Question on Trying to shoot a rock under water

[RoxnDox]

Hi everyone. First post here, so naturally it has to be a technical question... I am beginner in macro shooting, I love rocks, and a gravel side yard - so lots of subjects. I found one that has nice contrast and features, but only when wet. Submerged the rock in a dish of water, took a series of images, and used "Focus Stacker" on the Mac to produce final image.

I already know about one problem, my steps between shots are too wide and I ended up with out-of-focus bands in the final. Got it covered for next time.

What I wondered about, was whether anyone knows how to get shots without the tiny little air bubbles seen clinging to the rock's surface. do folks use mineral oil instead of water? Coat the rock with something to make it look wet and forget the water?

Thanks in advance,
Jim

Sorry about the yellow dish


Crop at 100% showing the bubbles:

[Pau]

Yes, oil will work nicely but could be difficult to remove once into the pores and it will make the rock surface much more reflective so you would need to be careful with illumination to avoid undesired reflections (and be sue that the rock is really dry before applying oil.

[enricosavazzi]

With moderately porous rocks, the bubbles that emerge from the rock surface may be air being replaced by water drawn into the rock pores by capillarity. These bubbles can be largely eliminated by submerging the subject in water, then applying a vacuum (or a substantially reduced atmospheric pressure) to the container for some time. The vacuum sucks out air from the rock pores, and most bubbles rise to the surface. When normal atmospheric pressure is returned, any small remaining bubbles on the rock surface are compressed to a much smaller size, and the pressure forces water into the rock pores.

A tensioactive substance (e.g. very diluted dish detergent) added to the water may also help to reduce bubbling.

If the problem is instead gases dissolved in the water, they can be eliminated by boiling the water under vacuum or very low pressure, then optionally letting the water cool down at low pressure before use.

One more thing worth thinking about is that photographing an immersed subject with the lens axis substantially inclined from the normal to the water surface introduces optical aberrations, including a form of transversal chromatic aberration. Also, the water surface near the edges of the container is typically not flat because of surface tension, which adds optical aberrations if shooting through these non-flat regions (up to a few mm away from the edges).

A simple technique sometimes used in geology is simply wetting the subject with water. This often causes reflections from the wet surface, which must be controlled by using a suitable illumination (easier said than done). To make one's life easier, it is generally better to photograph the subject when still wet but already drying up, rather than dripping wet.

[Lou Jost]

You might try photographing them dry but using cross-polarization (lots of onfo on the forum). For some subjects this eliminates surface reflections. It may not work for your rocks, I don't know, but this would eliminate all of the problems: the bubbles, the distortion, and the aberrations.

If you must work under water, minimize the depth and the angle of the camera with respect to the water surface (angle the rock, not the camera).

Try using ordinary boiled water while it is still warm. Water outgasses as it warms up, depositng bubbles on submerged surfaces. Soak your rock first so that trapped air goes out. Maybe boil the rock with the water.

If your subjects are less than about 4mm thick, you can use my new technique described here:
https://www.photomacrography.net/forum/ ... ueia+water

[RoxnDox]

Thanks for all the ideas and suggestions. I think I'll try using boiled water first, and drop the rock in for at least part of the time. The edges of the dish are far enough from the rock that surface curvature is negligible, depth of water at the shallowest over the rock was 2-3 mm. Setup is shown below.

No polarizing filters yet - if I keep shooting rocks I'll likely end up with some... Shooting with the surface wetted is the ideal, but the darn thing dries off way too fast. Maybe I can try a mister. Or rig up something to pull a partial vacuum (bigmouth thermos container, preheat with boiling water, drain most of it out and seal the lid?) Macro-scale geology was a lot easier to handle, photographically speaking!

Jim

[Lou Jost]

As I mentoined above, that is not a good arrangement. You should be shooting straight into the water if you want to minimize aberrations. Angle the rock, not the camera.

[RoxnDox]

Agreed, if I can rearrange things to do so...

[enricosavazzi]

A water aspirator connected to a running water faucet is probably the simplest and cheapest way to achieve a relatively good vacuum: https://en.wikipedia.org/wiki/Aspirator_(pump)