Correcting for Refraction (or not!) in alcohol

[Lou Jost]

Excuse me for re-awakening this thread, which interests me because I always have to shoot through alcohol when photographing preserved orchids.

People have often recommended using glycerin or other viscous liquids to keep a specimen stationary when submerged in water or alcohol. However, as the viscous liquid or gel mixes incompletely with the water, the difference in the refractive indices of these two liquids causes lots of wavy distortions in the medium (or rather, in the subject viewed through the medium).

I wonder if this might be the cause of some problems when people photograph through these kinds of mixes of liquids?

[rjlittlefield]

People have often recommended using glycerin or other viscous liquids to keep a specimen stationary when submerged in water or alcohol. However, as the viscous liquid or gel mixes incompletely with the water, the difference in the refractive indices of these two liquids causes lots of wavy distortions in the medium (or rather, in the subject viewed through the medium).

I wonder if this might be the cause of some problems when people photograph through these kinds of mixes of liquids?

This would definitely be a problem. Density variations that cause wavy distortions to your naked eye can cause severe loss of resolution for a wide-aperture macro lens or microscope objective.

You mention "gel", but I wonder, do you have something in mind that is different from the material & process discussed at http://www.photomacrography.net/forum/v ... hp?t=27714?

--Rik

[Lou Jost]

Rik, that's exactly the same gel that some orchid people use for their specimens...

[rjlittlefield]

Does it work OK for them?

Or are they using the gel in a position and for a duration such that it mixes with the liquid medium and drifts in front of the specimen to cause the wavy distortion?

--Rik

[Lou Jost]

They use it mainly to hold the specimen in place just long enough to examine and possibly draw it. They say it slowly dissolves in the alcohol/water. I haven't heard whether people also use it for photography.

[rjlittlefield]

They use it mainly to hold the specimen in place just long enough to examine and possibly draw it. They say it slowly dissolves in the alcohol/water. I haven't heard whether people also use it for photography.

That makes sense. It should work OK for photography too, over a short time frame. Mine definitely did dissolve, but the process was slow enough that I could get photos before it released the specimen or contaminated the medium.

--Rik

[Lou Jost]

So it didn't make blurry swirls in the liquid as it dissolved? When I add glycerine to alcohol, there is generally a region of swirly mixing at the bottom of the receptacle for quite some time. It does not mix well.

[rjlittlefield]

I see not so much "swirls" as slowly expanding density gradients, around the edge of the gel where mixing occurs.

But that's why I mounted the specimen with the gel behind it, in a position and orientation where any mixed medium would not be in front of the specimen.

I learned this technique from a fellow who shoots with a vertical setup, looking down, and that's what I used also. In this case the gel is underneath the specimen. The mixed medium is higher density than the unmixed liquid (water in my case), so the mixed stuff naturally stays behind the specimen even as it spreads out. At least that's the way I understand the theory, and it seemed to work out that way.

It is important to avoid disturbing the liquid. Air bubbles in the gel are also problematic, because if they break free and rise to the surface they will carry a streamer of gel with them.

--Rik