Red colour cast with Cokin filters

[oldmankit]

Hello,

I've been happily using my set of Cogin square filters (P series) which have helped me get the exposure right without the need for HDR processing. However on my latest trip I was met by an unpleasant surprise. On the second day's shooting only I noticed a strong red colour cast on all of the shots for which I used the filters. There was no such cast on the first day.

Here is a link to an example: http://peaceandtruth.com/blog/wp-con...y/IMG_5881.jpg

I'm kinda baffled about what the cause of this might be. I'm quite confident I was using the same filters both days (one neutral density gray, another graduated ND, sandwiched together in the Cogin holder). Any suggestions?

Thank you.
Kit

[Donald]

Kit

Are you aware of any changes in the lighting conditions in which you were shooting?

[Colin Southern]

Hi Kit,

It's caused by a different response to infrared ... So you get higher levels in the red channel.

It's something I get to a degree even with high-end Singh-Ray resin filters.

[Colin Southern]

Hi Kit,

It's caused by a different response to infrared ... So you get higher levels in the red channel.

It's something I get to a degree even with high-end Singh-Ray resin filters.

[oldmankit]

Hi,

Donald, I know the light was different, but I can't remember exactly how. It was a few weeks ago. I think it was basically brighter - mid-day in the Thai sun.

Colin, are there particular lighting situations where this infrared is more pronounced? I'd like to know how to predict it, as all of these shots are basically wasted (unless I go B&W), and I'd like to know a bit about this so I can stop it happening in the future - or at least know when it is going to happen.

Thanks both,
Kit

[oldmankit]

Correction. The photos aren't useless. I'd forgotten the power of raw to recover what would otherwise be useless.

[Colin Southern]

Correction. The photos aren't useless. I'd forgotten the power of raw to recover what would otherwise be useless.

It's hard to get the cast totally neutralised as it's different for different parts of the image - kinda like mixed lighting. with Singh-Ray filter it's usually not a big problem unless one stacks them (eg used something like a GND and a Morslo). Not sure what the Cokin ones are like -- generally, Cokin is known for "not being great quality" though

[oldmankit]

It's hard to get the cast totally neutralised as it's different for different parts of the image - kinda like mixed lighting. with Singh-Ray filter it's usually not a big problem unless one stacks them (eg used something like a GND and a Morslo). Not sure what the Cokin ones are like -- generally, Cokin is known for "not being great quality" though

Yes, I immediately saw it would require a lot of work to recover these images properly. Using Bibble 5 I made a rectanguar layer, heavily feathered (to work with the gradient of one of the filters), and pushed tint away from magenta. On immediate inspection the problem was totally removed, but it would require a lot of tweaking. Pushing the correction too far I got a very green, sick-looking image.

I will experiment. I tried reading-up but couldn't see any clear answers for why I would have a problem in one shoot and not others. If anything, I read that IR is more of a problem in the morning/evening, rather than bright mid-day shooting, as I had imagined.

On the whole I have been satisfied with my Cokin filters, which were not too expensive but definitely above the cheapest on the market. This is the first noticeable problem I've had with them. But I do know there ar emuch better filters out there!
Kit

[Dave Humphries]

Hi Kit,

Just a thought on "why I would have a problem in one shot and not others"; is it due to the sun shining onto the filter obliquely? (i.e. flare)

e.g.
with sun (even slightly) in front of lens (I assume using a lens hood isn't feasible) or with sun catching back surface, or even the egde, of the filter, gives the red effect
but with sun behind camera (and not striking filter) doesn't.

Have a look at a few more images (with and without effect) and see if you can confirm this by looking at shadow angles.

I could be wrong, but worth a look, may be even try some test shots.

If it is the cause and you need to shoot into the sun, or any bright light (compared to subject), make sure you take steps to shield the filter itself from being struck by extraneous light.

Cheers,

[oldmankit]

Hi Kit,

Just a thought on "why I would have a problem in one shot and not others"; is it due to the sun shining onto the filter obliquely? (i.e. flare)

e.g.
with sun (even slightly) in front of lens (I assume using a lens hood isn't feasible) or with sun catching back surface, or even the egde, of the filter, gives the red effect
but with sun behind camera (and not striking filter) doesn't.

Have a look at a few more images (with and without effect) and see if you can confirm this by looking at shadow angles.

I could be wrong, but worth a look, may be even try some test shots.

If it is the cause and you need to shoot into the sun, or any bright light (compared to subject), make sure you take steps to shield the filter itself from being struck by extraneous light.

Cheers,

Hi,

I think you may have got it there. It was mid-day, and I noticed visible flare in a few of the shots (but none from the shots the day before). Indeed, checking the shadows, I was shooting into the sun, even though it was high overhead.

Only one photo leaves me questioning. This accident:


You can see from the shadow where the sun was. Could it have been reflecting strongly off the water? The area in red was very shallow water.

It looks like I have an answer for why this day, and not others. I don't normally shoot with filters at mid-day, around water!

Thanks a million.
Kit