38 replies to this topic

[echeng]

I've heard a few people mention hyperfocal distances with regard to underwater photography. This doesn't really make sense to me, since you don't need the land-equivalent of infinity focus underwater. "Infinity" focus underwater can be approximated by 2x the diameter (4x the radius of curvature) of the dome port used, so with an 8" dome (assuming it's spherical), infinity focus would be at 16". Doesn't that mean that the underwater equivalent of "hyperfocal" would focus from some near distance to 16", and not some near distance to infinity? I'm speaking in terms of topside focus, as the markings on the lens indicate.

I want to make sure that my understanding is correct. I also want to prevent people from discovering hyperfocal distances, setting their lenses up, and plunging into the water only to come back with blurry shots.

Can someone confirm my calculations involved with UW "hyperfocal" distances?

For example, to find where you need to focus to get a sharp image from the closest point to 16" (infinity focus with a 8" diameter dome):

1) calculate hyperfocal distance with this formula. For Canon 1Ds, 15mm at f8, 8" diameter dome port, using 0.030mm for the Circle of Confusion, the hyperfocal distance (H) is 1077.5mm, or 3.54 ft.

2) plug H back into "Far distance of acceptable sharpness" formula with 16" or 406.4mm as Df (far distance for acceptable sharpness), and solve for s (focus distance) to get 298.11mm, or 11.7". That means that you need to focus at 11.7" to get "infinity" (16", with 8" dome port) focus underwater. (*note that this confirms the anecdotal evidence shared with me by more than one working pro: "use a 15mm lens, set focus to 1', and shoot away!")

3) plug focus distance s back into Dn (near distance for acceptable sharpness) to get 235.39mm, or 9.26".

For Canon 1Ds, 15mm at f8, 8" diameter dome port, using 0.030mm, if you focus the lens to 11.7", the picture will be sharp underwater to infinity, but the closest point of acceptable sharpness would be whatever the underwater focus distance would be if the lens were focused at 9.26".

Bear in mind that I used the horrible Windows XP calculator for some of the math, so there may be calculation errors.

[apete]

I played around with hyperfocal distance calculations. The formulas I used are slightly different from yours:

http://www.nikonians...yperfocal5.html

I have a feeling the ones you used are "more" correct. Using a dome port - focusing close - the difference may matter. (I don't have a dome port.)

As you noted infinity needs to be redefined u/w. Visibility is limited and with a dome port you're photographing a transformed image of the world.

Here's what I would do:

1) Calculate the hyperfocal distance (H) for the lens you plan to use.
2) Create a map that transforms between real distances and "dome" distances - and the reverse.
3) Use the H you just calculated with the formulas for Df and Dn to find a suitable focus distance bearing in mind that the lens sees the transformed dome world.
4) Transform Df and Dn back to "real world" distances.

With macro and/or dome port photography focus distances are short; errors due to approximations become relatively large. I don't have a dome port and don't use DoF estimations when shooting macro.

I think these calculations are only useful when the difference between Dn and Df is significantly larger than the error you make when you estimate distances.

With my Olympus C-5060 WZ I can get a DoF ranging from roughly 2 feet to infinity practically regardless of aperture. If I want to focus closer than that, and I can only use larger apertures, the numbers fast become less appealing.

A problem I have is that I don't know from what part of the camera/housing to measure/estimate distances. The only logical reference point would be the film/sensor plane. My camera can focus at 3cm. That would have to be measured from the front of the lens.

/Anders

[echeng]

2) Create a map that transforms between real distances and "dome" distances - and the reverse.

I would guess you can map this linearly: Assuming ideal lens placement, the closest focus point is the 1/2 the dome's diameter [edited], and infinity is 2x dome diameter. (this is also assuming that focus distance is the distance from the end of your lens to your subject).

Is that right?

[apete]

I really don't know this subject, but…

I too would guess it is a linear map.

How do you linearly map infinity to "2x dome diameter"? You can photograph objects that are at "2xDD" + 1cm distance. The "2xDD" rule must be deduced by some formula that tells you everything further away than "2xDD" can be estimated as infinity. I assume that formula will give you the linear factor you're after. I don't know what that formula is.

Good luck, /Anders

[echeng]

No, I mean that for the mapping land focus->underwater focus, 0.5xDD maps to 0.5xDD [edited], and 2xDD maps to infinity, since focusing at 2xDD focuses to infinity (and focusing beyond that gives a blurry image).

[echeng]

Also, I mean "linear" in terms of the focus distance -- not in terms of how far you have turn the focus ring (obviously, since that never linear).

Now will someone familiar with optics please step up and assert themselves authoritatively?

[apete]

I imagined that everything was just moved closer by a constant factor (and again, I don't really know anything on this subject).

0.5xDD => 0.5xDD *
1.0xDD => ?
1.5xDD => ?
2.0xDD => Really far away
2.5xDD => Even further away

*) Provided the lens can focus that close. This is on the dome surface, isn’t it? In terms of deducing a linear factor, this must be treated as zero, and all distances be reduced by 0.5xDD. (Just guessing!)

