Has someone tried to use extension rings instead of dioptres for wideangles behind a dome?

I would expect them to be more critical in terms of image quality when using with wideangles instead of macro lenses. But I do wonder if someone has tried?

regards,

Julian

I don't think one can find an extension tube thin enough for the job. Nikon made a very thin one in the K set but it will not work with modern bodies. Zooming and floating/rear element focusing designs may be a further complication since focus is optical rather than simply a variable physical extension of the lens.
Cheers!
Tom

Julian,

Can you clarify? You said extension RING. Can an extension ring that changes the dome position be used as an alternative to a diopter? I have heard that moving the dome closer can help w/ close focus...

Cheers
James

I assumed he was referring to rings that go between a camera body and lens because these are used to focus closer than what a lens can do otherwise. We have two kinds of extension rings in UW photography! Back when macro lenses only went to 1:2 one had to use an extension ring to go to 1:1 for example.
Cheers!
Tom

Yes, not a port extension but an extension ring fort the lens.



Controversially placing a dome far away increases close focus due to a greater (but optically not always correct) radius. Placing a dome too close decreases close focus (smaller effective radius).

The idea behind is: Our Sigma 12-24 does not allow a dipotre (you have the lens as well, right?). Don't have close focus problems (5" dome radius and accordingly placed! :-) ) but would like to do some tryout.

Tom,

thank you. Do you have an idea how to determine the required “thinness” as +3 dioptres are quiet commonly used behind a dome?
Has someone tried extension rings topside with a wideangle?

Thanks,

Julian

I agree with Tom, I dont think you it is possible to make one thin enough to work on a 12mm lens.

Extension Tube math.....

magnification = extension / focal lenghth

So take canon's 50mm macro, it focus at infinity and 1:2. At infinity it has 0mm of extnension and at 1:2 it has 25mm of extension built into the lens.

Now add a 25mm extension tube. At infinity setting you have 25mm of extension so the minimum maginifcation is 25/50 = 1:2. And at close end you have the 25mm of extension + 25mm built into the lens for a total of 50mm and and 50/50 = 1:1.

But with a 12 mm lens even if you found a 2 mm extension tube, infinity would now be at 1:6 or less than 10 inches away.....And good luck finding a 2mm extension tube.

Plus you would need an extension tube that would pass through the autofocus, aperture, etc.

Cheers
James

James, welcome to Canon world, tubes including EF lens signal transfer are available.

not 2mm, but 12mm. I will see what radius the dome has to have in order to work with the smallest available tube .... .

Julian

I was wondering due to the following description for the Canon AF Extension Tube EF 12 II Macro:
<<EF 12 II Extension Tube is especially suited for close-focusing with Wide-Angle lenses .... >>

A 12mm extension tube on a 12mm lens would bring infinity to 1:1. I suspect that the object would have to be touching the front element and that is the furthest the lens will focus.

When they say wide angle they mean something more along the lines of a 30mm lens.

For an extension tube to work with a 12mm lens you would need a tube no longer than 2mm.

ok, went thru the numbers, you are right, even 12mm is far too much.
mh, would have been nice :-)

acroporas, thanks,

Julian