The "virtual image"

Hi guys, Maybe someone in here can explain the virtual image created by a dome port in a way that a "physics challenged" person can fully understand.

My understanding is that when shooting with a dome port, that a "virtual image is projected" a few inches in front of the Dome, which requires a close-up filter with zooms that can't focus closer than the "apparant image". My questions, that I cant seem to find an answer I can understand are.

1: Why is the diopter ony required for a zoom lens? Or do you still need one if you are using a fixed focal length lens inside a dome port.

2: What if you put a 17-35 or such zoom lens in a dome port and did not want to zoom, but only use it for wide angle, is the diopter required?

3: If a dome of a certain size creates a virtual image 12" in front of the port, and if at that focus point "everything" is in focus, why can you not just set your camera for this point and leave it the whole trip? ( I know you can't...just dont know why?)

I have been doing this stuff for several years now, and I thik its time to try and really understand what is happenning down there.

Help the scientifically challanged amongst us.

The dome port converts infinity to some point a certain distance from the front of the dome, that distance being primarily a function of the radius of the dome. The ability of your primary lens to focus beyond that point ends up wasted. Objects that are closer than infinity will appear in front of that point, so the net effect is that the full range of focus gets compressed into a smaller range in front of the dome. It's essential that the primary lens be able to focus through the bulk of that range. Macro lenses may be able to do it. Very short primes and fisheyes can also. All others need the help of a diopter or possibly extension tube.

So:

1. Zoom lenses don't inherently required diopters, but they typically don't have sufficient close focus ability.

2. That depends on the minimum focus distance of that lens at the zoom setting chosen.

3. The virtual image you're talking about is for objects at infinite focus. Objects closer in will have virtual images inside of 12".

Here is a source of discussion on dome port optics and other things.

Thank you Craig.....that makes perfect sense.

And thanks for the web link.

Here's my "simple" explanation:

Start with a dome with an "infinite" radius - that's a flat port. There is no virtual image.

Now start curving it - give it a little curve - like a trashcan lid for example - the virtual image might be 10 or 15 feet out.

Now curve it a lot - like a 12" radius dome - the virtual image moves closer in. As the radius of curvature of the dome gets smaller, the virtual image moves in VERY close.

See - it's the radius of curvature of the dome that matters, NOT the diameter of the dome. That's why Ike's domes work - even though they're smaller in diameter.

So for small radius domes, the lens has to be able to focus close. Most primes from 14 to 24mm have no problem with this (they can focus at 9 inches in some cases) but zooms may be a bit trickier as only the REALLY expensive ones focus close.

Before housed autofocus lenses people would prefocus their prime on where they "thought" the virtual image would be and then shoot a test roll. Then maybe adjust the focus an inch or two and dial it in - then NOT change it. No need for autofocus for wideangle. Maybe SLIGHT need to adjust for CFWA vs shooting "way out" palegics - but that's only two settings.

Pretty cool eh?

Cheers

James

Ya, pretty cool James, but now I am lost again.....

Ikes dome ports require a diopter if your lens cannot focus down to 12" per his site. From what I understood of Craigs earlier post, I would assume that means the "apparant image is 12" where infitiy becomes in focus, and that at that point extra focus would be wasted.

Now, I am shooting a subject closer than the infinity point. Therefore the subject of the image would have a virtual image somewhere inside the 12" required. If I have a lens that focuses down to 12" would I not still need a diopter?

Told ya I was "Physics Challenged"

There is a distance, called the hyperfocal distance, that is important in this discussion. If you focus your lens at the hyperfocal distance, then everything at or beyond that distance will be "in focus". This explains how a lens can have "infinite" focus. Objects in front of the hyperfocal distance may also be in focus depending on depth of field.

When using a dome port and focusing on the "virtual image" you are really focusing on the virtual version of the hyperfocal distance. Since wide angle lenses typically provide large depth of field, focusing somewhat in front of the virtual image will provide great depth of field. Subjects sufficiently close will not be in focus when you do that, though.

While what James said was true, his language is a little dangerous. We all know that the diameter is two times the radius. What James said was that the diameter of the port is not necessarily two times the radius of curvature. Radius of curvature is what is important. The center of the radius of curvature needs to be at the nodal point of the lens and the nodal point moves around based on focal length. That's probably more than you want to know. :freak:

What James said and what I said are not in disagreement even if they appear to be. The dome compresses the overall field. The wide angle lens provides inherently great depth of field. If you combine them and set the focus in the right place, you can achieve focus that appears to range from infinity to quite close in.

Still, the answer to your original questions remain the same. The dome does not obviate the need for focusing. It only changes the range where focusing is important. If your lens can focus through the required range, then no diopter is needed. If it cannot, then a diopter will modify the focusing range to make it work. Guidelines for estimating the required diopter work because the focus range of a dome is determined by it's radius of curvature and the focus range of a lens will be determined by diopter strength. Match them up and you are set. I don't recall that guideline but it's been posted here before. I think it's best to ask the manufacturer for the specific recommendations.

Every thing above is correct. There are two important things to consider. One is the radius of curvature of the dome port. Note when Seacam or Subal say a 9" dome they are refering to the radius of curvature. The large the radius of curvature the further out the virtual image created by that dome port. Also there is less lenticular and chromatic aberation created with a longer radius of curvature. With the big domes you often do not need a diopter; even with a zoom lens.

The other thing you need to know is the close focusing distance of your lens. As previously noted often prime lenses focus closer than do zooms. My 17-35 focuses to about 8" (The stat sheet says 12" but if focuses closer than that) I have a 9" dome and don't use a diopter with it.

As noted above a small dome can have a longer radius of curvature. Subal makes a small dome port designed for the 20 mm lens that has a 6" radius. The Ikelite domes have a longer radius of curvature than their size might indicate.

In summary the key thing is the close focusing point of your lens. Lenses with an 18" or longer close focusing DX are of less use UW. All else being equal a dome with a longer radius of curvature would be better.

The lens should be placed so that it's anterior nodal point is at the focal point of the dome. People sometimes put the lens closer to the dome. The lens will focus on closer subjects if you do this but the optical quality of the shot will decrease.