Sigma 10-20mm f/3.5 EX DC HSM LENS w/ Sony A65 body

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Turf

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Location
Minnesota, United States
# of dives
50 - 99
Heads up for someone else who happens to have this lens or consider using it. There may be easier lenses to work with but this one works and I didn't want to buy an additional lens just for diving.

FYI: This is not a supported lens from Ikelite at this time but it works. All specs from Ikelite show that it should not fit and a zoom ring will not work either. Again, it does fit and with a small/extremely easy modification to the zoom ring components it works just fine. My guess is they allow for some fudge room the stated width allowed for port extensions (1 mm) avoid lenses not working in cases where measurements were not exact or help insure the standard included zoom ring will fit.

Facts for this lens setup in case you care to try a different set up (sample photo below).
- The entrance pupil or nodal point for this lens measures at 62-68 mm (10-20 mm zoom) from the base (ends up near the gold ring).
- 8" Dome that Ikelite said is a true 8" diameter on the inside if it were a full half dome.
- Port Theory says that as a general rule the entrance pupil should be at a distance of the dome's radius or 4" in this case. Port theory and reality of what looks good aren't always the same but is a good starting point.
-
I ended up using the 5510.16 extension at 51.76 mm length along with the 5510.50 extension ring at 32.66 mm. This is short of the optimum length but to go any longer means you'll see the port extension in the frame at the corners (see photo).

What needs to be done to make the zoom ring work:
Cut off the springs which hold the grabby part to the zoom ring because it's too fat with them. You can then hold the two remaining pieces to the lens with double sided tape if you like but not needed. I'm told there may be a low profile zoom ring component available as well. Alternatively, you can just use the right thickness of adhesive foam on the inside of the clear zoom ring (same as what comes with it).

Other extension dimensions per Brett w/Ikelite (thank you). This is the total physical length of each.
Extensions:
5510.16 = 51.76 mm

5510.22 = 67.64 mm
5510.24 = 71.13 mm
5510.28 = 93.31 mm

Extension Rings:
5510.50 = 32.66 mm
5510.54 = 44.95 mm
5510.58 = 58.14 mm

Sample Photo Showing my Test results - corner clarity can be improved when shooting closer to an F8 aperture. Keep in mind the optimum zoom for the last photo would have been somewhere between 10-20 mm and not right at the 10 mm I shot but I wanted consistency.
Side by Side 2.jpg

My calculations to find the needed extension ring:
For purposes of my calculations I always went off the inner radius of the dome vs. the outer, I have no idea which one is best.

The entrance pupil or nodal point for my lens measured at 62-68 mm from the base.
The dome is not a full half sphere so my best measurement puts the physical inner radius at ~67 mm to the outer flat base of the dome port. This means it's ~34.6 mm short of the true 4" (101.6 mm)
inner radius of a complete half sphere.


To find length of extension needed from camera body to dome I added the nodal point to the short fall of radius. EX: 34.6 mm (+) 62 mm (=) 96.6 mm of needed extension length but doesn't account for the extension overlap with the dome due to o-rings and threading.

Next, account for the fact that the port extension doesn't connect flush with the camera body like the lens.
My best guess is the entire physical port extension connects at a distance
~20 mm from the camera body.

EX: I take 96.6 mm (-) 20 mm = 76.6 mm. Then account for the overlap mentioned which was ~13 mm so 13 mm (+) 76.6 mm (=) 89.6 mm of total physical extension length needed at 10 mm zoom or 95.6 mm at 20 mm zoom.

Again, I opted not to go with the extension 5510.28 even though it's likely the right length because it was visible in frame at 10 mm zoom. It did provided more clarity then the other extensions tested but it wasn't worth it to me. You can see what I mean in the sample photo.
Hope this helps save someone some time.
 
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Update... I went to Bora Bora with my rig for the real test. Up next will be to save for strobes but here is a sample of one of my pictures. It worked fantastically and even in 70' of water it pulled off some pretty decent pictures without a strobe.
This is an un-cropped photo, specs below.
TCMASgallery.php

SONY SLT-A65V
f/6.3
1/400 sec
ISO-100
Exposure bias: 0
10mm
Max Aperture 3.6

Second image is cropped to show the far left edge of the original at 20mm zoom with the right edge at the center of the photo. Approximatly 50' down, no strobe, detail below.
TCMASgallery.php
ISO-100
f/5
1/400 sec
0 step
20mm zoom

I have to say I'm pretty happy with the results for being told by Iklite, a number of people on here and several retail specialists that this rig was not possible and to go with a different lens that performs worse in low light. Not to mention having to figure out port theory since no one could explain it (it's really much easier then the how the books explain it, see above). Hope this helps someone.
 
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https://www.shearwater.com/products/peregrine/

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