wlaidlaw Posted January 17, 2010 Share #1 Posted January 17, 2010 Advertisement (gone after registration) I have recently been trying to take infra red photographs with a 280mm Telyt + Visoflex III + M8 and 720 nano-metre IR filter. Whereas I have been pretty successful with shorter lenses on IR by using the rule of thumb; focus correctly and then back off towards close focus a touch, this is not accurate enough with the very shallow DOF on the Telyt. I searched on the internet, and to my surprise, was not able to come up with anything approaching a calculated focus correction formula. The best I could find was a way of making a correction table arbitrarily and by trial and error for one particular lens and filter, and then how to apply it to different distances. Any mathematicians or physicists out there who can given me a formula, hopefully one that can be transposed into Excel so that I can make a chart/table for different lenses/distances. Wilson Link to post Share on other sites More sharing options...
Advertisement Posted January 17, 2010 Posted January 17, 2010 Hi wlaidlaw, Take a look here Focusing formula for infrared on the M8. I'm sure you'll find what you were looking for!
ho_co Posted January 17, 2010 Share #2 Posted January 17, 2010 Wilson-- Not what you're hoping for, but Leica's recommended procedure: http://www.l-camera-forum.com/leica-forum/leica-m8-forum/72683-help-please-ir.html#post755244 Link to post Share on other sites More sharing options...
wlaidlaw Posted January 17, 2010 Author Share #3 Posted January 17, 2010 Wilson--Not what you're hoping for, but Leica's recommended procedure: http://www.l-camera-forum.com/leica-forum/leica-m8-forum/72683-help-please-ir.html#post755244 Howard, Many thanks for that link. I did have a look at it before I posted this thread I was really looking for something nearer to a scientific formula, taking the clipping frequency of the filter into account. I have tried modifying various parameters in the spreadsheet which is linked in that thread but sadly it is locked and password protected. Obviously the larger the nM number of the filter, the greater the correction factor will need to be. The big unknowable of course, it the extent of Apo-chromaticiy of the lens, as the greater this is, the less correction that will be needed. Also focus testing at 84 meters is not the easiest (300 X 280). Wilson Link to post Share on other sites More sharing options...
giordano Posted January 17, 2010 Share #4 Posted January 17, 2010 I don't think there's much chance of finding a formula that works for all lenses because every design has its own place on the spectrum between perfect colour correction and total mush. In real life this means IR focus correction of zero for super-apochromats (such as the CoastalOpt lenses that cover the range from IR to UV), zero or nearly for visible-light apochromats such as Leica APO lenses, and different though small amounts for other lenses. I imagine that the worst are simple achromats such as the old f/6.8 Telyts. Wilson, I've sent you a PM about that spreadsheet. Link to post Share on other sites More sharing options...
jaapv Posted January 17, 2010 Share #5 Posted January 17, 2010 In general, the two-stops down DOF marker will provide a good starting point- on lenses that have a DOF scale The best thing to do is to stick a strip of millimeter paper on the lens barrel and create your own distance scale for IR by chimping. Link to post Share on other sites More sharing options...
ho_co Posted January 18, 2010 Share #6 Posted January 18, 2010 Wilson-- I figured you had probably seen that post, but it's the only "Leica-official" pronouncement I know on the topic. Remember, at one time lens manufacturers had only the choice of "crown glass" or "flint glass." Like John, I think that with today's glass types, the only way you'd be able to calculate accurately would be to feed the optical formula into the spreadsheet. OT--Remember the LFI article on using the M8's IR sensitivity for false-color special effects? They suggested doing a focus series and using the LCD to check for the sharpest image, as Jaap suggested. IIRC, in one series with the 28-35-50 Tri-Elmar, I found that best 'near-infinity' focus point was about 3 m. Working with a more limited range of wavelengths as you're doing should simplify the process. Good luck. I'll keep fingers crossed for a formula or even just a rule-of-thumb. Link to post Share on other sites More sharing options...
SJP Posted January 25, 2010 Share #7 Posted January 25, 2010 Advertisement (gone after registration) I have a IR focus calculator spreadsheet that I can send. I used to have it on my website until google did some automatic conversion that fouled everything up. If you PM me or send to spicken59 at gmail.com I will mail it to you. Hopefully I can transfer my former website files to another host in due course. Link to post Share on other sites More sharing options...
russell c. greenberg Posted January 25, 2010 Share #8 Posted January 25, 2010 I have a IR focus calculator spreadsheet that I can send. I used to have it on my website until google did some automatic conversion that fouled everything up. If you PM me or send to spicken59 at gmail.com I will mail it to you. Hopefully I can transfer my former website files to another host in due course. Stephen, I would greatly appreciate a copy of your IR focus calculator spreadsheet also. You can send me a PM or e-mail me at rgreenbergphoto@aol.com. I also have trouble finding a reliable focus point, except with my 50 mm summicron (current version). I set the infinity point to the right DOF marker at F4, instead of the center mark on the DOF scale. With this lens, this works for me. I am not so successful with finding this point with the other lenses in my arsenal, so a IR focus calculator would be an effective tool for me. Thanks in advance Russell:) Link to post Share on other sites More sharing options...
giordano Posted January 25, 2010 Share #9 Posted January 25, 2010 In general, the two-stops down DOF marker will provide a good starting point- on lenses that have a DOF scale The best thing to do is to stick a strip of millimeter paper on the lens barrel and create your own distance scale for IR by chimping. That seems like the right starting point for lenses around 50mm focal length, but longer non-APO lenses are likely to need more, maybe much more. Or one can avoid the problem entirely by using one of a super-apochromat that maintains the same plane of focus from the near UV right through the visible range and well into the IR. A few of these are made, mainly for scientific and forensic-science use - but none in Leica mount that I know of. Link to post Share on other sites More sharing options...
brianv Posted January 26, 2010 Share #10 Posted January 26, 2010 APOchromat lenses are "good Enough" for near IR. I have a Pentax 85/4.5 Ultra-Achromat, and it extends well past what was required for a Silicon based CCD. It was originally used in a Spectrometer. They are not cheap. Mirror lenses also work well for Infrared. If you can get a T-Mount Mirror lens, that will work and not be too costly. I've used a 500mm F8 Reflex-Nikkor with my DCS200ir, without need to correct the focus for IR. 300mm F5.6 Mirror lenses are under $100, and typically use 67mm filters. Some have rear filter mounts. Macro lenses tend to be highly color corrected. I use my Micro-Nikkor 55/2.8 wide-open without correction for IR. Link to post Share on other sites More sharing options...
SJP Posted January 28, 2010 Share #11 Posted January 28, 2010 Found a free hosting site so these documents are available again for those interested. DOF stuff - pdf file DOF calculator + IR focus - excel spreadsheet enjoy;) Link to post Share on other sites More sharing options...
Woody Campbell Posted January 31, 2010 Share #12 Posted January 31, 2010 I shoot a great deal of IR. I generally use the 5.6 depth of field mark as an IR index - it's a pretty good approximation. Link to post Share on other sites More sharing options...
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