Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
08-20-2010, 01:45 PM   #1
New Member




Join Date: Oct 2009
Location: San Francisco, CA
Posts: 16
Can a manual-focus lens have back focus problems?

I've looked into back focus problems here and on the larger Web, and it's at the edge of what my brain can understand, but all the discussions seem to involve Canon/Nikon AF systems, and not much of that seems applicable. Can an A-series Pentax lens have back focus problems?

I have a Pentax K100D Super and a Pentax A series 50mm f/2.0 lens.

I keep taking shots that look like this:




(enlarged to show texture):




In the viewfinder, it looked as though the people were in focus. I wasn't trying to photograph the flowers. I get shots like this fairly often. The plane of focus seems to be about two feet (or 20 percent?) behind the thing I think I'm focusing on. Classic back focus problem, right? But can back focus exist in the lens? Doesn't back focus come from a discrepancy between the lens-to-CCD distance and the lens-to-focus-screen distance?

Settings for this shot were f/2.4, 1/60, Aperture priority -.5EV, ISO 1600, Anti-shake ON, hand-held and looking through the viewfinder. It was an evening barbeque; pretty dark.

I haven't noticed similar problems with my other lenses on this camera, but my other lenses start at f/3.5 and I use them mainly on auto-focus. My other lenses are 18-55 kit and 10-17 zoom fisheye.

I downloaded some focusing chart PDFs, but got frustrated and after a while I no longer trusted my own squinting eyeballs, so I got a little more home-brew, literally.

My roommate has an *ist Ds, so I borrowed that to see if the problem is me, my lens, or my camera. I set up beer bottles, some lamps, and a tripod in the kitchen. The 50mm lens has distance markings for 15ft ... 3m ... 8ft. I took pictures of a beer bottle 13ft from my camera lens, using the focus scale, NOT my own eyes, but here goes:

All shots at f/2.0 Wide open, tripod, Anti-shake OFF, 2-second timer release so I wasn't touching the tripod when it went off, ISO 800. (*ist Ds doesn't have anti-shake)

My camera, distance scale at 13 feet



*ist, distance scale at 13 feet



My camera, distance scale at 11.5 feet (halfway between 15 and 8)


*ist, distance scale at 10 feet (*ist at 11.5 image accidentally erased)



It seems to me that the lens's "real" focus is about two feet behind what the distance scale says. That jives with my real-world photos of the outdoor party.

I'd love for this all to be the fault of my $60 eBay 50mm, but I worry that it's a problem with my DSLR. What should my next step be? Many thanks.

08-20-2010, 01:58 PM   #2
Pentaxian




Join Date: Jun 2009
Location: GMT +10
Photos: Albums
Posts: 10,800
BF and FF are auto-focus specific issues, by definition.

You hit the nail on the head when you talk about your squinting eyeballs. What you may be seeing is an issue of your viewfinder dioptre needing adjustment to match the properties of your eyes (short sighted, long-sighted etc).
08-20-2010, 01:59 PM   #3
New Member




Join Date: Oct 2009
Location: San Francisco, CA
Posts: 16
Original Poster
After I posted the above, the forum program found me "related topics" that my earlier search missed:

QuoteOriginally posted by blu3ness Quote
I try to focus on the center of a subject using a M 50/f 1.7, the camera focus indicator would kick in, however, there's a little margin where that focus indicator indicates that the subject is in focus. Sometimes I would get a sharp picture but sometimes the focus's is a little off to the front or to the back. Is this just my inability to look properly through the viewfinder? I hope so..
To be clear: I take the red blips and the yellow hexagon under advisement, but I press the button when the intended subject of the picture looks sharp to me. Hrm.
08-20-2010, 02:15 PM   #4
Pentaxian
JohnBee's Avatar

Join Date: Sep 2008
Location: front of computer
Posts: 4,620
QuoteOriginally posted by PocketPixels Quote
To be clear: I take the red blips and the yellow hexagon under advisement, but I press the button when the intended subject of the picture looks sharp to me. Hrm.
Perhaps your focusing screen could use a little adjusting

08-20-2010, 02:32 PM   #5
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 27,291
A few things to keep in mind:
  • MF lenses do not have front/back focus issues though a particular body may have an incorrectly shimmed focus screen
  • The distance scales on your MF lenses are not intended as measuring devices and the precision decreases as distance decreases
  • The stock focus screen on your camera has intrinsically poor focus precision. This is difficult to explain except to say that there is exaggerated DOF relative to what is actually recorded on the sensor.
Focus issues with MF lenses are a recurring theme on this forum. The most common solution is to replace the stock focus screen with a split-image screen from various eBay merchants, focusingscreen.com, or Katz Eye Optics. I have been using a Katz Eye screen for a couple of years now and and quite happy with it.


