Domeport question

Is there a standard to domeport construction, so that the center of curvature is flush with the entry level to the port where it attaches to the housing? In other words are domeports usually or always half a sphere mounted to the port frame?

Usually dome ports are slighty less than hemispheres - so that the nodal point of the standard lens they are designed for (usually the Nikon 16mm for fisheye domes) is at the centre of the hemisphere they proscribe.

I would guestimate that many are between 80-90% of a full hemisphere.

Non-fisheye domes are usually much smaller section of a hemisphere.

Alex

If you want to measure a dome port's diameter, photograph it side on (making sure your are square on to it) with a ruler also square on and in the plane which bisects it. Then import the image into photoshop, draw a circle which fits the dome and drop appropriate guides to read off the diameter. Use a longish rather than short lens. Not a perfect way to do things, but close enough for jazz!

Boy, have I been living in the US too long - 'half a sphere' instead of hemisphere!

I guess now I'm obsessing about exactly where the front element of my lens should lie in relation to the entry of the port, and of course how critical this is when the front of the lens extends into the dome with zooming, like the 17-55.

If the front element of the lens moves in/out when zooming*, it could be a problem if the nodal point of the lens moves also.

*(the 17-55 has internal focusing; so, I don't think its front element moves in/out when zooming)

I have used the 17-35 with good results behind the 8" aquatica dome. (no problems, no diopter)

Take Care,

Chris

The 17-55 does move a couple of cm into the dome as one zooms to the wide end, if it is set up to be flush with the port opening at the tele end. I'm not sure how significant this is.

If the front element of the lens moves in/out when zooming*, it could be a problem if the nodal point of the lens moves also.

<snippage>

Take Care,

Chris

Small quibble: to the best of my knowledge, the entrance pupil, not the nodal point (neither front nor rear) is what should be at the center of the dome. Luckily, the entrance pupil is easier to locate: it is the plane in the lens where the image of the aperture blades appear (or can be located using a pano head). Many people (pano- and dome-heads) say nodal point when they mean (I think) entrance pupil.

All the best,

Chris in Red Stick

A dome can be any percentage of a hemisphere. If you want to know the radius you have to physically measure it with a lens clock.

The focus of the dome and lens you are using should be on the posterior nodal point of the lens system. This is where by definition all light rays are focused.

A rough a dirty way is to open your lens all the way. You should be able to see your reflection in the aperture of the lens. Your reflection's position should remain in the center of the lenses aperture no matter what angle you look at the aperture from.