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# Thread: How to "Focus" at Infinity

1. The "aperture plane"? You mean the one the exit pupil is in? Or the one the entrance pupil is in? Or where the physical diaphragm is?
None of the three is the correct thing to measure from.

You need to measure from the rear principal plane. Where that is located (and where the entrance and exit pupils are) depends on the lens design. When data are provided by the manufacturer, they usually state the position relative to other bits of the lens, like the last lens vertex. So you need to do some calculations (involving the exact focal length, not just the nominal focal length) to find a number you can use.

But that's all too technical. Not practical.
Cameras have a groundglass screen. Use your eyes.
Reduce the lens to film distance/bellows draw, until everything starts getting out of focus. Go back a bit, "et voila"! Infinity focus!

2. [QUOTE=CPorter;736422]
Originally Posted by Gary892
155 divided by 25.4 mm/inch = 6 inches-----------locate the aperture plane 6 inches from the film plane and you will have infinity.
I like imperial measurements as much as most people of my age and over who didn't have the metric system drummed into them at school.

However, why do a conversion? Why not just use the metric measurement?

Your conversion has already put it 0.1" out as 155mm is 6.102". 6" is 152.4mm.

I find that I use imperial for large things like building/restoring houses and metric for smaller engineering work.

Steve.

3. Originally Posted by Q.G.
The "aperture plane"? You mean the one the exit pupil is in? Or the one the entrance pupil is in? Or where the physical diaphragm is?
None of the three is the correct thing to measure from.
Well, I'm not going to pretend to know the ends and outs of lens construction and lingo--it's more than I care to think about. I can only tell you that I measure from where the aperture is and it works beautifully.

4. [QUOTE=Steve Smith;736541]
Originally Posted by CPorter
Why not just use the metric measurement?
IDK, I get there just the same and without delay.

5. When I wanted to find out where my lens was when focused at infinity, I took it outside at night and focused on some stars...

6. Originally Posted by EASmithV
When I wanted to find out where my lens was when focused at infinity, I took it outside at night and focused on some stars...
I simply found the distance from lens to film plane by measuring with a ruler just once, then noted the distance in mm from inside to inside of the front and back standards. Anytime I want infinity, I simply separate the standards by 126mm inside to inside on the monorail------very quick.

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