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jhelmuth

Member Since 13 May 2004
Offline Last Active Today, 01:16 PM
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Posts I've Made

In Topic: Rokinon 7.5mm MFT on EPL5 sweet spot and focus

Today, 06:17 AM

Hmmmm....   maybe I am not understanding all this, but I do believe that 4x gets me only to the front of the lens (that is where the base of the dome is - exactly at the lens front).  Unless I'm missing something, I also have to add in the distance from the front of the lens to the sensor.  If I add all that, I'm at ~ 10".

 

I'm not saying you're wrong. It's just what I've observed (and tried to learn and absorb) so far.

 

Again, I'll take time to do some more accurate and in-depth testing in the pool. I can lawn lay down a tape measure, and take more detailed tests.


In Topic: Rokinon 7.5mm MFT on EPL5 sweet spot and focus

Today, 05:25 AM

The virtual image at infinity will be at less than 20cm as that depends on the dome size the refraction index and the nodal point of the camera (unknown) so you should be looking at less than that as a starter around 0.6 ft or less

How did the 0.5 ft image come out?

 

Not nearly as sharp as the 0.8ft image (not even close).

 

I'm going to head to the pool sometime within the next few days and take more time to carefully measure exposures at various distances.  I'm not too concerned about having the infinite distance sharp (would be nice to have - but not a deal breaker).  I DO want the image to be sharp from as close as 6" (1.5cm) to about 2-3' (0.7-1.0m) from the front of the dome. These are the anticipated distances where I will likely "shoot" my subjects (at least that is what I am anticipating)  


In Topic: Rokinon 7.5mm MFT on EPL5 sweet spot and focus

Yesterday, 06:25 PM

Not M4/3, but APS-C : I use a samyang 8mm on Canon 400D. I set focus to minimum (30cm)

before closing the housing, then I try to close the aperture as much as possible.

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?

 

PS - Was able to use ISO 400, and 1/125th sec. shutter on 2x 2500 LED lights (reflected)


In Topic: Rokinon 7.5mm MFT on EPL5 sweet spot and focus

Yesterday, 05:21 PM

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)


In Topic: WTB - Sea & Sea ys90dx

25 July 2014 - 09:23 AM

He felt $100 was a reasonable seller price...

 

Not really unheard of either (there is some evidence of that - albeit a bit toward the rare side), but all evidence to the contrary - most ys90s (DX for sure) are actually selling at about $150 (Auto slightly more).