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Old June 17th, 2004, 07:26 AM   #1
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Proper White Balance

Wanting some input on white balancing given this scenario...I have three XL1s cameras for a wedding. Two of the cameras are in the back of the church in somewhat dark conditions and the third is up on the stage/alter with some very bright spots (albeit different color temp). I'm trying to decide where specifically do you do the white balancing for the two cameras in the back. Do you take them up on the stage and balance them there with the third camera since that is where they will be zoomed in to with the brighter/different light? Or white balance them in place?

Actually I went for auto white balance on all three for the shoot and only had to do minimal color correction in post.
Thoughts?
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Old June 17th, 2004, 07:58 AM   #2
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3 XL1s ? You re one lucky man :)
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Old June 17th, 2004, 07:46 PM   #3
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We use three xl1 cams in what sounds a similar situation to yours.
Place a white board on your stage/alter, then from your shooting positions zoom into white board, focus and press the white balance button. Presto all should be well.
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Old June 18th, 2004, 07:56 AM   #4
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Great idea Owen....thanks
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Old June 18th, 2004, 10:11 AM   #5
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>>Do you take them up on the stage and balance them there with the third camera since that is where they will be zoomed in to with the brighter/different light?<<

I do it the way Owen suggested if the white board is big enough/cams are close enough, otherwise I do as you suggested above. However you do it though, I would ask to make sure the lighting will be the same at the time of the white balance as it will be at the time of the event...I once did this and someone decided a door needed to be open (just before the event) spilling direct sunlight onto my subjects through the entire event. I got the dreaded blue spot (lens flare) on both cams...another scenario is someone deciding at the last minute that the overhead flourescents need to be on : (

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Old June 18th, 2004, 12:16 PM   #6
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You also have white balance memories on the S model. You can white balance the cams for different lighting conditions and switch between these. Its one of the really nice features of the S model.
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Old June 18th, 2004, 12:37 PM   #7
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I'm aware of the memories....however, I'm hoping to find an initial setup process that would allow me NOT having to change settings during the event if at all possible. The danger lies in forgetting to change one of the cameras in the midst of the flurry of the event.. For those who shoot under controlled conditions this may not seem like a big deal...however, during events such as weddings, we don't have the luxury of stopping the action to change camera setups/verifying checklists :-)
I know my XL1s(s) perform the best when the WB is set manually, however, I have spent countless hours in post fixing a WB problem when conditions beyond my control changed (like Randy mentioned...someone else changing parameters). Under these ever changing conditions, I have had the best luck with the cameras in auto WB and tweak afterwards...although I know this is not the correct way to do things....
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Old June 18th, 2004, 02:57 PM   #8
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There's a wonderful little gadget called an "Expodisc" which is essentially a frosty piece of glass guaranteed to be neutral in color (with 18% transmittance, BTW). One puts this over the lens and aims it at the light source (not the scene) and presses the color balance button. Anything subsequently shot illuminated by that light source with that balance setting, from whatever angle or distance, will be in balance. This will get all three cameras individually balanced but not necessarily together. To do the latter manually adjust exposure (with the disk still in place and still aimed at the light source) and pull a little tape. These frames will be overall gray at the correct exposure. Setting up filters to bring them to the same on the waveform monitor and vectorscope should bring all three cameras to consistency. I suppose it would also be a good idea to shoot a Macbeth target with all three so that chroma amplitude differences could be tweaked out as well.

Cheers, A.J.
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Old June 19th, 2004, 12:40 PM   #9
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This is the first I've heard of the Expodisc...looked at the website for the device....seems to be centered around still photography....has anyone used this with a video camera?
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Old June 19th, 2004, 03:16 PM   #10
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You probably won't be surprised if I say I do. It's on whatever lens is on the camera i.e. it does double duty as a lens cap attached by a string (part of the kit) to the strap on the handle. The first step in taking any shot is to power on the camera. The second is to point it at the light source and set color balance. My only concern is that some day I will forget that the Expodisc is off and point the camera at the sun. That will be a sad day and it's probably not a question of wheter but when.

