Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
08-12-2013, 07:35 PM   #1
Veteran Member




Join Date: Dec 2012
Photos: Gallery
Posts: 696
Can't autofocus beyond f/8?

On page 86 of the September 2013 issue of Outdoor Photographer, in an article about tele-extenders, it says:

"For a camera to autofocus, it opens to its widest aperature, compares the angles of two paths to set the focus, then closes back to the set exposure. Prosumer-type cameras can generally autofocus up to f/6.3, while pro DSLRs autofocus up to f/8. Depending on the lens and lighting conditions you're working with, when you lose stops with the extender, you may lose the ability to autofocus."

Really? I just bought a Pentax K-30, and, running around today with a 55-300mm lens set at 1/1000, f/11, and ISO 400... it seems to be autofocusing just fine. What am i missing here? Am trying to get the greatest depth-of-field possible combined with the fastest shutter speed possible, to see if I can use the telephoto lens for quick snapshots, as below... original at 300mm at left; 100% crop at right; cloudy day.

Attached Images
 
08-12-2013, 07:43 PM   #2
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 40,958
AF is performed with the lens wide-open, aka F4 and 55mm and F5.8 at 300mm. It doesn't matter what aperture you shoot at, because the lens doesn't get stopped down until the actual exposure.

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:

08-12-2013, 09:06 PM   #3
Site Supporter
jatrax's Avatar

Join Date: May 2010
Location: Oregon
Photos: Gallery | Albums
Posts: 10,287
QuoteOriginally posted by Adam Quote
AF is performed with the lens wide-open, aka F4 and 55mm and F5.8 at 300mm. It doesn't matter what aperture you shoot at, because the lens doesn't get stopped down until the actual exposure.
To expand on this, the article was talking about tele-extenders which reduce the amount of light available to use for AF. So if your 55-300 is at 300mm then the lens is at 5.8, which is totally fine but add a 2x TC and you lose perhaps two more stops making the amount of light seem as if you were at f/11 or so and that will prevent the AF from working. Same thing can happen when using ND filters. We associate the amount of light available for AF with the minimum aperture of the lens and in most cases that is correct but add a TC or an ND filter and the aperture is no longer the controlling factor.
08-12-2013, 09:16 PM   #4
Site Supporter




Join Date: May 2012
Location: Adelaide
Photos: Gallery | Albums
Posts: 1,081
More to the point, at 300mm, the autofocus is active with the lens wide open at F5.8 as others have said. Wack even a 1.4x teleconverter on, and with a stop of light loss, the camera would be asked to auto-focus at beyond F8 and also cope with the additional loss of contrast from incerting the TC into the light path. I think at that point, the AF would become decidedly hit and miss, unless the scene had lots of good sharp high contrast edges for the AF to find.

08-12-2013, 09:52 PM   #5
Veteran Member




Join Date: Dec 2012
Photos: Gallery
Posts: 696
Original Poster
Thanks guys! No plans for a teleconverter, but I will be using ND filter, and a polarizer -- so will go to manual focus and Live View for that ... for anything that cuts down that wide-open aperture light for the AF.

Greatly appreciate your fast responses... now I understand how autofocus works!
08-13-2013, 05:53 PM   #6
Site Supporter
jatrax's Avatar

Join Date: May 2010
Location: Oregon
Photos: Gallery | Albums
Posts: 10,287
QuoteOriginally posted by jon404 Quote
but I will be using ND filter, and a polarizer -- so will go to manual focus and Live View for that ..
Nothing is absolute though. I recently shot some with my DA 21mm and a 10 stop ND filter. AF locked on perfectly much to my total surprise. I could only see a vague impression through the viewfinder but the AF found something to lock onto.
08-13-2013, 07:04 PM   #7
Veteran Member




Join Date: Dec 2012
Photos: Gallery
Posts: 696
Original Poster
@jatrax -- it locked onto your cool logo!
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!
autofocus, camera, dslr, lens, photography
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
For Sale - Sold: SMC Pentax-F 28mm F/2.8 Autofocus Lens - Great Condition maomao Sold Items 2 08-09-2013 01:44 PM
HELP! New Sigma 17-50 can't autofocus?!? rzarector Pentax SLR Lens Discussion 27 06-27-2013 09:58 AM
Pentax 200mm f/2.8 autofocus not working? milka Pentax SLR Lens Discussion 15 02-26-2013 12:46 PM
For Sale - Sold: FA 77mm f/1.8 Limited Series Autofocus Lens jurysi Sold Items 2 11-05-2012 01:20 PM
For Sale - Sold: Pentax 50mm f/2.8 Macro Autofocus Lens momtog_kat Sold Items 6 03-19-2012 04:24 AM



All times are GMT -7. The time now is 11:59 PM. | See also: NikonForums.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