Magenta cast when using GND filters?

[mealers]

I've just got my first few sheets of Slide film back from the lab and I'm fairly happy with the results.
One thing that was obvious straight away was the heavy magenta cast in the skyline.
Could someone tell me why this is happening?
I'm using Hi-Tech filters with modern multicoated lenses with film that is not out of date. I've searched Google and it appears alot of people have this problem with different filter brands.

http://i477.photobucket.com/albums/rr134/mealers1/Whitsandsbay.jpg

[Paul O]

It's a commonly held idea that some filter manufacturers produce GND filters that introduce a strong colour cast - Cokin used to be the worst! Lee make perfectly neutral GND filters that do not introduce any cast - hence the cost! How old are the HItech filters you are using? The reason I ask is that I did use these filters a couple of years ago and never noticed any real colour cast - I was using GND combined with a slight warm-up. I used .3, .6 and .9 strengths. Only reason I changed to Lee was the bigger filter size they offer (150x100). If they are new then contact Formatt and ask to exchange. Nice pic by the way!

[Aender Brepsom]

Hi Mike,

if I didn't know (from the other thread) that this is Velvia 50, I would have guessed it could be Velvia 100. Sometimes, even with the very neutral Lee grads, I have seen a slight (not as strong as this one) magenta cast in some of those slides.
Could it also be due to the scanner?
Hopefully, you can correct the colour cast in Photoshop.

[David A. Goldfarb]

Some ND filters are less than neutral, but Hitech are usually pretty good, and you've got some magenta in the foreground as well, most visible on the right side of the frame, so I suspect another cause, particularly since you say this is your first time shooting LF color transparency film.

It could be a lab issue, or if you used a long exposure possibly a reciprocity issue. Check the data sheet for the film, since some color films are mainly designed for studio use with strobes, and even a 1 sec. exposure could cause reciprocity effects and color shifts.

[Dave - Landscapes]

Hi Mike,

FWIW I had a similar issue with a magenta cast across the top of some of my early shots with a Toyo 125VR and Velvia 50. The shots were boats on a lake early in the morning. I initially blamed processing but it turned out to be the bellows were not correctly locked in place on the bottom of the front standard hence light was leaking in. Not sure what camera you are using but if it has a bellows, it may be worth checking.

Also, nice photo!

Regards

Dave

[mealers]

Thanks for the suggestions.
I should of added that I've had the same results using Velvia 50 and Provia.
It would be easy enough to fix in Photoshop but I'd much rather get it right without any editing, the colour cast is also very evident on a light box so I can right off any scanning problems.
David, I never noticed the colour cast elsewhere in the image before you mentioned it, interesting.
Dave I think I can rule light leaks out aswell because my unfiltered slide looks as it should do.

I've just spoke to Formatt and they said it does sound unusual so I've sent the filters back for testing. The downside to that is that I have the week off work next week and was hoping to shoot more colour.

[Nathan Potter]

Some ND filters are less than neutral, but Hitech are usually pretty good, and you've got some magenta in the foreground as well, most visible on the right side of the frame, so I suspect another cause, particularly since you say this is your first time shooting LF color transparency film.

It could be a lab issue, or if you used a long exposure possibly a reciprocity issue. Check the data sheet for the film, since some color films are mainly designed for studio use with strobes, and even a 1 sec. exposure could cause reciprocity effects and color shifts.

My first guess would be as David suggested; It's a lab issue. Study the original for a uniform overall color shift to eliminate any scanner issues. This looks a lot like a temperature control issue that I've seen on occasion. It can even occur more non-uniformly along one edge of the film due to temperature variation vertically in a dip & dunk environment. It might be worth splitting a series of chromes between two labs as a personnel QC evaluation.

As I look at your chrome I really think the shift is too excessive to be ascribed to a filter shift.

Nate Potter, Austin TX.

[Charles Young]

I've come across a similar issue myself with a hard-edge 0.6 Singh-ray GND. It introduced a serious magenta color cast. I contacted Singh-ray, sent some examples, and they basically said 'that's the way it is' and 'that I was not using the filter properly', which is quite funny because i've also got a HiTech Soft-edge 0.6 GND and it doesn't introduce any colour casts at all...

Cheers,

Charles Young
paisagembrasileira.com (http://www.paisagembrasileira.com)

[CG]

Might be worth while to shoot a test mid day, sunlit only, nothing in shadow, non-mountainous area with neutrals as entire subject area. I'm guessing your filter isn't as neutral as the mfr would say and that you'll see some color cast when you've eliminated the non standard light and subject matter (mountain light is higher color temp, as is shadow lighting)

[aphexafx]

Just shoot a piece of white paper with your GND in the sunlight with your aperture opened up as far as you possibly can, under-expose by a stop - I'd be very interested in the results...

[mealers]

Just got a email back from Formatt where I bought my filters. They say they have tested them and have found no problem with them.
Its starting to look like it could of been a problem when I had them processed at the lab. I'm going to shoot some more slide this week and send them off to 2 different labs and see what comes back.
I'm also looking into developing my own E6, should save me alot of money,time and at least I will know that things are done right!

[Jim Michael]

Ask the lab for a copy of the process control plots for the date the film was processed. I'd be curious if the chemistry was in control. If they won't show you the plots that's a big red flag that they don't manage their processes. I don't see the cast in the bottom of the shot so I'm a little skeptical of processing as the cause, esp. when you say you've seen the same effect on other shots assuming processed at different times.