I decided that, if I”m going to be able to compare various lenses for hotspotting potential, I need to make the lighting repeatable. That means out with the old, natural light approach, and in with a completely artificially lit technique.
I started out by creating this target:
I printed it on a piece of C-sized Exhibition Fiber paper with an Epson 4900 and lit it with a Paul Buff Einstein strobe. Using the variable output control on the strobe, I made an aperture series with the 28mm f/1.4 Nikkor-D. When I looked at the images in Lightroom, I could see right away that I had a problem. The Epson 4900 inkset is not completely opaque to infrared light that the LifePixel standard IR filter responds to.
I found a crude solution; I covered the black dot with two pieces of gaffer tape. It wasn’t a perfect IR sink either, but it was a lot better:
In order to calibrate out some of the lens transmission differences — and, in the IR region, these very quite a bit across the samples that I’ve tested — I measured both the central black square, and the piece of gaffer tape on the right in Lightroom’s percentage with the sRGB tone curve.
Here’s the raw central data:
The Nikkor seems relatively immune to hot spotting. The CO lens might have a little at some of the wider apertures, but not wide open. The Zeiss lens starts out not so good, and rapidly gets worse as you stop down.
I linearized all the Lr gamma corrected values, took the ratio of the center gaffer tape readings to the right side gaffer tape readings, and plotted the result using a log scale :
Well, the results are different, but not in a way that heightens enlightenment, at least for me. There is one possible exception: you could say that the Zeiss results say that the lens is usable at f/8 and wider.
All in all, this is not a totally satisfying outcome. There are lots of things that could be done to improve the quality and the repeatability of the results.
- Go to a backlit target, with a piece of metal or something else truly IR-opaque occluding the central area.
- Develop algorithms to measure and compensate for lens falloff.
- Use light area rather than dark ones to normalize the illumination.
I know of no simple way to compensate for different light transmission vs wavelength characteristics in the lens,
I think this could easily turn into a project that’s a lot more trouble than it’s worth. For now, I think I’ll use this test as a first cut screen for hot spotting, to be followed up by field testing if the results are the least bit inconclusive.
By the way, although I did the above measurements after conversion to greyscale, I did look at the images in LR-converted false color. Here are thre
Note the spectral responses are wildly different in the light areas, They are also different, but in different ways, in the gaffer-tape-covers areas, whre the reflected spectrum in apparently different from the incident spectrum
you might find these posts by brianc1959 interesting. They are by Brian Caldwell designer of the CO60/4.
Thanks. That makes a lot of sense. I notice the tests in those threads mostly involve photographing a flat field. I’ve found that not to be very sensitive, although perhaps oversensitivity is a potential worry with the dark-center tests that I’m doing.
I am not sure if you followed up with any field testing as you indicate in this post but I would be particularly interested in your results for field testing of the CO 60/4.
I have also used a North blue sky around noon, making sure to use a lens hood, and incorporating a natural surrounding of well watered green tree cover around the periphery of the image. This last part is very important and I was glad to see it in your earlier hotspot tests of the 35/2ZF.2.. As I sometimes like to push my B&W 830nm images hard, any sign of a hot spot or IR flare, even if very faint, once the RAW file drops into ACR indicates signs of trouble. However, not displaying a hot spot in ACR (no processing other than exposure and black level adjustment) obviously doesn’t mean you are free and clear, so I then pass it through to Silver Efex Pro and just hit it hard with 024 Full Contrast & Structure and see how bad it is.
If you have any field tests for the CO 60/4, especially f/5.6 to f/11, I would be interested to see the “pushed” images, if you have the means to do so.
Shane, I’ll look into it, but it will take me a while. I’m just back from Alaska with 7000 images to process. And then there’s that book I’m working on.