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Old March 2nd, 2006, 09:34 AM   #1
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What the eff is Focal Length?

Okay, I'm sorta confused when it comes to FL. What does it really mean? I'm going to set up my GG, but I keep hearing you have to have a certain FL for it. My GG is in a Thorlabs Tube and I'm going either be using Canon FD or Nikon F(
I have them both) Does the GG have to be a certain length from in order for it to work properly?

How would you guess how far it is inside the tube? Would each thread in the tube represent a mm? Thanks in advance for any help.

Rich
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Old March 2nd, 2006, 11:01 AM   #2
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Yes, the GG does have to be a certain distance from the lens in order for it to work. The GG is what the image from the lens is "projected" onto, for lack of a better word. If that GG is set at the wrong distance the lens will be trying to "project" the image either too far ahead of the GG or too far behind it. Either way will screw your results. I haven't set up my own adapter yet, but all my parts are ordered and I should be assembling mine next week. From what I have found during my research, the lens is typically placed about 35mm away from the lens (+- 3mm). I'm not sure if the threads in the Thorlab tube represent 1 mm each or not. Maybe someone else can answer that.
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Old March 2nd, 2006, 11:39 AM   #3
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First your confusing focal length with flange distance. Focal Length refers to the lens itself. for example a 50mm lens is prob. one of the most common lenses. Well 50mm is it's focal length. The lower the number the wider the lens is and the higher the number the more tele the lens becomes. FL has nothing to do with the GG itself.

The flange distance is the measurement that is critical when dealing with placement of the GG to the lens. There are different distances for different mounts so you'll have to check to see what the distance is for your lenses. Here's a chart with some...

http://www.graphics.cornell.edu/~wes...-register.html
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Old March 2nd, 2006, 11:44 AM   #4
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Quote:
Originally Posted by Mike Oveson
From what I have found during my research, the lens is typically placed about 35mm away from the lens (+- 3mm)
Most of the 35mm SLR mounts are more in the 40-50mm range. The Canon FD lens is 42mm and the Nikon F is a 46.5mm.
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Old March 2nd, 2006, 12:16 PM   #5
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Keith, you are a genius! Thank you! Like I said in my post above, I haven't assembled my adapter yet. This should help greatly in calibrating the distance between the lens and the focusing screen I am using as a GG. That really is so incredibly helpful. I can't thank you enough.
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Old March 2nd, 2006, 12:22 PM   #6
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No problem. I had trouble understanding all the terms when I first started working on an adapter too, so I know how it goes. I look forward to seeing some footage and pics when you get working on the adapter.
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Old March 2nd, 2006, 12:32 PM   #7
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The interesting thing is that I have been trying to understand all of the technical concepts about photography for a while. While I understood some basics, I didn't really understand exactly how f-stops worked until I started working on an adapter and looking at lenses. I didn't understand the difference between a "slow" lens and a "fast" lens. I knew you could change the exposure and that higher f-stops meant less light, but by working with lenses I've got a much better understanding now on how it all works together. I'm no great photographer, I just understand the mechanics better. :D This flange distance concept is a lifesaver. What an easy way to calibrate your lens (theoretically). I wonder why I haven't ever heard of this before in the half million posts I've read already.
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Old March 2nd, 2006, 01:08 PM   #8
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The speed or f stop of a lens is simply a numerical symbol of the lens diameter divided into the focal length. For example a 6" diameter lens which has a focal length of 24" is a f/4 lens. In still lenses , where you change the aperature diameter , you also change the f stop rating. Always a lens rating is with the aperature wide open, i.e. a f/2 lens has a wide open iris diameter equal to one half of its focal length.So a 50mm f/2 lens has a focal length of 50 mm which means at wide open the iris is 25mm in diameter. Please correct me if I'm wrong but this is how I learned it . Kurth
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Old March 3rd, 2006, 12:43 AM   #9
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So what's the best way to measure you're FD? Using a measuring tape? If someone knows each thread on the tube(thorlabs) is measured, that would be pretty helpful.

Thanks guys for the help.

RH.
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Old March 3rd, 2006, 09:51 AM   #10
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Dennis Wood posted something here the other day in my Flange Questions thread in response to this very question...

Quote:
If you take a vernier caliper with a depth probe, sit this on your flange and drop the depth probe, you should contact the diffuser surface at the published Flange to GG distance. I'm sure you're aware that you need to adjust for GG thickness if the diffuser surface is at the rear.
If you don't have a "vernier caliper with a depth probe" see if you can have a piece machined to spec or cut at a plastics supplier to the exact flange length (like a 1.5" round cylinder) and then you'd simply sit the cylinder on a level surface inside your tubing, the tubing with the mount down on the same flat surface, place your GG ontop of the cylinder inside the tube, and affix the GG to the tubing interior (glue, or if it has threading, secure the thread ring).
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Old May 15th, 2006, 02:47 PM   #11
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Quote:
Originally Posted by Keith Kline
No problem. I had trouble understanding all the terms when I first started working on an adapter too, so I know how it goes. I look forward to seeing some footage and pics when you get working on the adapter.

I haven't been around in a while but I figured I'd throw this out there. I finally got my adapter finished and did a few random tests. If anyone would like to see the footage it's available here: (http://www.silentmountainstudios.com/ov35_imax.wmv) Please let me know what you think, good or bad.
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