Archived posting to the Leica Users Group, 2006/11/15
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]I have been conducting some experiments to understand the issue of IR contamination and filtration in digital cameras. I have posted some results at http://gallery.leica-users.org/v/MarkEDavison/M8infrared/ to show some comparative examples of IR filtration. I have included a Leica M8 shot where the filtration was done by applying a modified Phase One profile which is supposed to correct blacks under tungsten light. My conclusion is that the software filtration works surprising well on the blacks that are IR contaminated, but hardly affects the other contaminated colors at all, but you should look and come to your own conclusion. (Note: this is a cross post. There is an on-going discussion of software profile methods for IR filtration at http://www.leica-camera-user.com/digital-forum/9178-magenta-work-around-capture-one-workflow.html or http://tinyurl.com/y6cjx2 which gives more detail on the origins of the modified profile I used on the Leica M8 image.) Here's a description of the shots: The scene was shot with incandescent illumination from ordinary lightbulbs. The camera white balances were set to 2800 K except for the Epson R-D1, which was set to incandescent. (The Epson R-D1 does not allow you to set white balance in Kelvin.) The first example is the D200, which is very insensitive to IR. The colors in the first D200 photograph are a very accurate rendition of the way the scene appears to my eye. Take special note of the maroon and green pile blankets, the black Leica M lens, and the black pile jacket at the bottom of the photograph. The second photograph shows the D200 with IR cut filtration (via a Tiffen standard hot mirror filter). There is hardly any visible change in the colors. The third photograph is with the D200 and the IR pass filter (a Hoya R72), taken at the same exposure as the first two photographs. There is no visble IR at all at this exposure. The photographs continue in sequence for 3 more cameras: the Leica M8, the Epson R-D1 and the Nikon D2h. For each camera I show an image with no filtration, with IR cut, and IR pass, all at the same exposure. Note how much IR is recorded by the M8--it is the most IR sensitive of all the cameras. Note also how the IR contamination has completely bleached the green out of the green pile blanket, how the maroon blanket has shifted color, how there is a purple sheen on the barrel of the Leica lens, and how the black pile jacket has turned dark purple. The shot with IR cut filtration knocks down the purple sheen on the lens barrel, improves color saturation and contrast overall, but doesn't quite return the green pile blanket to the correct color. Note also that there was a glowing IR reflection from the "black" pile jacket on the bottom of the apple which is taken out by the IR filtration. Similar comments apply to the Nikon D2h, but the infrared sensitivity is weaker and the corrections with the IR cut filter look better to my eye. The Leica M8 shot which has been filtered by application of the profile Jamie Roberts supplied does have better blacks in the anodized aluminum objects, but the green of the pile blanket at the top has not been restored, and in general the colors of the pile fabrics look faded. More subtly, the IR reflection on the bottom of the apple has not been removed. My point is that IR contamination doesn't just affect synthetic black objects and dark anodized aluminum--it contaminates practially all synthetic pile fabrics that I can find in my house. So you can't just hunt down dark purple things and change their color. (By the way, if you shoot social events and students in classrooms in Seattle in the winter, you are going to encounter a lot of pile jackets and incandescent light, so this is not some obscure rare combination, at least for my use.) The Tiffen hot mirror filter which I used in these experiments is obviously too weak to restore all the colors (especially for the green pile fabric), so I have a better UV/IR cut filter on order--a Heliopan 8152. Some philosophical notes: I have been using these other cameras for some time now, and I always had more trouble getting indoor shots from the R-D1 and D2h to look "right". There was some indescernible purpleness about these photographs that reminded me of faded advertising posters. In comparison the D200 photographs looked rich and vibrant. Now I understand the source of the problem. I'll be using the IR cut filters on the other cameras when the situation warrants. I have also noticed that foliage never looks right in the IR sensitive cameras--it's always a funny spring green. I will wager anything that this comes from high IR reflections in plant leaves, even under daylight. One huge difference between a film and digital camera is that the spectral sensitivity functions of the digital camera R, G, and B channels are fixed forever. You can't change the spectral sensitivities by simply loading a different brand or type of film. Since this is the case, I think it would make life easier for serious photographers if digital camera manufacturers would measure the spectral sensitivity functions of their cameras and publish them, just as Kodak does with their films. With such a graph you can tell at a glance if the camera has significant IR or UV sensitivity, and you can pick your lens filters accordingly. Mark Davison