Thread: ND filters - colour cast

Sawdustmaker.jpgSorry mate, I couldn't resist the challenge so copied your image into PS and had a go. Did I get close?
Bob

Seems a bit too vivid a blue (aqua?)

Originally Posted by Oldgreybeard

It could be, when I was talking with Chris earlier this morning, he wondered if it was related to the filter in front of the image sensors. One would suspect that the 'professional' cameras could have a higher quality filter which could attenuate IR frequencies. I doubt that Nikon or Canon are going to admit that, but if the study goes ahead I could be cheeky enough to ask them to comment on why the colour caste is more pronounced in cropped sensor cameras. I won't hold my breath though waiting for a response.

Bob

The quality of filter in front of the sensor was a comment made elsewhere (at least I recall reading it somewhere)

I have a set of Cokin filters that I've been using for many years. Amongst these Cokin filters are some of Cokin ND filters, some around 30 years old, and others bought in the last five to ten years. The Cokin ND filters have in the past been known to cause colour casts, especially when stacked. I also have a few different brand screw-on ND filters of different sizes, and they also cause colour casts to varying degrees. It seems to be a problem with the manufacturing process for ND filters. When I was using film cameras, the colour casts used to cause all sorts of processing nightmares when trying to filter out the colour casts during darkroom colour printing.

Fortunately, now that most of us are using digital cameras, the colour casts can be easily removed, preferably during the photograph exposure process, or during post-processing using software (more easily done if you are shooting RAW digital, rather than jpeg).

The best treatment for ND filter colour casts, in my humble opinion, is prevention. I believe that the safest way of ensuring that you get consistently usable shots when using ND filters, is to do a Manual White Balance adjustment prior to taking the photograph sequence. If the light changes (e.g. a cloud moves across) then repeat the manual white balance adjustment. Setting the manual white balance requires different procedures on my Nikon DSLRs and on my old Canon DSLR ..... and my Sony digital video camera has a different procedure again - so read the manual for your camera to make sure you do the manual white balance procedure correctly. The terminology for "manual white balance" also varies between the brands of camera, just to add a bit more confusion. I use a sheet of A4 white copy paper when doing the manual white balance adjustment, but really any white surface will work - no need to buy one of those special "white balance cards" sold in some camera stores.

There has been some discussion elsewhere in this thread regarding whether the colour cast problem is caused by some digital cameras having more sensitivity to the infra-red light frequencies than some other cameras. We used to have the same discussions 20 and 30 years ago about different brands of film. I do know that DSLRs are delivered ex-factory with a near visible infra-red blocking filters already installed directly over the sensor, as there is a small industry out there that focuses on removing the Infra-red blocking filters from DSLR cameras in order to convert the cameras to being able to record images using infra-red light (similar to the days when some of us tried using Infra-red Film). Some of those Infrared Camera Conversion web sites can be seen at this link.

My opinion is that the colour casts resulting from the use of ND Filters are a simply visible light colour casts caused by the ND filters themselves.

I haven't purchased any ND Filters for a long time, so I haven't had a chance to try any newer technology ND filters. But, according to the Cokin website, their newly released Nuances range of ND filters are designed to "stack without colour casts". Sort of suggests that Cokin are aware that earlier versions of their ND filters caused nasty colour casts.....

The last point is that if you are using a Graduated ND Filter, then you'll find it nearly impossible to remove any colour cast, either using manual white balance, or during post-processing, as the colour cast will vary in colour and intensity in proportion with the Graduation in the ND Filter. The only solution that I know of for eliminating colour casts when using graduated ND Filters is to buy a filter that gives you colour casts that you can live with.

Hope that info helps.

Regards,

RoyG

Seems a bit too vivid a blue (aqua?)

Fair comment, I was working off my memory of the ocean colours around parts of Norfolk Island when we were there last November. The adjustments I made were using Curves adjustment and sharpening using the high pass filter which emphasised the aqua tones in particular. The aqua can be cut back with an adjustment to the hue and saturation as required.

Bob

Ok

I've found three simple methods (so far) of playing with the image in photoshop to rectify the colour cast.