I usually do not record frames with the Expodisc in place but a little playing around in FCP with the 3-way color corrector has got me thinking I might start doing that. Drag the sliders around until all the dots are from the Expodic frames are in the exact center of the vector display and you are balanced. You can even make up for goofs in setting color balance up if you do this or correct for errors the camera made in calculating the balance. If your exposure indicator was right on the center when you exposed the frames then R,G and B should all be right on 50 IRE. This serves as a check for the exposure meter.

I guess to sumarize what I am saying: I use the expodisc to rebalance the camera whenever I think the light may have changed i.e. moving from early AM towards noon, going into the woods, moving changing from mostly sun lit to mostly sky lit etc. In doing this I rely on the camera to take care of the balance. If, in addition, I pull a few frames with the expodisc in place I can use the vectorscope to check on how well the camera has done and correct for it.

WRT to your particular problem and assuming that the action up front is what you are interested in I'd take all three to the front, aim them at the light source and balance - then shoot some frames through the disk. Now if you were to pull back at some point to show the congregation you basically have to deal with two color temperatures - that of the spots up front and the (presumably) warmer lights over the rest of the church. In this case you might want to try setting up another of the memories for the balance in the back i.e. go sit in a pew, put on the expodisc, aim heavenwards and press the color balance button. I guessing that you wouldn't want to use the second setting much if at all because if you use the warmer balance for the rear to show the congregation but the groom's face appears in the scene it will be bluish (instead of the natural ashen color it should be if he knows what he's in for). Conversely, any shot of the action up front which includes part of the congregation which is balanced for the up front light is going to make them look ruddy and if you switch to shots that show only the congregation balanced for the light that falls on then they will look balanced whereas before they looked warm. I'm guessing that it's probably best to stick with one balance only and have that the right one for the three principals up front.

Just rambling here - interesting problem.

Cheers, A.J.
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Old July 17th, 2004, 08:16 AM   #11
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ExpoDisc's Reply

I wondered the same thing, can it be used for video since their website reviews only photo use. So I wrote them:

Will ExpoDisc work as a white balance tool for Digital Video cameras with the same filter size? Have you done white balance testing with Sony, Canon, or Panasonic DV camcorders?

Their reply:

The ExpoDisc works great with video cameras!! I use it on my little Canon ZR80 (DV Camcorder) and it works wonders...seriously. Since my camera lens is so small (30.5mm), I just got a 49mm and I hold it over my lens to set the white balance.

As long as your camera has a custom white balance setting tool, you should be able to use the ExpoDisc on it with no problems...in fact I use it on my video camcorder, and I still haven't tried it on my digital camera - I think the video is a lot easier. ;-)

Try it, and you won't regret it (but just in case you do, we have a 30-day 100% Satisfaction Guarantee).

Best regards,

Jessica Crozier
Customer Service
ExpoDisc, Inc.
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Old July 17th, 2004, 08:26 AM   #12
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Yes, and you can use bleached coffee filters with even greater success. This thread will be of interest to you. That thread also refers to a discussion on the Fred Miranda photo site and the images white balanced from the filters looked better than the Expodisc. The thread also outlines the differences and pitfalls between reflected and incident methods of white balancing.
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Old July 17th, 2004, 01:08 PM   #13
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Let me begin by expressing my amusement that guys are willing to quibble over a hundred bucks for an accessory for their 5K cameras and then go on to say that I don't make my living selling Expodiscs (or anything else for that matter). That out of the way, does it make sense that a coffee filter with no consideration given to its optical properties would consistently be more neutral (and that's what we require for this application) than a product that is designed with neutrality as its main requirement? Of course not but that doesn't mean it can't work out that way. When people say they like the results with a coffee filter better that means that coffee filters and Expodiscs were subjected to double blind triangle testing, yes? If not then the assertion is meaningless because the desire to save $100 (or come up with a clever idea or help fellow videographers/photographers or ...) most probably or at least possibly clouded the judgement of the "observers". This isn't meant as a poke at the people who expressed this opinion (I can't find the thread on the Miranda site). It's just human nature and that's why we have triangle tests and double blind protocols.

