotted around instead of the meaningful "pixel dimensions" or "number of pixels". Instead of saying something tractable like "The Pentax 645D has a 39.5 megapixels sensor" (which indeed it has) you'll hear someone say "The Pentax 645D produces 225 MB files!" Well, what do you mean by that? The Pentax 645D's raw files are in fact closer to about 50 MB in size and vary according to image content/ISO/etc., so that number is unnecessarily confusing.
- "Document Size" means "if you actually spread all the pixels above at a density of [Resolution] you will get an image that measures [Width × Height ][Unit]". Again, until you print the image, this whole section is meaningless and you can just ignore it altogether.
The bottom line is this:
- Image file quality and size are more meaningful in terms of the number of pixels.
- Unless you're having a discussion about printing, don't even bring up "resolution", "ppi" or "inches".
12:48 pmOff
Color Models vs Color Spaces
There is a widespread—though understandable, to a point—confusion between color models and color spaces. For example, we'll hear someone saying something like "CMYK is much smaller than ProPhoto RGB", which unfortunately doesn't make sense because it compares apples with oranges, even if there's a certain truth somewhere in there. First, let's look at the concepts.
Color modes in Photoshop
There are a number of ways to express color. If we want to express "maximum red" (whatever that arbitrary color means), we might use the RGB model and say "255 Red, 0 Green, 0 Blue", but we could just as well use the HSB representation and say "0° Hue, 100% Saturation, 100% Brightness". Both would express the same abstract color ("the most red that can be expressed"), just as we could do using the CMYK model (0% Cyan, 100% Magenta, 100% Yellow, 0% Key).
In other words, there is nothing inherently limitative in expressing colors using different color models, it just speaks differently to different people or contexts. Photographers often find the HSB model more intuitive, while graphic designers (and people closer to the print industry in general) tend to be more comfortable thinking in CMYK.
Unfortunately, things are not that simple, because "maximum red" means a different thing to each device and medium. A typical digital camera can capture a much richer array of colors than a printer can typically render, so a color model alone is not sufficient to describe colors. If the computer were to simply ask the printer to render "50% Green", the printer would naturally wonder "Well, what do you mean by that?", because no two printer/ink/paper combinations produce the same color when they spurt 50% of their green.
Which brings us to color spaces. Of all the colors in the full spectrum (well, the portion meaningful to the human eye/brain at least—see Lab), a color space defines which portion of it it can describe and how colors are distributed. The subset of the full color spectrum that a space contains is referred to as its gamut. Therefore, the "maximum red" of a digital camera, LCD monitor and printer/ink/paper combination are all expressed the same way as (for example, in the RGB model) "255 Red, 0 Green, 0 Blue" in their respective space, but they don't mean the same color, because all of their spaces have different gamuts. "Maximum red" on a given monitor might only be, say, "73% Red" of a given camera (which has a much more vivid "maximum red"), so when we want to have consistent color across a workflow, we need to know the color space of each device we're working with so that they can all "speak the same language".
A color space that describes the characteristics of a given device is also called a color profile. In a complete digital photography workflow, we would therefore need a profile for:
- The capture device (the digital camera—or scanner, if we're feeling nostalgic)
- The display device we're working on (such as an LCD monitor)
- The output device (such as an inkjet printer/ink/paper combination)
We will also need to decide the color space (and bit depth, which can be thought of as the precision level) in which we will be performing post-production (also called, for obvious reasons, "working" space).
Photographers typically work in the RGB model (unless they want to perform old-school transformations in Lab) and typically deal with a number of familiar working spaces—sRGB, Adobe RGB (1998) and ProPhoto RGB.
- sRGB is a relatively small space that is the common denominator/standard for web publishing, so it should only be used when an image is exported for the web (or to some commercial printer that asks for it).
- Adobe RGB (1998) has long been the de facto working space because it is significantly larger than sRGB. The truth of the matter is that Adobe RGB has been smaller than the color spaces of digital cameras for years (see this ooold article), so when exporting images from the raw processing software to the Adobe RGB space you are throwing away lots of information. Worse than that, even modern inkjet printers can render colors outside of the Adobe RGB gamut.
- ProPhoto RGB is huge. While one could make the case that ProPhoto RGB might be too large in certain situations, when working in 16-bits, these concerns are not warranted. (ProPhoto RGB also happens to be the working space used by Adobe Lightroom.)
So, to wrap up...
A color model is the way you express colors. It doesn't have a size, and it cannot describe "real world" colors. It simply says "I am expressing colors using Red/Green/Blue channels", or "using Cyan/Magenta/Yellow/Key (Black) channels".
