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Re: [OM] Binary DoF

Subject: Re: [OM] Binary DoF
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Thu, 23 Oct 2014 21:49:56 -0400
Rockwell's example <http://www.kenrockwell.com/tech/focus.htm> using the Leica 40/2 shows how to set the hyperfocal distance on a prime lens using the depth of field scale. He sets the infinity mark above the aperture he wants to use (f/8) at the right side of the scale. He then reads 10 feet above the f/8 mark on the left side of the scale. That's telling you that your depth of field ranges from 10 feet to infinity. The definition of hyperfocal distance is that focus distance that gives focus from half of the hyperfocal distance to infinity. Now you can say you understand if you've inferred that the hyperfocal distance at work here is 20 feet (double the closest distance). If you look at the Leica focusing scale again look at the triangle that marks the actual focus point on the scale. You have to do a little interpolation since the scale is not linear but I think you'll agree that the actual focus distance is 20 feet.

Part of the argument that's been going on here purports that the parameters on which this Leica scale is based are old and inappropriate for more modern films and digital gear. I can tell you that this Leica scale assumes a resolution of approximately 30 lines/mm. That's often sufficient for me. Want a more stringent parameter? Shoot at one stop smaller (f/11) while continuing to assume the 10 feet to infinity given by f/8 on the scale. That ups the assumed resolution value to 40 lines/mm. Still not good enough. Up it another stop to f/16 while still assuming the limits of the f/8 scale. That assumes about 57 lines/mm. But be careful now. As you increase the f/stop Yyu're starting to head into diffraction territory which will destroy the resolution you think you're going to get. Also, unless you're on a rock steady tripod your gear is never going to give you 57 lines/mm even though the lens be perfect.

Not yet discussed is that some labs limit print resolution to 250 dpi. If you plan the perfect parameters for a 300 dpi 10x15 print and your lab only prints to 250 dpi you're fooling yourself. No doubt Moose needs 400 dpi. I'm not sure how it's being done.

Chuck Norcutt

On 10/23/2014 12:08 PM, Sandy Harris wrote:
Ken Norton <ken@xxxxxxxxxxx> wrote:

The comment was made that DoF and Hyperfocal calculations are based on
a binary decision: It's either sharp (in focus) or it isn't. I highly
disagree with that assessment. ...

The history behind the DoF calculations (and lens markings) is based
on the viewing of a standard size print observed from a standard
viewing distance. It's not a binary thing, but extremely variable ...

I know he is far from a popular man on this list, but Ken Rockwell
has a page on this topic that seems to me (far from expert) to
make sense: http://www.kenrockwell.com/tech/focus.htm

Anyone care to critique that? Or point to a better resource?

A method I have used with a reasonably fast prime (f2.0 or
better) is to just pick something at a good distance (infinity
focus), open the lens as wide as it will go, shorten focus
until I can see blur, lengthen it a smidgen to eliminate the
blur, stop down to f5.6 and shoot. I'm reasonably happy
with results, but wonder if there is a better way.

Also, that method won't work for a slower lens. Does
anyone have a good suggestion for those?

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