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Lens and camera tests with sparticle and monochromator

60 replies to this topic

#21 Dmitry

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Posted 29 July 2018 - 10:01

You can paint cross marks or put metal grid to test lens focus shift.

#22 JMC

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Posted 29 July 2018 - 10:15

View PostJim Lloyd, on 28 July 2018 - 15:50, said:

Not sure Jonathan why the individual filters in your images look tonally quite similar ? Did you adjust tones ?
Jim, keep in mind that I am using a UV lens where the transmission doesn't drop with wavelength (atleast down to 300nm).
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#23 Jim Lloyd

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Posted 29 July 2018 - 10:25

I am not sure that is a big factor Jonathan - if my monochromator measurements are correct then the drop off with wavelength wasn't that different between the pinhole and the EL-Nikkor as below around 340 its the Bayer response that is dominating

#24 Andrea B.

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Posted 10 August 2018 - 15:46

Hi Jim -- Your explorations are always interesting. :)


A couple of quick comments:

1) I would really like to see a raw color patch beside the predicted color patch.

2) My results from the Edmund 340/10 do not seem to produce the green you have shown ??
http://www.ultraviol...-sunflower-sun/
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#25 Cadmium

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Posted 10 August 2018 - 22:23

I think the bandpass array tests above have their overall white balance derived from PTFE as seen through the UV-only lens filter transmission shot.
Jim is using a UG1 2mm + BG40 2mm stack, Jonathan is using Baader U, SEU MkII, LaLa U.
The white balance of these Sparticle tests is not based on any one of the bandpass filters, or even the totality of the bandpass filter grouping, but on the PTFE via the lens filter.
If you do the white balance through the 340BP10 alone, then the lens filtration bandwidth and white balance is limited and different than using the Baader U (for example).
Perhaps if you apply a Baader U PTFE white balance from the same scenario to a 340BP10 + Baader U stack lens shot, then the color of the shot would probably be closer to that of the 340BP10 in the Baader U shot of the Sparticle.

Edited by Cadmium, 10 August 2018 - 22:57.


#26 Andrea B.

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Posted 10 August 2018 - 23:34

White balance through a hard-coated 340fwhm10 cannot be done.
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#27 Cadmium

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Posted 11 August 2018 - 01:14

White balance using the Baader U (only), not through the 340BP10.
Essentially, the individual BP filters in the Sparticle groups when shot with the Baader U (or other) on the lens become a stack of sorts with the filter on the lens.
So each of the BP filters in the group is its own stack with the Baader U.
However, the white balance is done only through the Baader U, not through the Baader U + BP filter (s).
So if you want your 340..10 filter to have about the same color as the 340BP10 filters shown in the Sparticle groups in this topic,
then white balance on PTFE using the Baader U alone, then stack the 340 on the front of the Baader U and shoot a pic, then apply the PTFE Baader U white balance to the stacked pic.
The color should become more like what you see with the Sparticle 340BP10.

#28 Andrea B.

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Posted 12 August 2018 - 15:36

Gotcha !! Sorry I misinterpreted the first time through. :rolleyes:

I have lots of BaaderU white balance presets saved, so I will try this.
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#29 Jim Lloyd

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Posted 12 August 2018 - 16:24

Only just seen the latest posts here as I don't seem to be getting notification emails at the moment for some reason. - Also a bit slow as recovering from minor op - but will try to catch up ...


Andrea - Sorry what did you mean by:


" 1) I would really like to see a raw color patch beside the predicted color patch."

I haven't actually done the prediction for the exact filter used in the sparticle, but plan to do this shortly ...

#30 Cadmium

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Posted 12 August 2018 - 18:29

View PostAndrea B., on 12 August 2018 - 15:36, said:

Gotcha !! Sorry I misinterpreted the first time through. :rolleyes:

I have lots of BaaderU white balance presets saved, so I will try this.

Might work OK, but maybe best to use the same scene to shoot both the Baader U WB shot, and the Baader U + 340-10 stack shot.
The lighting might not be the same between some other Baader U WB shot and a new Baader U + 340 stack.

#31 Andrea B.

