Infrared focus and compensting for it.
Anyone using either Hoya R72 or Cokin 007 Infrared filters needs to be aware of the difference between the plane of focus of infrared light when compared to visible light, and the best way to demonstrate that is by showing it in a photo.
For this I drew a line chart, which is attached ( 1200 wide ), and can be printed on a sheet of A4 paper.
To demonstrate the effect I needed to have the shallowest depth of field I could get. As I am using the D100 for IR I chose to do the test with an autofocus lens so used an 85mm, set to f1.8 at near its minimum focussing distance. A Cokin 007 filter was used for the IR shots, but a Hoya R72 would have given the same result. I did not check the film filter but that is also probably in the same ballpark.
Method :
I firstly autofocussed on the centre mark and took a visible light pic.
Next, I turned A/F off to retain that point of focus, placed the IR filter on the lens and took another pic.
Lastly, I manually refocussed the lens to the IR index and took another pic.
All three were stitched together for the composite attatched.
For Canon users I am afraid I am not familiar with the lens markings, so assuming they are similar, I have attached a pic of Nikon lenses to give an idea of where a focus compensation mark may be found.
When using wide angle lenses its not that much of a problem. If you look at the focus scale on the 20mm lens in the pic you will see that f5.6 will cover everything from 2mtrs to infinity. It is a bigger problem though if you intend to do macro shots at 60mm plus, where depth of field is far shallower. Hence attaching the printable line chart so you can conduct you own tests, particularly if the lens has no IR compensation mark.
Don
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