Colour Temperature setting for photographing slides

[guyburns]

I am testing how to photograph Kodachrome slides. A friend has a Kodak carousel projector, lens removed, and in place of the lens he has a Nikon D700 (with macro lens) mounted on a plate and looking straight into the projector focussed on the slide. Works pretty well, but I'm not sure what colour temperature to set in the camera.

Our first try was to set the camera to 'tungsten' (naively thinking that 'tungsten' was the most suitable setting, given the projector's tungsten lamp). Silly me – I forgot about Kodachrome's blue cast, designed to counteract the yellow tungsten light when projected. As a result, I was overriding Kodachrome's colour correction: yellowish projector light through a blueish Kodachrome slide (which should give good colour), but which was then 'tungsten' corrected in camera to give a blue image.

Then I allowed the camera to set the colour temperature automatically, by using the projector's lamp (with no slide) as the light source. Images were still bluish.

Given that Kodachrome was designed with a blue cast to counteract the yellowish light from a projector, if you try and photograph a slide looking directly at the slide as it is being projected, what colour temperature should you set in the camera?

[digitaldog]

Given that Kodachrome was designed with a blue cast to counteract the yellowish light from a projector, if you try and photograph a slide looking directly at the slide as it is being projected, what colour temperature should you set in the camera?

I’d shoot raw so it wouldn’t matter, then I’d produce a custom WB in my raw converter (Lightroom/ACR) and build a preset. Apply to all, done.

[crames]

Given that Kodachrome was designed with a blue cast to counteract the yellowish light from a projector, if you try and photograph a slide looking directly at the slide as it is being projected, what colour temperature should you set in the camera?

It would be easiest to use the same color temperature as the tungsten bulb. That should give reasonably white whites. Remember that the blue in Kodachrome is non-linear and not correctable with a simple white balance, so to get the rest of the tones neutral at the same time as the whites, you have to apply different gammas to the color channels.

In effect, the color balance varies throughout the tone range. That's why Kodachrome's blue tint is tricky to get rid of, whether you use a camera or a scanner!

[Ellis Vener]

If you can locate a Kodachrome of a  gray card  or just one containing  a subject you know should be neutral gray that should help

Even better: if you can also also find a K'chrome of a ColorChecker you could then  use that to build a custom calibration "profile"   using Adobe DNG Profile Editor or Xrite's ColorChecker Passport

[guyburns]

Thanks for the suggestions. Even though I mentioned blue-cast, I didn't mean that I am trying to get rid of it at the photographing stage. That's not possible; but what I would like to do is obtain an image that is neutral. i.e. what is coming from the slide is captured by the camera without any colour correction. Maybe I've got this all wrong, but if I set Tungsten I'll get a certain cast; set Fluoro and I'll get a different cast. In this case, I want to try and capture the exact colours that are being projected.

One problem is: it's not my camera and the owner seems to know less about it than I do. Every time we do a test, I'm the one reading the manual. Is it possible for an upmarket camera such as the D700 to capture an image in such a way that there is no colour correction?

Even better: if you can also also find a K'chrome of a ColorChecker you could then  use that to build a custom calibration "profile"   using Adobe DNG Profile Editor or Xrite's ColorChecker Passport

I don't think this camera can be profiled to give acceptable results. My understanding says that profiling requires a consistent exposure, and because of limitations with the D700, this may not be possible. I have determined the dynamic range of the camera to be about 8.5 stops (about the same that dpreview.com says), so during initial testing I set the exposure at a certain value in the middle of that range (IS0 200, F11, 1/20 sec). But with my dozen or so reference slides, all of them a challenge to scan and most of them with a greater dynamic range than 8.5, I find that I get clipping at the extremes. And the D700 gives ugly results when blacks are clipped. Boy do they clip. I seem to get a better image by sacrificing the whites so that the blacks are not completely solid. I discovered this by comparing fixed exposure with auto exposure. For some of the troublesome slides, auto gave better results. The images looked better, even though some the highlight detail had been lost. They didn't have that scanned look (what I call the "chocolate look") when the contrast is too high.

So given that I can't use constant exposure to get the best results, I don't think profiling is an option.

The only reason I am playing around with the D700 is that it can peer into the shadows to a greater extent than the Coolscans or the Epson V700. Meaning: scan a slide with deep shadow detail, then photograph the same slide but overexpose it, and the detail will be captured by the camera, but not to the same extent by the scanners. The difference is quite startling.

One way around this problem of the camera not having enough dynamic range might be to use HDR – capture several images with the camera and combine them. However, my very first attempt introduced serious colour distortions in some areas, but that was only using two exposures. Three or four might work better.

Keep the ideas coming. Profiling is probably a non-starter, but other suggestions gratefully accepted. Next time I visit my friend with the D700, I hope to be able to set it up to capture colour without corrections. Any suggestions how to do that?

[Steve House]

The blue cast is introduced to remove the tungsten warmth to correct the white to one that matches the 'white' of the subject under the daylight illumination the film is balanced for.  It is intended to shift the warmth white of the tungsten in the direction of blue, ie, daylight.  After it passes through the slide, the 'tungsten white' becomes a 'daylight white.'  So set the camera white balance to daylight and see if that produces an acceptable result.

As has been mentioned, shooting raw lets you work with it in post and should simplfy the process.

[crames]

The whitest-white of Kodachrome, DMIN, doesn't have the blue tint. The three color layers have the same densities there, so the color of the projector light gets through. If your camera is color balanced to the projector lamp, then at least you will have the whites close to neutral.

With the whites taken care of, it's then just a matter of getting the mid and dark tones neutralized. If you use curves or gamma adjustments to align the mid and dark tones, it can be done without screwing up the already-neutralized whites.