Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
04-13-2017, 02:29 PM   #16
Pentaxian
photoptimist's Avatar

Join Date: Jul 2016
Photos: Albums
Posts: 5,113

Staff note: This post may contain affiliate links, which means Pentax Forums may earn a small commission if a visitor clicks through and makes a purchase. If you would like to support the forum directly, you may also make a donation here.


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
Site Supporter
Site Supporter
RobA_Oz's Avatar

Join Date: Nov 2008
Location: Tasmania, Australia
Photos: Gallery
Posts: 8,182
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
Veteran Member




Join Date: Dec 2016
Posts: 531
Original Poster
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
Pentaxian
photoptimist's Avatar

Join Date: Jul 2016
Photos: Albums
Posts: 5,113
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
Veteran Member




Join Date: Jan 2010
Location: Michigan
Photos: Gallery
Posts: 2,207
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
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
coatings, k-mount, lens, lenses, manufacturer, optics, pentax lens, production, question, return, rings, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
The Vintage Bokeh Polka 14 vintage lenses Compared Sliver-Surfer Canon, Nikon, Sony, and Other Camera Brands 1 02-07-2017 03:39 PM
Vintage-A vs vintage-K Jinnax Pentax SLR Lens Discussion 47 10-05-2015 04:58 PM
Abstract Vintage Camera for Vintage Cruisers jgblodgett Photo Critique 6 01-11-2015 07:21 PM
Vintage Camera for Vintage Cruisers jgblodgett Monthly Photo Contests 0 01-09-2015 04:01 PM
Thematic Vintage Feelings - Post your vintage shots schnitzer79 Mini-Challenges, Games, and Photo Stories 15 12-02-2014 06:50 AM



All times are GMT -7. The time now is 07:06 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top