02 March 2006

I like my sponge raw

A post at Pinguicula regarding the utility of saving digital images as RAW files prompted me to do some tests. I take pictures of snail shells and other small objects very frequently under fluorescent lamps. Although, digital cameras, including my Olympus E-500, have preset white balance (WB) adjustments for various types of fluorescent lamps, I have never had satisfactory results using those. So, what's one to do?

Here are some test results. First, on the left is the test subject, good ol' Spongebob, photographed using E-500's built-in flash with WB set at Auto as recommended in the camera's manual. Spongebob was on a white card which was on a gray card visible along the right side. The resulting picture looks fine as far as the colors are concerned: white is white and gray is gray. However, I normally don't like to use flash when I am photographing shells because flash creates harsh shadows and I usually have difficulty getting the exposure right.

Below is Bob photographed under one my fluorescent desk lamps with WB set at Auto. The result is ugly: white is yellow, gray is yellowish-brown. Next to it is the same picture after I readjusted the color balance in Photoshop. There is some improvement, but it is still not satisfactory.


Next is a couple of pictures taken with the color temperature set at 4000 K and 3200 K. (This was done using the custom white balance function of Olympus.) You can see that as the color temperature goes down, the yellow cast is replaced with a greenish cast. (If the color temperature is increased, the redness of the picture increases.) So, setting the color temperature manually doesn't correct the colors either, although the picture taken at 3200K is preferable to that taken at Auto WB.


Finally, below is a picture taken with WB set at Auto, but saved as a RAW file. Using Olympus Master, the software that came with E-500, I readjusted the gray point by picking up the eyedropper tool and clicking on the part of the picture showing the "gray" card that was actually yellowish-brown as above. When the program finished its adjustment, the correct colors were miraculously restored to the picture.


The ability to correct the WB to such an extent is, therefore, one of the advantages of saving images as RAW files. Now I have to always remember to include a portion of a gray card in all pictures I take under fluorescent lights.


Roger B. said...

An interesting experiment. Fluorescent lamps are probably the trickiest artificial light source to work with, so it's good to know that there's a way of correcting the problems they cause.

I'm sure there are considerable benefits in using the RAW format, but there seem to be a lot of variables to juggle when processing. It's going to take me a while to get to grips with them all.

I've now started shooting simultaneously in Jpeg and RAW. I'll save the RAW files for a rainy day when I've got time to play around with them.

Anonymous said...

On the RAW/corrected image the gray and white surfaces are neutral, but what about the colors on Sponge Bob?
The picture taken at 4ooo K is closer to the color temperature of the fluorescent household bulbs than 32ooK (tungsten bulb). In both cases the camera is adding green to compensate for magenta. Fluorescent bulbs come in different "shades" like coll white, daylight etc., all of which have different color balance. Another compounding problem: these neither have the complete color spectrum nor the ratio of the individual wavelengths present in "daylight". Remember the "warm" morning light? Even daylight is not uniform throught the day or seasons.


On the raw corrected image blue & red are fine, but yellow looks unsaturated in comparison with the actual thing. But that may be an exposure issue. In Photoshop, If I increase the saturation of yellow, without changing its hue or the color balance, the resulting picture looks better.