A color space (or color profile, or working space—all the same thing really, used somewhat interchangeably or depending on the context) is an actual description of a specific subset of the visible colors that maps—gives meaning to—the values used in a given model. Color spaces/color profiles/working spaces are all (typically) saved to disk in the form of ".ICC" files.
That being said, let's set the record straight on a number of color space related issues.
Using the camera profile as the working space
The camera's profile is quite large, but it is not the largest color space—not by a long shot. To give you an idea, here is a representation of the gamut of a Canon EOS 1Ds Mark II camera versus the ProPhoto RGB color space:
Comparison between the profile of a Canon EOS 1Ds Mark II and the ProPhoto RGB space.
Notice how abundantly larger the red mesh (ProPhoto RGB) is to the camera's profile (solid shape in the middle). The camera's profile is device-dependent to that particular camera and constrains colors to only the ones that that particular camera can capture, which has no bearing on how the artist would like to transform the image to bring it where he wants. The very bizarre idea of actually working in the space of the capture device is akin to caring that the final processed image will be "capturable" by the source device—it doesn't make any sense.
When working on an image, one should make abstraction of the source device. Imagine if a given photographer uses different cameras and wants to composite shots taken with different cameras in a single image? <head explodes>
The thing about CMYK
Because commercial printing processes typically work using color halftoning with CMYK separations, and because the default CMYK color profile used in North America is often U.S. Web Coated (SWOP) v2 (having a notoriously small gamut), we can be tempted to think that "CMYK is much smaller than ProPhoto RGB", which doesn't make sense. CMYK is a model, while ProPhoto RGB is a color space, so nothing can be said of the size of CMYK.
CMYK is not inherantly "smaller" than any other model, it's just that in practice, it is pretty much always true that any CMYK space used will be smaller than your working RGB space; let's just not confuse models and spaces.
Capture One
I find suspect the claim that "Capture One manages color better because it works in Lab". Let's understand that image transformations are mathematical abstractions that are only bound to a given space at the end of the process. Nothing prevents the internal image processing pipeline of a given software to juggle with imaginary colors of much greater precision than what can actually be displayed/exported.
Whatever happens behind the curtains, the "Lab" data has to be converted to a profile either to be displayed or to be exported to a baked file (e.g. TIFF, PSD, JPEG). In any case, it might very well be that internal algorithms used by Capture One manage to perform a "better" job with colors (I don't even know what it means to handle colors "better"—sounds rather subjective), but it would be for another reason than the fact that it does its math with L*, a* and b* coordinates.
6:55 pmOff
Mixing Flash with Ambient
Setting the shutter speed in the light meter
When using a Sekonic L-358 light meter to read a flash exposure (the same applies to other models, I am simply not familiar with them), one of the nice features it has is its ability to indicate which percentage of the total exposure came from the flash — the rest of the exposure being from the ambient light available in the location where the metering was done.
Here is a figure taken from the L-358's user manual:
Metering flash exposure
As you can see in this example, 70% — "Percentage of flash in total exposure" — means that the flash was responsible for 70% of the total light, while 30% came from the ambient light available. The total amount of light ended up requiring an aperture of f/5.6 and 3/10th (let's call this f/6.3).
If 30% of the total light gathered by the light meter during a reading comes from the ambient light, if you change the shutter speed (without changing the flash power), you should expect that percentage to change: the longer you expose, the more the ambient light will have an impact on the total light, since you will gather more of it (while still gathering the same amount of flash). On the other hand, if you expose for a very brief moment, you will gather very little of the ambient light (while still gathering the same amount of flash), so the ambient light won't have much of an impact in the total light.
You might very well end up in a situation where the flash is only responsible for a small fraction of the total light (say, 10%) or, at the other extreme, end up in a situation where the ambient light has no influence on the exposure (which would give a 100% flash exposure). For example, when working in a studio environment, we usually don't mind leaving the modeling lights on when shooting, because we know that the light they produce is insignificant in the total exposure compared to the flash power (we get a 100% flash exposure all the time regardless).
Now, coming back to a location shoot where we mix some ambient light with flash, we should often expect situations where the percentage of the flash in the total exposure will not be 100%. In the situation of the figure above, where 70% of the total light came from the flash, suppose we were to expose for 1/30 instead of 1/125 — that is two stops more ambient, giving us a brighter background. In this case, the flash would now only account for ~40% of the total exposure. What's more, since we're adding ambient light to the previous exposure (flash remained the same), we now have more light overall, so we must use a smaller aperture — we go from f/6.3 to ~f/9.
This makes the shutter speed a crucial parameter to provide to the light meter, otherwise the obtained value will be wrong.
