HeNe’s are gas discharge laser cavities…the precision needed for the cavities makes them expensive.
Ah you can do the same with that xenon lamp! That doesn’t require buying a green laserpointer. Linewidths are probably a bit broader than from a laser-diode, but that sure would be a multipoint calibration/check.
With an additional CFL you could crosscheck the multipont calibration. Yeah, that should do for a good enough calibration.
One more thing: mechanical backlash in the grating positioning. If you can, sweep the wavelengths from one direction. The closed loop steppermotor is good, but the mechanical angular positioning might have some slack. Turning the grating just in one direction reduces errors nicely. (Or have a go-to-pos command for the steppermotor driver that always arrives at the position from one side)
Ah do you have an estimate for your spectral bandwidth? So, how narrow can you make your input slit and have what wavelength bandwidth? If you have actual one to two nanometer resolution that would be rather cool for spending 250quid!
Erratum: maybe you need to take the CFL! looking at a pure xenon lamp spectrum, hmm, those bumps at 480nm might be too broad for calibration. In my head those were sharp lines!
I think the bandwidth is in the range of 1-2 nm, I got that impression while matching the HeNe laser. But I do not know for sure what the slit width is. Need to look at the datasheet of the monochromator and try to figure this out.
From my quick googling, xenon lamps apparently have a strong peak around 475nm, which confirms that my calibration is likely very close.
Erratum: maybe you need to take the CFL! looking at a pure xenon lamp spectrum, hmm, those bumps at 480nm might be too broad for calibration. In my head those were sharp lines!
Maybe. Might become more clear with a 1nm step rather than 2nm.
I’ll definitely try the CFL (this will only be possible January)
Takumar lenses have interesting coatings, with a more flat transmission spectrum than modern lenses (I think), so the results always look magenta using standard matrices.
Thats interesting. What type is it? Super multi coated? Pentax/Takumar has lots of different versions and combinations of coatings but still label them smc. The new HD coating is even more magenta in my experience. But thats just the result of “better” coatings right magenta being a mix of both ends of the spectrum?
Oh those transmission curves look everything but color-(wavelength-)neutral. I would not have expected such wavelength dependence and variability between lenses.
Basically, you need a new matrix/LUT for every lens! Wow.
Oh and the Helios should appear visibly tinted yellow to the eye.
Yes, the better transmission at the ends of the spectrum, the more magenta (relative to your average greenish lens).
The lens is Super Multi Coated. The coatings are noticably green, so it all adds up.
I think most modern lenses lines fall somewhere between my Helios44 and Takumar plots, closer to the Helios 44 (I find it produces quite neutral results with Adobe matrices). I don’t think it makes much difference in most cases, but with the Helios 85mm it becomes significant. Yes: it’s very yellow, and has very blue coatings.
It basically transforms daylight in to incandescent.
Ahh! Imagine having that sought after incandescent look in all images /joke
Im surprised to hear you expect contemporary coatings not to be flat. I mean that takumar is a sixties lens right. Coatings must have improved since then. I did hear somewhere that Pentax patented the shits out of the processes though. Perhaps thats still a blocker.
Unless there are advantages to filtering off the edges? Chromatic aberrations?
Edit: I have the K mount, Pentax branded, version of the 3.5/135 by the way. Same optics, different barrel, possibly adjusted coatings. It can give very nice feathered or brushed bokeh in some circumstances.
Blue/Purple fringing is an abberation from not perfectly correcting blue wavelengths to focus at the same focal length. So for the blue end (let’s say 450nm and below) it kind of makes sense to have some kind of roll-off to “hide” non apo-chromatic corrections, or better the errors that stem from non-apo-chromatic corrections. But those broadband large amplitude changes…that’s almost a stop of difference between certain wavelengths within the S-cone band.
In principle the same thing should exist at long wavelengths too, but at least for modern cameras there is the IR-cut filter which is has a rather steep highpass characteristic (OD5+ within 10nm or so).
Could those roll-off’s come from a spot-size shift at the sensor position? Meaning that because of a non-apo design the spotsize and thus flux-per-area changes wrt to the reference intensity measurement?
I’d love to see the transmission spectrum from a modern lens. I naively assumed that their transmission would be rather flat throughout the visible wavelength range.
Isn’t that Thorium doped glass that turns visibly Yellow? Blue or UV light curing might mitigate this.
Fascinating! The pentax has a lot of red and blue abberations, compared to my russian Tair 133mm lens, which barely has any, so you may be on to something.
Here’s a modern high end lens, an olympus 12-40 2.8:
Confirms those takumars were flatter than modern lenses! I think modern lenses try to maximise transmission as weighted by the luminance function, not equally for all wavelengths, so the results are brighter lenses (from the sensor’s POV) and are neutral enough. Also blocking UV could be advantageous? That’s my assumptions.
@Claes Well yes of course I have an industar 61L/Z! I have been taking my camera spectral measurements through that lens so far (as it seems quite neutral and I was not using it often). Will formally measure all my lenses tomrrw.
Every optic that is not truly neutral then would “stack” its filtration effect. This logic only works for cameras because you usually only attach one lens system. Attaching two lens systems (think lens+diopter or lens+anamorphic or lens+speedbooster) already would get you IMHO into colorimetric trouble. At least somewhat in the blue and red edges of the visible spectrum.
Now isn’t that a hellhole of complications to wrap ones head around. Broadband blues (blue chromaticities closer to achromatic) are going to be more affected by this blue-absorption-edge in terms of hue-shifts than narrowband blues! Narrowband-blues are “mostly” affected in terms of intensity but less so in terms of perceived hue-shift. At this point I’d say: never use a matrix profile for conversion of camera-rgb to whatever your connection space is. Use SSF derived LUT profiles if you can, and when in doubt, have an SSF profile for each lens.
Tried to measure some lenses today using a lamp, taking readings with and without the lens, but found the results weren’t repeatable enough. Will have to approach this differently. Might try a centered beam of light and involve some ground glass.
A diffuser would definitely help. How 'bout a piece of plywood with a 1/2" hole, with the diffuser taped to the opposite side of where the lamp shines…
Other listed cameras are coming over the next few days.
Lens data is coming once I have a repeatable measurement setup.
The camera response data and format will change as I improve the processing code until I believe I have controlled all possible factors - probably once I get lens measurement working, then camera data will be set in stone soon after.
Details of the calibration routine?
@HIRAM Calibration of what? Have you read the thread?
I’m trying stuff like this currently.
Does anyone have an idea how I could upload/back up the many GB of camera raw files? I don’t think it makes sense to do this on git. Each camera is 5-20 GB depending on compression.
/joke
