Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-06-2015, 07:47 AM   #1
Pentaxian
Wired's Avatar

Join Date: Aug 2009
Location: Edmonton, AB
Photos: Gallery | Albums
Posts: 2,494
focal point calibration concerns

Wondering if anyone else is seeing this problem...

For a while now I've been getting frustrated with the apparent softness of my K3. It's tack sharp most of the time, but every so often I get a slightly OOF image. At first it seemed random, so I started to determine it as user error. However, I noticed something on my last two shoots, it only seems to be when I'm using the furthest Focal points from center. The ones far left and far right.

anytime I use the center focal points the images are tack sharp. I use the outside ones though it appears to have some back focus. When I calibrate the center points they are dead accurate. But I haven't tried the side points.

its frustrating because all points should be calibrated the same no?

03-06-2015, 08:04 AM   #2
Veteran Member




Join Date: Jul 2009
Location: London
Posts: 856
Most lenses will be softer at the edges than the centre, so if that's where your subject is, and I presume it is because you're using the outer focal points, then it could be lens, not body?
03-06-2015, 08:19 AM   #3
Pentaxian
Wired's Avatar

Join Date: Aug 2009
Location: Edmonton, AB
Photos: Gallery | Albums
Posts: 2,494
Original Poster
QuoteOriginally posted by JohnX Quote
Most lenses will be softer at the edges than the centre, so if that's where your subject is, and I presume it is because you're using the outer focal points, then it could be lens, not body?


At f8?
03-06-2015, 08:20 AM   #4
Senior Member




Join Date: Dec 2013
Posts: 154
The 25 inner focus points are cross type, so they are sensitive to vertical and horizontal edges too. The 2 outer are vertical only (I think). Furthermore the center and one above and one below are F2.8 sensitive, so they are more accurate then any other focus points.

03-06-2015, 08:29 AM   #5
Pentaxian
Wired's Avatar

Join Date: Aug 2009
Location: Edmonton, AB
Photos: Gallery | Albums
Posts: 2,494
Original Poster
QuoteOriginally posted by 08amczb Quote
The 25 inner focus points are cross type, so they are sensitive to vertical and horizontal edges too. The 2 outer are vertical only (I think). Furthermore the center and one above and one below are F2.8 sensitive, so they are more accurate then any other focus points.
figured as much as that would be the cause.

Just didn't think with good light it would be that bad. They are pretty much useless for me right now.



this was the outermost to the right FP set on her eyebrow.
03-06-2015, 09:16 AM   #6
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,155
QuoteOriginally posted by 08amczb Quote
Furthermore the center and one above and one below are F2.8 sensitive, so they are more accurate then any other focus points.
More precise...meaning they are more likely to be accurate.

The other points have a focus sensitivity of f/5.6 which should be adequate at f/8. That being said, even the f/2.8 sensors will fail to satisfy if pixel peeping at a 24Mpx image. After all, there is only one true point of focus and DOF means little when viewing at full resolution.


Steve
03-06-2015, 09:47 AM   #7
Pentaxian
Wired's Avatar

Join Date: Aug 2009
Location: Edmonton, AB
Photos: Gallery | Albums
Posts: 2,494
Original Poster
QuoteOriginally posted by stevebrot Quote
More precise...meaning they are more likely to be accurate.

The other points have a focus sensitivity of f/5.6 which should be adequate at f/8. That being said, even the f/2.8 sensors will fail to satisfy if pixel peeping at a 24Mpx image. After all, there is only one true point of focus and DOF means little when viewing at full resolution.


Steve
true that. I print a lot, and often at 14x16 on matte or metallic. But I haven't seen how these images look at that print size because I haven't decided to waste money on that. But I'll print the above image and see if it meets my standards.