Steve
08-20-2010, 03:04 PM   #6
Veteran Member
Eruditass's Avatar

Join Date: Jul 2009
Location: Pittsburgh, PA
Photos: Gallery | Albums
Posts: 2,206
If you are using AF-confirm (green hex), that uses the autofocus sensors.

If you are going by the focus screen, it could have a shim issue.
08-20-2010, 03:13 PM   #7
Pentaxian
Moderator Emeritus




Join Date: May 2007
Location: Edmonton Alberta, Canada
Photos: Gallery | Albums
Posts: 10,648
QuoteOriginally posted by Eruditass Quote
If you are using AF-confirm (green hex), that uses the autofocus sensors.

If you are going by the focus screen, it could have a shim issue.
That's right and thus you can have the same BF/FF issues an AF lens has. Plus the AF display may have some variance if what's actually in focus. The little red squares in the VF are not always exactly where the sensor is either. The AF system was only designed with AF lenses in mind.

The only true solution is what Steve suggested and getting a split screen so you know for sure when the image is in focus.
08-20-2010, 03:20 PM   #8
Inactive Account




Join Date: Aug 2008
Location: Michigan, USA
Photos: Gallery | Albums
Posts: 7,485
QuoteOriginally posted by Peter Zack Quote
That's right and thus you can have the same BF/FF issues an AF lens has. Plus the AF display may have some variance if what's actually in focus. The little red squares in the VF are not always exactly where the sensor is either. The AF system was only designed with AF lenses in mind.

The only true solution is what Steve suggested and getting a split screen so you know for sure when the image is in focus.
..and you may still have to shim the screen. I haven't experienced a need to but I know others have. Don't know why the difference but we are talking very small tolerance differences here.



08-20-2010, 05:42 PM   #9
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
It's neither camera problem nor a lens problem, but a technique problem, combined with a design limitation.

The technique problem is that you are taking the picture when the subject looks in focus. But the viewfinder is far too small for you to really be able tell that - you may well think things are in focus that really aren't, simply because it's too small to say for sure. The trick is not to accept it as in focus when it stops look out of focus, but to make sure you are *the most in focus* you can be. And that pretty much requires comparing your subject against what is in front and what is behind. had you done this, you'd have seen the background was *more* in focus than the people.

The design limitation is that the focus screen on most modern DSLR's shows too much DOF at large apertures. So even if your eyes were better and you really could reliably tell when something was sufficiently in focus just by looking through the viewfinder, the viewfinder would be showing you things in focus that really aren't. The stuff really in focus would look in focus *also* - so it's not a matter of BF or FF - just too large a DOF. But with practice, it's possible to overcome that as well.

Bottom line: manual focus on a modern DSLR is not easy, but no, there is no problem with the lens or the camera. The only *possible* problem would be if the focus screen were not positioned correctly in the camera, but this would have a *tiny* effect on your picture - like moving the focus zone a few millimeters in the posted picture. Whereas you actually missed focus by a lot more than that.
08-20-2010, 06:56 PM - 1 Like   #10
Senior Member




Join Date: Jan 2010
Location: Netherlands
Photos: Gallery
Posts: 148
QuoteOriginally posted by Marc Sabatella Quote
It's neither camera problem nor a lens problem, but a technique problem, combined with a design limitation.

The technique problem is that you are taking the picture when the subject looks in focus. But the viewfinder is far too small for you to really be able tell that - you may well think things are in focus that really aren't, simply because it's too small to say for sure.
Sorry, but what an absolute nonsense, it is perfectly possible to use manual focus on a DLSR, nothing to it but setting the diopter right and looking trough the viewfinder.

The only problem TS has is a wrong shim.
What he simply needs to do is replace the shim with one of the correct thickness. No need at all for replacing the focus screen, that is, not for a FF or BF problem.

A lot of people even just uses small strips of post-it paper to ajust there focus screen. TS its problem is that simple, not more than some strips of paper will solve his problem.

If TS likes a split focus screen better than the default screen, he should buy one, but it won't do anything for FF or BF by default.

A picture says more than thousend words



The length of L1, L2 and L3 needs to be exactly the same length. L2 and L3 can be adjusted trough the menu. L1 needs to be adjusted with the right shim, the right spacer between the focusing screen and the pentaprism (or pentamirror whatever)
FF or BF with manual lenses is nothing more as not the correct lengt of L1

For some more explaining take a look at this link

Focusing Screen--How to adjust focusing screen--

Last edited by Sakura; 08-20-2010 at 07:12 PM.
08-20-2010, 07:35 PM   #11
New Member




Join Date: Oct 2009
Location: San Francisco, CA
Posts: 16
Original Poster
Thank you everybody for all this info. I'm realizing now that my beer-bottle test had two flaws:
1. It was testing the focusing scale of my lens. I seldom, if ever, focus with numbers. My real concern is getting sharp pictures the way I tend to shoot: looking through the viewfinder and focusing.
2. There was beer. Beer is not known for clarity.

