I need help with focus /hyperfocal distance

My mission today was to test the hyperfocal calculations for my E-330. I am testing some things out to make some long exposure shots in a cavern. I figured I would try the wide end of my lens so I manually set it at 14 mm. I would not be doing long exposures on these dives so I opened the aperture to 2.8 and set the iso at 200 (not supposed to have much more noise than 100). The calculated hyperfocal distance for my settings was 15.2 feet. The lens has no markings between 3 feet and infinity so I set the focus to auto - paced off 15 feet and focused a shot. I switched to manual focus without changing anything so it was locked at that distance. Below are the less than stellar results. There is no focus point at all and there should be focus from 8 feet to infinity. Even if my paces were off, there should be a pretty good zone of focus somewhere in the picture. My guess is two things may individually or combined be affecting the focus. First, the calculator is for air not the magnification of the water. The dome port should correct the magnification but may be the second source of the problem. The dome port's virtual image is within a foot of the camera so maybe I can't use the hyperfocal calculator for this virtual image. Can anyone shed some light on this for me?

The concept of hyperfocal distances doesn't really have a place in underwater dome port photography.

The dome-shaped interface maps objects at infinity (in real space) to a point approximately 3 dome radii away from its front edge (in virtual space). Objects in real space which are closer than infinity are in turn closer than 3 dome radii in virtual space... so the real field (zero -> infinity) is compressed into a virtual field (zero -> 3 x dome radii). This is explained in depth here.

Setting your camera lens to focus at any point outside of this virtual field will result in an unfocused image... as your shots have demonstrated.

I hope this helped!

Keri

My mission today was to test the hyperfocal calculations for my E-330. I am testing some things out to make some long exposure shots in a cavern. I figured I would try the wide end of my lens so I manually set it at 14 mm. I would not be doing long exposures on these dives so I opened the aperture to 2.8 and set the iso at 200 (not supposed to have much more noise than 100). The calculated hyperfocal distance for my settings was 15.2 feet. The lens has no markings between 3 feet and infinity so I set the focus to auto - paced off 15 feet and focused a shot. I switched to manual focus without changing anything so it was locked at that distance. Below are the less than stellar results. There is no focus point at all and there should be focus from 8 feet to infinity. Even if my paces were off, there should be a pretty good zone of focus somewhere in the picture. My guess is two things may individually or combined be affecting the focus. First, the calculator is for air not the magnification of the water. The dome port should correct the magnification but may be the second source of the problem. The dome port's virtual image is within a foot of the camera so maybe I can't use the hyperfocal calculator for this virtual image. Can anyone shed some light on this for me?

 

As Keri explains, the concept is that when you use a dome port you are actually taking a picture of the image of the scene that the dome creates. Forgetting the math (Craig forgive me) what happens is that the dome in the water acts like a big lens and this lens projects an image of the scene. You are attempting to photograph the image with your wide angle lens. Depending on the situation and the size of the port the focal point of the image made by the dome port will almost certainly not be at infinity, hence your problems.

 

Bill

So are you (way smarter than me) guys saying that there is no point in even considering hyperfocal distances when using dome ports? When I was in Rangiroa recently and wanted to shoot some video with the Canon 5D MkII, since it can't auto-focus (in video mode) I checked the hyperfocal distance of my 16mm lens, and tried to focus on something roughly that far away. The results were pretty good (seemed to have good "hyperfocal" depth of field), so I thought I had done the "right" thing.

I assumed that the camera was focusing on the dome's virtual image but was still "interpreting" that image with hyperfocal properties....but if I read you right, there's no point in even THINKING about hyperfocal distances with dome ports because they just don't apply(?).

Ahem,

Gentlemen, please let an "old dog" share some insights with you......

As all have stated for wide angle lenses, especially fisheye lenses, the "apparent image" on a dome port UNDERWATER may be in a range from let's say 7" - 1.1' for subjects physically 2' - infinity.

Many users from the "film days" like stellar photographer Ralph "Larry" Oberlander know in using a fisheye with great depth of field many times you can just set your lens at 11" to 1' or so and forget about focussing. He did this while snorkeling with the dolphins on a Bahamas Sheaarwater trip a couple years back and bagged his prize winning in focus image. Point and hold the trigger down for multiple shots! (I think it was on his Nikon D200.)

ZERO auto-focus lag with the preset focus method too. Yes, DOF (depth of filed) will vary a teensy bit depending on f-stop. but not that apparent with a Fisheye lens. A higher shutter sped of at least 1/125 is more important than f-stop.

As Bruce states using a Fisheye lens with the new crop of HD video capable dSLR cameras will work via the same process.

I have a Rebel T1i with the Tokina 10-17mm I'm testing in the pool Sunday in preparation for my Whale Shark excursion to Isla Holbox July 11th. The method of pre-focussing is how I will focus maybe 2 feet from my subject and roll footage. Canon bodies let you do this with your lens in AF with the * button. Once you focus and let go of the * button focus is locked at that distance (at least in video mode on all dSLR bodies to my knowledge.)

So don't over-think it. Set your lens at 1' behind a dome port and go out and shoot some test pics!

Or if you have a way to try slightly different focus distances try them and review the results on your computer.

Odds are you'll find the optimum for your particular lens and dome port and speed up the whole capture while still getting in focus shots.

YMMV

dhaas

Thanks for all the help!

I figured it would be the virtual image. I'll be getting out there and doing some practice shots before going to the cavern.