Damned customs - F 85 Soft

Bruce Walker bruce.walker at gmail.com
Wed Apr 6 10:20:41 EDT 2011

On 11-04-06 10:13 AM, Matthew Hunt wrote:
> On Wed, Apr 6, 2011 at 10:06 AM, steve harley<pdml at paper-ape.com>  wrote:
>> interesting technique, and thanks for sharing, but it doesn't seem intuitive
>> that spherical aberration alone can describe soft focus
>> a thought experiment: if you aimed a soft-focus lens at just the right
>> concave surface, would everything be sharp? i think there'd still be more
>> going on
> You're confusing spherical aberration with field curvature. Spherical
> aberration is an on-axis aberration; it's what you get from a
> spherical telescope mirror instead of a parabolic mirror. If you have
> rays of light from a point source on axis (i.e. at field center), rays
> of light at the edge of the mirror/lens come to a different focus than
> rays that pass near the center. So even a single star at the center of
> the field is blurred. It was the aberration that afflicted the Hubble
> Space Telescope at launch.

And we think Pentax QA is bad. :-)

So how did they fix that?  Send up a new mirror?


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