From: Wilba on
Paul Furman wrote:
> Wilba wrote:

>>>>>>> I don't think it gets within the inner 1/3 of an f/1.8 shot or even
>>>>>>> an f/2.8 shot.
>>>>>>
>>>>>> Why not?
>>>>>
>>>>> Because it's optimized for f/2.8, not f.1,8.
>>>>
>>>> Yeah, but the DOF is thinner... so sharp focus would be easier to
>>>> nail...?
>>>
>>> Not if the AF system can't see much wider than f/2.8.
>>
>> Um, are you saying that the DOF isn't thinner at f/1.8 than it is at
>> f/2.8? Or that the thinner DOF at f/1.8 is of no use to the AF sensor if
>> it gets it from the f/2.8 part of the exit pupil (i.e. not from the
>> extremes)? Or something else?
>
> The thinner DOF at f/1.8 is of no use to the AF sensor if it gets it from
> the fat f/2.8 part of the exit pupil.

OK, that's what I don't understand. :- )


From: Wilba on
Paul Furman wrote:
> Wilba wrote:
>> It's still an average 1.5mm front-focus, not back-focus.
>
> The average focus chosen by the AF system is behind the subject.

The average doesn't matter, since it's binary - alway either one or the
other.

> In the focus shift web page, he auto-focuses on the front eye but the
> final pic is focused on the back eye. I'm calling that 'back-focus'.

Right. That's the opposite of what's happening for me, it's front-focus.


From: Doug McDonald on
Paul Furman wrote:
> Wilba wrote:
>>
>> It's still an average 1.5mm front-focus, not back-focus.
>
> The average focus chosen by the AF system is behind the subject. In the
> focus shift web page, he auto-focuses on the front eye but the final pic
> is focused on the back eye. I'm calling that 'back-focus'.
>


One thing to remember: at least on my Canon 30D, the focus sensor
area is much larger than the little box that represents it in the viewfinder,
three times as large in linear dimension in fact, as far as I can tell
with tests. And I've never been able to tell exactly what it
tried to focus on ... the nearest object, or the average, or the one
with highest contrast.

What really is needed is a true live view focus system that allows you to select
the spot you want to focus on and blows it up on the screen to a size that
allows you to see individual pixels clearly.

I suspect that I will next buy a new camera when a full frame one comes
out with that feature.

Doug McDonald



From: Wilba on
Doug McDonald wrote:
>
> One thing to remember: at least on my Canon 30D, the focus sensor
> area is much larger than the little box that represents it in the
> viewfinder,
> three times as large in linear dimension in fact, as far as I can tell
> with tests. And I've never been able to tell exactly what it
> tried to focus on ... the nearest object, or the average, or the one
> with highest contrast.

Right, same with the ***D and ****D series. That's why the most reliable
tests use a flat target perpendicular to the lens axis with a nearby slanted
scale (Canon use that kind of setup for calibration).

> What really is needed is a true live view focus system that allows you to
> select the spot you want to focus on and blows it up on the screen to a
> size that allows you to see individual pixels clearly.

Looking at the numbers, the 450D probably comes close to showing the live
view at 100%, depending on what "10x zoom" actually means.

> I suspect that I will next buy a new camera when a full frame one comes
> out with that feature.

Sounds good. :- )


From: Chris Malcolm on
Doug McDonald <mcdonald(a)scs.uiuc.edu.remove.invalid> wrote:

> One thing to remember: at least on my Canon 30D, the focus sensor
> area is much larger than the little box that represents it in the viewfinder,
> three times as large in linear dimension in fact, as far as I can tell
> with tests. And I've never been able to tell exactly what it
> tried to focus on ... the nearest object, or the average, or the one
> with highest contrast.

> What really is needed is a true live view focus system that allows you to select
> the spot you want to focus on and blows it up on the screen to a size that
> allows you to see individual pixels clearly.

The Sony A550 has that. Makes it easy to solve those ambiguous focus
situations.

--
Chris Malcolm