www.photomacrography.net :: View topic - The use of green filters with achromatic objectives
www.photomacrography.net Forum Index
An online community dedicated to the practices of photomacrography, close-up and macro photography, and photomicrography.
Photomacrography Front Page Amateurmicrography Front Page
Old Forums/Galleries
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 
The use of green filters with achromatic objectives

 
Post new topic   Reply to topic    www.photomacrography.net Forum Index -> Macro and Micro Technique and Technical Discussions
View previous topic :: View next topic  
Author Message
Pitufo



Joined: 21 Jun 2015
Posts: 206
Location: United Kingdom

PostPosted: Sat Jun 09, 2018 10:18 am    Post subject: The use of green filters with achromatic objectives Reply with quote

As I understand it..a green filter on the light source can be used to improve image quality when using achromatic objectives, because these objectives are best corrected for spherical abberation at this wavelength. I have done this experiment and it seems to work.

My question is a simple one and yet I struggle to understand it.

Green light goes in (lets say this is a perfect 550nm - this may be the mistake) and passes through the sample. The image arrives at the camera sensor (and appears green) and I reset the white balance on the green background.

I now have a full colour image. How?

If part of the sample appears red (if it is solely acting as a red filter), this would appear to mean that red light has been allowed to pass through it to the detector. And yet, there was no red light to start with as this was eliminated by the green filter.

Please put me out of my misery... Smile
Back to top
View user's profile Send private message
Beatsy



Joined: 05 Jul 2013
Posts: 1391
Location: Malvern, UK

PostPosted: Sat Jun 09, 2018 11:45 am    Post subject: Reply with quote

Your green filter probably has a very wide bandpass centred around 550nm so some red(der) and blue(r) light is getting through too. Using the "green" to reset the white balance will change colour temp and tint, and massively reduce green intensity in the process - thus restoring the colour.

I suspect the downside will be a huge loss of dynamic range and you could start seeing banding in areas that originally had smooth colour gradients.

Edit: I just had a quick go to see this for myself. An old "coloured glass" filter does as you describe, it's possible to get back to a white background. But I also tried a green interference filter with a very narrow bandpass and, sure enough, the image made with that remains resolutely green.


Last edited by Beatsy on Sat Jun 09, 2018 11:58 am; edited 1 time in total
Back to top
View user's profile Send private message
Lou Jost



Joined: 04 Sep 2015
Posts: 2764
Location: Ecuador

PostPosted: Sat Jun 09, 2018 11:55 am    Post subject: Reply with quote

If you really want to use green light to improve resolution, and if you are fine with a black and white final image, you might consider using truly monochromatic green laser light. There will still be some signal on the red and blue sensels, but it won't cause loss of sharpness. You should convert it to monochrome at the end. The red and blue channels will be noisier than the green channel, but there will also be some signal there. Alternatively you could just use the green channel but you would be throwing away half your resolution. Best would be to get a monochrome sensor-cooled astrophotography camera. With that, you could improve resolution even more, by using shorter-wavelength blue or violet lasers instead of green lasers.

Note that any time you use laser light for photography, you have to despeckle it. I use dilute milk-filled diffusers for that.
_________________
Lou Jost
www.ecomingafoundation.wordpress.com
www.loujost.com
Back to top
View user's profile Send private message Visit poster's website
Pitufo



Joined: 21 Jun 2015
Posts: 206
Location: United Kingdom

PostPosted: Sat Jun 09, 2018 12:49 pm    Post subject: Reply with quote

Thanks for your answers Beatsy and Lou. That makes sense.

I've never noticed any banding artifacts or noticed a large reduction in intensity but it may well have happened and I haven't looked at any data.

Using a green filter did seem to improve the performance of achromats. This make sense if the longer and shorter visible wavelengths are mostly removed by the green filter - which I think would reduce the effects of both spherical and longditudinal chromatic abberation on the sensor.

However, if we are then removing most of our green light intensity when we reset the white balance on the background, aren't we then defeating the point of the process? (i.e. by effectively levelling the playing field and bringing all the wavelengths back to the same relative intensities).

Maybe this technique will only truly work with a green narrow bandpass interference filter as tested by Beatsy (or green laser suggested by Lou) and with a monochromatic output image.

EDIT: I found this comparison using a green filter and grayscale which seems to work http://www.microbehunter.com/microscopy-forum/viewtopic.php?t=2940

P.S. I don't really need to do this to improve my resolution, it is more something I would do if I was on an away trip with a portable achromatic microscope and wanted to make the images at least bearable.


Last edited by Pitufo on Sun Jun 10, 2018 3:02 am; edited 1 time in total
Back to top
View user's profile Send private message
rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 19327
Location: Richland, Washington State, USA

PostPosted: Sat Jun 09, 2018 2:25 pm    Post subject: Reply with quote

Pitufo wrote:
However, if we are then removing most of our green light intensity when we reset the white balance on the background, aren't we then defeating the point of the process? (i.e. by effectively levelling the playing field and bringing all the wavelengths back to the same relative intensities).

Sort of but not really. Resetting the white balance can only bring all three bands back to the same average intensity. It cannot do anything to equalize the relative abundance of various wavelengths within each band. The resulting playing field may be level on average, but still contain some pretty strong hills and valleys.

