jimcollum Posted August 31, 2007 Share #1 Posted August 31, 2007 Advertisement (gone after registration) When shooting color IR, some lenses will give a center 'hotspot', which is an area of lower contrast that gets worse as the lens is stopped down. Looking at the channel information, it turns out it's primarily in the blue channel, so it doesn't usually degrade b/w IR images. For those doing color IR work, i thought i'd start a thread that documents the lens type, and whether they have the hotspot. no hotspot Leica 21mm 2.8 pre-asph (E60 with pins) Leica 75mm 1.4 lux Zeiss 28mm 2.8 biogon hotspot Zeiss 21mm 2.8 biogon Zeiss 50mm 1.5 sonnar CV 15mm Heliar if anyone else can add info to the list, please do jim Link to post Share on other sites More sharing options...
Advertisement Posted August 31, 2007 Posted August 31, 2007 Hi jimcollum, Take a look here Lenses and IR 'hotspots'. I'm sure you'll find what you were looking for!
cliffordjim Posted August 31, 2007 Share #2 Posted August 31, 2007 Thanks Jim. I have been using my CV15 with a 89B IR filter and when I tried some tests at f22 there was a hotspot in the middle of the frame even in B+W. I couldn't figure out what was going on... be it the lens, filter or sensor. With this filter I usually shoot at f4.5 - f8 to keep it handheld so it is not usually a problem. Now I know to avoid going any higher. Link to post Share on other sites More sharing options...
jimcollum Posted August 31, 2007 Author Share #3 Posted August 31, 2007 Thanks Jim. I have been using my CV15 with a 89B IR filter and when I tried some tests at f22 there was a hotspot in the middle of the frame even in B+W. I couldn't figure out what was going on... be it the lens, filter or sensor. With this filter I usually shoot at f4.5 - f8 to keep it handheld so it is not usually a problem. Now I know to avoid going any higher. yea.. at least with the 15, you still get plenty of DOF at f8 this isn't strictly a Leica issue. you'lll find it with any other camera as well (70-200 2.8 IS for the Canon has one, 90T/S doesn't) How were you converting in B/W? if you go thru the channels in a color mode ( before conversion), you'll see each channel has more/less of that hot spot, and you might be able to minimize it more at smaller apertures jim Link to post Share on other sites More sharing options...
cliffordjim Posted August 31, 2007 Share #4 Posted August 31, 2007 That is very helpful with the color channels. Just did a test with my CV 28 Ultron and there is NO hotspot with a Hoya 072 filter at f 22 (this is converting to B+W as this filter doesn't allow much/if any of the natural daylight spectrum). I still have to acquire a 092 filter to try some hybrid shots... One other question concerning focus points. I am focusing on the subject and then turning the focus cylinder to the right 5.6 mark. It doesn't always prove successful. Any tips? Link to post Share on other sites More sharing options...
farnz Posted August 31, 2007 Share #5 Posted August 31, 2007 Hotspots on: 28 Elmarit ASPH 35 Cron ASPH 90 Elmarit pre-asph. However I noticed that the hotspot appears to be linked to aperture, ie the larger the aperture the larger and less-pronounced the hotspot. At f22 for all of the above lenses I see a small bright spot in the centre of all images. Then it occurred to me that it actually might be vignetting rather than a hot spot. I now shoot wide open, which minimises the effect, and use vignetting control in Lightroom or ACR to equalise (increase) the exposure at the edges by about 60% (2/3 stop?). My experience comes from using 072, 093, 89A and IRay infrared filters and all exhibit the same behaviour. Pete. Link to post Share on other sites More sharing options...
fefe Posted September 1, 2007 Share #6 Posted September 1, 2007 I get a IR hotspot on the 35 cron Asph too. Link to post Share on other sites More sharing options...
gravastar Posted September 2, 2007 Share #7 Posted September 2, 2007 Advertisement (gone after registration) 28/2.0 Summicron shows a hotspot of different color to the rest of the image, Heliopan RG715 filter. I've skewed the colors so you can see the difference. Bob. Link to post Share on other sites More sharing options...
jaapv Posted September 2, 2007 Share #8 Posted September 2, 2007 Probably the best lens for IR is the Summaron 35/3.5. Link to post Share on other sites More sharing options...
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