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Re: [OM] IMG: More Skippers

Subject: Re: [OM] IMG: More Skippers
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Mon, 25 Jun 2012 22:00:25 -0400
What!  Did you think I remembered the difference without looking it up?  :-)

Chuck Norcutt


On 6/25/2012 5:10 PM, Jim Nichols wrote:
> Thanks, Moose.  Good inputs, even if I did confuse "refraction" and
> "diffraction", which I blame on senility.  ;~))
>
> Jim Nichols
> Tullahoma, TN USA
> ----- Original Message -----
> From: "Moose" <olymoose@xxxxxxxxx>
> To: "Olympus Camera Discussion" <olympus@xxxxxxxxxxxxxxxxx>
> Sent: Monday, June 25, 2012 3:12 PM
> Subject: Re: [OM] IMG: More Skippers
>
>
>> 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
>>
>> --
>> What if the Hokey Pokey *IS* what it's all about?
>>
>>
>> --
>> _________________________________________________________________
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>>
>>
>
>

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