White balance

hello, community ... this is my first post, I wanted to consult a technical question to see if there is someone there who can help me.
I have a canon 7d with a tokina 10-17, in an aquatica housing with an 8-inch dome. It's about the custom white balance..how to do it with a white card ... it is impossible for the lens to focus the whole card with such a wide fisheye vision..even if I zoom, and make the card to the dome I can't grasp the whole card. Any advice or tip..or im missing something?..thanks in advance

Hey juanmag

A couple of thoughts. You don't necessarily need to fill the screen to white balance and you're right of course, pretty darn difficult with the 10-17.  

If you are photographing at a reasonably constant depth you could take one pic with your white card in the image. Then just keep shooting (preferably in RAW) with the same settings. In post-production you can use the eye-dropper (or similar) and select the white card on that image to set the corrected WB.  Then copy those settings to the other images. Reasonably straightforward if you use, say, Lightroom.

Another way is just to shoot RAW then in post-production just tinker with the WB until you get the tone/tint suits you.

I find that in general, if you are using strobes, setting the camera to Auto WB does a pretty good job. If you shoot RAW you can then tinker around but my experience has been that it's rare, certainly in blue water,  that I have to mess too much with the WB setting. If I do it's usually as I didn't have the strobes at the right power - generally underexposed.

Fixed up your text, I don't think it likes pasted formatted text.  On the WB, if you are doing stills with strobes agree custom balance is the way to go.  Video or ambient you'll get closer with a custom WB of course. 

I don't believe it needs to fill the frame for the WB to work, but it should show a dotted line in the viewfinder that the white image needs to fill, at least on the 7D mkI.  Another option might be a dome cover made of white ripstop nylon which should be translucent enough to work for ambient light - it won't work for flash.  You would have to make that yourself of course.

Thanks for  answering!!!! Ill try all that

No worries. If you get stuck, come back to us.