Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
07-01-2014, 01:51 AM   #1
Site Supporter
Dewman's Avatar

Join Date: May 2012
Location: Idaho
Photos: Albums
Posts: 3,243
What causes a lens to "search" while on AF?

I have a couple of lenses that seem to never quit searching for something to focus on. It becomes very irritating, often to the point that I'll switch to manual focus and continue on. Is this the lenses fault or do I have something set incorrectly?

Thanks,
Dewman

07-01-2014, 02:02 AM   #2
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 43,215
Normally it's the camera's fault (older Pentax DSLR AF systems are less sensitive to light), but lenses can also cause hunting due to low contrast, poor sharpness, slow aperture, etc.

For example, the K-5 II can focus more reliably in low light than a K-5. Thus, you might experience more hunting with the K-5 in that particular scenario.

Adam
PentaxForums.com Webmaster (Site Usage Guide | Site Help | My Photography)



PentaxForums.com's high server and development costs are user-supported. You can help cover those costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

07-01-2014, 02:11 AM   #3
Pentaxian




Join Date: Jun 2008
Location: Flyover America
Posts: 4,475
QuoteOriginally posted by Dewman Quote
What causes a lens to "search" while on AF?
Low light conditions and/or a "slow" lens - assuming nothing else is defective.
07-01-2014, 03:59 AM   #4
Site Supporter
Stone G.'s Avatar

Join Date: Jun 2010
Location: North Zealand, Denmark
Photos: Gallery | Albums
Posts: 1,516
QuoteOriginally posted by Dewman Quote
I have a couple of lenses that seem to never quit searching for something to focus on. It becomes very irritating, often to the point that I'll switch to manual focus and continue on. Is this the lenses fault or do I have something set incorrectly?

Thanks,
Dewman
You do not mention what camera you use, but as said above, no AF is perfect. Here is what an older manual (*ist DL) has to say about AF-troubles.
Hard-to-Autofocus Subjects
The autofocus mechanism is not perfect. Focusing may be difficult when taking pictures under the following conditions (a to f below). These also apply to manual focusing using the focus indicator in the viewfinder. If the subject cannot be focused automatically, set the focus mode lever to MF and use the manual focus mode to focus on the subject with the aid of the matte field in the viewfinder. (p.126)

(a) Extremely low-contrast subjects such as a white wall in the focusing area.

(b) Subjects which do not reflect much light within the focusing area.

(c) Fast moving objects.

(d) Strongly reflected light or strong backlighting (bright background).

(e) If vertical or horizontal line patterns appear within the focusing area.

(f) Multiple subjects in the foreground and background within the focusing area.
That said, a not-so-good lens may amplify the camera's problems with the 'Hard-to-Autofocus Subjects'. For example my older 18-200mm Tamron is rather hopeless at the wide end, where my DA 18-135 works flawlessly. My best 'remedy' here is to zoom somewhat in, focus and then zoom back again.

07-01-2014, 09:49 AM   #5
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 27,459
Some lenses have this as a characteristic due to low contrast and/or narrow maximum aperture. The problem can also become worse with use/age as the precision of the focus drive decreases with wear.


Steve
07-01-2014, 10:57 AM   #6
Pentaxian




Join Date: Oct 2006
Location: Chicago suburb, IL, USA
Posts: 1,535
Hi Dewman,

Some good responses here to your question, but I have to assume that you'd like some kind of work around other than buying a new camera body or lens. . .

One of the reasons a body/lens will continue to hunt despite repeated tries is that when the above mentioned conditions exist, the AF system consistently overshoots the focus plane, IOW, the focusing motor is changing the focus plane too quickly. going from completely out of focus near to completely out of focus far, passing the correct focus distance too quickly to pick up a difficult contrast border to lock on to. Try manually prefocusing the lens so you can make out your subject in the viewfinder, then actuate AF to lock final focus. The worse the conditions (low contrast, low light, slow lens, etc) the closer you will need to prefocus to get a reliable lock.

If your lens is a DA series with Quick Shift focusing, or has an AF/MF switch on the lens, then this is a pretty easy proposition. If the lens does not have an easy way to switch to manual focus, then you either have to use the AF/MF switch on the body or depress the lens lock button on the body to temporarily disengage the screw drive to manually prefocus (make sure that the lens is still locked after manually focusing if you use the second alternative).

Even on early Pentax DSLR bodies, you'd be surprised how well this works in even the most difficult AF situations.

Scptt
07-01-2014, 01:20 PM   #7
Pentaxian




Join Date: Jan 2011
Location: New York
Posts: 3,311
@Dewman, what camera and lens combination causes AF trouble?

Others have already said that low contrast and other factors can cause hunting. Here are my 2 most common AF problems:
  • Airshows and birds in flight: For my "normal" photography I use centerpoint AF and recompose as needed. If I try this with a telephoto lens and moving target, the lens locks onto the background whenever I make a slight tracking mistake. Solution is to enable more focus points to allow for operator error.
  • Wide angle (wider than 15mm) busy scenes: If there are a lot of small details in the image, AF sometimes give up if it's already close to being focused. It's not really hunting - AF doesn't move at all - but doesn't give an AF confirmation beep or viewfinder hexagon light. When this happens I can point away to something that's at a very different distance, AF on that, then repoint to my original target and AF again. Or I just switch to MF if I'm someplace where it keeps happening. The worst instance of this I've seen is the Philadelphia Masonic Temple. The interior walls are ornate with many tiny artistic details, plus dim lighting.
07-01-2014, 02:53 PM   #8
Site Supporter
Dewman's Avatar

Join Date: May 2012
Location: Idaho
Photos: Albums
Posts: 3,243
Original Poster
QuoteOriginally posted by DeadJohn Quote
@Dewman, what camera and lens combination causes AF trouble?



I'm using a K2000 and the two lenses that I seem to have the most trouble with are a Promaster 80-210mm and a Tamron 28-105mm. After reading Stone G's reply about the problem could be caused by:

"(a) Extremely low-contrast subjects such as a white wall in the focusing area.

(b) Subjects which do not reflect much light within the focusing area.

(c) Fast moving objects.

(d) Strongly reflected light or strong backlighting (bright background).

(e) If vertical or horizontal line patterns appear within the focusing area.

(f) Multiple subjects in the foreground and background within the focusing area."


.....I now realize that the culprit is one or all of those mentioned with the exception of "C." It's nothing that I can't overcome, but I thought I'd ask and perhaps learn something in the process.

Thanks to all for your help and guidance.

Dewman

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!
camera, causes a lens, focus, lens "searching.", lenses, pentax help, photography
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
What causes "normal perspective?" konraDarnok Pentax DSLR Discussion 36 09-03-2013 08:31 PM
What are your thoughts on "upgrading" to a K5II/s after reading the review? jpzk Pentax DSLR Discussion 90 07-05-2013 06:48 AM
Going from a "film" to a "digital" back to a "film" lens? 6BQ5 Pentax SLR Lens Discussion 9 06-18-2013 03:40 PM
Tired of Looking at Thousands of item's while doing a "Pentax" search on fleabay? Indecent Exposure General Talk 15 11-11-2011 07:27 AM
What is difference "APO" and "ED" glass in a lens? kathyk Pentax SLR Lens Discussion 4 03-06-2008 08:59 AM



All times are GMT -7. The time now is 02:42 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