For me, if I'm doing tear sheets and headshots for paying clients that are using these images to get work on their own, I want that eye to be as sharp as possible. I could care less about the skin, I smooth that anyways!
03-06-2015, 09:51 AM   #8
Veteran Member




Join Date: Jul 2009
Location: London
Posts: 856
QuoteOriginally posted by Wired Quote
At f8?
It's inherent in the optical design, not the focussing.

Have to say, however, that I'm not seeing an issue with the photo you posted.

03-06-2015, 09:54 AM   #9
Pentaxian
Wired's Avatar

Join Date: Aug 2009
Location: Edmonton, AB
Photos: Gallery | Albums
Posts: 2,494
Original Poster
QuoteOriginally posted by JohnX Quote
It's inherent in the optical design, not the focussing.

Have to say, however, that I'm not seeing an issue with the photo you posted.
Eyes are not sharp. Should have posted the original. But I applied pretty aggressive post processing sharpening masks to simulate sharpness.
03-06-2015, 10:05 AM   #10
Pentaxian
JinDesu's Avatar

Join Date: Jun 2011
Location: New York City
Photos: Gallery
Posts: 5,624
Could it possibly be the lens performance? I am not very well versed in the separation of signals for phase detect AF, but how does the light transmission perform on an off center part of the lens to the AF sensor?
03-06-2015, 10:18 AM   #11
Pentaxian
Wired's Avatar

Join Date: Aug 2009
Location: Edmonton, AB
Photos: Gallery | Albums
Posts: 2,494
Original Poster
QuoteOriginally posted by JinDesu Quote
Could it possibly be the lens performance? I am not very well versed in the separation of signals for phase detect AF, but how does the light transmission perform on an off center part of the lens to the AF sensor?
great question. But it does not seem to matter on which lens. I primarily only use the FA 31, FA 43, FA 77 over the past 6-7 months however.
03-06-2015, 11:30 AM   #12
Pentaxian
JinDesu's Avatar

Join Date: Jun 2011
Location: New York City
Photos: Gallery
Posts: 5,624
QuoteOriginally posted by Wired Quote
great question. But it does not seem to matter on which lens. I primarily only use the FA 31, FA 43, FA 77 over the past 6-7 months however.
Is it a consistent back focus no matter which end of the far focal points you are using?
03-06-2015, 11:32 AM   #13
Pentaxian




Join Date: Jun 2009
Location: GMT +10
Photos: Albums
Posts: 10,572
You should probably be focus-and-recomposing, rather than just selecting one edge focus point, the since the eyes are going to be close to outside of the AF frame. Pic crudely related:

03-06-2015, 11:36 AM   #14
Pentaxian
Wired's Avatar

Join Date: Aug 2009
Location: Edmonton, AB
Photos: Gallery | Albums
Posts: 2,494
Original Poster
QuoteOriginally posted by JinDesu Quote
Is it a consistent back focus no matter which end of the far focal points you are using?
correct.
03-06-2015, 11:40 AM - 1 Like   #15
Pentaxian




Join Date: Jun 2009
Location: GMT +10
Photos: Albums
Posts: 10,572
Live View AF would probably work better than the optical viewfinder for photo elements at the very periphery of the frame. You could also then, of course, zoom in right on the eyes to confirm focus before you take the shot.
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!
af, center, dslr, eyes, focus, frame, k-3, k3, pentax k-3, shot, tack
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help with K-5 Focal Point BobbyA Troubleshooting and Beginner Help 15 10-13-2012 06:08 PM
User Selected Focal Point not working myquelp Pentax K-5 12 04-25-2011 08:06 AM
Manual Focal Point adjustment- K20D heaven72501 Pentax SLR Lens Discussion 5 03-14-2010 01:06 PM
multi-point (11, 16, whatever) focal system—WHY?? WMBP Pentax DSLR Discussion 15 03-02-2010 10:51 AM
Setting up focal point for manual lenses. consider crop factor? D4rknezz Troubleshooting and Beginner Help 7 09-23-2009 01:00 PM



All times are GMT -7. The time now is 03:13 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