72mm dia. achromatic diopter with M35 at DVinfo.net

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Old March 14th, 2009, 09:46 PM   #1
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72mm dia. achromatic diopter with M35

I have a custom built M35 cine-adapter which I use with my SONY Z1U and a set of Zeiss lenses. A cheap 60mm dia. achromat which I mounted in an aluminum ring, screws into the camera's threads. I am getting pretty good results but there is a little softness at the edges of the images.

I believe that by replacing the achromat with one of those made by; Century Optics (now Schneider); Cinevate (Brevis) or Redrock may give me better results including hopefully, edge to edge sharpness.

Can anyone recommend one of these achromats for use with a Z1U?

Thanks,
Martin
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Old March 15th, 2009, 08:29 AM   #2
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Is your 60mm diam lens a true achromatic dioptre consisting of two pieces of glass bonded together or is it a single piece of glass?

Before recommending any achromat we should know whether your adaptor is a flip or non-flip product, ie., does the camera see the image upside-down or right-way up?

Generally, the achromat power used for non-flip adaptors is stronger, in the ballpark of about 10+ or maybe higher number. This is to keep the length of the appliance within sensible limits. The downside is that you can get soft corners.

Flip (also known as image erecting) adaptors, are more often found to have achromat powers of between 4+ and 7+.

This is because the flip or image erecting path folds the optical axis forward, sideways and back upon itself. This has the benefical effect of enabling the physical shortening the appliance. With a longer optical path, there is less likelyhood of soft corners.

Once we know whether your adaptor is a flip or not, then we are better positioned to advise you.

If your existing 60mm "achromat?" is a true achromatic dioptre, it should be adequate depending on whether your M35 adaptor has a condenser lens in it or not. How far forward of your camcorder lens is this 60mm dioptre mounted? You may discover you achieve a better result if the glass of this dioptre is mounted as close to the front element of the camcorder lens as you can get it. Take care not to let the two glass surfaces clash otherwise you will ruin both.

Before a trial fit up of the altered achromat ring, cut a piece of paper into a disk shape which is a loose fit within the diameter of the front of the camcorder filter orifice and place it in there. If it can slip around in the hole when you tip the camera upside down, after you have fitted up the achromat and screwed it fully home in the filter thread, then there is clearance between the two glass surfaces.

If the paper does not slip, or the thread has not screwed fully home, then you should not fit the lens up until you have re-adjusted things.

Last edited by Bob Hart; March 15th, 2009 at 08:41 AM. Reason: errors
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Old March 15th, 2009, 10:46 AM   #3
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I use a 72mm Schneider +3.5 achromatic diopter for underwater macro on my FX1 screwed directly to the lens threads.

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Old March 16th, 2009, 12:01 AM   #4
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Bob, my adaptor does not flip the image. It is a custom build based on the Redrock DIY kit which I made some modifcations to. On the whole it works well.

The achromat is a single piece of glass and it was cheap. It is mounted very close to the camera's lens, nearly touching it - not more than a millimeter away or two at most. I don't know what its power is, and I can't remember it's exact fl, but I acheive focus about 135 mm from the ground glass. I don't think it is a true achromatic diopter which is why I'm considering this purchase.

The Century Achromatic diopter seems to come in two magnifications +2 and +3.5. Also, Redrock sells one for about $400 which they claim provides edge to edge sharpness and an all-around better image. But so far I haven't been able to learn what its power or focal length are. I'm not sure what magnification I should be looking to get.

Dave, thanks for the info. Good to know the Schneider (Century) screws into the FX-1.
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Old March 16th, 2009, 06:54 AM   #5
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Martin. I am really only guessing here as there are other variables like how big a groundlgass area you are aquiring from.

However my guess is that if you are aquiring an approx 24mm wide image, you are in the ballpark of a 7+ if you are 135mm from the groundglass.

Dennis Woods and Wayne Kinney may be able to sell you achromats that have a known performance and are already able to be fitted up to your camera.

I myself used Century Optics 4+ and 7+ on my own flip adaptor. The 7+ is available only as a 58mm filter thread mount. The exit pupil is big enough to work with the FX1/Z1. It needed a 58mm to 72mm concentric step-up ring.

You need to be zoomed in nearly all the way with the 4+ for the 24mm wide groundglass image. This lens was also a 58mm filter thread mount but I believe you can have the 4+ in 72mm filter mount. A 4+ will mean your appliance will be rather lengthy.

With a true achromatic dioptre, you will observe an immediate improvement. The edges should not be soft if it is set up right and you will also observe better colour rendition and contrast.

Last edited by Bob Hart; March 16th, 2009 at 07:01 AM. Reason: error
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Old March 16th, 2009, 08:21 AM   #6
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Thanks for this Bob. I've already contacted Cinevate and Redrock and hope to get their advisement soon. I have to assume the latter should work as it's designed specficially for use with their production model adaptor, and I don't think my DIY version differs greatly. Having said that, it might be designed only for use with their new flip version.

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Old March 17th, 2009, 11:59 PM   #7
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Cinevate says their achromatic diopter is about a +7 but they didn't mention a focal length. Any idea approximately how far from the ground glass this achromat will acheive focus?

Sounds like I am already in that ballpark (as you put it) with my current set up.


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Old March 18th, 2009, 12:05 AM   #8
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And Redrock say theirs is a x8. Do you suppose that is the same thing as +8, and does it mean the focal distance to the GG is likely to be a bit shorter than the Cinevate?

Martin
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