I will do some more real-world tests and I will look into split screens (I know what those are; my dad's Olympus OM-1 had one where you lined up two hemispheres to focus) and focus screen shims (no idea, but that's why there are search fields.)

I had high hopes for the diopter suggestions, but the diopter is correct. I've got it all the way to the right, and that's the setting that gives me the sharpest view of the green numbers and the black crosshair pattern.

Many thanks. i'll be sure to post when I know more about what's going on.
08-20-2010, 10:15 PM   #12
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
QuoteOriginally posted by Sakura Quote
Sorry, but what an absolute nonsense, it is perfectly possible to use manual focus on a DLSR, nothing to it but setting the diopter right and looking trough the viewfinder.
Well, of course. I didn't say it was impossible - just that it does take a bit of skill. It's pretty common common for beginners to settle for an image looking "OK" in the viewfinder, but that's not nearly good enough.

QuoteQuote:
The only problem TS has is a wrong shim.
There is no evidence whatsoever there is any such problem. When I've played with shimming my focus screen, I never saw a change in focus that came even close to the discrepancy here.
08-21-2010, 12:54 AM   #13
Senior Member




Join Date: Jan 2010
Location: Netherlands
Photos: Gallery
Posts: 148
QuoteOriginally posted by Marc Sabatella Quote
Well, of course. I didn't say it was impossible - just that it does take a bit of skill. It's pretty common common for beginners to settle for an image looking "OK" in the viewfinder, but that's not nearly good enough.



There is no evidence whatsoever there is any such problem. When I've played with shimming my focus screen, I never saw a change in focus that came even close to the discrepancy here.
I always had a back focus problem with manual shooting with my K7 and tought it was me and my eyes. (even that while i never had that problem with my K100D and the same manual lenses)
I followed some (wrong as i later found out) suggestions of a new focus screen witch didn't do anything for my backfocus problem.
But i recieved 2 plastic shims with the screen. After a lot of trail and error i used both the plastic shims and removed the metal shim.
All my manual focus shots are right on the spot and the back focus problem disappeard totally.

Ok, manual focussing is something you have to learn to get right, but if it is you and not you're camera/shim, you should expect a more or less even number of FF as BF.

Last edited by Sakura; 08-21-2010 at 01:40 AM.
08-21-2010, 01:38 AM   #14
Veteran Member




Join Date: Jan 2009
Location: Budapest
Posts: 821
QuoteOriginally posted by PocketPixels Quote
I took pictures of a beer bottle 13ft from my camera lens, using the focus scale
Just a note: distance is measured from the film/sensor plane, not from the lens. So 13ft on your lens' distance scale means 13ft from the imaging sensor.
08-21-2010, 01:46 AM   #15
Pentaxian




Join Date: Jun 2009
Location: GMT +10
Photos: Albums
Posts: 10,800
If diopter adjustments didn't do the trick, I'd be reluctant to pursue any shimming until I was completely sure it wasn't a technique issue. OP needs to do more practice with manually focussing in low-light, and shoot a lot more scenes, to make sure it is not just a technique issue related to poor illumination, dim viewfinders and perhaps tired eyes.

If I was a eye doctor, I'd be reluctant to immediately advocate eye surgery if someone came to me and said my vision is sometimes a bit fuzzy. That's what some of these shimming and other suggestions sound like to me.
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!
*ist, anti-shake, camera, distance, dslr, feet, focus, lens, lenses, photography, scale, tripod
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Takumar manual/trap focus causes AF problems? FHPhotographer Pentax SLR Lens Discussion 5 06-29-2010 11:00 AM
K100D Super - Focus point locked to center in Manual Focus ? JGabr Pentax DSLR Discussion 2 01-25-2010 09:41 PM
would a manual lens experience front/back focus problems? blu3ness Troubleshooting and Beginner Help 8 11-04-2009 11:14 AM
Front focus/ back focus on Manual lens. Possible? WangJianWei Troubleshooting and Beginner Help 6 04-20-2009 07:50 PM
Focus problems for both manual and auto?! dkittle Pentax DSLR Discussion 32 08-20-2008 01:54 PM



All times are GMT -7. The time now is 12:46 AM. | 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