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PeteAtkinson

Member Since 25 Nov 2008
Offline Last Active Yesterday, 08:25 PM
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Posts I've Made

In Topic: Full Frame SLR wide angle corrector port tests

14 April 2016 - 12:31 AM

I am amazed! Concave side towards the dome!! I will try to find one at ADEX to look at it! Thanks.


In Topic: Full Frame SLR wide angle corrector port tests

10 April 2016 - 06:55 AM

Perhaps someone with this corrector lens could measure the curvature of both surfaces, to determine whether one or both are aspheric. The way I would do this is to scribe approx. curvature on a piece of thin card, cut it out, and see if it fits flush to the surface of the lens. Then do another, making adjustments till it's a perfect fit.

In New Zealand 20 years ago I had coated plano-convex dioptres made which were said to flatten the curvature of field, but B&W diopters always seemed to give better results.


In Topic: Sigma 15 fisheye, least parallax point anyone?

24 March 2016 - 01:55 AM

This helps a lot! And it shows why I need to ask, as my measurements are so bad, and yours, and Simon K's are virtually the same!

Many thanks to both of you. If you have NPP measurements for other lenses used underwater Randall it would be great if you could post them on Wetpixel where they should be pinned. Thanks again!

 

www.peteatkinson.com


In Topic: Sigma 15 fisheye, least parallax point anyone?

19 March 2016 - 06:14 PM

I asked, because I wanted to avoid the imperfect hassle of measuring it myself using lined-up pins and projecting those lines back into the lens. But I did it anyway...I prefocussed at 22cm since that should be about the virtual image of infinity with a 6" dome. I got a measurement of about 26mm in front of the mount surface. (The position varies with focus, but not that much.)

The apparent position of the iris as seen through the front element should also be about the "no" parallax point. I estimated it's position this way as 24.5mm in front of the mount.

I think the most accurate way to get the least parallax point is with a pano head, and the pano sites have data on many lenses, but not this one.

Anyone out there with a pano head to measure this?

Simon, how was your measurement done?


In Topic: Which +2 close up lens should I buy?

13 March 2016 - 07:12 PM

I used B&W diopters which seem excellent. However, using a diopter, you lose some angle of view. You may find you get crisper results by not using a diopter but just cropping the picture to the angle of view you would get with a diopter, losing the soft corners in the process. I'm not aware that anyone has tested this definitively.