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robski
18-01-09, 12:42
An exercise in White Balance.

Inspired by Andy153's thread and Joe's notion of attempts of a DIY version.
http://worldphotographyforum.com/showthread.php?t=4021

The main problem would be finding a material that was translucent enough to let plenty of light through but would completely diffuse the scene.
Tupperware plastic appears to be too transparent as it does not completely diffuse the scene. So I started to look around the house for cheap materials that may do the job.

The first piece of material I came across was a some white iron on tape in the sewing box. It was 50mm wide and about 2mm thick. Smooth like ribbon on one side and sort of velcro on the otherside. I suspect it was one of the wunder web hemming products bought from Woolworths many moons ago. It lets plent of light though, a bright white and completely diffused the scene.

I decided to use a piece of this material placed directly infront of the lens to conduct some tests.

My scene was lit by an electric eco lamp bulb. The camera was set to take RAW and JPEG in manual exposure mode.
The exposure setting was taken from a grey card 1 sec @ F8 ISO 400.

The first shot was taken with the camera set to AWB Auto white balance mode. This was not expected to give correct results as the scene was outside the working colour temperature range of this mode (3000 to 7000K). The "As Shot" reading given to this image is 3750K +27 tint.

Next I followed the Custom white balance proceduce outlined on page 68 of the Canon 40D user manual. The only difference being is that I placed the piece of tape over the front of the lens instead of photographing a white object such as a sheet of paper.

The next shot was taken using the Cutom white blance mode and this gave a "As Shot" reading of 2650K +16 tint.

The final shot was taken to include a Jessops Grey wedge to judge how good this method was. Note I have measured the Jessops Wedge on our works Spectrometer and I know what areas of it I can trust.

RAW Images were Colour Corrected only in Light Room 1.4 and Cropped and resized in PhotoShop.

Looking at the first attachment.
First image on the left is what the camera produced using AWB with no correction.
The middle image is using Light Rooms Auto Colour Adjustment, which is a step in the right direction but still too warm.
The image on the right is adjusted to the reading obtained using the Custom White Balance method.

The second attachment included the grey wedge so that an accurate assessment of results can be made.

Conclusions.

This method diffinately gets you in the right ball park if you have a sutable material to diffuse the light and not introduce it's own colour shift. Note many of these commercial products have options (filters) to give warm effects etc. Whether it beats including a grey wedge that professional studio photographers use is another matter. Using a Grey card at an indoor sports event is not really an option.

Trying to Colour Correct the JPG versions was pretty hopeless to get accurate results.
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Another potential material I bumped into was the white plastic pot from a individual microwave porridge product.

So has anybody other than Christine been tempted to make a Blue Peter version ?

Lello
18-01-09, 14:28
Hi Rob
I heard some people were using the early Pringles crisp tube covers (about 78mm diameter)
the early ones were opaque, and they worked quite well, but now the design has been changed to clear see through ones :(. because the cap is a good size to use (fits over my 18-200mm Nikon lens) I cut a circle of tracing paper and attached to inside of cap.
Unfortunately I have not done any tests with Pringle tube expodisc!! When I get time i will post some test results.

robski
18-01-09, 17:34
Hi Lello

Now you mention it does ring a bell folk using the pringles cap.

If I get time I'll retest with the porridge pot, The base is a few mm smaller than the pringles cap.

Joe
18-01-09, 18:29
Rob,
You have the wrong person.
I certainly haven't done any DIY in a while ;)
...great ideas though!
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