## Focusing on a refraction in water drop

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clarnibass
Posts: 111
Joined: Fri Jun 10, 2016 11:33 pm

### Focusing on a refraction in water drop

Hi

Someone on a macro group showed a photo where he had water drops on a sheet of glass with something below it, and the photo captures that in the drops (in that case it was an ipad showing a photo).

I'm trying it but have some problems. I've never tried this and I'm definitely doing something wrong.
I'm using a phone since I don't have an ipad.
I'm using a 105mm lens on a full frame camera. They used 60mm on a 4/3 camera. I don't know if this matters since our frames are different (at least I can't really tell the size of their frame).

For now I'm trying at around x1 magnification, three drops filling most of the frame. The phone is about 50cm under the glass. I don't get the refraction in focus in the drops anywhere in that range. If I raise the camera a bit I sort of have it, but not exactly, and I still leave the focus distance at x1.

Is there a way to roughly know the distances for something like this?

Thanks

rjlittlefield
Posts: 21406
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:
Take a look at https://www.photomacrography.net/forum/ ... hp?t=41424 . Notice that the small images formed by the drops are inverted. That's because they are real aerial images that are formed by light rays after they pass through the drop, on their way to the camera.

The location of the images will vary depending on the curvature of the surface of the drops. In general, the focal length in air of a lens with one flat surface and one spherical surface is approximately f = R/(n-1), where n is the refractive index of the lens medium and R is the radius of curvature of the surface. For water, refractive index 1.33, the focal length is about 3 times the radius of curvature.

Note that's the radius of curvature of the rounded surface of the face of the drop, not the circle where the drop meets the glass. Small drops will be almost spherical and have short focal lengths; large drops will flatten out so they have much longer focal length.

Summing up: the little images are formed on the camera side of the water drops, very close to the glass for tiny drops and farther away for bigger ones. You need to focus on the edges of the drops, and then even closer.

--Rik

Ref: formula 11 at http://www.physicsinsights.org/simple_o ... ses-1.html

clarnibass
Posts: 111
Joined: Fri Jun 10, 2016 11:33 pm
Thanks. I'll look at the formulas later and see if it's not too much

That example in the link is interesting in that the edges of the water drops look in focus, at least in that size. I finally was able to focus on the refraction and the drop edge was very out of focus. I'm wondering if that's because of magnification/aperture. I'll ask them about the parameters.

Thanks again

rjlittlefield