If you've never read John Shaw's "Closeups in Nature" you should. It
can be bought for a pittance as a used book on Amazon.
<http://www.amazon.com/Shaws-Closeups-Nature-Practical-Photography/dp/0817440526/ref=sr_1_1?s=books&ie=UTF8&qid=1292641036&sr=1-1>
He covers numerous way of achieving the same magnification using tubes,
teleconverters, diopters, stacked lenses, all or some in combination,
etc, etc. They all seem to produce a different amount of light loss and
at times seem to violate the laws of optics. A good read.
Chuck Norcutt
On 12/17/2010 7:29 PM, usher99@xxxxxxx wrote:
> AG writes:
>> The main reason why an internal-focus lens loses less light during
>> close focusing is two-fold. One reason is the lens will shorten in
>> focal length. The other reason is that depending on design, the
>> focusing is done by optically moving the rear nodal point through the
>> adjusting of lens elements in relationship to other lens elements,
>> whereas a traditional lens physically moves the rear nodal point by
>> physically moving the lens away from the film or sensor.
>
> Well it seems many macro lenses and many zooms with close focus ability
> do shrink the focal length to gain mag.
> Symmetric lenses at any given magnification won't gain any increase in
> effective aperture by changing focal length.
> Seems to be an easy way to get increased mag w/o having a giant
> extension required for the helicoid mechanism.
>
> Perhaps someone noticed the increase DOF of the longer FL lens in the
> toothwalker link with PMF of less than 1--
>
>
>> From toothwalker link:
> This is misleading!
>
>
> VWDOF 2.1---------input---------------------
> Format 24x36 mm 24x36 mm
> COC 0.030 mm 0.030 mm
> Focal length 60 mm 100 mm
> Pupil factor 1.0 0.7
> F-number 8 8
> Magnification 1 1
> ------------------output--------------------
> Depth of field 0.960 mm 1.17 mm
>
> Ha--this seems to violate the no free lunch rule of optics.. But NO!
> The effective aperture is governed by :
> effective aperture= marked aperture X (magnification/PMF +1)
> Thus the effective aperture # is 2.43 X marked aperture for the longer
> FL lens while the shorter is only 2X the marked aperture or F16.
> So when adjusted for Effective Aperture , only **Mag*** governs DOF:
> http://www.zen20934.zen.co.uk/photography/dof/dof.htm
>
> Thus there is NO free lunch.
>
> Why bother with this? Emperically AG is correct regarding at least
> moving the rear nodal point, I think. I have never
> seen the math behind this or how that works. In the wisdom of the OM
> archives, have seen others measure
> light loss with macro lens with IF on a bronica etrsi with a sekonic
> meter in reflective mode where the film would be demonstaring only 1
> 2/3 stops lost at 1:1 rather then 2 stops. This is about the same
> aswith the CV 125/2.5 Apo Lanthar using an OM meter.
> Seems to be an optics sleight of hand --- at least intuitively
> violating the NFL rule of optics.
>
> Mike
--
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