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Old Mar 15, 2003, 1:53 PM   #1
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Default Digicam zoom lens focus tracking

I'v noticed with my Fuji 602, that if I zoom then focus, the focus is not held as the zoom is returned to wide angle. From memory prof. TV cameras have zooms which track focus (clearly needed for TV).

I guess the issue is the precision of the optics, but does anybody else find this with their cameras, or have a more expensive camera and lens combinations where the focus does track?

Is this just a limitation of the price you pay for a pro-sumer cam? Lin ...is this one for you! Can focus tracking be measured in reviews or specified, to determine whether some cameras are better than others?
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Old Mar 15, 2003, 3:32 PM   #2
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Depends on the setting of the camera. If you have the Autofocus on continuously (fulltime) so it's always focusing, it will continue to adjust while the camera zooms. If you have the fulltime autofocus off, it will only focus when you hold the shutter down half way. If you have manual focus on, when you focus in on the zoomed in focal length, when you go wide angle it won't change until you change the position of the focusing lens.

Is that what you were asking?

Video cameras have full time autofocus (unless you set it to manual) so it's always tracking the subject.

While we're on the subject movie cameras don't have autofocus...there's someone there who has to keep an eye on the scene and adjust the focus of the camera; in the credits you might see the person listed as "Focus Puller". For instance in a scene where an actor is walking towards the camera the job of the Focus Puller is to adjust the focus of the movie camera while the actor is walking...the focus puller will put masking tape on the focusing ring with marks to signify objects along the actor's walking path...when the actor reaches a certain object, the focus puller has to have turned the focus ring to that mark, and the focus puller continues turning the focus ring while matching the marks to the where objects the actor is walking past.
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Old Mar 15, 2003, 6:09 PM   #3
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Mike, I thought it was possible on more expensive zoom lenses to manufacture them to a level of precision where once focused at max zoom, they retained focus at other focal lengths (without the electronic focusing gadgetry normally found in digicams to compensate).

On the movie cam front I was thinking of digi_Betacam, not film.
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Old Mar 15, 2003, 7:02 PM   #4
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With the advance of continuous autofocus camera, the manufacturers get away with varifocal. This is technically not a true zoom, a true zoom will retain the focus as the lens is zoomed in or out (ie your film SLR zoom). A varifocal is more compact in design, cheaper to built and is compensated by the continous AF of the camera. Vivitar used to make varifocal 'zoom' for film cameras that have high zoom ratios as well... but their distances scales are indicated by ondulating lines on the varifocal barrel (ie the focus distance scale needed to be adjusted by following the lines as one zoom in or out)

A high x4 'zoom' or more number usually indicate a varifocal design!
http://forums.dpreview.com/forums/re...essage=4550268
http://forums.dpreview.com/forums/re...essage=3048778
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Old Mar 15, 2003, 7:53 PM   #5
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NHL, this is all very intriguing. So slr zooms as I remember them would track focus, but digicams are now 'fly by wire' which means autofocus must be present and working to track zoom focus? The first dpreview post link actually put the same Betacam question - thanks for that link. These zoom lenses must be Cú4K so you'd expect them to be precision made lenses.

There was a recent post here re how a cam has changed from slr, real viewfinder/split wedge prism focus/simple exposure control to the digicam equivalent of all things electronic/inter-dependent and more to think about to take a shot. I'm tending to think this is true. Electronic processing makes it possible to manufacture cheaper components, but try uncoupling the electronic control which is now essential to the function, and the camera becomes harder to drive. It's getting more like cars, with engine management systems, every day!

Does this mean that a dslr with add on zoom lenses will track focus, independent of AF in the cam?
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Old Mar 16, 2003, 5:50 AM   #6
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Quote:
Does this mean that a dslr with add on zoom lenses will track focus, independent of AF in the cam?
A true zoom is a true zoom regardless where the lens is mounted! They just don't have a high zoom ratio though (ie < x4) http://www.stevesforums.com/phpBB2/v...amp;highlight= Some folks will be really upset with their SLR if after they zoomed in for the detail and zoomed out for the composition and the focus has changed in the meantime... :evil: :evil: :evil:

Unless of course someone come up with something funky like the 4/3 system where the focusing is done from the CCD sensor plane (a` la Contax) :lol: :lol: :lol: http://www.stevesforums.com/phpBB2/v...amp;highlight=

Quote:
... this is all very intriguing. So slr zooms as I remember them would track focus, but digicams are now 'fly by wire' which means autofocus must be present and working to track zoom focus?
Well not necessary... Check the two links on my previous post where the E10/20 and the D7 cameras actually track focus electronically even on manual focus to compensate for their varifocal design! 8) 8) 8)

It looks like the s602 does not have this feature... Anyone know about the 5700, or the F717?
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