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Posted 13 August 2018 - 05:38

The left side of this Edmund 340/10 photo has had a BaaderU white balance + D610 color profile combo applied. And there's the green! On the right side is the usual blue/yellow rendering. In strong sunlight between 10AM - 3PM white balance presets do not vary by much,if any. I've got scads of such presets to compare to.
Attached Image: helianthus_uvEdmund340x10_sun_20180612wf_10463wbCompare.jpg


[Off Topic] I still do not know if this Edmund 340/10 filter is recording properly. It is supposedly OD4. So why is the UV-absorbing area on the petal base not as dark as it should be? The filter was rear mounted, but I must still be getting some flare. [/Off Topic]


For the record, the 340/10 raw colors are shown in the next photo. For this photo the contrast and black/white points were adjusted because Raw Digger only applies the usual histogram scaling and "gamma" curving -- the net effect of which leaves the raw photo looking a bit flat.
Attached Image: helianthus_uvEdmund340x10_sun_20180612wf_10463rawCompPn.jpg
Andrea G. Blum
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#32 Andy Perrin

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Posted 13 August 2018 - 06:53

Quote

[Off Topic] I still do not know if this Edmund 340/10 filter is recording properly. It is supposedly OD4. So why is the UV-absorbing area on the petal base not as dark as it should be? The filter was rear mounted, but I must still be getting some flare. [/Off Topic]

Some thoughts:
- we know that both the sunlight and Bayer/microlens covered sensor sensitivity go down dramatically after 340. It’s possible that even at OD4, the blocking is not enough.

- or possibly the flower becomes somewhat reflective again in the short waves?

#33 Cadmium

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Posted 13 August 2018 - 19:51

Andrea, looks good to me. Same green color seen with the 340BP10 that the Sparticle shows when shot with a Baader U or the like.

Jim
http://www.ultraviol...dpost__p__22874

Jonathan
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Cadmium
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#34 Andrea B.

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Posted 13 August 2018 - 21:42

But in 12 years of using the BaaderU, I've never seen that green in any UV photos.
Andrea G. Blum
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#35 JMC

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Posted 13 August 2018 - 22:04

Andrea, as the wavelength shortens, the camera sensitivity and UV intensity drop quickly. So the relative contribution of the 340nm light compared to the longer wavelengths is much reduced, even though the filter still shows good transmission.

A while ago I shared this for my monochrome camera, showing his the sunlight and sensor sensitivity skewed the effective transmission curve for the filter (post #17);

http://www.ultraviol...dpost__p__19974

With a multispectral conversion, rather than a monochrome one there'll be even less contribution at 340nm as the sensor sensitivity curve drops more steeply below 400nm. Hence not surprising you don't see green. Hopefully that makes sense, if not I'll try and prepare a graph to show the effect more clearly.
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#36 Andrea B.

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Posted 13 August 2018 - 23:10

(note: I just accidently del-ed my post. Sorry. Will try to recreate.)

Yes, of course, that makes sense for the broadband BaaderU.

However, all that matters at 340nm is that there is enough 340 in sunlight for filters such as the 340/10 to be useable. Which apparently there is or I would not be able to make the sunflower photos above. :D

But I would like to let this topic get back on track now that I finally managed to apply the BaaderU settings to the 340/10 photo. :lol: :lol: :lol:
Andrea G. Blum
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#37 Jim Lloyd

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Posted 14 August 2018 - 10:34

Quote

But in 12 years of using the BaaderU, I've never seen that green in any UV photos.

Probably because we don't come across a source of 340 nm 10 nm width except in the artificial situation of sparticle images. Flower reflectance spectra for example tend to show relatively few peaks and troughs with most rapid changes around the uv/visible border with relatively little spectral shape between 320-380 nm. (based on Chittka's work).

So if you look at my second graph in the first post in this thread, one sees that you only get the green over a narrow band. Once the spectral content hitting the camera is broadened out (say consider a 330-360 source), then the red and green channels will tend to equalize producing a yellow.

