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Re: [OM] DoF Compared

Subject: Re: [OM] DoF Compared
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Tue, 10 Aug 2010 13:09:06 -0400
All of your rebuttal has nothing to do with my argument which I 
apparently didn't explain well enough.  My point is simply that, at 5 
meters, the depth of field at f/3.2 is sufficient that you can't 
guarantee (even using a magnified image) that you were focused on the 
same spot as the Zuiko.  Even assuming a much tighter CoC, it would be 
quite easy to focus beyond that point without being able to tell you had 
done it.

My point is simply that you haven't proven your point beyond a 
reasonable doubt.  And that doesn't even begin to address the 
performance of the individual lenses which might be able to account for 
it all by itself.

Chuck Norcutt


Ken Norton wrote:
>> The first is the performance of the lenses at f/4... an aperture much
>> more easily handled by the 4/3 lens which should be designed to shoot
>> high quality images at a couple of stops wider than the Zuiko 24/2.8.
> 
> I could have shown the F5.6 or F8 comparisons, but those are more
> nuanced and not so blatant because of the increased DoF at those
> apertures does include the distance objects and then at F8 you just
> start seeing the contrast loss from diffraction. I used F4 in this
> case because it was easier to see what I was pointing out AND it is
> consistent with other results when shot closer to an object at higher
> F-stops. Inotherwords, this is a good representation of real life
> results. Besides, I did this during lunch hour while eating tacos.
> 
> 
>> The second is the focusing accuracy achievable at 5 meters.  By your
>> calculations, DoF extends to 10.3 meters which implies using a CoC of
>> 0.015mm.  But that's at f/4.  Presumably you were focusing at f/2.8 with
>> the Zuiko and probably about f/3.2 with the zoom (which has an aperture
>> range of 2.8-3.5).  Since you're using a magnified image for focusing
>> I'll allow that the effective CoC is cut in half and tightens up to
>> 0.0075mm.  That would allow your actual focus at f/3.2 to be as far away
>> as 8.5 meters instead of 5 meters and still not detectable to the eye.
>> Then, when the CoC is relaxed back to 0.015mm and f/4 taking aperture
>> the far DoF can extend all the way to 70 meters.
> 
> I'm not sure I'm following your explanation. Each camera was focused
> to the same point--as close as live-view with 10x magnification on the
> L1 is able to achieve. I always do the back and forth routine to fine
> tune focusing to the halfway point between known defocuses. This is a
> technique I've used for decades and works well for me.  In the image
> files, the images appear to achieve maximum sharpness at the same
> point. Yes, I focused both wide-open. Neither of these two lenses
> experience problematic focus-shift with aperture adjustment that I've
> seen.
> 
> Again, this isn't my first comparative test and this one does reflect
> the results from the others.
> 
> The explanation that the exiting light cone is narrower is also my
> belief too. The Zeiss DoF/Bokeh article is a good reference in this in
> that it talks about the difference between Depth of Focus and Depth of
> Field. One is object side and the other is sensor/film side of the
> lens.
> 
> Let's think about a fictitious lens for a moment. The effective
> focal-length is 50mm on a 35mm camera/film format. But unlike our
> usual 50mm lenses, which, if symmetrical and non-tele or non-retro
> will place the convergence point 50mm from the film surface when
> focused at infinity. In this example, the exiting light rays are at
> the same angle as the incoming light rays. Now let's redesign the lens
> so the convergence point is placed 500mm from the film surface. The
> exiting light rays will be at a very narrow angle. Since the maximum
> amount of light-ray spread is only a fraction of that of the
> symmetrical non-tele/retro lens, the CoC of the sensor-side of the
> lens is greatly reduced.
> 
> Now, let's really blow people's minds...
> 
> Most zoom lenses aren't variable focal-length lenses. They're a
> wide-angle lens with a second lens group which zooms in or crops or
> optically selects a portion of the resulting image. They are actually
> two lenses in one. A 14-50 Leica when set at 24mm isn't a 24mm lens.
> It's a 14mm lens with a variable teleconverter added behind.
> (obviously it's a little more complicated than this as front elements
> are moved around too, but this is to correct for distortion, not
> magnification). There are a handful of lenses which are reverse--they
> are the longer focal-length with variable <1x optics. A pure varifocal
> zoom is a different animal, though, because whatever focal-length it
> is set at is true, not magnified. Actually, the Leica is a hybrid lens
> which possesses both varifocal optics and variable teleconverter. This
> is why it requires refocusing after zooming.
> 
> Now, to bring this back around to my fictitious 50mm lens with 500mm
> base... A "telecentric" lens is optically doing this very thing. It is
> taking a very short focal-length (like the DZ 7-14mm) and moving it
> away from the sensor plane. This is done for mechanical reasons, but
> also it is for optical reasons. By moving the exit pupil far from the
> sensor plane the exiting light rays leave at an extremely narrow angle
> in comparison to the subject-side light-rays.
> 
> I haven't even gotten into the subject of lens elements which aren't
> uniform in shape and what they do to out-of-focus light rays.
> 
> Just as an aside, the Zuiko 14-54 mk1 lens produced results closer to
> the OM-Z 24mm lens than the Leica. The Leica is in a different league
> when it comes to this kinda thing and it regularily gives an effective
> DoF of an additional stop. I've said it before and I'll say it again,
> this lens is definitely different than anything I've used before. In
> fact, when putting together the test results for this article I had to
> keep catching myself because I was trying to match up the wrong
> images. The Zuiko at F5.6 was extremely close to the Leica F4.0 image,
> and the F8 matched up to the F5.6 image. Others with this lens have
> said the same thing, so it isn't just me.
> 
> AG
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