This is the image (originally RAW and saved as is or was, ie no processing)

test II 2918.jpg

1st play
using the RAW editor in Photoshop
Simply adjust the white balance. for this example I adjusted the temperature to 3800 and the Tint to -64
Why did I use these figures? Well it was simply someone's idea which worked on their photos so I thought I'd start there. I probably need to play a little more but you get the idea.

result below

Test II Raw processing_2918.jpg

2nd play
using the JPEG image
make a duplicate layer
using the duplicate layer, go to the Filter menu, Blur and select average.
Create a curves layer, on this layer, select the grey eyedropper, click anywhere on the image field (it should change colour to mid grey)
make the second layer transparent (ie what was the duplicate layer)
viola the corrected image

result

test II average curves 2918.jpg

3rd Play
This method requires using a similar image (ie no colour cast) as a reference/control(?)
Using the reference image select the colour sampler tool and choose a neutral grey point and take a reading noting the RGB values
Back to the colour cast image again using the colour sampler tool choose the same point and take a reading and again note the RGB values.
Create a curves layer, select the red curve, select a point on the curve (if you don't do this it will turn out wonky) put the red reference value into the output box and the red colour cast result into the input box. Do the same process for the green and blue curves using their respective values.
Given my images are not identical I chose a point on the little rock between the two larger rocks and winged it from there.

result (and my control image)
test II individual curves 2918.jpg test II control_2902.jpg

here's a link to a video where I came across this idea. Interestingly he uses some of the more expensive filters in his examples - a B&W and a Lee.

Comments

These all seem to work and get somewhere close to what might have been without the ND filters influence.
the second method is by far the simplest to use. I imagine that it will really only work properly on a well balanced scene.
the Raw process is also fairly simple in execution but then again I cheated and used some other person's values.
I suppose with same playing around you could come up with your own values. The RAW process will only work on RAW images, so if you forget (as I did) to set for RAW this process is probably unavailable. (query - can you load JPEGs into the RAW editor?)
All my results seem darker than the control but this is easily explained by me most likely underexposing my shots - I aimed to get reasonable histograms when I was taking the images whereas the control was most likely aperture priority.

There are most likely other simple methods out there. In fact an Anne McKinnell suggests the plugin Topaz Restyle. A link to her comments here

RoyG

Thanks muchly for your comments.
I agree that setting the white balance pre-shoot is probably best. Even then I have noticed a couple of comments contrary during my web searches.
But whilst I had read something about ND colour cast in the past it hadn't stayed in the grey matter for this holiday and I really only noticed it when I saw two photos side by side, one with ND filters and one without. I didn't expect it to be this bad or obvious.
So I thought I'd raise it here so others were aware of what I found and what could (maybe) be done to save the photos.

I've heard that a polystyrene cup over the end of the lens is good for setting up a manual white balance.

ISawdustmaker1.jpg think my version of Photoshop (Photoshop cc 2015) is much easier to use than your earlier version when using curves adjustment to remove colour casts.

I have circled the eye dropper tool in red - when you click on it the cursor turns to an eye dropper and you select an area on the image that you wish to correct.
When doing your image I first selected the red channel and using the eyedropper selected the red on the rock in the bottom left.Once selected drag the cursor up or down to change the curve. I then repeated the process for the blue and green channels, but in these cases selected a lighter area on the top of the rocks.

It is pretty much a visual thing, just adjust the curves until the cast is removed. You may end up, as I did, with areas which require local adjustment to correct Hue, Saturation or Luminance, but these are not colour casts and further adjustments in Curves is unlikely to produce any better result.

In the screen capture I hope you will be able to see the curves for each of the channels which I ended up with.

Bob

Yeah it seems a bit different to this version

Maybe I could install the more recent version on this laptop
It's school hols at the moment and I get little access to the desktop with the updated version of photoshop on it

I suppose another issue is that I really should learn to use photoshop properly

this exercise has taught me a fair bit in itself

Originally Posted by Sawdust Maker

Yeah it seems a bit different to this version

Maybe I could install the more recent version on this laptop
It's school hols at the moment and I get little access to the desktop with the updated version of photoshop on it

I suppose another issue is that I really should learn to use photoshop properly

this exercise has taught me a fair bit in itself

Adobe have recently announced a special deal for photographers for their Creative Cloud 2015 version of Lightroom and Photoshop. Compare with their previous standalone prices I think this pretty good - I recently signed up.

