Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
05-26-2011, 02:23 PM   #16
Forum Member




Join Date: Apr 2011
Posts: 72
Original Poster
Thanks all of you for your replies. Whether the focus ring locks or not, it seems that everyone agrees that I would be able to notice if I had the wrong settings, and hence not break anything if I occasionally tried to gently turn the focus ring while in auto-focus mode.

To answer another concern that has come up in the thread: The focus motor spins faster than does the focus ring on the lens. Therefore, the gears between the motor and focus ring are reduction gears. If such a system is driven from the wrong end, by turning the focus ring, a much higher torque will be applied to the gears. The force required to turn the motor will be multiplied by the gears. This is definitely harder on the gears than normal auto-focus operation.

05-26-2011, 02:25 PM   #17
Veteran Member
dgaies's Avatar

Join Date: Oct 2009
Location: Maryland / Washington DC
Posts: 3,917
QuoteOriginally posted by Eruditass Quote
I just push the lens release button to MF.
QuoteOriginally posted by Blue Quote
What?
I've done that from time to time too. If you just need to quickly MF on something you can use the lens release button to disengage the AF-screw drive. Not really that much quicker than flipping the AF/MF switch, but it does work
05-26-2011, 02:34 PM   #18
axl
Veteran Member




Join Date: Mar 2007
Location: Nove Zamky, Slovakia
Photos: Gallery | Albums
Posts: 7,181
QuoteOriginally posted by Just1MoreDave Quote
I shift my truck's manual transmission without using the clutch but I wouldn't do this.
But you are not shifting gears in this case, it's more like pushing the car with 1st gear engaged. Would that ruin your motor?
05-26-2011, 02:48 PM   #19
Veteran Member
Eruditass's Avatar

Join Date: Jul 2009
Location: Pittsburgh, PA
Photos: Gallery | Albums
Posts: 2,206
QuoteOriginally posted by dgaies Quote
I've done that from time to time too. If you just need to quickly MF on something you can use the lens release button to disengage the AF-screw drive. Not really that much quicker than flipping the AF/MF switch, but it does work
Noticeably quicker for me since my left hand will be on the focus ring and I don't have to do the actions in serial (or flick it back to AF for that matter). Also, my right ring finger is always right by the button, while my left thumb is usually under the lens and doesn't feel natural to move back. The difference is less on small lenses where the focus ring is right by the body.

05-26-2011, 03:00 PM   #20
Veteran Member
creampuff's Avatar

Join Date: May 2007
Location: Singapore
Photos: Gallery
Posts: 3,955
There are small plastic gears in the lens and not disengaging the AF/MF switch and applying brute force to manually focus a non Quick Shift lens while the lens is set for AF is pretty dumb. A sure way to strip and damage the internal gears of the lens and applying unnecessary force. Why do it when a simple flick of the AF/MF switch to disengage the focusing motor will do?
05-26-2011, 03:23 PM   #21
Forum Member




Join Date: Apr 2011
Posts: 72
Original Poster
My question from the beginning was about what would happen if the focus ring was turned in auto-focus mode by accident. If it could be noticed, so that it is possible to stop trying to turn it before doing damage. I think that question has been answered.

I personally would not do it on purpose. Sure, the gears may be strong enough to handle it (but then again for how long?), but there will be more force applied to the gears, and that will wear on them. Whatever torque you apply at the focus ring, only a fraction will work on the motor (but with higher speed), and this fraction is what has to overcome the motors internal resistance to turning. When the motor is driving the gears, the torque from the motor instead gets multiplied before it has to drive the motion of the focus ring, and therefore a very small torque is required from the motor (and there is also no need for the gears to transmit the power necessary to overcome the motors internal resistance). I hope this helps to bring some clarity as to why manually turning the system from the wrong end will put more stress on it.

