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[OM] Re: Mini Macro questions

Subject: [OM] Re: Mini Macro questions
From: Moose <olymoose@xxxxxxxxx>
Date: Fri, 16 Mar 2007 23:10:21 -0700
Chuck Norcutt wrote:
> I would certainly choose the 105 based on working distance.  Whether f/2 
> vs 2.8 is an issue is hard to say.  Guess it depends on what and where 
> you intend to shoot.  
Coastal redwood forest, much darkness. And with an E-500, so the 
viewfinder isn't ideally bright anyway.
> But if you're talking OM gear and bellows the bellows is calibrated for 50mm 
> and not for 105.  Maybe that would make a difference to someone.
>   
The specific discussion at hand was 4/3 system macro, ZD 50/2 and Sigma 
105/2.8
> Even though both have the same DOF at equivalent apertures and 
> magnification I was thinking that the 105 might ultimately be more 
> preferable for DOF if the larger lens diameter allowed a smaller 
> aperture to be used before diffraction limits set in.  
According to cambridgeincolour 
<http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm>, 
there in no difference:

"Since the physical size of the lens aperture is larger for telephoto 
lenses (f/22 is a larger aperture at 20 mm than at 100 mm), why doesn't 
the size of the airy disk vary with focal length?  This is because the 
distance to the focal plane also increases with focal length, and so the 
airy disk diverges more over this greater distance.  As a result, the 
two effects of physical aperture size and focal length cancel out.  
Therefore the size of the airy disk only depends on the f-stop, which 
describes both focal length and aperture size."
> In searching for the answer to this I discovered the previously unknown to me 
> phenomenon of diffraction at the pixel sites on the sensor.  Make perfect 
> sense given that diffraction is caused by light bending around *any* edge 
> (including the lens mounting cell) but I hadn't stopped to think about it 
> before.
>
> Interestingly, the 8MP 4/3 sensors are diffraction limited beyond f/5.6 
>   
Whoa! So my recent advice to Scott to stop down further in aid of DOF 
will soften the overall image even as it brings more into focus. Maybe 
that's the reason for the rumors of FF sensors from more manufacturers.
> and the 5D sensor is diffraction limited beyond f/11.  
That I haven't noticed, but it may be subtle at f16-22 and lost in other 
factors like DOF, focal accuracy and motion.

Moose

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