Auto and Manual white balance

I'm trying to get to grips with manual white balance. I feel a real novice in this area.

Armed with a Canon A95 I've tried shooting both on auto WB as well the custom manual WB to see what the difference is. The white balance is set from a white plastic card at the same depth of 16 meters.

Would anyone care to comment on why the effect is so drastic. The effect is less pronounced in the shallows but still shows quite a drastic effect. These shots are in a UK quarry where the water is very green. The shots are straight from the camera with no photoshop adjustments to colour.

Hi Trevor,

The results of the manual white balance are drastic because the colour cast (green) is very strong in the first place. A manual white balance attempts to correct colour casts, so the stronger the colour cast, the stronger the effect. This explains the red torchlight beam.

When using your strobes, I would suggest that you stick to auto WB. If you want to take natural light shots with manual WB, you will get more pleasing/natural results if you incorporate a filter in your setup. When diving in green water, a cut magenta gel filter on the inside of your housing front port should give you good results.

Cheers,

Mat

Hi Trevor,

The AWB assumes that you have neutral colors in your picture and tries to balance the image accordingly. I don't know the exact startegy for AWB used by Canon but my guess would be they look for the maximum intensity and set this white and look at the darkest and set this black. This is probably why your first picture is not color corrected correctly though the modeling light of the subject is oK.

By manually white balancing you set what a neutral color is. Most of the time (not true in shallow waters) there is a lack of red in the lightning so the camera bring some back while interpolating the pixels color from the adjescent ones (remeber that you have two green for one red and one blue pixel on the RGB chips). So on your second picture red has been added to balance the colors, unfortunately the light beam was already white so it became red.

This also explain why there is less differences between AWB and custom WB in the shallows.

The next thing I've done is to try and reduce the extent of the colour shift and set the WB at a shallower depth than I'm taking the phots at.

After a bit of trial and error in the same green water quarry I've set the WB off the same plastic white slate but done it at 5 metres depth before then taking these shots at 18 metres.

The first shot is at 18 metres with manual WB set at 18 metres

The second shot is at 18 metres with the manual WB set from the start of the dive whilst at 5 metres.

I'm happy that the divers torch beam colour looks OK on the second shot and I've lost a bit of green in the water but not gone too blue.

I guess shooting in very green water is tough going. See what you think - any comments?

My main motivation here is photography without strobe lighting.

I've not tried a magenta filter. The main reason being that I have not found any easy way to attatch a filter. On my Canon housing there is no space between the Inon wet fit lenses I like using and the front housing port. On the inside of the housing there is only a miniscule gap between the front of the camera lens and the front inside port. This has precluded any of the thick UR Pro type filters.

The only filter I have tried on the inside of the housing is an Alex magic filter bcause it is so thin and was easy to fit. It sounds promising for blue water but after some tests it did not prove too useful in this green UK quarry.

Perhaps there are other very thin magenta filters I can get? The other thing I do not really understand yet is - could I set the white balnce off a 'cool' card to tone down the severity of the colour shift?

Hi Trevor,

I figured out you wouldn't have much space in front of the port, that's why I mentioned a cut gel filter in the front. These can be purchased easily on internet, e.g. from B&H at http://www.bhphotovideo.com/bnh/controller...tlist&sku=26385

The Kodak CC30M are large enough that you should get at least 4 filters from one sheet.

Cheers,

Mat

Thanks for your comments and thanks for the link Mat. I was aware of magenta filters like the Kodak CC30M. Having never seen them I was not sure how thick they were if space to fit them was tight. They sound worth a try for very green water.

Trev