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Re: [OM] Mea Culpa? [was Eeeek!]

Subject: Re: [OM] Mea Culpa? [was Eeeek!]
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Sat, 11 Aug 2012 17:17:51 -0400
I'm glad to hear that there's apparently nothing not understood now.  I 
didn't say anything at the time but when you first posted this "problem" 
I wondered why I could never recall seeing anything similar with either 
my Minolta A1 or Samsung WB650 which both, of course, have CDAF.  And, 
in terms of digital technology years, the Minolta is already ancient 
history.

I suppose it should also show the mid-range focusing problem where it 
focuses on whatever shows greatest contrast but not necessarily the 
correct depth.  But perhaps I've never seen that on these small sensor 
cameras with their very large depth of field.  Oops.  Wait a minute. 
With the Samsung racked out to its 350mm equivalent focal length I do 
sometimes get a mis-focused image.  I attributed that to operator error 
but now it occurs to me that, despite the small sensor, depth of field 
is not so great at 350mm equivalent.  Still operator error I guess but 
of a different kind than I expected.  I'll have to pay more attention to 
where the focus actually lands and why.

Chuck Norcutt


On 8/11/2012 3:54 PM, Moose wrote:
> On 8/11/2012 6:56 AM, Carlos J. Santisteban wrote:
>> Hi Moose and all,
>>
>>
>> From: Moose <olymoose@xxxxxxxxx>
>>
>>> but where it's CDAF will focus in a complex, 3D subject is just 'different' 
>>> than with Canon PDAF.
>
> I still find this to be true.
>
>>> Example: A bee on the dark center of a sunflower. With the Canons, 
>>> centering the central focus point on the bee would focus on the bee. The 
>>> OM-D and 14-150 focused on the surface of the flower.
>
> Here, I believe I may have misunderstood the effect of operator error. The 
> way the E-M5 indicates an attempt to focus
> closer than the lens will go is somehow different than I was used to. I had 
> the camera set to take a shot even if focus
> could not be achieved, as I had been playing with how OOF shots looked with 
> this lens/camera.
>
> I now think that what happened with these particular shots was that the bee 
> was just slightly too close for the lens to
> focus on, it focused as close as it could, I didn't properly note the lack of 
> confirmation beep/flash, and the above
> setting let me go ahead and shoot out of focus.
>
> I've now got the camera set to refuse to fire without focus confirmation. :-)
>
>> The great difference between both AF systems is that PDAF, much like a
>> human with a split-screen, usually "knows" if a particular object within
>> the AF area is in focus, back-focused or front-focused. It seems they're
>> designed to focus on the closest object inside the AF area, leaving the
>> rest beyond the focus plane.
>
> Yes, although often moving the central spot around and refocusing will get 
> what I want. That depends, of course, on
> vision acute enough to see the effect in the viewfinder.
>
>> On the other hand, CDAF (which works more like a matte screen) has no idea
>> at all -- it just moves the focusing mechanism back and forth and keeps
>> comparing the apparent sharpness of whatever lies on the AF area. If the
>> measured sharpness/contrast is going _down_, it reverses the focusing
>> direction, until contrast goes down again -- supposedly after reaching the
>> focus point.
>>
>> But if the objects inside the AF area are located at different distances,
>> the one who contributes "more" to that perceived sharpness (be it because
>> of contrast, pattern, colour, orientation... whatever) will "win" the CDAF
>> quest...
>
> Yes. As a result, it will sometimes focus at a random appearing depth in a 
> complex, deep, 3D subject. I've run into that
> on a few occasions. Again, not necessarily 'bad', but different from PDAF.
>
> On the up side, I discovered S-AF[MF] focus mode. A half press of the shutter 
> release, or press of a function button set
> to AF, focuses, and fine tuning may be done with the MF ring.
>
> This is very much like the nicer Canon lenses that allow MF adjustment 
> without switching out of AF mode.
>
> I like!!
>
> MF AF Moose
>
-- 
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