shooting through water? (was: image stacking and stitching?)

[ChrisR]

Is this why underwater photographers use a hemisherical window?
Though they also use flat ones (eg Nikonos).

The Wikipwedia article alludes to the point but doesn't, errm, go very deep.
http://en.wikipedia.org/wiki/Underwater_photography - see paragraph next to the picture of the underwater housing.
I have a feeling we're talking about something we'll discover was all worked out 100 years ago...
Were it not 5am I'd look further.

[rjlittlefield]

A question was asked offline about my setup.

Here it is:



What you're seeing here is a water bath in which the front is an old haze filter about 2 mm thick, followed by about 1 to 50 mm of water depending on where I slide the chunk of wood.

Shooting through the thin layer of glass introduces only a smidgeon more aberration than shooting through a correspondingly thin additional layer of water. (Refractive index for water ~ 1.33, glass ~ 1.5, air ~ 1.000)

In exchange the glass gives an optically flat interface with no hassles from tiny ripples, meniscus, etc.

--Rik

[rjlittlefield]

Is this why underwater photographers use a hemisherical window?

Sure. Note especially the part about "often designing them to be used with specific lenses to maximize their effectiveness". The hemispherical window essentially becomes part of the optics, resulting in a lens system that is well corrected when immersed in water. Putting a flat surface in front of a wide angle lens results in a lens system that is not well corrected, and the error is large enough to matter.

I have a feeling we're talking about something we'll discover was all worked out 100 years ago...

Very likely. But just because something was worked out 100 years ago doesn't mean that it's easily accessible or readily applicable. I don't think it would be very helpful if we recommended to Drbluethumb that he fabricate an optically precise hemispherical window to photograph his coral through!

--Rik

[Chris S.]

Very interesting test, Rik! And setup.

Is it too early (you said you're "chipping away") to postulate what is causing this form of aberration? Camera angled with respect to the to the glass/air and glass/water interfaces? Difference in amount of water the light travels through?

Your sense seems to be that the effect would be pretty much the same if your test setup were through water only, with no glass filter. With no particular reason to disagree, I'll still ask: Is this something that has reason to be checked empirically?

----

Drbluethumb, how sensitive are these corals to disturbance? Would such a coral be harmed or visually changed by being temporarily moved to a dedicated photographic aquarium, where the rock on which it grows might be gently clamped just under the surface of the water?

If none of this would hurt the coral, I'd be temped to position its rock sideways, so that it could be shot from above, but would appear to be shot from the side. Lighting would be key here, but pretty easy: Since our brains expect light to come from above a subject, whatever direction the light is coming from is going to look like "up." So it might be easy to avoid the appearance of having shot downward at the coral.

If this is possible, you would be shooting through no glass, shooting through as little water as possible, be able to use very clean water, and have a lot of control over your light. This is what I ended up doing with my pond weeds.

----

Let's assume one would like to build some little glass containers/aquaria (inch dimensions) for photography of aquatic critters, is there, apart from Chris' suggestion to use photo filters, any glass material available that will introduce minimal distortion and should hence be the preferred building material?

My own--as yet untested--solution has been to make tiny aquaria with microscope slides for the back, cover slips for the front, and layers of silicone sealant for the sides. I made a few of these just after winter set in here in Ohio, and have them in wait for the spring--am thinking of fairy shrimp and other creatures. Will shoot through the cover slip portion, but be able to light through the other sides. Not sure it will work, of course, but I have hopes.

Cheers,

--Chris

[rjlittlefield]

Is it too early (you said you're "chipping away") to postulate what is causing this form of aberration? Camera angled with respect to the to the glass/air and glass/water interfaces? Difference in amount of water the light travels through?

Surely it's the angle, which affects the amount of water the light travels through. These are all center of the sensor, using optics that are known sharp in air.

