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dome port and nodal point effects

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Please excuse my ignorance on this point, but what effect does moving the dome port closer to the lens have on the closest focus distance of the lens, if any? Of course, I know that the optical quality will suffer at the edges when the port isn't at the optimal position, but I'm not sure what the close focus will be. I ask because I have a tokina 12-24mm behind a Ikelite 8 inch dome with the proper extension ring, but would like to know how I can get it to focus closer, if possible. Thanks.

 

Scott

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Maybe, a +2 diopter* and use the correct port extension. (moving the port would just mess up the edge focus)

 

*note: with the diopter you will be able to focus closer. But, you won't be able to focus at infinity.

 

Take Care,

ChrisS

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Maybe, a +2 diopter* and use the correct port extension. (moving the port would just mess up the edge focus)

 

*note: with the diopter you will be able to focus closer. But, you won't be able to focus at infinity.

 

Take Care,

ChrisS

 

I'm pretty sure you can focus at infinity UW with a +2 diopter, just not topside. I believe an 8" dome will be optimized with a +3 diopter, which would adjust the lens' focus range to that of the virtual image created by the dome port. Dual element diopter's are better, I use a Canon 500D which is a +2 i think. There are a few old threads about this topic and a good article on Dome Theory in the articles section.

 

like ChrisS said, make sure you got the proper port extension first. Cheers,

 

Chris

Edited by Undertow

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Scott

 

Your question is loaded! Theoretically, if you know that you are going to use a lens at a close focus point then in a perfect world, the dome should be positioned so that it is optimised for that lens at the close focus point. This is because the lens components do shift as the lens focuses closer and alignment should be made with the components in this position.

 

However in practice, and especially with a zoom lens, you have already made a whole load of compromises with the dome position in relation to the lens, but.....

 

If you want the lens to focus closer the answer is extremely simple - use a higher powered diopter! Whilst either a +2 or +3 should help realign focus with a correctly set up 8" port, you can happily use a +4 which will curtail the maximum focus distance but should allow you to focus closer at the close end. Be aware though that the corners will suffer the closer you get and that, depending on the close focus of your lens, it might be possible to focus on the port itself, or if not the port then closer than the virtual image it creates, which will not look good!!!

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Scott

 

Your question is loaded! Theoretically, if you know that you are going to use a lens at a close focus point then in a perfect world, the dome should be positioned so that it is optimised for that lens at the close focus point. This is because the lens components do shift as the lens focuses closer and alignment should be made with the components in this position.

 

However in practice, and especially with a zoom lens, you have already made a whole load of compromises with the dome position in relation to the lens, but.....

 

If you want the lens to focus closer the answer is extremely simple - use a higher powered diopter! Whilst either a +2 or +3 should help realign focus with a correctly set up 8" port, you can happily use a +4 which will curtail the maximum focus distance but should allow you to focus closer at the close end. Be aware though that the corners will suffer the closer you get and that, depending on the close focus of your lens, it might be possible to focus on the port itself, or if not the port then closer than the virtual image it creates, which will not look good!!!

 

Thanks to all of you for the quick replies. Firstly, can anyone suggest a good brand and source for diopters? And , secondly, on a somewhat related note, how close can you focus with a tokina 10-17mm behind an 8 inch dome with the proper dome extension? I may look into one of these.

 

Scott

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Firstly, can anyone suggest a good brand and source for diopters? Scott

 

Any 'good' manufacturer - B&W, Hoya, etc. I'm personally using a Marumi +3 with excellent results at the moment.

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And , secondly, on a somewhat related note, how close can you focus with a tokina 10-17mm behind an 8 inch dome with the proper dome extension? I may look into one of these.

 

Scott,

 

I just started using a 10-17 with an Ikelite 8" dome. You can get within a couple of inches of your subject (dome to subject distance) and still focus. I thought I was going to hit some of the subjects I was photographing.

 

Regards

 

Jon Bertsch

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how close can you focus with a tokina 10-17mm behind an 8 inch dome with the proper dome extension? I may look into one of these.

 

Scott

 

I've never taken it UW but i know the tokina 10-17mm can focus ridiculously close, about the same as my sigma 15mm FE which can focus on air bubbles on the dome port. This makes both extremely useful at close range and the reason why I traded in my nikon 16mm FE for the sigma. My sigma, like the tokina affords the ability for 'macro wide-angle' shots of any subject that will let you poke a dome port in their face.

 

1805043520_f5f5a7fb22_o.jpg

 

this lionfish was only a couple inches from my dome and i have others where he's even closer, but without the wreck which really gives the 'macro wide-angle' feel. . Try getting corners like that with a rectilinear wide lens! If you want close focus, forget about the 12-24mm, get the 10-17mm FE or the sigma 15mm FE. Cheers,

 

Chris

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I've never taken it UW but i know the tokina 10-17mm can focus ridiculously close, about the same as my sigma 15mm FE which can focus on air bubbles on the dome port. This makes both extremely useful at close range and the reason why I traded in my nikon 16mm FE for the sigma. My sigma, like the tokina affords the ability for 'macro wide-angle' shots of any subject that will let you poke a dome port in their face.

 

1805043520_f5f5a7fb22_o.jpg

 

this lionfish was only a couple inches from my dome and i have others where he's even closer, but without the wreck which really gives the 'macro wide-angle' feel. . Try getting corners like that with a rectilinear wide lens! If you want close focus, forget about the 12-24mm, get the 10-17mm FE or the sigma 15mm FE. Cheers,

 

Chris

 

Nice shot Chris. A picture IS worth a 1000 words. I really liked doing this kind of work when I shot with film and a sigma 24mm close-focus "macro". It was a cheap, fairly sharp lens that focused close enough to touch the dome on the subject. I think the tokina 10-17mm is just what I want for this time of work, and even better because even on the 1.6 crop it's wider than the old 24 on a 35mm film body.

 

Thanks to you all for the great advice.

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