EPL5 with Rokinon 7.5mm fisheye MFT - sweet spot and focus setup

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jhelmuth

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Location
Clearwater, FL
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Hey guys (and gals),

I've got an Olympus EPL5 (housed in the Oly PT-EP10) and have recently added a new port and lens combination (currently shoot mostly the Oly 9-18mm zoom in a Zen 100mm port) in the Athena 100mm dome for the Lumix 8mm fisheye along with the Rokinon 7.5mm fisheye. Since these are very close to the same focal length and physical size, I do believe this combination will work well.

So... since the 7.5mm fisheye is a fully manual lens, I want to set this up for nearly any circumstance where I would be shooting say from 1ft out.

From reading, I've decided that the aperture sweet spot should be right about f8.0 (I've read that it could be from a low of 5.6 to a high of 11.0, with most agreement on 6.8 to 8.0). So I've experimented just a bit (all on land so far) and found that if I set my focus ring infinity index mark to align with the aperture ring index mark (between 3.5 and 4.0) that I seem to get very good all around focus on shots from about 8" and out (again - this is @ f8.0).

My question to you is: [A] does this seem correct to you? any suggestions? [C] How might this change when I place this in the Athena dome?

I'll plan to follow-up with a review (including whatever I learn) on this combination...

Thanks in advance,

Jim

PS - any tips on dual strobe positioning for fisheye?
 
Yes, excellent point... (and thank you for the article, as I did not have that as a reference). That was why I chose 12"... 12" was/is the "rule" as that was my reference point for any dome (which I now know may not be correct). I think my take-away from that article (again - thank you) is that I might want to push this to say f11 (the lens will go to f22, but I understand that the m4/3 format has a preference for an aperture of 5.6) and shift the focus ring more toward the closer distance - say 6-ish inches.

I'm going to do some pool work early next week with the combination and see how this translates into the real world...


Thanks again!

Jim
 
So it would seem to me that the virtual image DOF is "squished" - correct? That would make the DOF more narrow? (IE - the actual distances the lens "sees" at the virtual focus point out to infinity)

If I am absorbing this correctly, I'm thinking the virtual image is at 10" from the Image Sensor (true focal distance). I arrived at that by 50mm (radius) * 3.03 (factor) = 151.5mm (6.01") in front of the dome vertex (v) + ~100mm from (v) to Sensor (fp) = ~ 251.5mm (9.99").


So I set the focus ring index to 0.8 ft (~10") which also coincides with the infinty index DOF mark. Took some test shots (bathtub) from ~ 6" out to about 2'+ (from the dome front) and did the same for 0.5 ft (focus ring setting)...

results: the images set at 0.8ft were all mostly in focus (short, med, and long - the long shot was slightly out-of-focus)

Comments? Suggestions?

(shot @ ISO 400, and 1/125th sec. shutter with 2x 2500 LED lights [reflected])
 
OK... I still did not get to the pool (and I might get some time in tomorrow), but I did get some time in with better testing in the tub! (just and old fiberglass bathtub).

Here is what I did...

> Used an Inon D2000 strobe as my "subject" since it had several qualities that I thought would help me decide on focus clarity.

> Used 2x BigBlue 2500 lumen LED torches as my ambient lighting (which afforded me an ISO of 400 and shutter of 1/100 - 1/125 at various apertures)

> Snipped several bit of old measuring tape (the type that is stiff and rolls into the case by itself) so I could measure distance, but also because the fine lines and graduations would give me a better objective to judge focus.

> Tested distance of 2", 4", 6", 8", 10", 12", 18", 24" (inches) all at f11 (I've been able to prove to myself that this seems to be the aperture sweet spot), but different focus settings (0.3ft, 0.4ft, 0.5ft, 0.65ft, 0.8ft)

> I wanted to test 2" and 4" shots, but I'm weighting them quite low as I do not intend to shoot 4" and in. Still, I took them as a reference to help me figure out what was going on

> As a final test, I used several objects set in background and foreground and tried f16 and f22 at the 0.65ft focus, as well as f11 (again) and f8.

Here is what I found...

* 0.3ft (minimum focus) and 0.4ft were all pretty much rubbish except at the 2" and 4" distances (actually, even the 4" distance was rubbish with the 0.3ft focus)
* 0.8ft looked best only beyond 6"
* The 2 contenders for best DOF seemed to fall to the 0.5ft and 0.65ft shots.
* As one might suspect, the background focus was slightly better at the 0.65ft shots, while the foreground focus was decidedly better with the 0.5ft shots
* f11 was a clear winner in the final test shots (I think this is the sweet spot of the lens overall)
* @ f11 and 4" - 6" - 8" - 10", 0.5ft & 0.65ft were virtually identical - I could not call a winner
* @f11 and 12" (and beyond) 0.65ft was more clearly the "winner" as the distances became longer.

Final Results...

So now I considered f11 focused at 0.65ft (which was actually as close to exactly between 0.5ft and 0.8ft as I could guesstimate) to be my decided "winner"
Now I wanted to "see" what f22, f16, an f8.0 would produce... and f11 was the clear winner.

Conclusion: In my limited testing, I would choose f11 focused at as close to exactly between 0.5ft and 0.8ft on the focus ring as my correct hyperfocal setting for anything 4" and beyond (excepting 2" and 24"+ - in which case I'd choose 0.5ft for the 2" shot, and 0.8ft for the 24"+ shot)

sqm9gx.jpg

ISO 400 1/100th f11 (2" with focus set to 0.5ft)

34gpqir.jpg

ISO 400 1/100th f11 (6" with focus set to 0.65ft)

ve14yo.jpg

ISO 400 1/100th f11 (12" with focus set to 0.65ft)

aemteq.jpg

ISO 400 1/100th f11 composite

PS - I do get the tinyest mechanical vignetting with this Dome/lens combination (again - Athena 100mm fisheye dome for Oly PT-EPxx [designed fromLumix 8mm] with a Rokinon 7.5mm fisheye)
 
https://www.shearwater.com/products/swift/

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