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07-26-2010, 02:17 AM   #1
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Mercury spectra with K10D and K20D

I wanted to know if a light yellow filter would be helpfull in surpressing purple fringing.
The special filter of B&W (415 or 420) are quite expensive, so I want to be sure what I can expect.
I did have a UV light source from my EPROM erasing days, and a VLP (looks like avery big CD) to use as crude grating.

What to expect :

405nm : violet
436nm : blue
546nm : green
577nm : yellow/orange

First K10D :


Thats almost what I see in the viewfinder, except the violet part, which is much brighter here.

I have a lightyellow Nikon Y44 filter, which should block above 440nm.

K10D+Y44:


Results, violet is gone, blue is dimmer.


Well, I also have a K20D, so lets see what that camera/sensor does.


K20D without filter :


No violet ! So I can expect less purple fringing with the K20D compared to the K10D ?
And another observation : the orange part has become green !

K20D with Y44 :


The filter has a similar effect on the blue part as in the K10D case.

Leaves the question, what happened to the orange part.

Does anybody, from datasheet or experiment, have a complete spectrum sensitivity of the Pentax used sensors + aliasing filter ?

07-30-2010, 07:14 AM   #2
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Interesting post.
07-30-2010, 08:23 AM   #3
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I tested this a while ago with the 420 filter on a K10D.
It works, but does not remove it completely in severe cases.



When I have some time I will repeat the test and give some more examples.


Btw. here is the transmission curve for the B&W filters:
07-30-2010, 10:01 AM   #4
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I found that when I used a Light Pollution filter on a 135mm lens that the PF around some stars were smaller and even removed in some places. This got me to thinking that maybe the UV and near-UV was contributing to the PF.

I did a lot of reading about UV filters and finally did some testing with the Hoya ones I had already. I found that using a filter that has a hard cutoff at around 400 nm seems to work very well at reducing the PF on some of my lenses.

The fellow at this site did a lot of spectral analysis for several filter brands and the Hoya ones seem to test very well. UV filters test - Introduction - Lenstip.com

Hope this helps!

08-01-2010, 05:09 PM   #5
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Original Poster
QuoteOriginally posted by blende8 Quote
I tested this a while ago with the 420 filter on a K10D.
It works, but does not remove it completely in severe cases.


Btw. here is the transmission curve for the B&W filters:
Yes, but I am afraid that for the K20D the gain will be less, because it already blocks more violet than the K10D.

I already found the transmission curves of the B&W filters, but I have no idea what the curve for the nikon Y44 is, except that it is around 440nm.

I will try to make a better spectrum setup, to test also the red end.
08-01-2010, 10:25 PM   #6
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QuoteOriginally posted by blende8 Quote
I tested this a while ago with the 420 filter on a K10D.
It works, but does not remove it completely in severe cases.
Maybe I am confused about what is being discussed, but the color aberration (purple firing) shown in the picture happens *inside* the lens, not *outside* and no filter can fix that.
08-01-2010, 10:59 PM   #7
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stanjo, the way that the filter works in this case is that because the wavelengths of light that the lens does not bring to a focus properly are reduced, then the PF is not as strong.

If the filter was not in place, then the lens doesn't bend the incoming colors properly and thus the unfocused ultraviolet light shows up as PF.

This was well documented in black and white film astrophotography times when people would shoot stars while using a yellow or red filter. By only allowing one wavelength of light to pass into the lens, the resulting star images would be much sharper and lens abberations would be minimized.
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