[Leica] Some IR filtration experiments

[philippe.orlent at pandora.be (Philippe Orlent)]

Thanks for both posts. This is really in depth and very informative.
Philippe



Op 15-nov-06, om 19:45 heeft MARK DAVISON het volgende geschreven:

> 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
>
>
>
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