I just put on my "Test it Moose" t-shirt. But the math does give you
some starting points and boundaries.
Chuck Norcutt
On 6/25/2012 4:12 PM, Moose wrote:
> On 6/24/2012 9:52 AM, Jim Nichols wrote:
>> Does that mean I have to expose at f/5.6 to avoid refraction problems when
>> fully extended?
>
> Not necessarily. While all the numbers and equations the various Drs. are
> throwing around are accurate for thin (i.e.
> single element) lenses, they are only useful as guidelines or starting points
> for the real world. They don't address two
> important factors.
>
> 1. Complex lenses have different DOF characteristics, so the trade-off of
> resolution in the focal plane and DOF may vary
> by lens and diffraction effects may be more or less obvious at any given
> aperture.
>
> 2. Many contemporary lenses, going back to some Zuikos and others with moving
> elements for close focus back at least 30
> years, mess all this up. They change focal length as they focus closer. When
> your 100 mm macro lens becomes 60 mm at
> 1:2, you need to rework the equations. But - nobody publishes this data on
> their lenses.
>
> The upshot is simple. Put up a 3D subject that approximates the things you
> are shooting. Set up with tripod, focus
> carefully, in your case with E-510 live view, and shoot a small series of
> shots at various focal distances you use at
> apertures from say f4 to the smallest the lens will do. Remember to check
> focus for each shot, as many lenses change
> focus at least slightly with aperture.
>
> It immediately becomes apparent where the sweet spots are.
>
> I have done this, and the practical results don't always agree with the
> theory.
>
> Another thing to remember is that the 'theory' isn't pure math/optics. It
> started based on studies of human visual
> acuity. They showed lots of B&W prints to lots of people to determine at what
> point, for the average person, viewing a
> specific print size at a specific distance, differences in sharpness could be
> seen. The size of the Airy disks chosen
> for all the fancy calculations are all based on this type of research. If it
> had all been based on my vision, the
> numbers would all be different. :-)
>
> We are now viewing in color, sometimes in a web image, sometimes at 100%,
> often images made with significantly more
> complex lenses. At 100%, diffraction effects are going to be easier to
> notice. At 800-1000 pixels wide on a computer
> monitor, they will be less noticeable.
>
> Test It Moose
>
--
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