number of af screw turns

derrdaniel · 04-27-2011, 06:03 AM

Hello,

the rotational speed of the af drive should be equal for all screw driven lenses on the same camera. Therefor the number of turns needed to focus from infinity to close focus should be proportional to af-speed.

The number of turns needed can be counted very easily by taking the lens of the camera and turning the screw drive carefully (e.g. with a small screwdriver) while counting the turns.

In my opinion focusing speed is an important attribute of a specific lens and should be easier to find out on the internet.
In this thread you can post how many turns your lenses need to focus from infinity to the nearest setting in order to make it easier for newbies to find an overview over the speed of the different lenses.

in case a similar comparison exists, where can i find it?

please excuse my English

cheers,
derrdaniel

**Lowell Goudge** · 04-27-2011, 06:43 AM

I think you are partially correct and partially incorrect.

The AF focusing will depend upon 2 things, which may actually fight each other.

If you consider the number of screw turns from infinity to minimum focus as analogous to focus throw (measured in degrees) for manual focus lenses then my comment will become much clearer.

Lets say your lens has a 90 degree focus throw or a low mumber of AF screw turns from minimum focus to infinity. Sure you will be able to get close to correct focus very quickly, BUT the precision is lacking to lock poerfect focus.

Lets now take the other extreme, of having almost 360 degrees focus throw from min focus to infinity or a very high number of turns of the AF screw. It will take forever to aquire focus due to the much longer travel, BUT it will be very precise.

The trade off has always been there, and in general, you can have quick, or you can have accurate, it is rare to find the perfect balance that delivers both.

Adam · 04-27-2011, 08:04 AM

Also, on some lenses, more "stuff" has to be moved than on others, so the rotation speed will vary from lens to lens.

**Lowell Goudge** · 04-27-2011, 07:01 PM

Originally posted by Adam

Also, on some lenses, more "stuff" has to be moved than on others, so the rotation speed will vary from lens to lens.

This again may helP or hurt. More mass slows the response to change probably more than slowing down maximum speed. As a result the lens may overshoot the first attempt from a big focus error but may then get there from close focus nicely depending on the focus servo response This may change from camera model to model

04-27-2011, 06:03 AM	#1
derrdaniel New Member Join Date: Apr 2011 Posts: 4	number of af screw turns Hello, the rotational speed of the af drive should be equal for all screw driven lenses on the same camera. Therefor the number of turns needed to focus from infinity to close focus should be proportional to af-speed. The number of turns needed can be counted very easily by taking the lens of the camera and turning the screw drive carefully (e.g. with a small screwdriver) while counting the turns. In my opinion focusing speed is an important attribute of a specific lens and should be easier to find out on the internet. In this thread you can post how many turns your lenses need to focus from infinity to the nearest setting in order to make it easier for newbies to find an overview over the speed of the different lenses. in case a similar comparison exists, where can i find it? please excuse my English cheers, derrdaniel

04-27-2011, 08:04 AM	#3
Adam Administrator Site Webmaster Join Date: Sep 2006 Location: Arizona Photos: Gallery \| Albums Posts: 51,595	Also, on some lenses, more "stuff" has to be moved than on others, so the rotation speed will vary from lens to lens. Adam PentaxForums.com Webmaster (Site Usage Guide \| Site Help \| My Photography) PentaxForums.com server and development costs are user-supported. You can help cover these costs by donating or purchasing one of our Pentax eBooks. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, KEH, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

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04-27-2011, 06:43 AM	#2
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,888	I think you are partially correct and partially incorrect. The AF focusing will depend upon 2 things, which may actually fight each other. If you consider the number of screw turns from infinity to minimum focus as analogous to focus throw (measured in degrees) for manual focus lenses then my comment will become much clearer. Lets say your lens has a 90 degree focus throw or a low mumber of AF screw turns from minimum focus to infinity. Sure you will be able to get close to correct focus very quickly, BUT the precision is lacking to lock poerfect focus. Lets now take the other extreme, of having almost 360 degrees focus throw from min focus to infinity or a very high number of turns of the AF screw. It will take forever to aquire focus due to the much longer travel, BUT it will be very precise. The trade off has always been there, and in general, you can have quick, or you can have accurate, it is rare to find the perfect balance that delivers both.

04-27-2011, 07:01 PM	#4
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,888	Originally posted by Adam Also, on some lenses, more "stuff" has to be moved than on others, so the rotation speed will vary from lens to lens. This again may helP or hurt. More mass slows the response to change probably more than slowing down maximum speed. As a result the lens may overshoot the first attempt from a big focus error but may then get there from close focus nicely depending on the focus servo response This may change from camera model to model