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How accurate are lens distance markings?
Old 11-29-2005   #1
daveozzz
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How accurate are lens distance markings?

I'm about to embark (again) on adjusting my bessa rangefinder which is missing focus close up.
Just a thought - can I really trust the lens barrel markings to be accurate? If I set the lens focussed at 1m according to the distance scale how confident would you be that the actual focus point is 1 metre from the film plane?
Anyone got any experience of this?

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Old 11-29-2005   #2
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You could test this by placing a piece of ground glass at the film plane and focusing on an object one meter distant from the film plane. You can also use a piece of Scotch Tape fixed at the film plane (the tape should be the kind with the frosted surface). Once you have the object in focus look at the lens barrel index mark and see if it lines up with the one meter mark on the lens.
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Old 11-29-2005   #3
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Lens distance markings are reasonably to very accurate, but it always pays to make some test-shots before trusting them completely. DOF markings are not more than an indication, as DOF is dependent on the end use of the photo and varies with the focussing distance. for instance, at your distance of 1 m. the DOF markings will be very much off, especially in front of the plane of focus.
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Old 11-29-2005   #4
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Thanks just tried the scotch tape suggestion - v useful along with a magnifier of some sort - so at least on my CV nokton the lens marking don't seem to be particularly accurate. I'll stick to checking the scotch I think....
now to work out how to get close focus to work without knocking out infinite focus.. grrrrr....
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Old 12-01-2005   #5
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Quote:
Originally Posted by daveozzz
....
now to work out how to get close focus to work without knocking out infinite focus.. grrrrr....
Although I never did RF alignment on a Bessa, I've done so on Hexar RF ( http://www.rangefinderforum.com/foru...ead.php?t=1937 )

What I noticed was that getting alignment correct at close distances is much more difficult than at infinity. When I started re-alignment based on infinity, the camera would appear to align correctly at infinity, but would screw up considerably at anything less than 3 meter (10ft).

Once I started doing it the other way round, that is align at 1 meter using a ruler and the lens barrel markings, I found that as a result inifinity focus was spot on as well. It's apparently easier to mark out an object at exactly 1meter than something at infinity. The moon may appear to be a good object for infinity, but it's so bright compared to a completely black night sky, that exact line up of the double image is difficult to judge.

As part of testing the re-alignment outcome, I also noticed that the lens barrel markings for distance are quite accurate, at least they were on the CV90/3.5 apo-lanthar and the M-Hexanon 50/2. Within the centimeter of each other..
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Old 12-01-2005   #6
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"Infinity" isn't Star Trek type infinity and focussing on the moon.

The distance of a city block or so is infinity for our purposes.
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Old 12-01-2005   #7
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I had this problem on the Canon 7 that I was using to set the J-3's. I ended up aligning infinity at ~300ft. With that, the focus was right-on at 3'. That was checked by focussing the Nikkor 5cm F1.4 using a loupe and film strip at the filmgate and then checking that the RF image lined up.

There are more complex adjustments that can be made on RF's besides simple alignments, and it is possible that an RF that is correct at infinity will not focus close-up. For those adjustments, popping the top plate and/or removing the RF mechanism is usually required. I had to do such an adjustment on my Retina IIIS, and the original repair manual was required.

Last edited by Brian Sweeney : 12-01-2005 at 05:05.
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Old 12-01-2005   #8
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Yeah I gave up - couldn't match up at 1m and infinite at the same time. For a while I thought it would be a good idea do make it accurate close up but live with out of alignment at infinity (after all there's no question about where infinity is on a lens) but when I got this working on one lens it'd be out on another one! Impossible. So I've *really* given up - off the the ForSale section for me!
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Old 12-01-2005   #9
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Quote:
Originally Posted by Jon Claremont
"Infinity" isn't Star Trek type infinity and focussing on the moon.

The distance of a city block or so is infinity for our purposes.
Kirk: 'Spock, why can't we blast those Klingons off of the surface of their moon?'

Spock: 'Captain, it appears that one of targeting rangefinders is not aligned..'

Kirk: 'How can it be Spock?'

Spock: 'Captain, logic dictates that one of the maintenance crew aligned it using an object a city block away.. It can however be corrected if required. You will find the correct procedure outlined in the ship's repair manual.'

Kirk: 'Lt Sulu, set the targeting rangefinder to infinity'

Sulu: 'Targeting rangefinder set, Sir'

Kirk: 'Lt Sulu, align the double image'

Sulu: 'Image aligned, Sir'

Kirk: 'Lt Sulu, aim the proton blaster at that moon'

Sulu: 'Proton blaster aimed, Sir'

Kirk: 'Lt Sulu, fire that blaster'

Sulu: 'Yes!'

All in good humor of course
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Old 12-01-2005   #10
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"Make it so."
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Old 12-01-2005   #11
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One of the Klingon Battle cruisers was named the "Hektor".

Cap'n, The Hektor, Elmar, and Summar are all closing down on us!

Quick! Use the Neutral Density Field Generator.
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Old 12-01-2005   #12
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'My Hero' (a British TV show) has a character from the planet Ultron.
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Old 12-01-2005   #13
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I'll stick to checking the scotch I think....

I think I see the problem!
I test 1 meter, then infinity for me is a telephone pole about 300 ft away.
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Old 12-01-2005   #14
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Quote:
Originally Posted by pvdhaar
Kirk: 'Spock, why can't we blast those Klingons off of the surface of their moon?'

Spock: 'Captain, it appears that one of targeting rangefinders is not aligned..'

Kirk: 'How can it be Spock?'

Spock: 'Captain, logic dictates that one of the maintenance crew aligned it using an object a city block away.. It can however be corrected if required. You will find the correct procedure outlined in the ship's repair manual.'

Kirk: 'Lt Sulu, set the targeting rangefinder to infinity'

Sulu: 'Targeting rangefinder set, Sir'

Kirk: 'Lt Sulu, align the double image'

Sulu: 'Image aligned, Sir'

Kirk: 'Lt Sulu, aim the proton blaster at that moon'

Sulu: 'Proton blaster aimed, Sir'

Kirk: 'Lt Sulu, fire that blaster'

Sulu: 'Yes!'

All in good humor of course

ROTFL!! Best laugh I've had all day

Klas
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Old 12-01-2005   #15
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Picard: Number One, lock on to Kirk and zap him into #2

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Oy - I have marked my calendar when a Star Trek dialog became a thread at RFF.
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