On 09/04/2011 8:27 PM, Paul Stenquist wrote:
I can't help with the Kx, as I don';t have one, but I would strongly advise
against using any of the focus charts out there in favour of adjusting the
focus against a real world (and real world distance) object.
Unless you are using a flat field lens, (you aren't) a focus chart is
guaranteed to give a false front focus indication.
Why is that? Haven't heard that before. I used a homemade chart to adjust all
my lenses at a distance of six to eight feet, checking focus in the center of
the lens. Seems to me that would be accurate. But I'm no expert at these things.
It was one of the things that occurred to me when everyone on
ForumsNeurotica were dumping all over the K5 because it had difficulty
resolving correct focus in near darkness with very fast lenses.
A couple of points to consider when using a focus chart, which for the
purpose of discussion would be one similar to the famous Nikon D70 focus
chart (view-able here in small size) http://pinterest.com/pin/4898728/ are:
Is the chart absolutely perpendicular to the lens axis?
If it isn't it will give a false read.
This is easy to see, since unless the lens is decentered, the degree of
misfortune should be the same on either side of the chart.
So, the way to use one of these charts is to focus on the center line
(in the case of Yvone Borques chart, the knife edge between black and
white, but his chart is useless for technical reasons unrelated to lens
curvature) and then read the degree of front focus in millimetres some
distance from the point of AF.
If the lens has field curvature, and most do (macro lenses and enlarger
lenses tend to be flat field), then field curvature is introducing some
error, and unless the degree of curvature is known, the user has no way
of knowing exactly how much error is being introduced.
I use a similar method to yours for dialing in lenses. I focus on an
object (in my case a clock dial) at some distance from the camera using
center point AF, and I then adjust AF by how well what I was focusing on
is in focus.
This is very different from using the center focus point for focus and
then looking at something that would be in the region of one of the far
side points.
--
William Robb
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