Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
04-30-2008, 12:57 PM   #1
Site Supporter




Join Date: Apr 2008
Location: Central Ohio (formerly SF Bay Area)
Posts: 1,490
50mm f/1.4 Focus Issues

I'm new to the DSLR game -- just last week I purchased a K20D with the well-regarded Pentax 50mm f/1.4 prime lens, and it's my first SLR. (Before that, I found myself using manual, shutter-priority, and aperture-priority modes on my pocket camera more than I expected, so I knew I needed more.)

Anyway, I have a new baby, and given my fairly limited funds, I initially wanted a lens that would be good for non-flash portraits. The 50mm lens, combined with the K20D's high-ISO performance, both seem to work great for that purpose.

But I am seeing some autofocus anomalies I hope someone can shed some light on. Since I have only that one lens, I don't know whether I am seeing a lens problem, a body problem, or something within the range of normal behavior. I'm getting poor focusing results, and if I try to compensate for it using the lens-specific AF calibration function, I can get it to be accurate at either short range or long range, but not both.

Here's the background. While playing with the combo and trying to see how shallow the DOF was at f/1.4, I tried to take a picture of my cat in front of the fireplace, using the single center-point autofocus mode, with the focus obtained from the cat's collar. (All of the photos in this post, by the way, were taken in Av mode at f/1.4.)

As you'll see below, the cat is quite out-of-focus, but the fabric on the baby swing to the left (a few feet in front of the cat) is pretty clear. You can see all the faint Winnie-the-Poohs without too much trouble. The cat is maybe 12 feet away from me, and the seat is 2-3 feet closer, so on a percentage basis that's a pretty big error. Here's a cropped reduction:



So I figured I'd try to adjust it with the autofocus calibration custom setting. At close range, I need to set it to -9 (out of a possible -10!) to get it about right. Here's a (cropped and reduced) focus chart without AF calibration, focused on the middle line -- as you'll see it has actually focused a significant distance in front of the line, a little short of the 20mm mark:



And here's the same shot with AF calibration at -9 -- much better.



BUT now at longer range (e.g. the cat shot, from before) I get focus WELL BEHIND the desired location. Here's the basket where the cat was standing, focused on that basket, with compensation at -9 -- the subject is out of focus:



And again, but with the focus centered on the baby swing -- the swing is out-of-focus, but the basket is much clearer:



This doesn't seem right to me. I know to expect some softness at wide-open apertures on this lens, but I'd at least hope for a shallow plane of *accurate* focus centered on my desired location. Suggestions, anyone? I'm using 90% of the available calibration scale just to fix this lens at short distances, and doing that seems to OVERcompensate for longer distances.

So, whaddayathink? RMA? User error? Unrealistic expectations?

Thanks for taking a look.

04-30-2008, 01:19 PM   #2
Veteran Member
bc_the_path's Avatar

Join Date: Mar 2007
Location: Ankara, Turkey
Photos: Gallery
Posts: 390
I think you should repeat the calibration again.
If the lens is back focusing now, as you think, the chart photo should also show that.
It is not easily conceivable that back focusing / front focusing to occur only at certain ranges.
Are you sure you made the test at 45 degrees?
And are you sure that the camera was able to save it as you adjusted? (I remember my K10D refusing to change the RAW file type; a firmware refresh helped!).
04-30-2008, 01:23 PM   #3
Veteran Member




Join Date: Mar 2008
Location: CT / NY
Photos: Gallery
Posts: 822
I've read a lot about these issues on the K10D and from what I read the AF test is indeed good for close range... but as you distance yourself from the subject, the issues "reappear"


One thing to point out: make sure your AF sensor is right at the subject, because as you move AWAY the chances of getting the surroundings in th AF range increases. BUT, in your specific case, the baby swing and the basket, for instance, are pretty far apart.


I am not sure how the adjustments work in K20D, but do you have an option for short AF x long AF?


sorry, i am not helping much.
04-30-2008, 01:43 PM   #4
Veteran Member
bc_the_path's Avatar

Join Date: Mar 2007
Location: Ankara, Turkey
Photos: Gallery
Posts: 390
I also suggest doing the tests/adjustments under either daylight or halogen light. Just in case...

04-30-2008, 02:00 PM   #5
Site Supporter




Join Date: Apr 2008
Location: Central Ohio (formerly SF Bay Area)
Posts: 1,490
Original Poster
QuoteOriginally posted by BBear Quote
I am not sure how the adjustments work in K20D, but do you have an option for short AF x long AF?
No, but that would probably help. I only get a single +/- 10 setting per lens.

I'll repeat under daylight and improved test conditions (basket vs. swing just happened to be there; I'm sure I can do better by printing some targets and putting them on chairs outside). But daylight will have to wait until the weekend -- gotta work for a living!
04-30-2008, 03:41 PM   #6
Veteran Member




Join Date: May 2007
Location: York Region Canada
Posts: 642
Long shot, but is your diotor set for you

Dave
05-01-2008, 03:08 PM   #7
Site Supporter




Join Date: Apr 2008
Location: Central Ohio (formerly SF Bay Area)
Posts: 1,490
Original Poster
I don't think diopter has anything to do with it, as these are all autofocus. Diopter is strictly for the viewfinder, right?

