Lenses and IR 'hotspots'

[jimcollum]

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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

[cliffordjim]

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.

[jimcollum]

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

[cliffordjim]

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?

[farnz]

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.

[fefe]

I get a IR hotspot on the 35 cron Asph too.

[gravastar]

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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.

[jaapv]

Probably the best lens for IR is the Summaron 35/3.5.