Flash WB problem

[xenomer]

For about a year I used halogen lights for my home studio. This

Christmas as a present I got a studio flash kit. Even at my first

portrait shots I discovered that the color tones are quite different

from the shots I made with the halogen. Even with setting the WB

manually in the RAW converter, I weren't able to reproduce the same

tones I had with the halogens.

 

Yesterday I made a few object shots with a softbox attached to one

of the flashes. I used a white cardboard as a background, I set up

the flash with the softbox at the side of the object. I shot RAW.

During the post processing I was unable to set a correct WB for the

whole background. Even when I used the RAW converter's WB picker

tool. If i clicked on one end of the background the other end's WB

was wrong. If I click the other end, then this end is not white. Any

suggestions for the possible cause? (The only other light in

the "studio" was the modeling light of the flash dimmed to be just

strong enough for the camera AF function to work, so I think that

the reason is not other the effect of other lights)

[xenomer]

Is it possible that the reflecting surface (a silver colored alumnium foil) I used at the other side of the setup caused the problem? If yes, what kind of reflecting surface should I use to avoid such problems?

[xenomer]

Or is it possible that the material of the cheap softbox is not consistent and that causes the problem?

[Ed_Ingold]

Halogen lights operate at a lower color temperature (2800K to 3200K) than photofloods (3400K), so you may be used to "warmer" tones in your shots. Flash operates at a color temperature of 5500K to 6500K, and has a much higher UV content. Different studio flash (and different power levels) give significantly different results. Your problems are mainly due to inexperience in lighting and color management.

 

More expensive studio flash units have better voltage control, and maintain a more consistent color temperature at various power levels. With cheaper units, you may have to adjust the color balance for each particular setup. You can minimize the effect of modeling lights by using higher shutter speeds, or simply turning them off when not needed.

 

Most of the time, using the built-in "electronic flash" WB works for me. For a simple "calibration", include a grey card in one of the images. You could prepare a custom WB in the camera (Nikon?) with the grey card, or use the eyedropper in Photoshop to set establish a neutral mid-tone balance.

 

For more demanding work, use a Gretag-MacBeth Color Checker (or Color Checker SG) card and software to create a custom profile. I use "InCamera" as a Photoshop plugin, or Eye One Match (SG card only). The advantage of using a Color Checker card is that an accurate curve can be drawn for each primary color over a much larger dynamic range than using a simple grey card. As a result, the shadows and highlights are in better control.

 

I don't find autofocus has a place in studio photography, or auto-anything for that matter. There's nothing "average" about studio work - you have to take control.

[xenomer]

Thank you very much Edward for your detailed explanation. You're right I have a little knowledge about the theory and a very little experience in color management. In this case I thought that the completely white background paper could serve as a WB reference (instad of shooting a grey card) I still not understand that why was this theory wrong. If I use Canon DPP-s White balance clicker it gives me different WB settings depending on the part of the background paper I click with it. That's why I think that the WB is not consistent on the photo.

The reason I use AF is, because it provides better fogusing results then my eyes combined with the viewfinder of the 350D. Could you please suggest a better method for focusing?

Thanks again for the detailed response,

regards,

Xenomer

[xenomer]

sorry for the misstypings....

[bill c.]

Xenomer-- The above suggestions might prove to be very helpful, however there are some other factors which may play a part.

 

(BTW, voltage control per se isn't what modifies the flash's color temp-- the ONLY thing that can to that is a coating on the flash tubes. Xenon will always glow at exactly the same temp no matter what the post-ionization voltage is. However, variations on the TIME duration of the flash itself can have an effect on the effective color balance in the image, due to reciprocity and other factors. High-end flashes have flash times consistently in the slower ranges to limit this effect.)

 

On to the other factors. First, what is your shutter speed? If you use a shutter speed that allows the ambient light to have an effect on the exposure, that can radically alter your color balance by allowing warm modeling lights or green florescents or super-blue window light to read. It is also possible (though unlikely) that the ambient could be strong enough to have an effect on your exposure even at your fastest sync speed, so you might want to check that.

 

Second, some papers can flouresce from the effect of the flash. If the paper has a lot of starch in it-- and most super-white papers do-- then the paper can have a bluish glow to it from the flash. This can be mitigated with the use of a UV filter over the flash (not the lens). Many high-end flashes will be UV balanced, but few on-camera flashes or low-end monoblocs are. However, if the paper has a strong UV response, then there's little that will help. Hold it under a black light and see if it glows. If it does, then try using another material as your backdrop.

 

Happy shooting. -BC-

[xenomer]

Thank you Bill for your valuable input. My flash uses a 150W modeling light and in this case it was at 1/4 power and I used a 1/200 sec shutter speed with aperture F/16 so I think that the effect of the modeling light is not significant in this case.

Probably the solution is in the last paragraph of your answer as the background paper was a super white semi-glossy cardboard. As the flash was located at the side of the setup it is possible that due to the slightly different lighting angle and intensity on the background the UV response of the background paper was different at different areas of it. I'll check it in practice as you suggested. Thank you very much again for the answere. I think it will be a great help to me.