If you are looking for good FREE Photoshop and Lightroom tutorials, this lady is not only the senior evangelist for Adobe, she is a great teacher - have a look here

Bob

Originally Posted by Oldgreybeard

...

If you are looking for good FREE Photoshop and Lightroom tutorials, this lady is not only the senior evangelist for Adobe, she is a great teacher - have a look here

Bob

Thanks heaps

Here's my quick, no thought way to fix white balance issues with Photoshop -

Go to Image - Adjustment - Levels (or Control l )
In the dialogue box, select the White Point eyedropper (far right eyedropper)
Click this eyedropper on the region that is the most overexposed/oversaturated (ie what should really be your white reference point)
Bingo! Colour cast gone.

I clicked on one of the highlights in the surf to make the adjustment below.


Quick photoshoppery.jpg

This is a commonly used approach and can also be used in Curves and the White Balance control in Lightroom. I tend not to use it as it is dependent upon there being a "true white point" in the image. If the area you sample should be an "off white" the result may not be as expected. While it works well in most cases with RAW files, I have had less success with Jpeg images, but that could be just me.

In my photography, I tend to include selective tonal adjustment(s) in my processing workflow for most images, therefore it is easier to just open the Curves adjustment panel and do both adjustments there.

Bob

Mr Splinter
thanks for you input.

Unfortunately I think your approach is still leaving a colour cast, albeit a bit less than the original

I think this is closer to natural

Test ND pprocess_2930.jpg

Originally Posted by Sawdust Maker

RoyG


I've heard that a polystyrene cup over the end of the lens is good for setting up a manual white balance.

Within reason they do, but for really finicky stuff, not quite right.

In another life I printed colour negative and colour transparency films (slide film) in an industrial environment. Plus I also ran my personal films, both colour negative and colour transparency in my home darkroom. I developed the films and also printed the colour prints, mainly colour negative. With film, getting correct colour is just the same as with electronic cameras, it is just a bit more difficult time wise and hip pocket wise with film.

The Wallace Expo Disc, was invented by George Wallace as a means of turning the light meter inside your camera from a reflected light meter, to an incident light meter. An incident light meter measures the amount of light falling onto the subject, or the dome/disc of the meter.

Extremely critical manufacturing was employed from the start, recipients of the Wallace Expo Disc received a calibration certificate to show the deviation of colour for each Expo Disc, and what filtration had been inserted in a sandwich arrangement between the filters themselves and the inner and outer Expo Disc surfaces to get each disc correct.

As a result the Expo Disc rendered a perfect neutral grey light to the camera meter, this was a revelation to the photographers who used one of these Expo Discs and made life in the lab so much easier and extremely accurate for those who had to print from the films.

Essentially one placed the disc onto the camera, pointed the camera towards the light source (more or less) then with the camera metering system set to automatic, one frame of film was exposed, this became the neutral grey frame for light density measuring and colour setting. One simply used that frame in the enlarger, placed the colour analyser probe on the base board then changed the colour head controls to get a perfectly correct colour balance. It really did work and still does.

George passed away about 12-13 years ago, his daughter (I think) realised that cameras with electronic sensors instead of film as the medium, had more problems getting correct colour balance. The terminology used by the electronic camera people is white balance, but in effect they are measuring for a neutral grey, as far as I understand it.

To get a perfect white balance with a friend and his Olympus electronic camera, we placed one of my Wallace Expo Discs on the end of his lens, he then did a white balance exposure (or whatever it is called) then proceeded to take a few shots. The shots were of a mixed light sourced scene, there was Tungsten, natural light and Sodium lights. Even I was amazed at the immediate difference, he was flabbergasted. The upshot was that he purchased his own Expo Disc (Wallace is now dropped from the name) and has been using that same expo disc for about 7-8 years with great success.

I have had my two Wallace Expo Discs since around 1988-90, whenever I shoot colour film I still use them to get my darkroom neutral grey negative frame.

There are many references to the Wallace Expo Disc and it successor, here is one:

Expodisc

Mick.