38 replies to this topic

[echeng]

Actually, I use a 1Ds, 15mm fish-eye lens or 20mm lens or 16-35mm lens, and the 9" superdome. What I'm looking for is a sheet of paper or graph with air focus distances, and the virtual equivalent with near/far sharp values for a certain COC...

[acroporas]

Ok, I took my ikelite dome port and calculated the radius of curvature. It is 3.33 inches. Or a diameter of 6.65 inches.

For anyone wondering or anyone who would like to verify my measuremtns and calculations, here is how I calculated the radius.
I took a picture of the dome taking care that the dome was level and square with the camera. In photoshop I measured A, B, and C (see picture). I then measured C'=1.75" with a ruler. To convert the photoshop units to inches for A and B, I called these new measurements y, and x.

y=C/C'*A
x=C/C'*B

Then to calculate the radius I derived this equation from the equation for a circle.

r=(x^2 + 4 y^2) / (8 y)

[james]

That will give you the OD of the dome.

ID = OD - 2(t)

Cheers
James

[acroporas]

I agree that for this application it is important to specify ID/OD probably need to have both to be as accurate as possible. So here we go...

I dont know any nifty tricks to measure the thickness but it is pretty thin. I would guess it to be somewhere close to 1/8" and would give IR/ID = 3.2/6.4

So james are you implying that when manufacturers give the diameter it is the inner diameter? And do you think that when they say it is an x inch dome that they are really accurate to within +/- 2t anyway. Would Ikelite use the ID and call it a 6.40" dome, the OD and call it a 6.65" dome. I would bet that they would just round it to 6.5"

[scorpio_fish]

You think too much. I use hyperfocal distances on land when appropriate. I have to have them written on cards. They are quite handy when you have lens choices and can vary your subject distance (the old foot zoom) or choose any old f/stop you want because your camera is sitting on a tripod.

Focus on your close subject, stop down as much as feasible for your ambient exposure, and let the background go fuzzy if it has to. Not many will notice your soft background, but everyone will notice a fuzzy/soft well lit foreground.

[echeng]

Perhaps.

However, if you are diving with very fast animals -- at night, for example, and need a one-setting focus point for the highest odds of a sharp photo, we need to know these numbers.

I spoke with Ernie Brooks at the Boston Sea Rovers clinic. For the last 40 years, he has used one camera, one lens, one f-stop, and one focus point (three ticks out). Pretty amazing results from settings like that.

Also, where do you focus on your subject? I always focus on the eyes, but perhaps it is appropriate to focus slightly behind the eyes to get more of the body in focus, while keeping the eyes sharp.

I'm not saying that I'd consult a graph every time I was about to focus the camera. But it would be nice to see some numbers to have an idea.

[Jolly]

Thanks William, now I have an impression of that piece. Is that the 5503.15 for fisheye?

Eric,
first I have created a "hyperfocal uw chart" for the 15mm with 9" dome. circle of confusion assumed with 0.03mm (theoretically your 1Ds sensor would be able to deliver more, even under 0.02mm).

It gives the distance to set on the lens in order to have the biggest depth of field while maintaining infinity sharpness.



I hope I did not make mistakes with the calculations. So no guarantee for sharp pix :roll:

Julian

[echeng]

Julian -

You should charge a dollar per calculation.

I think we should get some of these out into an article, with some common setups that people might have. Are you up for doing some more, if I send you a list of sensor sizes, f-stops, dome-diameters, and COC values?

[Jolly]

Eric,

in general yes. But it can take some time. I also have to make sure that the stuff is correct and I did't forget something (school time is far behind) :-)

you have my email adress out of the forum?

[acroporas]

Julian, no it is 5503, I just removed the shade so I could make the measurements. But in your post about 5503.15 Ike said that both are cut from the same sphere just mounted closer to the housing so I would assume that that means the black part has a steeper angle on the 5503.15

[acroporas]

I think we should get some of these out into an article, with some common setups that people might have.  Are you up for doing some more, if I send you a list of sensor sizes, f-stops, dome-diameters, and COC values?

Why not just add a a little web program to the site that allows you to enter the infromation and it will return the data. It should be very easy to program.

[Jolly]

thanks william. yes, the 5503.15 should be the same size.

a little program? ouh, JAVA etc. ...... don't ask me :-)

[ikelite]

Ikelite dome was 3" inside radius / 3.218" outside radius. These could have changed slightly when mold was polished and/or resurfaced twice over the years............

[acroporas]

Ikelite dome was 3" inside radius / 3.218" outside radius.  These could have changed slightly when mold was polished and/or resurfaced twice over the years............

ThanksIke, that makes me proud. I was pretty darn close, 3.3 vs slightly >3.2.

[Jolly]

thanx ike, wasn't to hard, was it?!?!

[apete]

Julian -

You should charge a dollar per calculation. :)

I think we should get some of these out into an article, with some common setups that people might have.  Are you up for doing some more, if I send you a list of sensor sizes, f-stops, dome-diameters, and COC values? ;)

Please specify (and explain) the formulas used.

[apete]

Now I've done this for my Olympus C-7070WZ with a WCON07 and an Athena dome.

Excel document with calculations

Look at the Dome DoF sheet.

If I set the focus to 27cm then everything will be (appear) sharp from at least 0,5m to infinity regardless of aperture. That seems a good general purpose setting.

With the smallest possible aperture, and focus set to the closest possible, the range starts at 4cm and goes to infinity.

[Paul Kay]

Hmmm. Just back from a trip actually taking photos. I've thought about this topic on and off for a long time. Despite a technical background, I've decided that there are too many variables and too little time to work it all out. This said, it would be interesting to see just how much differential there is in the frame corners (ie loss of focus at different apertures) from the frame centre if anyone is prepared to calculate this.

My reckoning is that above a 90 FOV you can show that corners will probably remain soft on most closely focussed subjects - but hang on, don't we know this emprically already?!

[dhaas]

Amigos,

Not sure about UW but the great Lazaro Ruda has used this exclusively I believe to get his over / under Gator pics witha big dome port. You could try to PM him here or over at DDN.

dhaas