For example, if your green filter passes some yellow but completely blocks deep red, then things that started off being deep red will still appear black even after resetting the white balance. Things that started off yellow will have their color altered, but the new color will still be clearly distinguishable from gray so the appearance of full color will be retained. Likewise for a filter that completely blocks deep blue/purple while still passing some blue-green.

It is usually the extreme wavelengths that give the most trouble with chromatic aberrations, so getting rid of just those can still make a significant improvement even though the final image may appear to be full color.

--Rik
Back to top
View user's profile Send private message Visit poster's website
Pitufo



Joined: 21 Jun 2015
Posts: 206
Location: United Kingdom

PostPosted: Sun Jun 10, 2018 2:59 am    Post subject: Reply with quote

Quote:
Sort of but not really. Resetting the white balance can only bring all three bands back to the same average intensity. It cannot do anything to equalize the relative abundance of various wavelengths within each band. The resulting playing field may be level on average, but still contain some pretty strong hills and valleys.


Thanks Rik - I think that is the important point, which I had completely overlooked.

I need to look into how white balance works exactly.

Thank you for helping me understand Smile


Last edited by Pitufo on Sun Jun 10, 2018 3:49 am; edited 1 time in total
Back to top
View user's profile Send private message
enricosavazzi



Joined: 21 Nov 2009
Posts: 1076
Location: Stockholm, Sweden

PostPosted: Sun Jun 10, 2018 3:39 am    Post subject: Reply with quote

rjlittlefield wrote:
...
It is usually the extreme wavelengths that give the most trouble with chromatic aberrations, so getting rid of just those can still make a significant improvement even though the final image may appear to be full color.
...

Exactly. Amateur astronomers have been using for many years "semi-apo" filters, which transmit only the middle parts of the red, green and blue bands and cut all the rest, for exactly the same purpose. Astronomik also markets L1, L2 and L3 filters that cut UV and IR as well as both extreme ends of VIS. L1 has cutoff points in the UV and IR, L3 has cutoff points slightly within the VIS range. These are dielectric coated filters with very sharp cutoff and roughly flat bandpass, potentially useful if achromatic aberration is highest at the ends of the VIS band.

Astronomy filters that selectively cut various VIS narrow bands (including "trouble" bands caused by some types of artificial street illumination) are also available. If you know which bands have the most troublesome chromatic aberration you could use one or more of these filters to cut these bands.

Using one (or more) of these filters is more likely to give satisfactory results that shooting with a green filter and trying to restore WB in post-processing. Of course, selectively eliminating some wavelength bands does alter the color rendering of the subject in ways that cannot be restored in post-processing, there is no way around it. But if sharpness is more important than an exact color rendering, these filters have their uses.
_________________
--ES
Back to top
View user's profile Send private message Visit poster's website
JH



Joined: 09 Mar 2013
Posts: 1171
Location: Vallentuna, Stockholm, Sweden

PostPosted: Sun Jun 10, 2018 5:11 am    Post subject: Reply with quote

For some subjects (ants) I have had problems with CA even if the lens is quite well corrected (Nikon Fluor).

Here is a single picture. The left one is EPI (WB 3200 K) and the right one is with the green filter (GIF) (WB in camera).
http://www.photomacrography.net/forum/userpix/3238_formica_epi_and_epi_green_ii_1.jpg

Here is a stacked example, the right one is the GIF filter.
http://www.photomacrography.net/forum/userpix/3238_formica_epi_and_epi_green_1.jpg

I have also tried a baader fringe killer filter. Here is a picture of a butterfly scale, with and without the filter
http://www.photomacrography.net/forum/userpix/3238_baader_fringe_killer_1.jpg

Here is a stacked picture with the Baader filter:
http://www.photomacrography.net/forum/userpix/3238_ps_20160119221731_ZS_retouched_1.jpg

I like to use the GIF-filer from time to time. The effect of the Baader fringe killer on CA is not enough for me, at least not with my fluor objectives. I have found the Baader fringe killer filter more usefull for UV light photos;

http://www.photomacrography.net/forum/userpix/3238_ps_PMax_00830197_1.jpg
http://www.photomacrography.net/forum/userpix/3238_UV_pollen_1.jpg

Pictures was from this threads
https://www.photomacrography.net/forum/viewtopic.php?t=29536
https://www.photomacrography.net/forum/viewtopic.php?t=29764
https://www.photomacrography.net/forum/viewtopic.php?t=34098
https://www.photomacrography.net/forum/viewtopic.php?t=36982

Best regards
Jörgen Hellberg

Edit filter specifications:

https://www.nikon.com/products/industrial-metrology/support/download/brochures/pdf/2ce-kxqh-7.pdf


https://www.baader-planetarium.com/en/baader-fringe-killer-filter.html


_________________
Jörgen Hellberg, my webbsite www.hphoto.se
Back to top
View user's profile Send private message Visit poster's website
Pitufo



Joined: 21 Jun 2015
Posts: 206
Location: United Kingdom

PostPosted: Tue Jun 12, 2018 1:32 am    Post subject: Reply with quote

Thank you Enrico and Jörgen - very interesting stuff.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    www.photomacrography.net Forum Index -> Macro and Micro Technique and Technical Discussions All times are GMT - 7 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group