Best regards,

Jonas
05-26-2011, 05:09 PM   #22
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
QuoteOriginally posted by axl Quote
He's applying torque in the same places just the power is coming from the wrong end IMO.
The comment about the different gear ratios may be relevant here, although I can't say for sure. But I'll reword this in terms of resistance. The amount of resistance encounter by the gears when they are by the focus motor and they in turn drive the focus ring on the lens is fairly small. But the amount of resistance encountered by these same gears when they are driven by the focus focus ring and they in turn drive the focus motor is much greater. This can't possible be good for them. I'm sure they are designed to withstand doing that a few times here and there, but I wouldn't be so sure they were designed to do this regularly, so I think it is it right to be concerned.
05-26-2011, 06:33 PM   #23
Veteran Member




Join Date: Aug 2009
Location: New England
Photos: Albums
Posts: 689
QuoteOriginally posted by hjb981 Quote
If it could be noticed, so that it is possible to stop trying to turn it before doing damage.
I think you'll notice right away, especially if you've ever spent even a moment doing manual focusing on a particular AF lens with the drive disengaged. The difference between the drive engaged/disengaged is significant. I have a lot of AF lenses, and none of them have quick-shift, and I've only accidentally tried to manually focus with the drive engaged a couple times in the last four years.

05-26-2011, 08:13 PM   #24
Veteran Member




Join Date: Mar 2011
Location: Arizona
Photos: Albums
Posts: 330
QuoteOriginally posted by selar Quote
There is a safeway to use Quickshift on a FA lens without stripping the screw drive or the AF motor. You will need to half press and hold the lens release button while you adjust the focus. this will disengage the screw drive. AF will resume when you release the lens release button, so its better to take the shot while the button is still half depressed.
If you are so concerned about stripping gears by turning the focus ring why aren't you concerned about this lens release button? Ever used a power drill with a screw bit and only had the bit partially engaged? Fastest way in the world to strip a screw. What happens if you didn't get the button pushed in all the way and the slot partially engages then the motor starts? Or your finger slips and the motor starts?

QuoteOriginally posted by selar Quote
Ok buddy thanks for letting us know how you look after your equipment.
I don't know Aussie; but "look after" in US English means maintenance/ take care of/ check on... And I have never had to do any maintenance to the AF system.

QuoteOriginally posted by excanonfd Quote
I don't know why some people continue to perpetuate the myth about screw drive AF lens without QS.
Because they have no understanding of how of how it works. I have no problem if people don't understand but if you do not understand do not make a comment on it.

QuoteOriginally posted by excanonfd Quote
Focus ring do not 'lock'
Finally someone who understands.

QuoteOriginally posted by Marc Sabatella Quote
Might not "strip" anything, at least not right away, but you're applying torque in places the system wasn't designed to withstand, so I doubt it's good for the camera or lens.
Ever heard of a safety factor? Been doing it for 3yrs, not a problem yet. Best dampening to the focus system in the world. Yes I am putting torque in ways that are not normal, but they are obviously not out of design limits since it still works. I don't even want to get started on how many things everybody uses all the time which are not the intended use but us engineers know it will happen so we design for it.


QuoteOriginally posted by axl Quote
But you are not shifting gears in this case, it's more like pushing the car with 1st gear engaged. Would that ruin your motor?
This is how you start a car with a manual transmission when the battery is dead and nobody is around to jump you. Done it at least 10 times.

QuoteOriginally posted by creampuff Quote
A sure way to strip and damage the internal gears of the lens and applying unnecessary force. Why do it when a simple flick of the AF/MF switch to disengage the focusing motor will do?
This can easily be the difference in time between having a publishable shot and having a POS. Hey if it doesn't last me but one more time I don't care because doing this has paid for my equipment 10 times over. I am hard on my equipment because I have to be. I shoot climbing/mountaineering/skiing/adventure photography. If it gets me the shot I will do it. Many of the shots are a once in a lifetime chance to get it.

QuoteOriginally posted by hjb981 Quote
If it could be noticed
If this is your concern you will notice it

QuoteOriginally posted by hjb981 Quote
Whether the focus ring locks or not
Bet you $30,000 USD that it doesn't lock. It will rotate.

Last edited by atlnq9; 05-26-2011 at 08:27 PM.
05-26-2011, 09:41 PM   #25
Veteran Member




Join Date: Jan 2010
Location: Vancouver, BC
Posts: 1,421
QuoteOriginally posted by creampuff Quote
There are small plastic gears in the lens and not disengaging the AF/MF switch and applying brute force to manually focus a non Quick Shift lens while the lens is set for AF is pretty dumb. A sure way to strip and damage the internal gears of the lens and applying unnecessary force. Why do it when a simple flick of the AF/MF switch to disengage the focusing motor will do?
Brute force? I don't think you understand what brute force is. When a screw drive AF lens hunts for focus, the focus ring turns at speed in one direction, then it reverses and speed in the opposite direction until accurate focus is achieved. Sometimes this quick and abrupt changes in direction can go on for a few cycles depending on the subject, and each time the focus ring changes direction like that, you can hear and most times feel the changes in direction through your DSLR.

