Universal achromatic close up lens specs? at DVinfo.net

Toenis Liivamaegi · December 18th, 2006, 10:41 AM

I tried to search the forums but ended up with confusing results.

What are the best/universal specs for powerful achromatic close up lens?

Those with minimal center to edge thickness diference and with shortest focal lenght? By shortest I mean focal lenght between 100-200mm.
Would 90% of clear aperture from 2" diametre be enough?

Which would result in sharpest edge to edge image?
What should be the distance from the (frensel) focusing screen, 50mm?

I have a source for high quality 2" achromats that are corrected for visible light but I should know the requirements first. Unfortunately those are more expensive than SGpro`s and therefore have no commercial value.

Thanks in advance,
T

Daniel Schaumberger · December 19th, 2006, 06:31 AM

Hi Toenis,

for me I found this specs are ideal.

Achromat +10 (focal length = 100mm)

90% from 2" is too less - for HD you definitely need a 72mm diameter achromat for bigger sweetspot.

distance from gg to camcorder should be 35-45mm

Ive got a source for cheap achromatic triplets from Zeiss but unfortunately they are also 2" in diameter.

best regards
Daniel

Wayne Kinney · December 19th, 2006, 08:23 AM

Quote:

Originally Posted by Daniel Schaumberger

90% from 2" is too less - for HD you definitely need a 72mm diameter achromat for bigger sweetspot.

Thats not really true. I have tested both the Redrock and brevis achromats, which are 72mm, against the SGpro R2 achromat which is 51mm dia. Both the RR and brevis achromats had a hard time getting good edge to edge focus.You could get the centre in focus but then the edges would be blurred, or move focus so the edges are sharp but the centre is then blurred. Yet the SGpro achromat keeps focus the same across the frame, with peaking set on the camcorder, all the area of the frame 'peaked' at the same point on the camcorders focus ring.

Its more to do with the design of the achromat then overall size.

Dominic Jones · December 19th, 2006, 09:00 AM

Hi again Wayne,

That's interesting - I've got the older 55mm Redrock Achromat, which is pretty good edge-edge but not perfect. What do you reckon the chances would be of retro-fitting my M2 with one of your Achromats?

I can see there might be problems with this as the M2 GG is quite a way further away from the rear casing of the unit than the SGPro GG appears to be, but it would be great if it is possible...

Wayne Kinney · December 19th, 2006, 09:04 AM

Dominic,

Bens SGpro is the Revision 1. The SGpro R2 has a greater distance between the GG and achromat/camcorder.

72mm black anodized rings are currently being machined, and I will be selling the SGpro's R2 achromat with threaded aluminium rings for £99. It will be a direct replacement for your current achromat im sure.

Dominic Jones · December 19th, 2006, 04:38 PM

That sounds great Wayne - be sure and let me know when they're finished!

December 18th, 2006, 10:41 AM	#1
Toenis Liivamaegi Major Player Join Date: Nov 2005 Location: Tartu, Estonia Posts: 579	Universal achromatic close up lens specs? I tried to search the forums but ended up with confusing results. What are the best/universal specs for powerful achromatic close up lens? Those with minimal center to edge thickness diference and with shortest focal lenght? By shortest I mean focal lenght between 100-200mm. Would 90% of clear aperture from 2" diametre be enough? Which would result in sharpest edge to edge image? What should be the distance from the (frensel) focusing screen, 50mm? I have a source for high quality 2" achromats that are corrected for visible light but I should know the requirements first. Unfortunately those are more expensive than SGpro`s and therefore have no commercial value. Thanks in advance, T

December 19th, 2006, 09:04 AM	#5
Wayne Kinney Major Player Join Date: Jun 2005 Location: Brighton, East Sussex, UK Posts: 938	Dominic, Bens SGpro is the Revision 1. The SGpro R2 has a greater distance between the GG and achromat/camcorder. 72mm black anodized rings are currently being machined, and I will be selling the SGpro's R2 achromat with threaded aluminium rings for £99. It will be a direct replacement for your current achromat im sure. __________________ Thanks, Wayne.

December 19th, 2006, 06:31 AM	#2
Daniel Schaumberger Regular Crew Join Date: Sep 2006 Location: Atlantis Posts: 58	Hi Toenis, for me I found this specs are ideal. Achromat +10 (focal length = 100mm) 90% from 2" is too less - for HD you definitely need a 72mm diameter achromat for bigger sweetspot. distance from gg to camcorder should be 35-45mm Ive got a source for cheap achromatic triplets from Zeiss but unfortunately they are also 2" in diameter. best regards Daniel

December 19th, 2006, 09:00 AM	#4
Dominic Jones Major Player Join Date: Mar 2005 Location: Brighton, England Posts: 225	Hi again Wayne, That's interesting - I've got the older 55mm Redrock Achromat, which is pretty good edge-edge but not perfect. What do you reckon the chances would be of retro-fitting my M2 with one of your Achromats? I can see there might be problems with this as the M2 GG is quite a way further away from the rear casing of the unit than the SGPro GG appears to be, but it would be great if it is possible...

December 19th, 2006, 04:38 PM	#6
Dominic Jones Major Player Join Date: Mar 2005 Location: Brighton, England Posts: 225	That sounds great Wayne - be sure and let me know when they're finished!