/Anders

[echeng]

Oh -- you're right. I meant 1/2 DD, and not DD. Replace all instances of that in my last two posts.

But more than 2X DD shouldn't be in the list, since it's past UW equivalent of infinity focus, right?

[apete]

I was proposing the idea that "really far away" and "even further away" are both considered to be "infinity". Mathematically they would have to be mapped to two different numbers - one larger than the other - but both large enough for you to consider them too large for practical use.

If the map has a singularity at 2xDD it can´t be linear!

If nobody else joins this thread you need to buy a book. ;-)

[echeng]

Hehehe. I may just have to buy a book.

I'm confused at what happens when you focus past 2xDD. If 2xDD is a singularity, then we know what happens as we approach 2xDD -- but that lies beyond it is what? Undefined?

[echeng]

I think the answer to this thread is: focus at 12" and stop thinking any more about it.

[apete]

I really do think the map is linear, and there is no singularity!

2xDD is a practical limit.

[james]

Eric:

Jump in the water with your setup and swim away from the dive platform.

Get a focus lock on the dive platform and take a shot.

Turn off your camera.

Dry off everything and get it opened up on the camera table.

Mark with fingernail polish on the lens focus ring and-or distance scale (is it inside the lens???).

Tell us what it says...:-)

Shouldn't you be able to line up the marks next time? Better yet, can you take a shot of the distance scale on the lens and print it out for future reference?

Then repeat as needed for close focus.

Cheers
James

[echeng]

Swim platform? You mean, in the gym pools here in New York City?

What I want is to focus somewhere on land to get UW equivalent of hyperfocal. I want it to be sharp from X to infinity! My calculations say 11.7". James Watt says 12", through experience alone. Pretty close.

But yeah. At some point I may do some pool tests where I focus on a variety of things and see what the land-focus equivalents are. You know, in all my spare time. Then, I'll just graph the thing and carry it with me.

[james]

Yep, in a pool.

I'm an engineer so I like to calculate things.

But I also do construction management - so I like to TRY it and make sure it works first too...:-)

Ike has reported that hyperfocal underwater distances change depending on the density of the water. Truk Lagoon and Red Sea require slightly different settings.

Cheers
James

[echeng]

Then why has Ike been so silent? IKE! Speak up? We need your expertise!!

[Peter Schulz]

Forever I have wanted to use manual focus with my 5050 with a WAL and INON dome port focused on the hyperfocal plane. From calculations and experiments, the best I could figure was that the hyperfocal plane was at 31", if it in fact it exists for the INON WAL/dome port setup i.e. the dome port may not be a spherical section and/or the section may not lie on an arc with the lens.

Based on calculation and trial and error experiments I figured the distance at 31". And at 31' I got some pretty good pictures. For samples see Group 4 at the following URL.

http://www.splashdow...ple_gallery.htm

The beauty of these settings is that there is virtually no shutter lag AND no focusing light is needed for night dives or dark engine room shots.

But sometimes the pictures just didn't seem as well focused as the 5050 can do in auto so I stopped messing around.

Then I saw a forum entry that explained how I could let my camera do the math by letting the camera calculate and display the focus distance at various actual distances.

http://www.digitaldi...y;threadid=5769

Shooting from 3 to 5 feet actual I got readings of 24 to 30 inches. Shooting at infinity the camera saw something much closer to 2.6 feet than 10 feet. While these measurements are not precise, I concluded the following.

1. For my 5050 setup with an INON WAL and dome there is not a true hyperfocal plane.

2. But now when I do use this approach, I feel comfortable with 31", or maybe a few inches less, as the best I can do.

YMMV

[Jolly]

Hi Eric. The mapping would not be linear.
I've calculated some distances to make a table for your 8" dome.
This is just roughly. I did not take into consediration:
glass thickness, outer and inner curvature, exact refraction index of a certain dome (acryl for example would have something around 1,49).
Water was assumed with standard 1,33. By the way, the influence of the variable water index is very little compared to the dome refraction and curvature values. But I don't think there is any manufacture who delivers these values.
Ikelite doesn't even give the radius/diameter which I have asked for. top secret company information for future buyers :-)



cheers,

Julian

[echeng]

Julian - that is great!

Now, is it possible to get that same chart, with closest and furthest values for acceptable sharpness, given a COC value?

And...

uh, how did you figure that out?

[Jolly]

Eric,

yes, it is. But here the lens comes in. what lens you use with this dome? I guess on you 1Ds?

For the time being I try to figure out what housing / dome for my Canon 10D / EOS30. Since no housing builder is able to give information on the dome/size/focus stuff I asked a german engineer who is publishing a lot of uw photo books over here.

I have bought two (paper) files and he has supplied me with a lot of mathematic background based on Cousteau's calculations.

Julian