The only situation in which we could ignore the shutter speed would be in a pitch-black room where the only light is the flash — in this case, the ambient light (or lack thereof) would have no impact in the reading whatsoever.
Changing our perception of flash color gels
If you're mixing flash with ambient light, you know that you can (must?) correct the color of the flash so that its light color is well harmonized with the ambient light color, lest you get a quirky result.
But you can also use color gels not necessarily because you want to make the two light sources the same, but because you want to add an effect, such as warming up the subject so that it contrasts more with a colder background, for example.
In the image below, both the foreground and the background lights are of the same neutral color:
Foreground and background light have same color
Using a CTO gel (orange) on the flash aimed at the subject, you can warm up the subject, which will further isolate it from the background (which will now be of a relatively colder color than subject, even if it is the same as in the first shot):
Foreground light is warmer than background light (note: a strong color was used to make the illustration obvious)
Indeed, while the background remained the same, it can be said that the subject is warmer.
You will notice that I also (conveniently) included a gray card in the frame. If we take the picture above (the same exact file), but that we color correct using the gray card, here's what we get:
Image above, but white balance corrected with the gray card
You will notice that the subject is now perfectly neutral — exactly back to what we had in the first frame — but that the background turned blue (colder)! In this case, can it be said that the subject is warmer than in the first shot? Absolutely not — in fact, they are exactly the same, after correction, even if a "warming" gel was used.
If you're going to include a gray card (or GretagMacbeth chart) in your frame, you might want to do it while you're not using a gel on the subject, otherwise, balancing the color with a neutral reference will remove any color you might have thrown on the subject and, rather, affect the background. If you're going to include a gray card in your frame while you are using a gel, the way you should look at it is that you are in fact changing the color of the background in the opposite direction than that of the color gel (e.g.: if you're using a warming color on the subject, you're actually not changing the subject, but rendering the background colder!)
The fact that we are using a "warming" gel has no bearing on the final look of the image, because it depends on the white balance selected. A "warming" gel might therefore be more meaningfully seen as a "gel that makes the background colder"! That is why, when you work indoors under tungsten lights, you use a warming gel: not to warm up the subject (you still want it neutral), but rather to make the background colder.
8:11 amOff
On White Balance
So you want to get accurate white balance. There are many ways to get there, if you know why you are doing it and what you want to achieve.
If you're shooting raw, you're probably aware that you don't technically need to worry about it at capture time, because it's not set in stone — whatever setting you use won't definitely affect your final image; you'll easily be able to change it in your raw processor. (Of course if you're shooting JPEG, then you'll definitely want to nail it as close as possible to your objective to minimize the amount of manipulation required. But you're not shooting JPEG anyway, are you?)
So you're shooting raw. Why even bother performing your white balance at capture time, then?
- One reason might be to get a better approximate histogram, so that you can better judge your exposure. Indeed, you might think that one of the channels is clipping when, in fact, it's the color cast that was throwing off the histogram. But then again, if you're evaluating the exposure another way (say, with a light meter), then this won't matter to you.
- Another reason might be to get a better approximate image preview, so that you can better judge the colors. Of course, you can't really rely on the image preview on that little LCD for accurate color evaluation, but surely, if the color balance is completely off, it might be quite misleading. But then again, you might only use the image on the LCD to judge the composition, pose and other aspects of the image, keeping in mind that color will be dealt with afterwards, so this, too, might not matter to you.
- Yet another reason might be so that when the unprocessed images pop up on your computer screen, the client watching you work will have a better idea of what the images will look like right away. Indeed, the client might not respond well if there is an annoying color cast in the images, even if you know you'll fix it later. But if you're not shooting tethered with a client watching over your shoulder, this might not matter to you either. (Note that some raw processors, such as Capture One, can be configured to automatically apply a certain white balance to all imported images, so that the images are immediately corrected, regardless of how they were shot, which is another reason why performing in-camera white balance might be irrelevant to you.)
Only you can know how important it is for images to be properly balanced at capture time, but if you want to do it, there are many ways to go about it.
- One way is to use the Auto White Balance feature of your camera. Sometimes, in rapidly/wildly changing light situations, this setting might be more efficient at getting a decent approximate white balance than having to fiddle repeatedly with your camera's parameters. If you're not working in such a difficult situation and the color of the light is not changing — say, you're working in a studio — then this setting might, on the contrary, produce unpredictable results.
- To get consistent results, another way is to use one of the white balance presets your camera offers — things like "sunny", "cloudy", "tungsten", "fluorescent", etc. This usually won't give you an exact result, but you'll likely get reasonably close to your target for your histogram and image preview to be very usable.
