How does the image from the leaf back differ from my mamiya ZD back with IR filter ( the optional IR filter that allows to make IR images with a ZD back). I really love IR, do a lot with my Kodak Proback without any filter but was wondering...
My mamiya ZD-IR back does not get out that often as i relly love the square kodak format.
Me too (Kodak Proback 645M & Mamiya 645AFD). That's why I had to ask exactly what the design is, as I was wondering just how special or novel this Leaf dual IR/vis camera back is. It's been 10 years since the original Kodak Pro Back for Hasselblad/RZ, with the click-on, click-off removable IR filter...(My Kodak back tethers fine, BTW.)
This summer I was hoping to go one better than the Leaf, and match the Mamiya ZD. As Bart has said, the best place optically for the IR blocking/passing dielectric filters is back near the sensor. (Having the IR-passing filter at the back also means that you can
SEE through the viewfinder when you are shooting in IR). I have a spare (damaged coatings) rear IR-blocking filter for my Proback, which I was going to have stripped to bare glass, then recoated as an IR-passing filter with an 820nm cutoff, so that I would have equal IR response in all pixels (this being a feature of the Kodak chips), and the image would not need de-Bayering. I never got around to it unfortunately, but still intend to, once I find a suitable optical house to do the work.
No doubt the Leaf 33MP back gives a better visible light file...at a price of course.
In the IR, I'm not sure
any of the Leafs listed just now by Steve will be outstanding. Let me explain why.
There is an old joke here in Ireland: a lost tourist sees a farmer coming towards him, so he stops his car to ask for directions to his destination. The farmer replies: "Well first of all, if I was going there, I wouldn't start from here!".
In the same vein, if I was going to make a back for IR, I wouldn't start from here; here being a Leaf, Dalsa-chipped back (or for that matter, the Mamiya ZD, Dalsa-chipped back).
The attached graphs show the spectral response of the Dalsa sensor in the Leaf 33MP back; and a carefully overlaid comparison by me of the Dalsa sensor in the ZD 22MP back and the Kodak sensor in the Kodak 16MP Proback series (Dalsa traces are thin R/G/B lines; Kodak ones are thick R/G/B lines).
Both the Dalsa sensors exhibit
- near-zero response above 700nm, from the Green and Blue pixels
- the 1-in-4 Red pixels, which are carrying almost the entire IR effort, are themselves on a sharp decline to low response.
- there might possibly be some recovery past 800nm, but Dalsa didn't see fit to show it, do the rational conclusion is that it continues to decline towards zero between 800nm and 900nm.
The Kodak sensor (and its other Kodak MFD brethren are all pretty much the same) exhibits
- a resurgence in response above 700nm, from the Green and Blue pixels; they soon converge up with the Red
- a response in Red which is systematically higher than the Dalsa, and which (along with the Green and Blue) doesn't begin to decline until over 100 nm
further into the IR.
- equally high response in Red, Green and Blue pixels above 820nm - the Bayer pigments become completely transparent - which makes a perfect B&W monochromatic IR image possible, without interpolation!
So the Kodak is a high-ISO back in the IR. You can handhold IR pics easily, and compose them in the normal visible-light viewfinder. The degree of luminance interpolation required is far lower (indeed, none at all past 820nm), so the per-pixel sharpness can be much higher.
What we need are more
Kodak-sensored backs to be built in the way that this Leaf back has been built.
Ray
