Custom White Balance

[Grouch]

I have been taking some shots (under photofloods) and first took a white card to allocate for custom white balance.

The manual says you can use any white balance setting for the initial shot. However, when I tried a range of white balance settings (resulting in differently tinted white card shots), the results when I shot the subject varied according to which image I selected for the white balance. It seems to me that I need to shoot the white card up front with no white balance correction, but there isn't an option for that.

Finally, I selected RAW but found the white balance still affecting my images once downloaded - is there a way to circumvent any white balance correction in camera and do it all in PhotoShop?

Apologies if this has been covered elsewhere or is simply a silly question! ;)

K

[gyelland]

When in RAW mode there is no white balance applied.
This is why you have to set it in the conversion software. So this is best if you want to do it manally.

I have used the custom white balance, at it seemed ok but I am not really sure how this is working.

Another alternative is to use a colour temerature meter, this will measure the colour temerature in Kelvin and then (depending on your camera) you are able to set this on the camera, or enter it in the conversion software as a precise figure.

[Roger_Cavanagh]

I think you're supposed to shoot the custom WB shot using AWB making sure the white/gray card covers, at least, the partial focus area of the viewfinder, if not more.

Then you choose custom WB on the control and select the test pic you just shot

It doesn't matter whether there is a tint; that's the point: the camera knows the test picture is supposed to be neutral and adjusts accordingly. The test image doesn't need to be in the camera once it has been set, so you can keep "samples" on a separate CF for re-use.

You raise an interesting point about WB during conversion. I guess there is no way to avoid choosing a WB of some kind.

Regards,

[Grouch]

Thanks for replies to date.

I have tested the EOS 10D using a range of preset WBs (including AWB) when I take the white card. Each produced a different white and a different result when chosen for the custom WB for the final shot. If the original pic is a blue or red tint (due to me choosing presets), surely the camera can't work out a correction curve that is independant of that tint... or am I being incredibly slow. ;)

Also, I've read, on this and other forums, that RAW does not contain any WB information. How come that my RAW pics converted using Canon's software, vary depending which WB setting I choose (and indeed, which setting I choose when I shoot an image for custom WB)?

K

[bluebomberx]

The RAW images do use WB settings. They are a jumping off point to work with in the converter. If you shoot an image at 7000K, it's going to show up as 7000K when you open it up in the converter. As you already know, you can tweak it to your liking if you wish in the converter.

-Richie

[tonygamble]

>

The RAW file of the image never differs, but the WB you chose comes with the file as a flag.


>

Because your RAW conversion software is following the instructions of your flag.

The camera does not know whether you are in Daylight, Shade or Tungsten. You are telling it and the flag is telling your RAW converter what you told the camera.

When your flag is AWB the WB info is read from the recorded image and also added to the RAW file. So when you opt for AWB as your chosen WB on your coverter it reads the data you have given it from them camera and applies a suitable algorithm.


Tony

[Grouch]

Thanks for that explanation. So, if I ignore the flag, I can calibrate the RAW data (once converted to TIFF) in PhotoShop later.

Just leaves me with the question about custom WB - i.e. how can I stop the WB correction applied to an initial white card shot affecting the final WB when I use that shot for custom WB?

K

[Roger_Cavanagh]

Grouch wrote:
Thanks for that explanation. So, if I ignore the flag, I can calibrate the RAW data (once converted to TIFF) in PhotoShop later.

You can only ignore the WB flag by choosing another WB.

Just leaves me with the question about custom WB - i.e. how can I stop the WB correction applied to an initial white card shot affecting the final WB when I use that shot for custom WB?

You choose another WB preset or colour temperature during conversion, but there is no way not to apply a WB of some sort using the normal converters. The fact that you get different colours when you choose different presets indicates that the converter is doing its job properly. When you take a picture of, say a white card, it will look white in some lighting situations and not in other. Our brain can automatically compensate for lighting differences, so, by and large, we will look at the white card and see it as white.

So what you are doing with a WB is telling the camera: the card in this picture is SUPPOSED to be white (actually neutral). It doesn't matter whether it looks red, blue, or green or whatever; the camera or converter can work out how to adjust the colour balance so that the final image of the white card really does look white.

If you cannot get WB balance, one thing that may be wrong is that there are specular highlights on the white card, i.e., it is being over-exposed in one or more channels. If this happens, you can either use a gray card or deliberately under-expose the white card. It doesn't matter, if it looks gray because the key thing is to have a neutral colour. That's why some experts recommend a gray card because it's hard to over-expose.

Regards,

[Grouch]

Many thanks Roger (et al).

I now understand what I need to do, but am finding the process defeats my (clearly inadequate) logic skills. I guess I'm thinking the camera sees what it thinks should be white and applies a curve to rectify it based on the white card image. Because this original image can vary so much (depending on my WB selection when I took it) it seems odd to me that the camera can still work out what's going on - I can only assume it's because of the flag on the original image. Still, it works and I'm happy with that.

On the RAW front, what would you recommend I do to take pics for publication that are, as correctly as possible, white balanced? I have access to a 18% grey card... but I'm still confused as to the best path to get pics through RAW, to TIFF with minimal risk of mucking up the colour balance.

Sorry for the simple questions - have just moved from consumer camera taking snaps to EOS taking the odd shot for publication.

Once again, many thanks for taking time to answer. :)

K

[Roger_Cavanagh]

Grouch wrote:
Many thanks Roger (et al).

I now understand what I need to do, but am finding the process defeats my (clearly inadequate) logic skills. I guess I'm thinking the camera sees what it thinks should be white and applies a curve to rectify it based on the white card image. Because this original image can vary so much (depending on my WB selection when I took it) it seems odd to me that the camera can still work out what's going on - I can only assume it's because of the flag on the original image. Still, it works and I'm happy with that.

A couple of extra thoughts: when you use a custom WB image, you are telling the camera this should be neutral. It looks the picture and says "aha, RGB values are 100, 110, 120. I'll change them to 110, 110, 110". Obviously, not that simple, but you get the picture. When you then choose another WB preset, the camera is not using the info' that the card should be neutral. It's using the fact you've told it that you took the picture under tungsten lighting or whatever, so it's making colour adjustments using different criteria, which is why you get different results. It thinks "if this were an average scene taken under tungsten, what adjustments would I have to make?" and does that. Obviously, different light means different adjustments.

On the RAW front, what would you recommend I do to take pics for publication that are, as correctly as possible, white balanced? I have access to a 18% grey card... but I'm still confused as to the best path to get pics through RAW, to TIFF with minimal risk of mucking up the colour balance.

Sorry for the simple questions - have just moved from consumer camera taking snaps to EOS taking the odd shot for publication.

Once again, many thanks for taking time to answer. :)

K

Well, you're asking someone who uses AWB most of the time and fixes things afterwards, if they look funny. :) But the simplest thing to do is just take a picture with a gray card or something like the Macbeth Colour Checker in it. So long as the card is illuminated by the same lighting you can use it afterwards to make adjustments either in Photoshop or in converting from raw because all the converters allow you to use a custom WB one way or another. If you use a custom WB in-camera, you aren't committed to anything because, as we know, you can change most things after the fact with raw format. It may make life marginally easier, but not all that much.

As a final point: sometimes you might not want to change the effect of light. A golden sunset is the classic example, if you colour correct that so that white looks white, your beautiful sunset is gone.

Regards,