[SIZE=4][B][FONT=Arial]Choose your lens for near infra red photography with care[/FONT][/B][/SIZE]

I have long been interested in infra-red photography and have even used infra-red film in the past. Digital cameras open up new opportunities for infra-red photography and I decided to get a spare Olympus Pen E-PL1 Micro Four Thirds compact system camera infra-red converted a couple of years ago. This worked really well and I published an [URL="http://dpnow.com/7993.html"]article on digital infra red photography[/URL] it at the time.

PS Our IR Converted Olympus Pen E-PL1 is available for hire and there is a [URL="http://e-group.uk.net/forum/showthread.php?t=27274"]special offer discount until 16th June[/URL].

In all this time I had not encountered the problem of 'hot spots' when photographing infra-red, but I was recently alerted to this problem by member of the forum at our [URL="http://e-group.uk.net"]Olympus UK E-System User Group[/URL] site.

It seems to be a phenomenon that is specific to certain lenses and it has nothing to do with the age of the lens or the sophistication of its design. In fact one of Olympus' newest and most impressive lenses, the m.Zuiko 75mm f/1.8, is a good example. Here is an un-adjusted image using this lens and our converted E-PL1:


[IMG]http://dpnow.com/files/blog/P6060004.jpg[/IMG]

Clearly visible in the centre of the frame is a lighter area or hot spot.

The theory is that while lenses are designed to minimise extraneous internal reflections through the use of lens coatings and even the blacking of the cut edges of lens elements, these measures may only work properly for visible light. To make things more confusing, some lenses are a lot more prone to this problem than others. I had used a variety of lenses with my E-PL1 and never encountered the problem but a little online research reveals that many lenses, some of them very expensive, are prone to hot spots.

[IMG]http://dpnow.com/files/blog/P6060003.jpg[/IMG]

Interestingly, with the 75mm f/1.8 I was able to eliminate most of the hot spot effect by shooting wide open (above). The problem with this particular lens is at its worst when stopped down. The first image on this page above was shot at f/22.

[IMG]http://dpnow.com/files/blog/P6060005.jpg[/IMG]

At a more normal shooting aperture of f/7.1 (above) the hot spot is there but not excessively obtrusive.

The effect of a hot spot is often more noticeable in a photo than on the camera's screen and the usual post-processing for infra red images can worsen the effect.

No doubt I will be trying out different lenses from our collection and noting which are prone to hot spots and which are not over the coming days and weeks!