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Re: [OM] IMG: More Skippers

Subject: Re: [OM] IMG: More Skippers
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Sun, 24 Jun 2012 22:17:18 -0400
See, Mike doesn't realize how he's being used.  I just utter some 
nonsense and then he'll come along and fill in all the hard techie stuff.

Dr. Diffraction



On 6/24/2012 9:27 PM, usher99@xxxxxxx wrote:
>> The equation accounts for it perfectly... it's just that you have to
>> compute the effective focal ratio.  Not hard actually if you refer to
>> your macro lens's exposure table.  If it says to increase the exposure
>> by a factor of two at a given extension then the effective f-stop is
> one
>> stop smaller than indicated.  If it's reading f/8 then it's actually
> f/11.
>
>> Chuck Norcutt
>
>
>
>
> Jim
> Chuck wanted to keep things nice and easy to demonstrate the principle
> and prolly didn't wish to rouse Dr. Diffraction from his
> usual aestivation.  He usually appears in the depth of cold dark
> winters when we have more detailed discussions of
> effective apertures and DOF.
>
> Yes it is the effective aperture not the lens setting that determines
> dof.  A complicating factor that Dr. Diffraction
> has discussed in the past to some degree is pupillary magnification.
>
> Effective F-Stop = F-Stop x (1 + Magnification / Pupil Magnification)
>
> Thus
> Total DoF = 2NC(1 + M/P) / [ M^2 - (CN/f)^2 ]  (thin lens approximation
> and of course ignores very complex full wave solutions)
>
> where: N = aperture, C = circle of confusion diameter, M = subject
> magnification f = focal length
>
> Assuming a symmetric lens P=1 and thus going from infinity to 1:1,  it
> is easy---lose 2 stops.
>
> Most telephotos the P is <1 and thus actually "gains" dof at cost of
> more light lost.  Retrofocus lenses usually have P>1.
>
> I believe the P may change with mag for a given lens as well.  Then
> there is the focal length shortening that often occurs as mag increases
> in many lenses
> as well as other floating element shenanigans.  Big Foot loses about
> 1/2 stop less than it should at 1:1 with its internal machinations
> compared to just adding extension.
> I have never  come across any formulas of just how that works  and
> didn't quite believe John Shaw until I empirically proved it to be
> correct.
> In any event don't be too afraid of some diffraction if you really need
> dof. If you develop a liking to focus stacking, by all means avoid all
> diffraction.
>
> Oh, here is an OK link I think is worth a quick look:
> http://www.cambridgeincolour.com/tutorials/macro-lenses.htm
>
> IIRC, many Nykon cams actually display the effective aperture--not the
> case with any of mine though it really doesn't matter all that much
> from a practical
> POV.
>
> Uh Oh, I might have awoken Dr. D,  Mike
>
>
>
>
>
>

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