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marknorton · 02/10/08, 06:01 AM

You say your lens will focus to 200 yards (let's call it meters) but not to infinity. The question is the, how much out of adjustment is the lens?

To a first approximation, lenses follow the basic formula:

1/f = 1/u + 1/v

At infinity, the lens is exactly "f" in front of the focal plane, 50mm. At 200metres, it moves slightly further forwards to 50.0125mm, or by 1/80mm. To a first approximation then, your lens is sitting 1/80mm too far forwards.

The Noctilux focussing scale is 9cm long and in moving from one end to the other, the lens moves forward by about 3mm from its infinity position. That means your infinity stop position is off by about 9/(3/(1/80)) or 0.375mm. The 50/1.4 ASPH focussing scale is only half as long AND because the lens focusses closer, to 0.7m, the travel is 4mm; in that case, the infinity stop is off by a mere 0.015mm.

It may very well be that these are within the limits of how accurately the infinity stop can be set; in any event, even at f1, 200m is well inside the available DoF.

02/10/08, 06:01 AM	#11 (permalink)
marknorton Erfahrener Benutzer Join Date: 11/12/02 Posts: 5,620	Re: How Important is Infinity Focus? You say your lens will focus to 200 yards (let's call it meters) but not to infinity. The question is the, how much out of adjustment is the lens? To a first approximation, lenses follow the basic formula: 1/f = 1/u + 1/v At infinity, the lens is exactly "f" in front of the focal plane, 50mm. At 200metres, it moves slightly further forwards to 50.0125mm, or by 1/80mm. To a first approximation then, your lens is sitting 1/80mm too far forwards. The Noctilux focussing scale is 9cm long and in moving from one end to the other, the lens moves forward by about 3mm from its infinity position. That means your infinity stop position is off by about 9/(3/(1/80)) or 0.375mm. The 50/1.4 ASPH focussing scale is only half as long AND because the lens focusses closer, to 0.7m, the travel is 4mm; in that case, the infinity stop is off by a mere 0.015mm. It may very well be that these are within the limits of how accurately the infinity stop can be set; in any event, even at f1, 200m is well inside the available DoF. __________________ Mark Last edited by marknorton : 02/10/08 at 06:04 AM.