flat port optics

Hey all.

I thought I had a website with reference for this, but can't find it now.

When you put a lens behind a flat port, underwater, the effective angle of view is reduced by some factor. What is the commonly accepted factor?

Also, the apparent distance is changed.

So, if I have a lens focused at 20 inches, in air, when I put it behind flat port, and in water, where will the point of sharp focus be? Is it pushed out? By what factor?

We have a "dome port" theory article here, but no "flat port" theory...

Thanks,

Don

Check out this link:

http://wetpixel.com/forums/index.php?showtopic=7566

Older UW photobooks go into this in more detail. In-Water Photography by Mertens, 1970; published by Wiley, in particular. It is replete with formulas and graphs.

Tom

Yup - thanks Tom. The actual physical focus distance is moved out, and the magnification remains unchanged.

Err, the thing I didn't see what by what amount.

I.E. If I have a lens focused at 20 inches, (on the lens barrel), what will the actual phyiscal distance be, corresponding. Obviously, the apparent distance will be 20 inches.

Thanks!

Don

Don,

1.33 times 20 or about 27 inches. This is measured from the film plane.

Hope this helps

Tom

Just to make it a bit more difficult... What is the effect of the distance between the lens and the inside of the flat port.? Does it matter if the port is a bit long for the lens and if so how does it effect the optics? As most lenses change length in focusing or zooming this may be a factor.

Too long a port could vignette like using too long a lens shade. Effects of dispersion increase with distance from the glass-air interface too but may be too slight to detect.