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Zooming and Focusing

 
 
Fedman Kassad
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      01-17-2004
Sorry for the maybe silly question, but I noticed that when I zoom in or out
with my Fujifilm S602Z the image goes out of focus while the zoom is
actually working, only to go back to normal when I'm done zooming.
Now is this normal behaviour for my camera (or any other digicam that has a
zoom for that matter) or is it maybe defective in some way? I can clearly
remember that in my optical SLR when the focus is ok for a certain distance,
it stays the same even if I change the zoom factor (and all the way through
the whole zooming process).
And if it's just normal for digicams to do so, can somebody explain to me
why the different behaviour compared to analog cameras?

Thanks in advance for any insight...

Col. Fedman Kassad


 
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Philip Rawson
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      01-17-2004
"Fedman Kassad" <(E-Mail Removed)> wrote in message
news:qG3Ob.44330$(E-Mail Removed)...
> Sorry for the maybe silly question, but I noticed that when I zoom in or

out
> with my Fujifilm S602Z the image goes out of focus while the zoom is
> actually working, only to go back to normal when I'm done zooming.
> Now is this normal behaviour for my camera (or any other digicam that has

a
> zoom for that matter) or is it maybe defective in some way?


Yes it's quite normal...

Both my 602 and Oly 2100UZ do it, except the 602 gets back into focus
quicker.

--
Philip Rawson

www.flymodels.co.uk


 
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Bill
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      01-17-2004
My S602 does the same thing. I'm sure it's normal. As to why it's different
than your film camera in this way, I'm not sure. Maybe because the sensor is
much smaller than the film area in a 35mm camera, so focus is more criticalm
but that's purely a guess.

Bill

"Fedman Kassad" <(E-Mail Removed)> wrote in message
news:qG3Ob.44330$(E-Mail Removed)...
> Sorry for the maybe silly question, but I noticed that when I zoom in or

out
> with my Fujifilm S602Z the image goes out of focus while the zoom is
> actually working, only to go back to normal when I'm done zooming.
> Now is this normal behaviour for my camera (or any other digicam that has

a
> zoom for that matter) or is it maybe defective in some way? I can clearly
> remember that in my optical SLR when the focus is ok for a certain

distance,
> it stays the same even if I change the zoom factor (and all the way

through
> the whole zooming process).
> And if it's just normal for digicams to do so, can somebody explain to me
> why the different behaviour compared to analog cameras?
>
> Thanks in advance for any insight...
>
> Col. Fedman Kassad
>
>



 
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Don Stauffer
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      01-17-2004
This is a design issue, and is one of the criteria the lens designer
uses when he designs. There is a sort of curve one can make while
analyzing a design. If the design is in focus at one (or two, or even
three) points in the focal length range, you can plot how far out of
focus it is at other focal lengths. Best designs have a sort of 'S'
shaped curve, say in focus at one extreme, curve bends away, returns to
focus somewhere near middle, curves in other direction, and returns to
focus at other extreme. Cheaper designs hold focus at two points,
cheapest is in focus only at one focal length, off for all others. It
is almost impossible to get the curve truly flat, equally good focus at
ALL focal lengths when focused at one.

Fedman Kassad wrote:
>
> Sorry for the maybe silly question, but I noticed that when I zoom in or out
> with my Fujifilm S602Z the image goes out of focus while the zoom is
> actually working, only to go back to normal when I'm done zooming.
> Now is this normal behaviour for my camera (or any other digicam that has a
> zoom for that matter) or is it maybe defective in some way? I can clearly
> remember that in my optical SLR when the focus is ok for a certain distance,
> it stays the same even if I change the zoom factor (and all the way through
> the whole zooming process).
> And if it's just normal for digicams to do so, can somebody explain to me
> why the different behaviour compared to analog cameras?
>
> Thanks in advance for any insight...
>
> Col. Fedman Kassad


--
Don Stauffer in Minnesota
http://www.velocityreviews.com/forums/(E-Mail Removed)
webpage- http://www.usfamily.net/web/stauffer
 
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Samuel Paik
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      01-17-2004
"Fedman Kassad" <(E-Mail Removed)> wrote:
> Sorry for the maybe silly question, but I noticed that when I zoom in or out
> with my Fujifilm S602Z the image goes out of focus while the zoom is
> actually working, only to go back to normal when I'm done zooming.
> Now is this normal behaviour for my camera (or any other digicam that has a
> zoom for that matter) or is it maybe defective in some way? I can clearly
> remember that in my optical SLR when the focus is ok for a certain distance,
> it stays the same even if I change the zoom factor (and all the way through
> the whole zooming process).
> And if it's just normal for digicams to do so, can somebody explain to me
> why the different behaviour compared to analog cameras?


It is probably normal for THAT digicam model. A zoom lens which
doesn't maintain focus while changing focal length can be less
expensive than a zoom lens which does. Since autofocus is the norm
for many point and shoot cameras, many manufacturers skimp on the lens
lere.
 
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Al Denelsbeck
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      01-17-2004
"Fedman Kassad" <(E-Mail Removed)> wrote in
news:qG3Ob.44330$(E-Mail Removed):

> Sorry for the maybe silly question, but I noticed that when I zoom in
> or out with my Fujifilm S602Z the image goes out of focus while the
> zoom is actually working, only to go back to normal when I'm done
> zooming. Now is this normal behaviour for my camera (or any other
> digicam that has a zoom for that matter) or is it maybe defective in
> some way? I can clearly remember that in my optical SLR when the focus
> is ok for a certain distance, it stays the same even if I change the
> zoom factor (and all the way through the whole zooming process).
> And if it's just normal for digicams to do so, can somebody explain to
> me why the different behaviour compared to analog cameras?



It all depends on the design of the zoom lens. Some are designed so
that they maintain that same focal distance while the focal length changes,
and are called 'Constant focus'. Those that don't are called 'Vari-focus'.

Vari-focus lenses tend to be smaller and easier to make, and the
demand for constant focus isn't high in the market for cameras with built-
in lenses.


- Al.

--
To reply, insert dash in address to match domain below
Online photo gallery at www.wading-in.net
 
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Fedman Kassad
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Posts: n/a
 
      01-18-2004

"Al Denelsbeck" <(E-Mail Removed)> ha scritto nel messaggio
news:Xns9473AD0838424sandalsatwadinginnet@65.32.1. 7...

> It all depends on the design of the zoom lens. Some are designed so
> that they maintain that same focal distance while the focal length

changes,
> and are called 'Constant focus'. Those that don't are called 'Vari-focus'.
>
> Vari-focus lenses tend to be smaller and easier to make, and the
> demand for constant focus isn't high in the market for cameras with built-
> in lenses.
>
>
> - Al.
>
> --
> To reply, insert dash in address to match domain below
> Online photo gallery at www.wading-in.net


Thanx for the quite complete answer. It all really make more sense now.
Too bad that for getting a smaller (and I bet cheaper too) zoom assembly,
one ends up having to wait for refocusing lags...

Thanx again and see you around...

Fedman Kassad


 
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