Your sense seems to be that the effect would be pretty much the same if your test setup were through water only, with no glass filter. With no particular reason to disagree, I'll still ask: Is this something that has reason to be checked empirically?

Well, as a matter of policy I think that everything needs to be checked empirically. As a matter of practice, I have other things that seem more important to check. For my own interests, a relevant question is "How should I shoot 5X and up through 5 mm of glycerin?" It turns out, according to what seems like a good calculation, that using an NA 0.25 objective is a really bad idea while NA 0.1 should not be disturbed enough to matter. Since NA 0.25 is known to be significantly sharper without the glycerine, one might then ask where is the crossover point where the two objectives are "equal"?

Based on what I know now, if I were interested in highest resolution and least distortion, I would probably not adopt your strategy of shooting down through a liquid surface. This is based on risk of ripples & meniscus. On the other hand, it's hard to beat the convenience of a thin layer of water, and indeed I've used that strategy myself on occasion such as HERE, which I believe you're familiar with.

--Rik

[Chris S.]

Based on what I know now, if I were interested in highest resolution and least distortion, I would probably not adopt your strategy of shooting down through a liquid surface. This is based on risk of ripples & meniscus.

I'd note that in my experience, ripples subside very rapidly, and meniscus seems only to be a problem at the edges of a water container--so that by using a sufficiently large container and staying away from the edges, it isn't a problem. My photographic aquarium is probably a 10 gallon one--a quick search shows them for around $13 new, but mine cost $2 at a local thrift shop. I doubt one would need anything nearly this large to avoid meniscus, but it's what I have.

--Chris

[Chris S.]

Just posted here is an approximately 1:1 underwater image made using the "shoot it from the top" strategy." This project was delivered to the client six years ago, and most of the images are now in deep storage on DVDs. . .somewhere. However, I found that this particular shot--neither the worst nor the best of the project--somehow resides on a hard drive currently in my computer.

Attempts at a similar image of this pond weed, taken through the aquarium glass, looked notably worse.

--Chris

[Craig Gerard]

Would it be of benefit to extend the title of this thread so as to encompass the expanded discussion?

Rik, I like your mini aquarium I hope you are intending to dunk some other subjects in future projects?


Craig

[Peter De Smidt]

Corals can be moved without harming them, assuming they aren't attached to anything to big to move. What you have to do is move them without exposing them to air.

I like the top down idea. In a small, specially built aquarium, place the coral on a shelf such that it is close to water's surface when it's polyps are extended. Photography down through the still water and avoid glass altogether.

[rjlittlefield]

Attempts at a similar image of this pond weed, taken through the aquarium glass, looked notably worse.

Good to know. I'm undoubtedly biased by working with a piece of clean glass that was intended to be photographed through. Certainly a water surface has the advantage that it's easy to clean and never scratches. It also provides less opportunity for reflections.

My sensitivity to meniscus again is due to dealing with small subjects at higher magnification, e.g. bristles sticking up through a thin layer of glycerin. At larger scales it wouldn't be an issue.

--Rik

[AndrewC]

Would it be of benefit to extend the title of this thread so as to encompass the expanded discussion?
..

... or even just run a spell check on the current one !

[rjlittlefield]

Title fixed...

(It's funny -- I actually didn't understand Andrew's comment about spell-checking until my edit picked up a red-line.)

--Rik

[ChrisR]

just because something was worked out 100 years ago doesn't mean that it's easily accessible or readily applicable

Was there a suggestion that it would necessarily be either of those things?

I don't think it would be very helpful if we recommended to Drbluethumb that he fabricate an optically precise hemispherical window

To suggest that, would have deserved a derogatory riposte, I'm surprised my comments were taken as meaning that.

Underwater photography has been around a long time, only one development was hemispherical windows.
I am suggesting that some basic issues are likely to have been analysed over a long period, and a search for publications would be worthwhile.

[rjlittlefield]

I'm surprised my comments were taken as meaning that.

