How to calculate focus point for small dome port lens?

I read on the forums that the hyperfocal length of a camera with a dome port lens is always 2xDiameter. But this doesn't seem right because we can't seem to get them in focus underwater. It is made harder because the camera I am working with here at work, you have to take the housing apart to adjust the focus. The underwater subjects will be from 2-6m from camera. Our dome port lens is 3" wide with a radius of 1.6". Other specs are 7.4x7.4mm CCD, with 6.5mm lens with 1:1.8 aperature. So how should I focus this?

Thanks,

CP

Hi CP,

Sounds like an interesting project you got there. I don't know how accurate my knowledge is, but I'll give it a go.

Physical objects at infinity appear in the virtual image created by the dome at a distance from the dome of 3.03 * r, where r is the radius of the dome.

Your 'r' is 1.6" or 40.6mm, giving you a distance from the dome to the virtual image of about 123mm. Add onto that your air gap and you're looking at something in the order of 160mm from the entry pupil. I'm assuming you have the entry pupil lined up ok with the centre of the dome.

How does that tally with the focal distances you've been trying?

A subject at 2-6m is not far from infinity as far as these optics are concerned, but you might want to dial a slight front focus to get the best results. Your biggest issue may be trying to get the curved image in focus to keep the edges sharp. Thinking about it, you're looking at a field of view similar to a 25mm lens on 35mm format, so edges should be fair but worth watching.

HTH,

Martyn

Very intrguing!

Maybe a slightly simpler way to look at it - infinity object appear exactly as Martyn says, at 3.03 x radius of the dome from the front of the dome = 123mm.

The other important piece of info you need is the distance behind the dome of the sensor plane (Most cameras have a mark on the body to show this, often a circle with a line running across it, the line marking the position of the sensor plane). Measure the distance from the sensor plane to the dome and add this distance to the 123mm you already know and that is the distance to set on the lens for a subject at infinity.

As your subjects are at 2-6m the image of them the dome creates will be a bit closer than this maximum distance, but not much, again as Martyn says, so set the focus a bit closer.

I know it'll be a apin, but I'd run a test with the focus set at the maximum calculated distance, then a bit closer, then a bit closer again until you get sharp images over the 2-6m range.

Good luck!

So if I focus on land, do I focus at infinity or at the 160mm range? Or do I focus to infinity on land and u/w everything from 160mm on will be infinity?

Answer #2

Focus at just short of 160mm in air. That will cause the system to focus at about infinity underwater. That's the best guess starting point, then tweak once you get enough in focus to evaluate.

Good luck,

Martyn