Auto WB

I have a simple question: does anyone know why auto WB on camera is not accurate sometimes but when you do an auto WB in ACR on the same picture, it gets much closer to the correct WB? Why camera's internal software is not capable of doing the same thing?

Thanks!

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Originally Posted by Alis

I have a simple question: does anyone know why auto WB on camera is not accurate sometimes but when you do an auto WB in ACR on the same picture, it gets much closer to the correct WB? Why camera's internal software is not capable of doing the same thing?

Thanks!

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I'm assuming you are using one camera only in your analysis because different manufacturer's may use a different value to represent the color temperature under different lighting conditions. The differences will be minor but it can have an effect on exposures. Another thing to consider is that for any given light source, the intensity of that light (daylight) changes throughout the day.

And AWB needs a good reference point in the scene in order to set a suitable value.

An average street scene in good and stable light, for example, will provide plenty of reference areas for AWB; but problem scenes with difficult colours or lighting means that your camera has to guess what is the best setting.

That is why setting a Custom White Balance can be so helpful with these problem scenes. Failing that, using one of the pre set camera options often works reasonably well and better than AWB.

Thanks, guys.

I shoot RAW almost exclusively now, so the camera is always on AWB. The same image I download and in ACR set the WB to Auto and there is a big different between auto and as shot. I was wondering what the ACR software does that the camera's firmware can not do.

Use your gray card ... it'll be more accurate than either "auto" options! :)

I forget where I read it, but one photo reference recommended to never use AWB and ever since I chose to follow this recommendation I've had, at least, stable white balance results.

In AWB mode, the camera's processor reviews the captured image and guesses what element of the scene should be white and it then uses that value in subsequent image processing.

When you select a fixed white balance, the camera will not make any guesses for you and the processing is based on the chosen white point. Even if it's wrong for the scene, a constant white point means all the exposures of a shoot will carry the same white balance and when you do your color correction, the same adjustment will, or at least should, work for the whole batch.

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Originally Posted by Colin Southern

Use your gray card ... it'll be more accurate than either "auto" options! :)

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Sir, this was just a technical question, just out of curiousity.

As for using the card, I have been using it but for example at a birthday party in a big gym for kids, depending on where I was in the room, there were different lighting setting, and it was not that useful to use the card. But at home for example, it works very well!

The camera must process the image in a limited amount of time. For example if a camera can shoot continuously at 5 frames per second then there is 1/5 seconds available to demosaic, white balance, sharpen, jpeg, image storage etc. ACR could use the entire 1/5 of a second to white balance and you would never notice it. The computer running ACR has more processing capacity and less time constraints.

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Originally Posted by Alis

As for using the card, I have been using it but for example at a birthday party in a big gym for kids, depending on where I was in the room, there were different lighting setting, and it was not that useful to use the card.

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By the way, you can still use a gray card in that situation - you just need to use something / someone as a "transfer standard".

I know what you're saying though - personally my best guess is that they just use different algorithms.

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Originally Posted by AlanP

The camera must process the image in a limited amount of time. For example if a camera can shoot continuously at 5 frames per second then there is 1/5 seconds available to demosaic, white balance, sharpen, jpeg, image storage etc. ACR could use the entire 1/5 of a second to white balance and you would never notice it. The computer running ACR has more processing capacity and less time constraints.

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Thanks, Alan! That was my guess too, but then my question always has been, why does the camera need to do all the things you mentioned in-camera? Specially when shooting RAW, wouldn't it be easier to just leave all of that to the PPD or ACR and focus on transferring more and more data to the card?

When shooting RAW the camera still has to process an image for reviewing in camera.

Another thing to consider is that the original scene is reflected light but the computer screen is transmitted light. Take a look here: http://en.wikipedia.org/wiki/White_balance