Help with checking my reasoning for the extension I need!

[Turf]

I recently ordered a underwater setup for my Sony Alpha 65 along with a lens. The dome and extension setup that was suggested when I called for this lens was Ikelite STANDARD 3.5" BODY WITH 8" DOME B&H# IK5511.2 to go with the lens I ordered which was the Sigma 10-20mm f/3.5 EX DC HSM LENS f/SONY B&H# SI102035S.


The problem I have is that that there is no way the extension is even close to an appropriate length based on all the theories out there as my lens extends WAY into the dome (~1") so it will need to be replaced. I contacted Ikelite but they were not a huge help as their stats show my lens doesn't fit inside the extensions even though it clearly does.


Please check my reasoning/math as I've only just started learning this and the extensions are far to expensive to just order a bunch and throw them away when they don't work.


My calculations to find the needed extension ring:
For purposes of my calculations I always go off the inner radius of the dome vs. the outer, I have no idea which one to use, do you?
The entrance pupil or nodal point for my lens is measured at 62-68mm from the base.
The dome is not a full half sphere as my best measurement puts the physical inner radius at ~67mm or ~34.6mm short of the true 4" inner radius.
So I added 34.6mm (+) 62mm (=) 96.6mm of needed extension length from camera body to dome not counting the threading/o-rings on extensions yet.


But the extension doesn't obviously connect flush with the camera body like the lens so again my best guess is it connects at a distance from the camera body of ~20mm (now accounting for the ~9mm lip of the extension o-ring).
Thus I take 96.6mm (-) 20mm (+) 13mm (13mm is for the threaded o-ring on the other side of the extension) (=) 89.6mm of total physical extension length needed (including the o-rings)
Per Ikelite 5510.16 extension (which was originally suggested to me) has a total length of 51.76 mm which is 39mm short or 1 1/2"! That seems like alot. It should also be noted that any extensions for similar length lenses use much longer extensions.


Based on all this it would say that Ikelite extension item 5510.28 with a physical length 93.31 mm would fall right in the middle of the nodal point distances if using the inner dome radius. I would love for someone to confidently answer if they agree or disagree as I'm not getting anywhere with anyone else.


Thank you so much for your help and sorry for the long message but details seem to be a missing factor in this process given what's been suggested thus far.

Ryan

 

[DelrayDM]

The Sigma 10-20mm f/3.5 is way too fat to fit in an Ikelite port body.
Zen Underwater makes an large diameter alumimum port body for Ikelites, it is about 89 mm inside diameter. So it theoretically might pass the 87.3mm diameter lens you have.

The Sigma 10-20mm f/3.5 is only 7mm longer than the Sigma 10-20mm f/4.5-5.6 lens and they have the same 24 cm minimum focus distance (measure from the film plane), so maybe the 'optical center' of these lenses is comparable enough that we can infer that the 3.5 inch port body would be correct, or at least a good starting point for testing. The Zen Underwater alumunimum port body is only around 43 mm long, so that is no where near long enough for you.

The ultimate answer is to do some real pool testing and judge the results for yourself. For pool tests we often try 3 or more different extensions for each lens/port combination as in this example.

Unfortunately, I do not see how you are going to do this without spending a lot of time in the machine shop.

 

[Turf]

Delray thank you for your reply and I will take that into consideration. You are correct on paper that my lens will not fit but in actuality it does. I have an extension... it fits fine but is the wrong length.

 

[Larry C]

Have you tested it? Ikelite specifies that extension for the f4.5-5.6 which has the same nominal length of 3.5" and the same minimum focal length of 9.4", but they specify that it won't accommodate zoom and must be used at a fixed focal length.

 

[Turf]

I modified the zoom ring so it works.

 

[Turf]

*Edit* Below is a cropped photo to show the left edge of my suggested setup. I seemed to get vignetting at all focal lengths and most were more pronounced then this one taken at 20mm. I did just read the the shade can cause vignetting so I will try again without it. At this point I still do not understand why the nodal point would be far closer then the radius of the dome and no one seems to know enough to suggest if what I think I've learned on port theory is correct or not.

On a very simple level I would like to know if having my lens extend into the dome well over an inch seems normal. I've gathered it is not in which case this would clearly be the wrong extension and far too short. I also have to challenge this because Ikelite says my lens will not fit but it does. They say my lens cannot zoom but it does. So why should I blindly believe the port extension they suggest when no one can support or justify it?

https://drive.google.com/file/d/0B0B-x_dlPHiBQVR4ZEpUVUNieUU/edit?usp=sharing

 

[Turf]

Update: After substantially more reading there are a few similar lenses that suggested one size up from the extension I have so it should be here this week. My lens would still extend into the dome a bit but who knows it might work. Otherwise, as coincidence would have it, the larger extension I was originally considering is only a couple mm's shorter then the maximum length I could put on my housing before the port becomes visible in frame. It would also put the front of the lens rim nearly flush with the inner flat surface of the dome.

 

[Turf]

Problem solved and correction to my last. The 5510.28 extension at 93.31 mm length easily had the best results for corner sharpness at all lengths of zoom but the extension tube is visible at 10 mm in frame. Adjusting the aperture did noticeably improve the sharpness for the first two extensions around F8-F10 but nearly all apertures worked reasonably well with the longer extension.

I'm opting for the to stay with the original 5510.16 extension at 51.76 mm but then adding 5510.50 extension ring at 32.66 mm to leave me just a bit short of what is likely the optimum length however the extension will no longer be visible at 10 mm zoom. Since every length of extension continually improved until the longest I may loose a little bit of sharpness but will certainly be far better then what was originally suggested. After that I can simply adjust the aperture to clean it up more as I did with the others.

This shows only the left side of each photo side by side for reference.

 

[Turf]

I just updated another thread showing photos from my dives with this DIY rig out in Bora Bora. http://www.scubaboard.com/forums/so...0mm-f-3-5-ex-dc-hsm-lens-w-sony-a65-body.html