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Having had my EOS 20D converted to Infrared, I am gaining experience in photographing and processing infrared. I have discovered that the performance of lenses for infrared (strictly near infrared) does vary a lot. I have found some suggestions on the web of lenses to avoid and some to use but few tests have been carried out. One of the well known problems is hot spots.
Of my four lenses, I have found the following:
a. Canon 10-22mm EF-S - recommended and I find very good performance and sharp images
b. Sigma 12-24mm not recommended on one site but recommended on another, I find very susceptable to IR flare and edges always very soft (almost unusable)
c. Canon 24-105mm seems to be one of the most recommended for IR and my experience is excellent performance sharp images
d. Canon 70-200mm f2.8 IR L - not recommended on one site as having a hot spot but I have not used mine for IR yet
I would be interested if anyone has first hand experience of lens performance for IR. Even if it is not the lenses I use, it may be of value to others.
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Last time I tried infrared I think I used a 300D with the 18-55 kit lens. The flare was horrific.
I've found the Canon 50mm f/1.4 gives a noticeable hot-spot, but the Canon 28-135 IS seems fine (used for all the IR shots in my gallery).
I'll be trying the Sigma 100-300 next time I pop up to Whipsnade ...
There's a list of IR recommended lenses here.
here is another list. With a few more on.
From my very limited experiments to date I think some of the hotspots, colourbands are reflections from the viewfinder. On a side note it helped explain one of the problems I had when I borrowed the big stopper.
If you cover the viewfinder they diminish. From what I've read some of the older primes are better, as they dont have coatings that optimise the visible spectrum.
Nick thanks for that. The link is one of those I had found which has my 70-200 under the hotspots category but this one has the Sigma 12-24 in the good category though another site says it is bad and my experience is that all shots were unsharp apart from the centre.
You mentioned covering the viewfinder and this was certainly an issue when I originally used my 20D with an R72 filter. In this case the the R72 blocks visible light at the lens and the sensor was not very sensitive to IR because of the internal IR block filter. I used to have to use typically 30 seconds exposure times. The high level of visible light from the viewfinder could thus be very significant. With the modified camera the internal IR block filter has been removed and a 720nM IR pass filter added which blocks much of the visible light (not only from the lens but any other source). The exposure times are now similar to when the camera was used for visible light as the sensor is just as sensitive to near IR as visible. I had not heard of any problems with converted cameras and no longer use a stopper. Are you using an external filter or has your camera been modified for IR?
A hotspot on the image can't be the viewfinder - that's blocked for you when the mirror flips up to take the shot.
Quote: A hotspot on the image can't be the viewfinder - that's blocked for you when the mirror flips up to take the shot.
Well I cant think of another explanation - I did before and after in several locations and in every instance the results with the viewfinder shielder were infinately better - I created a post on it. If you cant get internal reflections why do camera manufacturers provide viefinder caps - the D700 & D3 from Nikon have them built in.
This is the topic see 2/3 of way down the page.
After I had the problem it the same phenomena and results are stated on several technical websites (sorry didn't bookmark them).
Dave I missed the last part of your question. At the moment I'm using an external Hoya R72 but I'm awaiting delivery of my old D80 which I'm having converted. Weve just set up an IR group where we are trying to get more interest in IR
They supply a viewfinder cap to prevent light entering the viewfinder and upsetting the metering. This can be a problem if you set up a shot then take it without your eye blocking the light into the viewfinder.
Chris is almost certainly right that it is not a hot spot because the light causing the problems is not coming through the lens. However, it could indeed cause flare just by extraneous light from the Vewfinder getting past the mirror. Normally light leaking past the mirror would be relatively very small compared with the light through the lens but, for an unconverted Camera using an external R72 filter the leaking light may be significant. Before my camera was converted the IR light recorded was 11 stops less than normal visible shots but the leakage would remain the same level.
If you follow the link - when the viewfinder was uncovered I got the effect - when I covered the viewfinder there was none (see example its really obvious) and could be seen instantly on the review screen.
As an aside, I've just realised I've solved a problem I had years ago on night shots. Your right it is probably miniscule amouts of light leaking ffrom the sides of the mirror - the violet colour gives away that the light source is from the visible part of the spectrum, which would be shifted far more to the Visible / UV part of the spectrum - I think with the D700 the difference in eV is ca 13ev (or stops) caused by the highly effective UV filter. I had a similar problem with night shots (about 10-20 min exposures) when I didn't cover the viewfinder.
Note: I also tend to use true mirror up on all but seascapes. And I'm in manual mode 95%+ of the time. In the example I had it seemed to be in direct correlation to the shutter speed - I was running some at about 3 - 4 minutes (from memory I think the unfiltered shot was about 1/6 to 1/10sec, but could be wrong).
Quote: If you cant get internal reflections why do camera manufacturers provide viefinder caps - the D700 & D3 from Nikon have them built in.
Usually because the meter is located between the viewfinder and the mirror, in the pentaprism housing - so light through the viewfinder can affect the exposure time even when the mirror's up. It's not an issue if your head is blocking the light but can be if it's not.
That which you showed in your link wasn't what is generally referred to as a hotspot - those are literally a roughly circular spot in the centre of the image, frequently varying in size/intensity according to the lens used and the aperture.
I would agree that your issue was due to the slight light leakage and the long exposures you were using. Heating of the sensor due to the length of exposure could also come into play.
Chris I dont think heat would be an issue - as its a top end dSLR - and I dont get the problem normally with long exposures - also just thinking out loud it would still heat up with the viewfinder covered. The problem with IR with unconverted cameras is you are dealing with extreme exposures, exposing for part of the spectrum the camera wasn't designed for. So any "normal" light entering would be exacerbated. Since posting the thread I was looking for other examples - there was one poor image on flickr that demonstrated it They covered the lens had a 30 sec exposure and shone a maglight in the viewfinder and got a very similar result. - not something I want to repeat!
I wasn't meaning to imply that normal hotspots were due the viewfinder, they show reflections from the internal lens elements (normally) - I was trying to explain that some examples I've seen do not conform to the typical hotspots, but are more linear and may just may be similar to what I have seen. They could also come from light leaks if a slot in filter is used (I was using a circular Hoya screw on filter) - So as you can see I did go through quite a few scenarios, before I drew my conclusions.
I still havent found the website where I originally found identical results to mine.
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