Wrong triangle, I think. Hard to explain in just text. And you
changed what I said a little bit. Did not say anything about a center
subject at 10 feet.
A different way to think about it. Use a protractor and draw a 10 cm
semicircle. Now draw a straight line tangent to the semicircle at the
90 degree point in the middle of the arc. Assume that you want your
center of sharp focus at 45 degrees on the circle. Now when you
reframe so your subject is to the side your 10 cm focus setting is
still on the semi circle. But you have a straight line that
represents the plane of focus and your subject you focused on is in
front of it.
Winsor
Long Beach, California, USA
On Aug 24, 2005, at 2:20 PM, Chuck Norcutt wrote:
> Eh? I will certainly agree with you that there is a difference in
> distance between a subject at the center of the field (10 feet
> away) and
> another subject on the same plane 5 feet to the side. In this case
> the
> trig tells us that the distance to the subject at the side of the
> frame
> is 11.2 feet.
>
> However, if I choose the subject at the side of the frame as my focus
> point the camera will be focused at 11.2 feet. If I then reframe to
> incorporate a subject on the same plane and directly in front of me
> the
> subject directly in front will be at 10 feet and 1.2 feet ahead of the
> plane of focus. But the original (and presumably main) subject is
> still
> at 11.2 feet and still in precise focus. If I wanted the center
> subject
> in focus that's where I should have focused in the first place while
> recognizing that the DOF has a better chance of incorporating the
> subject 11.2 feet away since the DOF is greater behind the plane of
> focus. Or I should be more careful and check the DOF to be sure.
>
> So, I disagree with the premise you started with and the math of your
> correspondent. Did I miss something?
>
> Chuck Norcutt
>
>
> Winsor Crosby wrote:
>
>
>> I set the center focus sensor on my D100 as well, but am thinking
>> about changing it back. I saw a really interesting discussion that
>> showed the errors in focus using a center focus device. Most of the
>> time it is not noticeable, but if your your desired center of sharp
>> focus is off center, which it usually is, the distance to the subject
>> will be off. For instance if the focused distance is 10 feet and you
>> reframe you now have a plane of focus 10 feet away and the off center
>> subject will be in front of the plane of focus. Trigonometry. It
>> does not make any difference in most shots because of depth of field,
>> but it will certainly explain why that eye is not sharp in a portrait
>> or why that macro shot looks different than you remembered it. I
>> think that is why some photographers on the list some time ago went
>> to all matte screens on their OMs, so they would not be fooled into
>> using the center focusing aids.
>>
>> The reason for so many focus points is not to completely take you out
>> of the equation. Some camera makers call the 4 way rocker switch on
>> the back of the camera the "Focus point selector". It is your way of
>> selecting your sharpest point of focus in the picture.
>>
>>
>>
>>
>> Winsor
>> Long Beach, California, USA
>>
>>
>>
>>
>> On Aug 24, 2005, at 10:22 AM, alfredo pagliano wrote:
>>
>>
>>
>>> I agree completely
>>>
>>> Simpy cannot understand al those AF points on modern DSLRs.
>>>
>>> Someone uses them though...
>>>
>>>
>>> On Aug 24, 2005, at 3:30 PM, Walt Wayman wrote:
>>>
>>>
>>>
>>>
>>>> Scott,
>>>>
>>>> The autofocus setting was the first thing I changed on my E-1. In
>>>> fact, I had made up my mind to do so before I even got my hands on
>>>> the
>>>> camera. Before I bought, I downloaded the manual from the Oly
>>>> website, and upon reading "Normally, the camera measures the
>>>> distance
>>>> to the subject using the 3 AF frames in the viewfinder and selects
>>>> the
>>>> most appropriate point," my immediate response was, "Bulls**t!
>>>> I'm in
>>>> charge here and I'll be the one deciding what's the most
>>>> appropriate
>>>> point."
>>>>
>>>> So, before I took even the first shot, I set the autofocus to the
>>>> center frame only, and that's where it's going to stay. And
>>>> this is
>>>> even more fitting now that I've changed to the Katz Eye screen,
>>>> which
>>>> doesn't have all those little rectangles, just a split prism with a
>>>> microprism collar, a lot like what I see in the viewfinder of an
>>>> OM.
>>>>
>>>> And, of course, being in control at all times requires that
>>>> simultaneous AF and MF modes be set so the human in charge has the
>>>> authority to occasionally fine tune the auto focus, whether it
>>>> needs
>>>> it or not, as a reminder to the little man in the camera about
>>>> who's
>>>> really the boss of this outfit. You don't want him getting too
>>>> uppity. :-)
>>>>
>>>> Walt
>>>>
>>>> --
>>>> "Anything more than 500 yards from
>>>> the car just isn't photogenic." --
>>>> Edward Weston
>>>>
>>>> -------------- Original message ----------------------
>>>> From: "Scott Gomez" <scott@xxxxxxxxxxxxxxxxxxxxxxxxxx>
>>>>
>>>> [snip]
>>>>
>>>>
>>>>
>>>>> First, the autofocus is making me nuts. It's not at all clear
>>>>> to me
>>>>> how
>>>>> one tells just what the hell it's focusing *on*. From the
>>>>> manual, it
>>>>> appears that there are three "focus zones" outlined in the
>>>>> viewfinder,
>>>>> any one of which is selected by the E-1 as the focusing point.
>>>>> There's a
>>>>> little viewfinder doohickey that even shows you which of the three
>>>>> targets was used (I've updated to firmware version 1.4; I believe
>>>>> that
>>>>> the feature was added in firmware ver. 1.3). However, depending
>>>>> on the
>>>>> scene being photographed, the zone encompassed by any one of the
>>>>> focus-outlines can be a target-rich environment. Just how does one
>>>>> know
>>>>> where the actual focus is, with any reliability? What are the
>>>>> pitfalls
>>>>> of forcing the camera to use only one focusing zone (as it
>>>>> appears is
>>>>> possible to accomplish through settings)? Having *never*
>>>>> extensively
>>>>> used a camera having autofocus, I'm finding this quite
>>>>> frustrating.
>>>>>
>>>>>
>>>>
>>>> [snip]
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>>>
>>>
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