Tuesday, September 1, 2009

Focus Range for 150mm lens

Infinity stop of 150mm lens on Byron, is at the end of the rail, and 2.5mm beyond. If a 150mm lens is mounted, better with a lens board who is 2.5mm thicker than normal one, then all you have to do is pull the front stand to the end of rail, that is its infinity stop.

Now, let's talk about its focus range.

For Polaroid 110B camera, moving range for flat bed rail is about 20mm, good for its default lens-127mm Ysarex, focus range from infinity to 3 feet. Even better for Symmar-S 5.6/100mm and Angulon 6.8/90mm, which can focus as close as 2.5/2 feet.

But for a 150mm lens, 5 feet. for most condition, it is too far to focus, we need it closer.

We need it go beyond that, say, how about 4 feet?

Carefully measured, rail moving range should be expanded from 20mm to 26.5mm, to reach 4 feet focus. First thought coming to my mind is modifying the flat bed, extend rail's moving range to 26.5mm. After weeks of drawing and planning, talked to my mechanist, finally we gave up this way, it costs to much, no worth it!

Then we found another way.A way to compromise with existing conditions and real needs. 

What we have is only 20mm of moving range for lens, then we should put this range into the focus range we want the most. If the moving range starts from infinity, then it will end at 5 feet focus point.

Now, let's make the range starts from 10 feet, instead of infinity, then focus range will ends at about 3.5 feet. not bad, eh?! 

How to do it? make a protruding lens board! about 10mm.

So the infinity stop will backward a little bit, not at the end of the rail. When you want to focus other than infinity, pull the front stand all the way till reach end of rail, now the lens is focus at about 10 feet, and 20mm rail moving range gives you 10~3.5 feet focus range.

What we lost?
Now we have focus range from 3.5~10 feet and infinity, but we miss the range beyond 10 feet. But that's OK, since very rare we need to use that range. 

Got it? a better way than modifying flat bed.

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