Opticallimits

Any colour filter experts here? I know the trend is to correct many things in post, but there are times that can't be done or has other limitations.

 

Today I started dabbling with videos of my marine aquaria. http://vimeo.com/115072506 This is my first "serious" video attempt, but don't expect much as there's a lot of noob mistakes in there.

 

That aside, one of the things I need to fix is the white balance. The lighting used on these aquaria is typically very high kelvin. I have no idea what mine is rated to, but ones on another aquarium which aren't as blue are rated at 25000k for example. Suffice to say this really confuses the camera metering/AWB with clipped blues and red/greens looking rather off. What's detected by camera doesn't match what the human eye perceives at all in this respect. As a workaround I could switch temporarily to daylight lighting, but this then doesn't really set off the phosphorescence.

 

I'd ideally need say two strengths of yellow filter. One to totally cut blue as far as is practical. One to attenuate it a fair bit, but not remove it totally. The dominant peak for the blues is usually around 460nm if it helps. The attenuator would be to tame the blues, the eliminator would allow the phosphorescence to be shown by itself.

To answer my own question, I kinda decided to get two filters roughly equivalent to Wratten 8 and 12 to 15. I quickly found Hoya make the former and ordered one of those to try. Getting the stronger filter proved more difficult until I found Tiffen do a 15, but they didn't call it Wratten so it never turned up in my early searches. That's also been ordered, along with an unspecified characteristic Cokin yellow.

Quote:I know the trend is to correct many things in post, but there are times that can't be done or has other limitations.

 

 

I found that out the hard way while trying to digitize color film negatives. Without a filter, it's very easy to blow out the red channel or completely underexpose the blue and green channels. I now use a B+W KB-12 (blue) filter to partially compensate the orange mask. 

 

Regarding your aquarium light: If you're an Adobe user, you could try to add a custom camera profile with a skewed white balance using the DNG profile editor. I have such a profile for my infrared images (>720nm), because the normal range of white balance adjustments is not enough to get rid of the red cast in the images. Dunno if it works, though. And it obviously doesn't help at all for video 

 

Ultimately, I think it's only in such extreme situations that color filters are still useful in digital photography. Most of the time one can correct things in post. 

I've received the Hoya, Cokin and Tiffen filters now. Not had time to play with any of them... white balance in any software I tried just doesn't have enough adjustment range if I have the blues on without filter. I either clip the blue channel massively, or grossly underexpose red/green. I suppose if I don't care about the blue channel, I could let it blow and remove it in post. But with a yellow filter, I get better metering and real time feedback when taking the shots.

Quote:white balance in any software I tried just doesn't have enough adjustment range if I have the blues on without filter. 

 

That's exactly what a custom camera profile with skewed white balance will give you. Here's how it works for IR: http://www.davidclapp.co.uk/blog/view/calibrating-white-balance-to-process-infrared-images-in-lightroom 

Your case is basically the same just with a massive amount of blue light instead of red.

 

However, I guess if the exposure is completely off there is a limit to what you can recover with camera calibration.