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auto focus in digital cameras any different that auto focus in film cameras?

 
 
nospam@wtez.net
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      11-03-2005
Is there any basic difference in the autofocussing technology in
digital cameras vs film cameras? I have an Olympus 750 and it seems
like it takes longer to achieve autofocus than its cheaper brother a
stylus point and zoom shoot film camera.. Is the digital camera just
trying to do a better job, does the autofocus have to work harder in a
digital camera or is the difference that I perceive not really there?

Especially in low light this problem is aggravating and its not helped
by the fact that the manual focus mode is not very easy to use. Boy
I'd love to be able to grab a ring and turn it to get focus. As it is
its a lot of button and menu button selection. I guess that's the
price you pay for a low end digital camera.

tks
Don

 
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David J Taylor
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      11-03-2005
http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:
> Is there any basic difference in the autofocussing technology in
> digital cameras vs film cameras? I have an Olympus 750 and it seems
> like it takes longer to achieve autofocus than its cheaper brother a
> stylus point and zoom shoot film camera.. Is the digital camera just
> trying to do a better job, does the autofocus have to work harder in a
> digital camera or is the difference that I perceive not really there?
>
> Especially in low light this problem is aggravating and its not helped
> by the fact that the manual focus mode is not very easy to use. Boy
> I'd love to be able to grab a ring and turn it to get focus. As it is
> its a lot of button and menu button selection. I guess that's the
> price you pay for a low end digital camera.


Google helped me find this:

Point-and-shoot cameras can be slow because they use the contrast
detection method:
http://www.cs.mtu.edu/~shene/DigiCam...uto-Focus.html

Some cameras (e.g. Nikon 8400) have an additional sensor to speed things
up.

DSLRs use the phase detection method which is quicker:
http://www.a1.nl/phomepag/markerink/af_expla.htm


http://en.wikipedia.org/wiki/Autofocus

David


 
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Andrew Crabtree
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      11-03-2005

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Is there any basic difference in the autofocussing technology in
> digital cameras vs film cameras?

Digital SLRs use the same autofocus as their film counterparts. Digital P&S
cameras use an abomination known as Contrast Detect. It is universally
known for being crappy and slow, especially in low light. I can't speak for
your particular camera, but there are some tricks that will sometimes help
Contrast Detect cameras find focus. Most importantly, attempt to focus on
something that has high contrast. Human faces have low contrast, and that
is what we largely focus on. Try clothing instead, although make sure you
find something the same distance away. Also, some cameras use an algorithm
that is tied to contrast in a verticle direction. For some circumstances,
rotating the camera 90 degrees will allow it to work.

Cheers!
Andrew


 
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Malcolm
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      11-03-2005
> Especially in low light this problem is aggravating and its not helped
> by the fact that the manual focus mode is not very easy to use. Boy
> I'd love to be able to grab a ring and turn it to get focus. As it is
> its a lot of button and menu button selection. I guess that's the
> price you pay for a low end digital camera.



I too have a C750. In low light, I switch to manual focus, estimate the
distance and set to that distance. Because of the great depth of field of
these cameras this method works very well. I don't try and focus by looking
at the screen. The resolution is just too low.

Malcolm


 
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nospam@wtez.net
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      11-04-2005
tks everyone.
I'll just have to set a "program" in for manual focus and use that and
I agree Malcolm looking at the viewfinder/screen does not help.

I wonder if the film Olympus PS Stylus uses something different. It
doesn't seem as susceptible to the problem.

Maybe it doesnt' try/work as hard.

Don

 
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Dave Martindale
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      11-05-2005
(E-Mail Removed) writes:

>I wonder if the film Olympus PS Stylus uses something different. It
>doesn't seem as susceptible to the problem.


Most P&S digital cameras use contrast measuring methods using images
read from the main sensor, because it's cheap (no extra hardware).

Film P&S cameras don't have this hardware, so have to use some
additional hardware for autofocus, and typically choose active infrared
rangefinding. It's fast and works in the dark. A few digital P&S
cameras use this too.

Both film and digital SLRs tend to use phase detection, which is fast
and works for a wide variety of lenses.

Dave
 
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