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Old December 7th, 2005, 01:10 PM   #1
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Achromat Question

I tried searching already, but have still come short of the answer(s) I'm looking for.

First, when I am looking at achromats they are often listed like this: OD is 54mm. Focal Length is 180mm. What does this mean exactly? In other words, how do I know if that is the right one for what I want? Does the focal length=180mm mean that the lens has to be 180mm from the object it is focusing through? Or does it mean something else entirely?

Also, what is the difference between an achromat and a macro lens? Is there a difference? I have a +10 macro filter for my camera, do I need an achromat instead? Do I need both?

Another question, do I need a condenser AND an achromat? Or do I just need one or the other?

One final question, I am working on an image erecting piece for my adapter also. I found a good source of inexpensive prisms, but I'm unclear as to the size I need. Is there an easy way to calculate the physical size of the sides of the prism?

I guess I just need to find a good book on optics :) Thanks in advance for any insight and answers you can provide.

Thanks,
Larry
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Old December 7th, 2005, 01:45 PM   #2
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180fl means infinity focus on your camera will let you focus on 180mm away, so you can focus on anything 180mm or closer to your camera.
Achromat and macro lens does the same function: let your camera focus on the GG. achromat which has 2 pieces cemented together has better chroma correction. You only need 1, not both.
Condenser is used in Letus35 but not all adapters, so you can decide if it needs to be used.
I can only give you advice on prisms if I know your plan. ex: 2,3 or 4 RAs, 1 roof prism and 2 mirrors...
My question for you. Where is the source?
Thanks.

Quyen
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Old December 7th, 2005, 04:22 PM   #3
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Quyen - thanks for the explanation of the focal length, I think I understand now. So if I use an achromat with a focal length of 180mm then I can have it anywhere between 0mm and 180mm from the front of the camera lens and still focus at infinity, right?

I'm still slightly confused on the condenser. I understand that it is not necessary, I can use it or not. However, what exactly is the purpose of the condenser? What functionality does it add to my design?

I was going to use two right angle prisms and simulate a double porro abbe prism (http://en.wikipedia.org/wiki/Porro-Abbe_prism). I purchased two porro abbe prisms, but not understanding size I misread it and got these little tiny (16mm, I think) prisms that are too small to work. So instead I was going to try using two right-angle prisms, like this: http://www.surplusshed.com/pages/item/l2001d.html

My thought was to have one placed horizontal and the other vertical causing the light path to travel the same as in a porro abbe prism (I've seen this done with four front surface mirrors). Unfortunately, I am realizing every day how much I DON'T know about optics...so I don't know if the right angle prism will work or if I should by four front surface mirrors and do it that way.

Anyway, any thoughts on if it will work? And, if so, how do I determine the size prism I need?

The source I found for inexpensive prisms, mirrors, optics, etc, etc is http://www.surplusshed.com I think they get lots of equipment from everywhere, so there is no guarantee that they will always have a particular item. But they have really cheap prices on the stuff they do have, including a lot of prisms, mirrors and lenses.

Thanks again,
Larry
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Old December 7th, 2005, 04:39 PM   #4
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Actually the achromat will be close to your camera lens and your camera now can only focus from ? to 180mm where ? will depend on camera. Some camera has shorter minimum focus range while other has longer range.
Condenser's job is to help eliminate vignetting.
With prisms from surplusshed, I don't think they have enough prisms to do the flipping job, thanks.

Quyen
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Old December 7th, 2005, 04:51 PM   #5
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What achromat would you recommend for the dvx100a? (72mm threads) The macro I bought on ebay is terrible.
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Old December 10th, 2005, 03:48 AM   #6
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Lots of people recommend the +7 from Century Optics , which I'm sure you already knew. But I've also heard that you need 2 +10's for a dvx100a, because it can't focus properly without them (but I can't verify that.... search the forums)

I'm looking at a B+W macro from Schneider Optics, the company that owns Century. I have to imagine they're good, but I haven't heard anyone speak about them before....


Info...
http://www.schneideroptics.com/info/...ose_up_lenses/
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Old December 10th, 2005, 10:28 AM   #7
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http://www.bhphotovideo.com/bnh/cont...ughType=search

This one?

Of course, I have a DVX100 so this won't work then I guess...
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Old December 10th, 2005, 01:25 PM   #8
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He's talking about this, although the same company makes both:

http://www.bhphotovideo.com/bnh/cont...list&sku=11209

Looks like it's worth a try for sure.
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Old December 10th, 2005, 09:35 PM   #9
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I just noticed that that one is not front threaded so I'm not sure how you would be able to use 2 together easily...always something I guess. =(
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Old December 11th, 2005, 01:44 AM   #10
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Achromat works or not really depends on the distance of the camera to GG. I use 170mm achromat (about +6) with my newly developed unit (which records image upright) with DVX100 and it works fine. I think what matters the most is using the achromat at the right distance, thanks.

Quyen
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Old December 11th, 2005, 10:39 AM   #11
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Yup, distance is the key.... but are we measuring from the slr lens, or you camera lens to the gg?

Luke, according to Schneider Optics, these are stackable, but they don't suggest using more than two.

Last edited by DJ Lewis; December 11th, 2005 at 06:22 PM.
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Old December 11th, 2005, 10:46 AM   #12
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If you don't mind me asking, where can I get a 72mm 170mm focal length achromat?
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Old December 11th, 2005, 10:48 AM   #13
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I'll look around for you, but from what I've read, most people are using stepdown rings, and zooming in a little.... Will look though.
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Old December 11th, 2005, 10:52 AM   #14
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Matthew,
I can get you a 50mm 180mm FL achromat sourced here in the UK. Would be happy to send one to you since you sent me that microwax a while back. Email me if interested.
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Wayne.
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Old December 11th, 2005, 12:36 PM   #15
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I just bought a couple of 72mm ones on ebay to test them out. They probably won't be the greatest but if my setup works I can then upgrade them to something else.

Now the trick is to find some way to mount Canon FD lenses...
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