Archived posting to the Leica Users Group, 2007/10/12

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Subject: [Leica] George: 28mm Summicron IR problem
From: s.dimitrov at charter.net (slobodan dimitrov)
Date: Fri Oct 12 18:30:18 2007
References: <7FA6647D-A36C-4A86-84B3-A4756DA8FB77@comcast.net> <673C7DEC-A9C1-490A-8F6F-821246CF27FC@comcast.net> <00db01c80d21$88b0d030$6101a8c0@jimnichols> <9E328A82-BC89-4914-AAA9-151AB6E16214@mac.com> <BFC87FAB-5BC4-4265-9C1E-61414D3C4227@comcast.net>

Here is an interesting artifact from the early Canon 18-55mm.
It is said the new one (II) is more resistant to such optical bravura.

http://www.laweekly.com/news/news/jimmys-final-gasps/632/

s.d.



On Oct 12, 2007, at 5:51 PM, Leonard Taupier wrote:

>
> 3. IR and Lenses
>
>
> In addition to the sensitivity of the sensor the quality of digtal  
> infrared depends on the characteristic of the lens.
>
> For example, the Canon EF-S 18-55mm f/3.5-5.6 typically produces a  
> pronounced hot spot as can be seen in the sample picture to the  
> left. The hot spot is a a result of internal reflections within the  
> lens produced by the lens' coatings. Some types of coating are not  
> transparent to near-infrared wavelengths.
>
>
>
> The above was taken from a web site dedicated to Infra Red digital  
> photography. Quite a few lenses are listed which produce these hot  
> spots. The author did not include Leica lenses in the recommended  
> or hot spot producing lists.
>
> Len
>
>
>
>
>
> On Oct 12, 2007, at 6:59 PM, Lottermoser George wrote:
>
>> Having attempted to deal with these "hot spotted" images in a  
>> number of different ways; I think it would be a poor use of time  
>> to attempt to correct it with blocking in the optical path.
>>
>> For a brief period I considered the possible use of a center  
>> weighted filter. But once you see how "hard" this hot spot is; and  
>> without contrast; it becomes fairly evident that any available  
>> center filter would not work. You'd have to go through mind  
>> numbing tests; looking at all sorts of various "donut"  
>> configurations in the image to come anywhere close to a "block"  
>> correction. And even if you could get there and correct for the  
>> "over exposure" you'd still have to deal with the lack of contrast  
>> in that  "spot."
>>
>> Regards,
>> George Lottermoser
>> george@imagist.com
>>
>>
>>
>> On Oct 12, 2007, at 5:45 PM, Jim Nichols wrote:
>>
>>> I've been following this thread, but since I have none of the  
>>> equipment involved, haven't said anything.  In looking at Tom's  
>>> comment, it occurred to me that it might be interesting to try a  
>>> blocking center spot applied to the filter, similar to the  
>>> approach used with one of the early Leica lenses.
>>
>>
>> _______________________________________________
>> Leica Users Group.
>> See http://leica-users.org/mailman/listinfo/lug for more information
>
>
> _______________________________________________
> Leica Users Group.
> See http://leica-users.org/mailman/listinfo/lug for more information


In reply to: Message from len-1 at comcast.net (Leonard Taupier) ([Leica] George: 28mm Summicron IR problem)
Message from tomschofield at comcast.net (Tom Schofield) ([Leica] George: 28mm Summicron IR problem)
Message from jhnichols at bellsouth.net (Jim Nichols) ([Leica] George: 28mm Summicron IR problem)
Message from imagist3 at mac.com (Lottermoser George) ([Leica] George: 28mm Summicron IR problem)
Message from len-1 at comcast.net (Leonard Taupier) ([Leica] George: 28mm Summicron IR problem)