#38 Jim Lloyd

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Posted 18 August 2018 - 13:13

Update:

Continuing with the plan:

First fitted continuous curves through my sensor response data, by fitting two Gaussians to the data points using the solver function in Excel to minimize sum of squared errors:

Attached Image: Capture fitted responses.JPG

Just to recap - this is the measured RGB response (normalized to source irradiance) of Nikon d3200 with physical BG40 2 mm filter and Nikkor EL 80 mm f/5.6 old style lens and modeled UG1 2mm filter, modeled sunlight and modeled white balance (that is normalized so that the area under each curve is the same)

Then used data that Jonathan has kindly provided on the sparticle filter transmission. In each case I approximated this by a Gaussian fit (just adjsuted parameters manually in Excel until the curve matched the data)

Then convolved filter transmission and RGB response for each filter and summed to get predicted response (i.e. at each wavelength between 280-450 multiplied transmission and response and then summed over all wavelengths)

Then compared sparticle image of filters with predicted colour from the model.Sparticle image was obtained with same camera/fileters as described above and raw converted / white balanced using photoninja using reference to ptfe tape within same image

It is immediately apparent that although the colours looked approximately correct, the luminance was low at the short and long wavelengths end. So luminace was adjusted manually to get a match by eye.


Results
Attached Image: Capture final sparticle v model result.JPG

Discussion

A reasonable match between the measured and predicted colours can be obtained, but only if the luminance is manually adjusted to obtain that match.

I am unsure why this is. I think it is most likley due to non-linearity of sensor response with respect to source irradiance. In the original measurements using a monochromator, the output irradiance was reasonably independent of wavelength but the exposure time was varied to avoid over saturating the sensor. The exposure time at 400 nm was roughly 1/20 of that at 340nm. _ the response was then scaled in proportion to exposure time and output irradiance (aperture and iso were not adjusted) . The underlying assumption here is that response is proportional to exposure, but in reality it is likely that have a sigmoid shape.

I have used a modeled solar spectrum rather than measured. This is also likely to impact the results since not only does the absolute UV level vary with sun poistion and time of day, but the ratio of shorter to longer (say 350 v 400) may also change considerably.

The colours and image appearance vary a little depending on what software is used to demosacic and white balance. This will also have some impact on the relationship between model and image.

There could be some inaccuracy in the filter transmission measurements, although I think this would only be small and have limited impact compared to the points above.

Conclusion

The work appears to show promise and heading in the right direction, although quite a few issues need to be addressed. The bayer/sensor response shown appears to have roughly the correct RGB ratios at each wavelength, but probably the overall responses are overestimated at the peak and underestimated at the short and long wavelength ends.

This is probably as far as I will take this for now as I think it would require a bigger investment in time than I have available to compete properly. I feel I have done enough to illustrate the principle and ideally will keep on the look out for someone else to work with to take this forward. My main thrust has I think to be more on the art side now.



Be interested in feedback ...

Edited by Jim Lloyd, 18 August 2018 - 13:19.


#39 Jim Lloyd

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Posted 19 August 2018 - 15:16

The issue of non-linearity (of sensor response v exposure) is described very well in this paper:

Garcia et al determine spectral sensor response in two steps: firstly they determine the response v exposure and then fit a curve so that they can work back from a measured response to a linearised exposure (the exposure that would have given that response if the system was linear) (described here in detail). Then they determine linearised relative response as a function of wavelength.

In my work with the monochromator I haven't taken this requirement for linearisation into account. Then there is the same issue with the sparticle images. As shown by Jonathan here the transmission through the 396, 405, 406 filters is about 2.5x that of the others filters. Also the solar irradiance is lower at shorter wavelengths - so overall exposure at the shorter wavelengths may be a 1/4 of that at 400.

Looking at the response curves from Garcia (see below) suggests to me that the response at 1/4 peak might be around 2x what would be predicted if the response was linear - which would fit with the fact that my sparticle images look brighter than I predicted..It is interesting that the response-exposure curves have slightly different shapes for different channels which suggest that not only will the absolute intensity of response change, but so to will the (false) colour.

Of course in photographic processing we tend to play around with the exposure and maybe reduce highlights and boost shadows anyway, suggesting that aesthetically we do not want a linear response. But for any quantitative work, the non-linear response needs to be considered.

Any thoughts ?

Attached Image: journal.pone.0079534.g001.png

Edited by Jim Lloyd, 19 August 2018 - 15:18.


#40 Jim Lloyd

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Posted 19 August 2018 - 15:40

Just a thought to add to the above ...

It would be important to take linearity into account when comparing the spectral response of two different cameras - as the observed spectral response difference could actually be due to different exposure/response curve shapes.