Unwilling to set up such tests myself I thought I'd have a partly qualitative look at how neutral coffee filters (Safeway brand) were compared to Expodiscs so I put each in a spectrophotometer. You can see the results at http://www.pbase.com/image/31446058. The most interesting feature is the bump in absorbtion at 675 nm (red). This particular piece calibrated at the factory to + .02 Green and + .01 blue (I assume those numbers refer to equivalent CC filters) and I don't question that because the tristimulus response at 675, especially as weighted by Illuminant C per the NTSC practice, is quite a bit down from it's peak). The other interesting thing about the Expodisc is that the absorption zooms at 410 - 420 nm - still in the visible but again the tristimulus values are modest at these wavelengths.

Now looking at the coffee filter it is seen to be amazingly flat i.e. no ripples but it does have an appreciable trend towards increased absorbtion at the blue end. This isn't a whole lot but as it is smooth and monotonic it is going to push the color balance of the camera to increase the blue gain (or decrease the red and green gains) and things photographed based on that balance will this probably appear cool.

The bottom curve shows tha absorbtion of a PEC pad - something that is probably in every photographers bag anyway and I don't drink coffee. It is nice and flat (no trend) but it does show increased attentuation at the blue end. This is probably close enough to the end of the visible that it probably would not have much of an adverse effect on the balance (again because weigted tristimulus is realtively small at these wavelengths).

As one of the advantages of the Expodisc is, to my way of thinking, that it protects the lens from dust and as the PEC pad can be used to remove dust I'd say use PEC pads instead of coffee filters. They're whiter too!

What I would really like to see on these cameras is a raw mode in which the CCD data is not matrixed, converted to YUV or processed in any other way. The numbers right out of the A/D's go directly to the recording medium. This is commonly done on still cameras and the result is that you don't worry about whether you have color balance correct or not because color temperature is set in post. I expect that this is basically impossible in DV because the standard for DV requires YUV (to the best of my knowledge). It would be nice, at least, to have more than two color temperatures available from the color balance control but I have to admit I never thought to list that on the XL2 wishlist. OTOH it's certainly easy enough to pop the Expodisc (or PEC pad or coffee filter) in front of the lens, aim at the light source and push the balance button.

A.J.
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Old July 17th, 2004, 01:24 PM   #14
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Huh? I'm pretty sure its illegal to say "tristimulus" out loud, but I'm not sure what it means.

For an Alabama reader, did you just say the ExpoDisc was good or bad? Do I put the coffee filters back on the shelf? And since we don't have Safeway around here can I assume the ones at the Piggly-Wiggly are the same?
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Old July 18th, 2004, 05:22 PM   #15
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Yes, I think the Expodisc is a good product but I think a PEC pad would work just as well if getting color balance was the only issue. As I said in my first post I use it as a lens cap and a means of checking exposure as well. It's made to have transmission that should give you 50 IRE in R,G and B if you shoot a few frames with it in place and the light meter is working correctly.

It occured to me that it would indeed be pretty simple to do a triangle test right here! All you have to do is go to http://www.pbase.com/image/31492875
http://www.pbase.com/image/31493209 and
http://www.pbase.com/image/31492974
and look at the three frames shot with incident light balance derived from an Expodisc and from Safeway coffee filters (2 layers). Two of the frames are the same WRT color balance and one is different. They are labeled Frame1, Frame2 and Frame3. Send me an e-mail at ajdel@cox.net and tell me which one of the 3 is different and then whether you like it better than the other two and why. The more people who respond, the more meaningful the result will be.

For best comparison the files should be loaded into something like Photoshop which should be told that they are in NTSC space and then converted to your working space for examination on a calibrated monitor. Playing them out of FCP or a similar program onto a studio monitor should also serve.

Obviously this isn't a full up double blind test because I know which is which and I'm not presenting AAB to one group, ABA to a second, ABB to third and so on but it will be interestin to see what happens.

Cheers, A.J.

PS: No, I don't think you can assume that Piggly-Wiggly and Safeway filters are the same (though they might be) and that is, I think, one of the shortcomings of this method and why I suggest PEC pads if you don't want to buy an Expodisc.
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