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04-13-2017, 02:29 PM   #16
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QuoteOriginally posted by tvdtvdtvd Quote
QuoteOriginally posted by dcshooter Quote
You can have the AF, or you can have the well-damped, fine-threaded "perfect" focus helicals. The two are mutually exclusive. Pretty much everything else is describing the FA and DA (without aperture ring) Limited series lenses.
That is my dream lens in a nutshell, physics be damned. You could even forego the modern optical design, though
modern coatings would be nice.

Come on, don't tell me it can't be done; science it!
LOL! And hell ya!

You certainly can "science it" in at least three ways:

1) Lots of down-gearing between the screw drive shaft and the smooth helicoid to amplify the torque from the motor. AF might be extremely slow but would be very accurate.

2) Connect the AF directly to an internal lightweight plastic focusing assembly and also have a damped exterior metal barrel helicoid connected to the AF assembly via a clutch so that the AF motor doesn't have to move the helicoid. Such a design would give the user the nice greased helicoid feel although in not offer quite the accuracy of focus if the clutch has some play in it.

3) It's just a firmware upgrade! Something magical in firmware! And it could be installed in every digital camera ever made! And it would provide continuous tracking AF for photographing border collies chasing butterflies! ;-)

04-13-2017, 03:47 PM   #17
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QuoteOriginally posted by tvdtvdtvd Quote
That is my dream lens in a nutshell, physics be damned. You could even forego the modern optical design, though modern coatings would be nice.

Come on, don't tell me it can't be done; science it!
It doesn't need science, just two separate focusing drive mechanisms. I haven't dismantled an FA* lens, but they decoupled the MF ring to allow the AF screw drive to operate. They're certainly heavy enough to have two drive mechanisms, although I suspect there was only one.
04-13-2017, 04:13 PM   #18
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http://www.china-ebay.com/images/other-tool/excellent-quality-8x350mm-lead-s...ut-10551_2.jpg
I was wondering something like this, directly engaged to the body coupling and a gear coupled to the helicoid. It cant be used for zooms, though
04-13-2017, 06:15 PM   #19
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QuoteOriginally posted by dcshooter Quote
The question would be where the heck to put all this down-gearing. Many of the current lenses are already using a spur/pinion of 8-10 teeth to an inner ring gear that's the entire circumference of the lens barrel already and are still functioning at the limits of what the screwdrive motor can handle. Yeah, you can get around this by making the teeth even smaller to increase the ratio, but then you are getting into watch-gear tolerances and creating a service nightmare.
I was envisioning a spur gear driving a ring gear but then a cylindrical cage of perhaps 3 to 5 step-down gears (with axles oriented radially WRT the lens axis) and a second ring gear about the same size as the first that revolves at perhaps 1/3 to 1/5 the rate of the first ring gear.


QuoteOriginally posted by dcshooter Quote
This is basically how the lenses with the push-pull AF sleeves (e.g. the FA85 1.4) work already, and it's kludgy, sloppy, and prone to breakage if the AF tries to push the focus while manual is still partially engaged, even without a damped MF ring. This is what happened with a lens I recently worked on where this occurred:
Yes it is a kludge.

QuoteOriginally posted by dcshooter Quote
Hence my suggestion of completely decoupling the two and going focus by wire.
Now that's a great idea. There's still the issue of the available power coming from the camera body (unless one adds a battery to the lens) but it does give the designer more freedom to put the AF motor and gearing anywhere inside the lens.

04-13-2017, 07:40 PM   #20
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This one is a vintage Computar 105mm ~ 150mm variable focal length lens.
https://app.box.com/s/ftfro5plttrrqlz91qfi
It is not a zoom lens as we know it these days.
The focal length is set by the camera rack moving the whole lens,
and there is a scroll with trunions to move the front group to focus.

Or, as I used it mostly, just set to 135 mm with the front to infinity,
and focus conventionally with the rangefinder.

I can imagine big brass gears and a pico, but sure will never get around to doing that!
Here is the mount getting adapted to the Speed.
https://app.box.com/s/gxfofae5bu7mhmgupt97

Maybe I get it out if the weather ever warms up here.
The lens can be sharp and contrasty,
but i see last time I used it the colors are a bit ... whimsical
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