This quick and abrupt change in direction is far more forceful than what I do to manually turn the focus ring on an AF engaged, motor stopped lens. I am slowly increasing the torque to turn the focus ring until the resistance is overcome, whereas the AF system is stopping the focus ring's turning motion so abruptly that you can feel the jarring motion vibrating through your DSLR, instantaneously reverse and speed off in the other direction at the same speed.

If these screw drive AF lenses can withstand the rigours of this start, stop, forward and reverse motion at what seems to be a constant speed and torque, I think an occasional, accidental or even purposeful, manually turning of the focus ring while the AF is still engaged, will hardly make any difference to the life of the lens.

Thanks,
05-26-2011, 11:04 PM   #26
Veteran Member




Join Date: Jan 2010
Location: Southern England
Posts: 495
I have 3 points:

If the gear system is of the "worm-drive" type, then it is effectively self-locking in the reverse direction. Mind you, from what's been said, I doubt any of the lenses use this type of system.

The inertia of the motor seen at the focus ring will be multiplied by the square of the gear ratio. If you're at all rough, very large torques may occur, which obviously are undesirable. (It's the "pushing the car in first gear" analogy.)

If you stall the autofocus motor, large currents can flow - again very undesirable, but of course the camera's electronics ought to be able to protect against this.
05-26-2011, 11:26 PM   #27
Pentaxian




Join Date: Nov 2008
Location: Bangalore
Photos: Gallery
Posts: 2,448
QuoteOriginally posted by hjb981 Quote
What would happen if I put an auto-focus lens without quick-shift on a modern body (e.g. the K-5) and turned the focus ring with the lens set to auto-focus? Would I feel that the focus ring was stuck if I only tried to turn it very carefully, and hence be able to stop before damaging something? Or would something (what) break very easily?
I wouldn't advice that - that could damage the lens or body. Use lens-release if needed.
05-27-2011, 02:51 AM   #28
Veteran Member
selar's Avatar

Join Date: Jul 2007
Location: Sydney, Australia
Photos: Gallery | Albums
Posts: 2,035
QuoteOriginally posted by atlnq9 Quote
I don't know Aussie; but "look after" in US English means maintenance/ take care of/ check on... And I have never had to do any maintenance to the AF system.
Thanks muchly for the lesson in English, Guv, but this cobber thinks in Strine that you're a few sheep short of a Paddock.
05-27-2011, 05:03 AM   #29
Veteran Member




Join Date: Aug 2009
Posts: 2,311
Maybe somebody has pointed this out, or not... but here i go...

There is a switch on the camera, that everyone can reach without taking the eye from the VF. It can be done with the left hand. The switch is near the lens mount, and is made for changing between AF, C.AF and MF. Its fast and reliable and works with all AF lenses, Quickshift or not.

I think this function is very nice to have. It shifts av AF lens to MF very quickly....

05-27-2011, 05:04 AM   #30
Veteran Member
dgaies's Avatar

Join Date: Oct 2009
Location: Maryland / Washington DC
Posts: 3,917
QuoteOriginally posted by the swede Quote
I think this function is very nice to have.

So that's what that switch is for
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!
auto-focus, focus, k-mount, lens, pentax lens, primes, quick-shift, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Quick shift: video <--> RAW ? Jaboney Video and Pentax HDSLRs 3 03-10-2010 06:45 PM
Quick Shift, for Canikon? systemA Non-Pentax Cameras: Canon, Nikon, etc. 7 01-27-2010 03:40 PM
Permanent quick-shift? LeDave Pentax DSLR Discussion 12 12-27-2009 12:41 PM
Kx DA L quick shift dilemma! Flopass Troubleshooting and Beginner Help 4 11-25-2009 01:06 PM
k-x with quick shift lense? p0opstlnksal0t Pentax DSLR Discussion 2 10-21-2009 04:08 AM



All times are GMT -7. The time now is 11:16 AM. | 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