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"Video AF" in digital cameras?

 
 
Nostrobino
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      08-08-2004
All of my Minolta digital cameras use "video AF" according to the specs
given in the manuals. How is this different in principle from autofocus
methods used in 35mm cameras?

I'm asking because I've had a lot of unexplainable autofocus misses with
these digital cameras, where such misses were never (or very, very rarely)
experienced with 35mm AF cameras, either SLR or point-and-shoot. The misses
appear to be related to the use of wide-area AF, and seem (so far, fingers
crossed) to disappear when spot AF is used instead. But wide-area AF is the
default in these cameras.

Indeed, the very popular little X-series Minoltas have no provision for
switching to spot AF. Fortunately, they don't seem to need it either--I've
had no AF misses with my Xt and Xg cameras that I can recall.

It does seem to vary according to camera model. The S404 and S414 are
somewhat notorious for AF misses according to other users (and I have one of
each and they both sometimes do it), though as mentioned, switching to spot
AF apparently fixes this. At least one user has complained about AF misses
with the 7i, but mine has given no problem with this even in wide-area AF.
My F300 in wide-area AF mode in one case gave a bizarre result: a vase way
over to the side, well outside the indicated AF zone, was in perfect
focus--while the girl across the table from me, in the center of the
picture, was out of focus.

I have seen similar complaints about high-end Olympus cameras (like the
C-5060) which have wide-area AF, so I presume this is not a rare phenomenon
and occurs across many if not all makes.



 
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Roland Karlsson
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      08-08-2004
"Nostrobino" <(E-Mail Removed)> wrote in news:wquRc.894$ZC7.551
@newssvr19.news.prodigy.com:

> All of my Minolta digital cameras use "video AF" according to the specs
> given in the manuals. How is this different in principle from autofocus
> methods used in 35mm cameras?


I assume that Minolta means that the video output from the CCD
is used for auto focus. That is the most common auto focus method
for non SLR digital cameras, i.e. cameras with live preview.

> I'm asking because I've had a lot of unexplainable autofocus misses


Yepp - that is a problem.

Cameras without live preview, e.g. film based ones, must have a
mechnism that is specially built for auto focus. This mechanism is,
depending on price range for the camera, very cleverly built. It is
often fast and accurate.

Now - for cameras with live preview you have another option. You can
use the data from the live preview for meassuring in the auto focus
system. This has some advantages.

1. It is (almost) for free, i.e. you save money.
2. You focus exactly on the sensor, i.e. no need for
expensive aligment between the auto focus sensor and
the picture sensor. The camera becomes more robust.
If you drop it, it will probably still focus correctly.
3. You don't obscure the optical path to the sensor.

But it also has some disadvantages.

1. It is slow. Specially built small sensors are much faster.
2. It is too simple. Specially built auto focus sensors can
be made to detect phase differences, by using some prisms
or sensors in several planes etc.

The result of both the speed issue and the too simple construction
is that it is generally less precise. It needs rather much light
(as the sensor gets slower when it is darker) and also a rather
good pattern with high contrast, often also aligned along some
axis, e.g. vertical.


/Roland
 
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Nostrobino
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      08-09-2004

"Roland Karlsson" <(E-Mail Removed)> wrote in message
news:Xns953FEFD938388klotjohan@130.133.1.4...
> "Nostrobino" <(E-Mail Removed)> wrote in news:wquRc.894$ZC7.551
> @newssvr19.news.prodigy.com:
>
> > All of my Minolta digital cameras use "video AF" according to the specs
> > given in the manuals. How is this different in principle from autofocus
> > methods used in 35mm cameras?

>
> I assume that Minolta means that the video output from the CCD
> is used for auto focus. That is the most common auto focus method
> for non SLR digital cameras, i.e. cameras with live preview.
>
> > I'm asking because I've had a lot of unexplainable autofocus misses

>
> Yepp - that is a problem.
>
> Cameras without live preview, e.g. film based ones, must have a
> mechnism that is specially built for auto focus. This mechanism is,
> depending on price range for the camera, very cleverly built. It is
> often fast and accurate.
>
> Now - for cameras with live preview you have another option. You can
> use the data from the live preview for meassuring in the auto focus
> system. This has some advantages.
>
> 1. It is (almost) for free, i.e. you save money.
> 2. You focus exactly on the sensor, i.e. no need for
> expensive aligment between the auto focus sensor and
> the picture sensor. The camera becomes more robust.
> If you drop it, it will probably still focus correctly.
> 3. You don't obscure the optical path to the sensor.
>
> But it also has some disadvantages.
>
> 1. It is slow. Specially built small sensors are much faster.
> 2. It is too simple. Specially built auto focus sensors can
> be made to detect phase differences, by using some prisms
> or sensors in several planes etc.
>
> The result of both the speed issue and the too simple construction
> is that it is generally less precise. It needs rather much light
> (as the sensor gets slower when it is darker) and also a rather
> good pattern with high contrast, often also aligned along some
> axis, e.g. vertical.


Yes; I've noticed that when I try to test whether the camera is focusing
properly or not--by shooting objects with clear, crisp areas of contrast,
lettering etc., so that I can judge the sharpness after taking the shot--the
AF usually gives perfect results. But the same camera often fails to give
sharp focus on people pictures. The S404 gave me a lot of mildly
out-of-focus shots in bright sunlight, always with people in the middle
distance as the subjects. Shots of houses, boats etc. were perfectly sharp.
Maybe such subjects have more edge acutance than people and are easier for
video AF to focus on for that reason?

What is surprising in very, very low light is that the camera often SIGNALS
that it has established focus, even though on the LCD monitor it is plainly
way out of focus. That seems strange.

Thanks for the info.

N.


 
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