From what I'm reading in myriad other threads on this and other forums, this is simply a known issue when using autofocus with wide-open apertures on fast lenses. Not just on Pentax, either.

There are a lot of differing opinions, but it appears as bc_the_path suggested above that the lighting does play a role.

So I'll try again in sunlight. Maybe my lens does need a little tweak, but less than the -9 I'm seeing under relatively dim indoor compact-fluorescent conditions.

I'll post results when I have some.
05-01-2008, 05:48 PM   #8
Veteran Member
creampuff's Avatar

Join Date: May 2007
Location: Singapore
Photos: Gallery
Posts: 3,955
Check focus point dial

I don't believe the issue is backfocusing at all.
First of all what is the AF point dial set at?
The camera default to select the focus point is Auto - where the camera sets the optimum AF point even if the subject isn't centered.

I suggest you either select the center focus point or manually select the focus point by turning the AF point dial to SEL and using the four way controller to select the desired AF point.

05-02-2008, 09:13 AM   #9
Site Supporter




Join Date: Apr 2008
Location: Central Ohio (formerly SF Bay Area)
Posts: 1,490
Original Poster
Oh, AF has been set to center this whole time -- so I can at least rule that out as a cause!

I did a little more testing yesterday evening in natural light (and with flash), and at this point it looks like I'm getting reliable front-focus unless the AF compensation is set to -10 (maximum!), regardless of range.

I want to take a few more shots at infinity tonight, then I'll put up some more samples. Right now I'm leaning toward the lens neeeding calibration.
05-02-2008, 09:34 AM   #10
Veteran Member




Join Date: Sep 2007
Location: West Virginia
Photos: Gallery
Posts: 338
Apart from your chart test, the other shots seem to me almost designed to produce sub-optimal results. At f1.4 DOF is so thin that even with SR on you could have been OOF even with a 'beep'. I suggest a new test with different object(s)................
05-02-2008, 11:12 AM   #11
Site Supporter




Join Date: Apr 2008
Location: Central Ohio (formerly SF Bay Area)
Posts: 1,490
Original Poster
Agree that the lighting was bad -- and also that I had some difficult focus targets.

Regardless of the state of my lens, I'm coming around to the position that if I want to use this lens under these conditions, maybe careful manual focusing (via Katz Eye) is the way to go.
05-03-2008, 07:34 AM   #12
Veteran Member




Join Date: Dec 2007
Posts: 923
The FA 50mm f/1.4 is a tricky lens to focus between f/1.4 ~ f/3.5.
The first thing I did when I bought the lens was to do a proper focus test with the card - it was OK.
With wide open aperture, especially from f/1.4 to f/2.8, the DOF is so shallow, any case of the camera focusing on the wrong object will leave your subject quite blur. Its often the case with this lens that what people think is a front or back focus issue is really not the case.

When I use this lens, I try as much as possible to stay clear of any nearer object close to the line of sight of the focus point. Just bear in mind the actual center focus point is very much larger than the pin-point focus light.

Many times I found that the camera has locked in on the nearer object rather than what I want.

Having said all that, I love the FA 50mm f/1.4 , especially at f/2.8. Just a superb lens.
05-05-2008, 10:54 AM   #13
Site Supporter




Join Date: Apr 2008
Location: Central Ohio (formerly SF Bay Area)
Posts: 1,490
Original Poster
Thanks for the reassurance. I'm getting more comfortable with it every day, simply by taking lots of pictures. As you say, at f/2.8 and higher, it's pretty easy. And I'm getting some nice ones at f/2.0 and 1.7, provided I take *plenty* of shots and pick out the good ones. (For example, when taking pictures of my baby, by moving the focus point around from nose, to eye, to lips, etc.)

As you say, f/1.4 is really hard though, and the inherent inaccuracy of AF combined with the razor-thin DOF is probably to blame for most of the bad shots I was experiencing -- and I wouldn't discount the possibility of the sensor grabbing focus on something I didn't intend, even when I use only the center point. It varies so much -- light level, light type (incandescent vs. fluorescent vs. natural), distance to subject, whether focus is going in or out -- I have to believe that it's largely, if not entirely, a function of the large aperture.

Is there a photographic equivalent to what they say with respect to computers -- PEBKAC? That is, Problem Exists Between Keyboard And Chair?

In any case, I'm going to keep playing around with it, and after I have more experience (and a few more lenses!) I'll be able to make a better judgment. That's why I have a warranty. :-)
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, basket, calibration, cat, f/1.4, feet, focus, front, k-mount, lens, pentax lens, range, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
New K-5- I need help with focus issues Kini62 Pentax K-5 27 10-25-2010 10:12 PM
focus test issues, FA 50mm 1.7 & DAL 18-55mm kxr4trids Pentax SLR Lens Discussion 5 07-03-2010 12:29 PM
Focus issues VincenTC Troubleshooting and Beginner Help 5 02-09-2010 01:49 PM
Focus Issues aesch Pentax DSLR Discussion 0 03-06-2009 05:52 PM
help FA 50 1.4 Focus issues qksilver Pentax SLR Lens Discussion 7 06-23-2008 06:21 PM



All times are GMT -7. The time now is 08:58 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