Diffraction - Workaround

[Graham_of_Rainham]

Not wanting to spoil Ian's latest Headline Article on "Diffraction Limiting", how about another approach.

Regardless of what camera is used, diffraction affects colours differently. The longer the wavelength, the more diffraction will scatter a given colour of light. With blue light producing much less diffraction than red light, can we use a blue gel on the flash and adjust the WB to compensate.

Fitting a blue filter to the lens would have a similar effect but the additional air/glass interface effects would reduce the gains made buy reducing the diffraction.

Now all that's needed is a set of test shots to see if the idea is sound.

[Ian]

Re: Diffraction - Workaround

Are we talking about light 'scattering'? Not sure about that. But if you filter to blue light and the circles of confusion become smaller then that would work. Interesting.

Ian

[Graham_of_Rainham]

Re: Diffraction - Workaround

I finally got round to testing the idea of illuminating the subject with a blue light to reduce the effect of diffraction.

I pressed to service my ever co-operative muse for this crude set of tests



With the camera set to f/22 and manual focus, I took 3 pictures.
The one above, with the bare flash, one with a Red gel over the flash and another with a Blue gel





I then removed the colour from the RAW files, examined them and cropped an area which I think best shows the effect.

RED Light


Blue Light


The filters used are very saturated "effects" filters and beyond any colours that you would get from natural light, but serve to show how the longer wavelength of the red light is diffracted more than the short wavelength blue light.

From a practical viewpoint, this is useless because controling the colour of the light is rarely possible and it has huge effects on the final image, depending on how the subject absorbs and reflects the different coloured light.

As we all know when using filters on the lens for B&W photography, they alter the contrast and other aspects of the image, so again not a practical solution.

However what is interesting is just how good an Olympus 14-42 kit lens is at f/22

[Bob Ross]

Re: Diffraction - Workaround

Graham's ideas here may have answered a question from the IR mess around Leica's M8. When it was found that the solution would be IR cut filters for the front of those fine lenses, a wail went up. After a few of us got our free filters from Leica, we noticed that the images were sharper (JPEG & DNG) with the filters and some of the wailing subsided. Questions were posed and suggested answers were not very clear.
Based on what Graham said the really long infrared wavelengths would have more diffraction than the visible bands, The IR cut filter would not only cut the IR-light but also the diffraction it carried into the image. Those wavelengths may also bring diffraction into the images at wider apertures like the wide open ones that the mythical Leica shooter would use....LOL
I seldom take the IR cut filters off, but some like to do so when shooting B&W and, of course, IR work. I have never looked at the images from the diffraction contamination as a partner to the IR contamination angle.
Thanks for sharing, Graham.
Bob

[Graham_of_Rainham]

Re: Diffraction - Workaround

Hi Bob,

I'd forgotten about that "discussion" on IR-Cut filters...

It also went further with the pros & cons of reflective vs absorptive types of filters, in addition to the extra air/glass interface effects.

I guess when you pay that much for a lens, expectiations are equally high.

It would be interesting to see how such filters effect the images on the new Leica M Monochrom.

[John Perriment]

Re: Diffraction - Workaround

Very interesting thread, Graham, I missed this first time around. Your test shows a very obvoius difference between blue light and red light. You suggest that this might not have any practical benefits, but I think that perhaps it does.

Take the situation of warm, reddish light around sunrise or sunset. Your results suggest that diffraction my be more of an issue than during "normal" daylight and it's useful to be aware that at the extremes of the day diffraction may start at a larger than expected aperture.

Conversely, at dawn or dusk when the sun has no influence and blue tones predominate, it may well be possible to stop down further than normal without risking diffraction. It's knowledge that may be used to advantage in the right conditions. In fact, shooting landscapes with a pale blue correction filter (rather than conversion filter) then adjusting WB in camera or raw processing may be beneficial at anytime, particularly if you normally use a clear protection filter anyway (obviously don't stack the filters).

This has also set me thinking about APO telephoto lenses. In addition to reducing chromatic abberations do they also help reduce the effects of diffraction at smaller apertures?

As I say, very interesting!

[Bob Ross]

Re: Diffraction - Workaround

Hi Bob,

I'd forgotten about that "discussion" on IR-Cut filters...

It also went further with the pros & cons of reflective vs absorptive types of filters, in addition to the extra air/glass interface effects.

I guess when you pay that much for a lens, expectiations are equally high.

It would be interesting to see how such filters effect the images on the new Leica M Monochrom.

Hi Graham,
When Leica finally did the M8, I was sitting on my 4 old lenses and I was so overjoyed with being able to use them digitally, that all the discussions and work-a-rounds were mostly fun to me. I do have a 39mm green filter, so maybe I should get on the Monochrom preorder list to see if it might affect diffraction .......
Bob

[Graham_of_Rainham]

Re: Diffraction - Workaround

<snip>Take the situation of warm, reddish light around sunrise or sunset. <snip>

I enjoy this type of light a lot. I also like pictures to reflect the subject and will often soften an image (by various means) to produce a "glow" that adds to the warmth and appeal...

Using a super high quality lens, such as my Zuiko 14-35 often recieves comments from models, that it shows up all their imperfections...

Which all goes to show that, by understanding the way the light will work with your kit and the subject, all adds to the chances of producing better end results. This was so much more important in the days of film, when we didn't have the luxury of the instant result, but having the knowledge is a huge advantage when working on fleeting subjects, in fading light or models that charge by the hour

[John Perriment]

Re: Diffraction - Workaround

I enjoy this type of light a lot. I also like pictures to reflect the subject and will often soften an image (by various means) to produce a "glow" that adds to the warmth and appeal...

Using a super high quality lens, such as my Zuiko 14-35 often recieves comments from models, that it shows up all their imperfections...

Which all goes to show that, by understanding the way the light will work with your kit and the subject, all adds to the chances of producing better end results. This was so much more important in the days of film, when we didn't have the luxury of the instant result, but having the knowledge is a huge advantage when working on fleeting subjects, in fading light or models that charge by the hour

I guess that a little difraction is not a bad thing to preserve the models' vanity, especially when using that 14-35mm!