MFDB for Infrared photography

[chiek]

Hi all.
I'm trying to my personal work, MFDB for Infrared photography.

as We know, 1st needed is remove factory IR filter. and attatch black IR filter. (87, 89 etc..)

I tried my own MFDBs, Leaf, Phase and now Hassy CF back.

Leaf back is best choice for IR photo, It can easy removed, only unscrew bolts.

but IR filter from phase and Hassy backs are glued by silicon... so I'm worry about position of CCD (focusing)



I've tried easier DSLRS even over 12MP, but I needs more detail and adjustments.

I'm trying to buy cheap older MFDBs for IR photo only.


Have anyone tried this? Please share your exprience.

[bradleygibson]

Phase will build you a back without the IR filter (IR-transparent glass in its place) if you request it.  You will probably need to wait a bit longer to receive your back and there may be a fee involved.

Without the IR filter, it's quite sensitive to IR.

B+W makes clear IR-blocking filters (#486, if memory serves) you can put on your lenses if you wish to do visible light photography.

You probably know all about IR focusing technique, so you'd be good to go!

I had one a number of years ago, but the #486 filters weren't out yet (the M8 hadn't shipped --pehaps I was simply unaware).  The only options I was aware of for visible light photography were using IR-blocking filters that had a color cast, and it wasn't practical--in addition to needing color correction, they ate precious light.

HTH,
-Brad

[MartinWidmann]

As far as I know, with every digital back from Sinar you can remove the filter with a Screwdriver. But be aware, that the sensor is more prone to scratches than the filter glas. You can have the IF filter replaced by a pure glas in the factory.

Martin

[Jack Flesher]

My personal .02 after experimenting with a wide variety of IR solutions.  The main issue is the relatively wide breadth of the IR band, and the relative inadequacy of conventional optic's abilities to keep that band all focusing to one point on the image.  IOW, we need a IR band "APO" lens to take advantage of current sensor resolution capabilities to render an image sharp enough to stand up to current sensors, and most readily available DSLR or MF camera lenses simply cannot.  So at the end of the day the best you can get is a high MP count but overall soft image.  With this in mind, I would suggest that somewhere around 12-16MP is about the maximum practical resolution point for IR capture.  So a converted one or two generation DSLR makes a superb dedicated IR cam.

The second issue is focus point shift in the IR band.  It's relatively easy to adjust for in a DSLR, but significantly more difficult to nail reliably with a MFDB -- you or your camera cannot "see" it on the GG.  Alternatively, it's super easy with a camera that uses inter-pixel sensor contrast at the sensor to confirm and auto-focus with, like most micro 4/3rds cameras do.  Whch is why that was my most recent choice.  I had a Panasonic GF1 converted to IR (715nm) and it exposes, WB's and AF's dead on with the included kit lens or any other lens I mount on it. (Full details with a lot of IR image examples here: http://forum.getdpi.com/forum/showthread.php?t=13852 )

Finally, some other considerations.  First is exact IR cutoff point choice. Do you want only monochrome IR or do you also want some false color capability, or only false color capability?  Next, visible light lenses, especially those with aspheric elements do often "hot spot" when used for IR capture, so it's best to research this too before you make your final system conversion choice.

Cheers,

[darylgo]

A Phase One back with a B+W 092 filter works well.  The back is not modified and exposures are long but a tripod solves that issue.  This won't work for all situations but for landscape shooting it gives great results. 

[ondebanks]

I'm trying to buy cheap older MFDBs for IR photo only.

Have anyone tried this? Please share your exprience.

I do it with my old Kodak ProBack 645M & Mamiya AFD. This back is ideal because the IR filter is deliberately user removable. It clicks on and off without tools. So one moment you're shooting with normal visible light and 30 seconds later you can be shooting IR. Then switch back to the visible spectrum in another 30 seconds if you wish. Two cameras in one. Magnifico!

To be honest I haven't done a great deal of terrestrial, pictorial IR shooting with it. I am more interested in the massive gain in signal that removing the filter gives me in astrophotography. Compare these two fixed-camera shots of Sagittarius over the French campsite where we holidayed this summer (on different nights but from almost the same camera position and pointing). Both taken with a Mamiya 80/1.9 lens at f1.9. The IR shot was actually the shorter one (6 seconds, vs. 8 seconds for the visible light) but despite that it has around 2.5 stops more signal amd the Milky Way is much better defined with bright star clouds and dark molecular/dust clouds. Another nice thing about this back is that even with the IR filter on, nebulae are properly reddish, which means it transmits the 656nm Hydrogen line well - something that no stock DSLR does.

[Tomas Johanson]

I do it with my old Kodak ProBack 645M & Mamiya AFD. This back is ideal because the IR filter is deliberately user removable. It clicks on and off without tools. So one moment you're shooting with normal visible light and 30 seconds later you can be shooting IR. Then switch back to the visible spectrum in another 30 seconds if you wish. Two cameras in one. Magnifico!

Do you replace the IR-filter with another filter in front of the sensor or just put the right IR filter on the lens?

[ondebanks]

Do you replace the IR-filter with another filter in front of the sensor or just put the right IR filter on the lens?

Hi Tomas,

At the moment I don't use either; in the example I attached above I let the full visible + IR spectrum through. This is best for photon-starved astrophotography.

But I will probably soon pick up a lens-mounted filter, to play around with real deep-IR landscape stuff. Above 850nm, the RGB CFA filters are all equally transparent, so I should be able to do true B&W photography without even needing to do any Bayer interpolation in RAW processing (rather like the PhaseOne P45+ Achromatic back).

Ideally, one would have an IR filter cut to fit over the sensor, instead of over the lens, with the advantages of
- you can see what you're shooting
- no filter changes with lens changes.
This would be my preferred solution for daytime IR, and it's something I'll aim at doing in the longer term.

One might also think that, with the right refractive index and thickness, a new filter over the sensor would preserve the original optical path length, so that AF would still be accurate.
But since the AF is sensed in visible light while the image is formed in IR light, AF would not be accurate, unless the lens is apochromatic right down to the IR - as Jack has already alluded to above. Jack also makes a very good point about how cameras which use their imaging sensor for their AF, like the micro 4/3rds, inherently avoid this problem and reach their best IR focus.

Nor does one necessarily need a filter over the sensor to reach infinity with the lenses; you know that little red mark for IR infinity that every lens has? It moves the lens forward from the film/sensor. If instead one removes the filter from the front of the sensor, one achieves much the same change to optical path length, so that one then uses the normal visible-light infinity mark for IR.

The corollary is that in a completely unfiltered system, if the IR component is satisfactorily focused, the focus for the visible light component (actually a small fraction of the total light transmitted) is not optimal. I am of course aware of this softening of net focus.

However this doesn't concern me unduly because it is important to be aware of yet another effect which softens IR images - an irreducible effect which cannot even be fixed with the right filters, optical path lengths and uber-hyper-APO (or all-reflecting) lenses! It is the "Diffusion MTF issue", explained here -  http://bit.ly/9mCrvb . Basically, IR photons penetrate so deep into sensors designed for the visible spectrum that the photoelectrons they release wander (diffuse) all over the place and end up in the wrong pixels.

You know the way that, up to a point, things scale in imaging? - i.e. smaller sensors have fewer mm but the lenses designed for them tend to resolve more lp/mm, so quality scales reasonably well? Well, Diffusion MTF breaks this scaling in the IR. The degree of electron diffusion is more like a non-scalable absolute; in fact, if anything, it's a stronger and worse effect in small-pixel sensors. This is one of the main reasons why a larger (MF) sensor is best for IR photography.

Ray