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Re: [OM] Focus Accuracy Test - Oh what misery we bring upon ourselves

Subject: Re: [OM] Focus Accuracy Test - Oh what misery we bring upon ourselves
From: Wayne Harridge <wayneharridge@xxxxxxxxxxxxxxx>
Date: Tue, 17 Aug 2010 13:31:14 +1000
You need to use a "messraster" focussing screen:

http://anstendig.com/Messraster/death.html

...Wayne


> Ken Norton <ken@xxxxxxxxxxx> wrote:
> 
> For the sake of argument (or non-argument), I'll agree with you,
> Chuck. We'll make the assumption (which may or may not be true) that I
> misfocused for the original test. Since I didn't have usaf charts in
> the scene, I was at the mercy of the details in the scene--which in
> this case is the grass. If I did misfocus, this does point out just
> how haphazard our focusing really is and we're just getting it close
> or maybe not-so-close most of the time. I did think my "real world"
> landscape shot with ultra-detail was a good indicator of lens
> performance, though.
> 
> As to the 5D Mk2's live-view--it is my understanding that this camera
> doesn't use all pixels for live-view and skips pixels. This, again,
> would get you in the neighborhood but is anything, but exact. To
> differenciate a focus point at 5 vs. 6 meters with a "normal" focal
> length, the difference with most lenses is a whisper of a nudge. Like
> C.H. says, 0.5mm or so. The problem is that wide-open, there is
> usually just enough CA or other issues with the lens which prevents
> precise focusing to this level. As mentioned, stopping down the 24mm
> F2.8 lens to F4 acually make precise focusing more feasable. The
> line-pairs on the usaf lit up like a Christmas Tree when in focus. I
> could conclude, therefore, that it is possible the L1 has a superior
> live-view than the 5Dmk2.
> 
> Here's the rub. Let's say we adjust the focus point to, say, 3 meters.
> This is a nice point close enough to the camera that focusing should
> be easier.  But wait!  What is the accuracy of focus? If you can't
> differenciate between 5 and 6 meters, then at 3 meters your accuracy
> is probably no closer than 5 cm. This would throw off the test too.
> 
> Therefore, I can only conclude that it is impossible to focus any lens
> to any degree which would allow us to compare DoF, Bokeh or any other
> trait. We're pretty much hosed. We're obviously deep inside the margin
> of error.
> 
> BTW, a nifty trick for focusing an SLR in the dark. Shine a laser
> pointer through the viewfinder. When the dot on your subject is sharp
> you are in focus.  :)
> 
> So, here we are--a lens conundrum. How to test these lenses in a way
> that either proves or disproves the theory. Empiracal evidence is no
> evidence at all. The emperical evidence shows that there is something
> beyond the simple algebraic calculations which assume simple optical
> traits. But Jan, Chuck and I'm sure others are screaming at their
> computers right now saying "just trust the numbers".
> 
> I don't think I've ever claimed that one lens has more DoF than
> another, I've stated pretty clearly, I hope, that one lens has an
> apparant DoF greater than another. What is causing this, I don't know.
> That's what I'm trying to figure out. But then there is a side
> question of "What is DoF?" If DoF calculations are based on a
> subjective analysis of a printed image and the "apparant" increase in
> DoF is visible in the printed image doesn't that therefore defeat the
> existing formula for calculating DoF? Again, if the formula is
> subjective analysis based and the increase is also subjective, then
> the formula is wrong.
> 
> The formula is wrong or defeated by diffraction. Also, how does the
> formula change in relation to the position of the nodal points to the
> film plane? Most formulas use the front nodal point. One reason why I
> selected such a long distance was to cover up the variations in the
> nodal point positioning. Closer focusing will greatly alter the
> position of this front nodal point to the level that it will mess with
> any usable conclusions. The 50mm tests were done closely enough that
> the change in front nodal point was enough to change subject size.
> 
> Recently I did a test of 50mm focal lengths. It was quite amazing the
> differences between the size and shape of the bokeh at identical
> apertures. Some of the lenses with aspherical elements folded the OOF
> highlights in on themselves. Instead of a symmetrical growth pattern,
> it instead remained small. If you could develop a lens with multiple
> aspherical elements which successfully folded the OOF highlights back
> to a point then you'd have a deep-focus lens. In fact, check this out:
> 
> http://www.freepatentsonline.com/1627892.pdf
> 
> There is obviously then a way to defeat the DoF calculations with
> aspherical elements and other nifty tricks. A cross-section and ray
> analysis of some of our modern lenses may show deep-focus lens traits.
> 
> But unless we can figure out how to accurately focus our lenses, we're
> pretty much hosed in ever knowing.
> 
> AG
> -- 
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...Wayne

Wayne Harridge

http://lrh.structuregraphs.com
-- 
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