- If this is not good enough, a more sophisticated way to set the white balance would be to manually enter a kelvin temperature (which, unfortunately, only affects the blue-yellow axis, not the green-magenta color), or further tweak the result with the "white balance shift" tool. (Have fun. Who wants to spend time doing that?)
- Finally, the most accurate in-camera result you can achieve would be to use the "Custom White Balance" feature.
Bear in mind that whatever approach you choose, none — not even the custom one — will give you a surgically exact result. The only way to get a better result would be to include a neutral reference in a photo under each light situation, and later use that reference to synchronize the white balance for all the images shot under this type of lighting.
For the most accurate readings, you might include a tool like a WhiBal gray card, which is good to take care of the white balance, or go one step further with an X-Rite ColorChecker Passport, which includes a good ol' Gretag MacBeth color chart as well as the software required to generate camera calibration profiles for Lightroom — serious business!
If you don't want to spend the money (or carry these around, or shoot those test shots), for fairly good results, many common objects will be good enough to get you close to your target and allow you to work without having to worry too much about color. This might be a piece of white paper, tissue, gray hair, etc. It would be pretty hard to judge if these references are absolutely neutral (unlikely), so you understand that those are not perfect solutions. (For example, stuff usually increasingly turns yellow as it gets older.)
But keep in mind that few situations require critically accurate color — you be the judge. (Sometimes, even a perfect white balance won't be enough and colors will have to be manually tweaked in Photoshop with a virtual reference anyway (such as a Pantone swatch), and yet even then, color accuracy will likely take another hit when the image gets to the printer, so this discussion is at least partly moot.)
Now, in those situations where extreme color accuracy will not be an issue, you will have to determine if accurately reproducing the scene is what's important to you. Often, perfect color balance will not yield the image that will look better to your eye — you might very well decide that warming, cooling or otherwise coloring your image will produce something that is more pleasing, whatever light was there the moment you took the picture. You might even decide to convert your images to monochrome, and maybe even to tone them. Yet another reason why you might not need to worry too much about technical accuracy, but more about achieving an image that speaks to you.
For those reasons, on a more practical/pragmatic note, unless critical color accuracy is expected, I will usually save time and use one of the camera presets to get reasonably close results in-camera (so that I can still get useful information out of my histogram and image preview), include a WhiBal in the first shot if there are no other useful neutral references around, and tweak the results in post anyway. If I know in advance that I will deliberately change the look of an image, I might use a different preset (such as using a "cloudy" setting on a sunny day to make the image warmer); this will give me a better instant feedback, even if it won't have any definitive effect on the image until I make the final decision/adjustment in post.
If you still want to perform a custom white balance to get as close as you can in-camera, note that Canon and Nikon employ dramatically different procedures to achieve this (I am not familiar with the way other brands work):
- With a Canon camera, the procedure requires a ridiculous number of steps (which explains why I usually don't bother). You first have to shoot a picture of a neutral reference, filling the center part of the frame with it. You then have to go to the "Custom WB" menu option. You then have to point the camera to the picture of the neutral reference you want to use for the calibration (usually the one you just shot), and then confirm your choice. You then have, each time, to dismiss a non-removable reminder that tells you to change your white balance setting to "Custom". You then have to go ahead and change the white balance setting to "Custom". (Still there?) If this wasn't enough, you'll also be left with an otherwise useless frame, on your memory card, that was shot just for this, which you might want to delete. (Or you'll keep it and use it to perfect the correction in post...)
- With a Nikon camera, the procedure is decisively more efficient (take a hint, Canon). Set the camera to the "Pre" (preset) white balance setting. Hold the white balance button for two seconds until "PRE" starts blinking. Shoot your neutral reference.
If the procedure fails because the camera doesn't want to take the shot, this is because you're in a "one-shot/AF-A" autofocus mode that won't allow you to shoot until focus is obtained. Contrary to what you might have been told, performing a custom white balance does not require the camera to be set to manual focus, but since the neutral reference is usually a plain card, the AF system simply won't be able to achieve focus, which is the reason it will appear not to work. Just focus on something else or set the lens to manual focus — it doesn't really matter, as long as the center part of the frame contains the neutral reference. (If you're shooting in a continuous AF mode, this won't be an issue.)
If the procedure fails with the camera telling you it couldn't perform the calibration, that's because the exposure was completely off: the camera cannot read the color information out of a clipped highlight. Make the exposure average, or use a semi-automatic mode just for that shot (such as aperture-priority).
Make sure you actually do your reading based on the actual light conditions you'll be shooting in... This is especially relevant when you're working with flash — you don't want to perform your studio calibration based on the ambient light, or your location calibration on incorrectly gelled/randomly bounced mixed-light and get results that don't make sense.