I was, of course, attempting to make a joke.

I am suggesting that some basic issues are likely to have been analysed over a long period, and a search for publications would be worthwhile.

I'm sure it doesn't count as a "search", but I did reach to my bookshelves for three conveniently available references:

• Paul Harcourt Davies' book, "Small Things Big"
• Kodak Publication N12-A, "Close-Up Photography"
• Williams & Becklund, "Optics: A Short Course for Engineers & Scientists"

The third one is a bit arcane for this discussion, but I can quickly summarize the recommendations and discussions of the first two.



In words: avoid reflections, shoot through the glass, and build custom tanks to constrain your subjects.

No mention is made of the issues addressed by Chris S. or myself, other than a brief warning that "The camera back needs to be parallel to the water surface."

Details can be read here:
SmallThingsBig_p108
SmallThingsBig_p109
SmallThingsBig_p112
KodakN12A_p79
KodakN12A_p80

If you have access to better resources, I'd certainly appreciate learning what they say.

--Rik

[Chris S.]

Glycerine and higher magnifications would surely make meniscus much harder to deal with. But at about 1x, shooting top down in water without intervening glass, I have not found it to be a difficult issue. Simply start with the specimen as high up in the water as possible to minimize the amount of water between camera and subject, but lower it a bit if meniscus problems show up in a test shot. This discard from my 2006 project illustrates a subject that is not quite low enough in the water, and shows concentric rings around several branch tips. Other shots, with the specimen lowered a millimeter or two lower, were fine.

Discarded shot uncropped, about 1x-3x* (image from before my stacking days, and so DOF is limited):



Pixel-level crop of the region too close to the water's surface, where meniscus effects are evident:


This plant is Ceratophyllum demersum, the genus being commonly known as "coontail." The bubbles are being released by the plant, which was rapidly photosynthesizing under my flash and focusing lights. (Nikkor AF-D micro 105 f/2.8 at f/22 on a D200. Image taken as part of a study under a grant from the Geauga Park District in Geauga Country, Ohio.)

. . .working with a piece of clean glass that was intended to be photographed through.

I'd note that my aquarium glass, on the side through which I tried shooting before switching to the top-down, glass-free approach, was quite clean; I had previously cleaned it as I would any optical element I was planning to shoot through. No telling how big a problem it was that this glass was not intended to be photographed through. I suspect the big bugaboo was that this aquarium's glass is about 3.6mm thick, which, at around 1x-3x* magnification, seemed to impose a penalty (spherical aberration?) that I could not get around no matter how carefully I placed my camera and lights.

Based on a quick search, I see Hoya lists 1mm as the thickness for one of its UV filters, and ceteris paribus, I'd expect that less thickness means less penalty--so the filter in Rik's test rig should be much better than the glass in my aquarium on that score alone. A cover slip is typically about 0.17 mm, and can be even thinner--hence my choice of this item for really small stuff I intend to shoot from the side. But if one's image permits it, I'd strongly consider "no glass at all."

The situation I was solving for with these pond weeds seems reasonably close to that of corals between the size of a dime and a ping-pong ball, so perhaps the protocol I used may work for Drbluethumb. Rik's situation with subjects in glycerine at 5x seems quite different--and much more difficult. (Actually, it's simple, Rik--just rinse the glycerine off the specimens and float them in liquid helium! )

--Chris

*Since posting this, I've edited two magnification references from the original "1x" to "1-3x," to reflect my fuzzy memory of shooting this six-year-old outtake. The specimen was smaller than most coontails, and my sense is that 1x is pretty close. But in the absence of notes, I'd like to indicate some level of imprecision. The original specimen was subsequently mounted on an herbarium sheet, but this sheet is being curated by a university in another state, and so is not convenient for me to measure. Nonetheless, my tests during that project covered the 1x-3x magnification range pretty thoroughly, and what I observed about image degradation from aquarium glass seemed consistent across that range.