Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
11-16-2007, 09:37 PM   #1
Inactive Account




Join Date: Oct 2007
Location: Moscow, ID
Posts: 21
Manual lens focusing: what is causing this?

OK, deep breath. I've been having issues with manual focus on a recently acquired SMC-M 50/1.4, used on a K10D. In short, the point I've been focusing on (based on both visual appearance and the green focus indicator) is consistently blurry, with the resulting plane of focus well behind the subject. I shot the below test targets to make sure I wasn't going nuts. Aperture of f/1.4, OEM focus screen, diopter adjusted correctly, manual focus mode, M exposure mode, shake reduction off, and mounted on a tripod. Oh, and corrected vision (contacts) for my 27 year old eyes. Based on recommendations from others on this forum, I've attached results showing differences in focus based on the direction from which you approach the focus point.

Focused 'from the left', from near to far until green focus indicator lights:
View Picture EXIF
Name:  SMC-M 50-f1.4-1.jpg
Views: 594
Size:  185.6 KB

Focused 'from the right', from far to near until green focus indicator lights:
View Picture EXIF
Name:  SMC-M 50-f1.4-2.jpg
Views: 546
Size:  183.8 KB

In both cases, focus looked SPOT ON to the line of text, which actually reads "This text should be perfectly in focus." How is this happening??? Some have mentioned the need for a better screen, but these results are WAY off. Are all users of manual focus lenses using a split focus screen etc., or is this an equipment problem? Lens, or camera?

My previous experience with manual focus lenses was in 4x5 with a ground glass, where everything was at it seemed... so this problem really has me stumped. As it is, this lens is unusable at wide apertures, which is of course it's intended use. I can see how there might be a range of focus, resulting in the differences seen based on direction of approach, but NEITHER of them contain the actual point of focus!!! Any ideas what might be causing this?

Kerey

11-16-2007, 11:43 PM   #2
Veteran Member




Join Date: Sep 2006
Location: West Chester, PA
Photos: Gallery | Albums
Posts: 1,420
I guess I didn't answer this clearly enough the first round...

There is an inherent DOF with the focus screen (and hence the viewfinder). It is deeper than the DOF created by your 50mm prime at f/1.4. Therefore, you can move the focus of the lens without any apparent change through the viewfinder.

That's all, there is nothing defective about your equipment other than it isn't designed for manual focus with fast lenses. But as I said before none of the other manufacturer's DSLR bodies are either. There are two OEM exceptions that I know of - Minolta made a dedicated manual focus screen for their 7D and Canon makes one for the 5D. The Minolta required service at the center to replace the focus screen, I'm not sure about the Canon.

It isn't impossible to do, just sometimes frustrating. I manual focus 99% of the time.

Good luck,
Sean
11-17-2007, 12:00 AM   #3
Veteran Member
Fritz's Avatar

Join Date: Jun 2007
Location: Tillamook, OR
Posts: 1,168
Are you having this same back focusing problem with your other lenses? I would suspect that you might, but it wouldn't be as noticeable on a slower lens. I have a M-50mm f1.4 and it focuses fine on my k100d, maybe a slight front focus.

From what I have read you could send the camera to Pentax to have the focusing recalibrated. Use the search feature on this board. There are a ton of threads with k10's and f1.4 lenses that are not focusing right.

Here is a link that has some information too. Although I'm not sure how much would apply to your k10, it might give you some ideas.

I'm also using a split prism screen that seems to be very accurate. I believe it would focus right even if the auto focus system (including green hex) is not.

HTH
11-17-2007, 12:04 AM   #4
Veteran Member
Fritz's Avatar

Join Date: Jun 2007
Location: Tillamook, OR
Posts: 1,168
QuoteOriginally posted by carpents Quote
I guess I didn't answer this clearly enough the first round...

There is an inherent DOF with the focus screen (and hence the viewfinder). It is deeper than the DOF created by your 50mm prime at f/1.4. Therefore, you can move the focus of the lens without any apparent change through the viewfinder.

That's all, there is nothing defective about your equipment other than it isn't designed for manual focus with fast lenses. But as I said before none of the other manufacturer's DSLR bodies are either. There are two OEM exceptions that I know of - Minolta made a dedicated manual focus screen for their 7D and Canon makes one for the 5D. The Minolta required service at the center to replace the focus screen, I'm not sure about the Canon.

It isn't impossible to do, just sometimes frustrating. I manual focus 99% of the time.

Good luck,
Sean
That all sounds reasonable to me!

11-17-2007, 02:23 AM   #5
Junior Member
dancel's Avatar

Join Date: Oct 2007
Location: Canada
Posts: 49
I find manual focus more fun cause i feel like i have more in control, I actually just purchased the eyecup magnifier for my k10d and it helps a lot.
11-17-2007, 05:14 AM   #6
Veteran Member
Finn's Avatar

Join Date: Feb 2007
Location: Phoenix
Photos: Gallery
Posts: 1,056
Also keep in mind that the focus indicator is a little imprecise. Not inaccurate, just imprecise. And at f/1.4 (especially if your subject is close), getting even a split prism to focus precisely is a tall order.
11-17-2007, 05:54 AM   #7
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
one thing everyone needs to remember is that depth of field IS NOT a linear function.

Assuming you are correctly focused on a subject (the line in this focusing chart as an example) the amount of text in front of the line is much less than behind the line. it will be in a ratio of approximately 1/3 in frong, 3/2 behind.

This is evident when you look at the DOF markings on a lens.

I have the SMC-P-50mm f/1.4, very similar to the lens used for these shots. If you look at the focusing scale, and set it to focus at 5 feet, the F/16 markings show the range as 4 feet to 7 feet. (1/3 in front, 2/3 behind).

I also note that you are not "square" to the focusing grid or properly centered in the frame, both of these will influence the focus.

re-try this test, keeping the camera correctly alligned (left to right) and the focus grid square to the camera (line horizontal) I would bet you are much closer than you think.

Editing Note:

I reviewed the test setup and focusing target. Based upon the images shown, it would appear that the lens was roughly 0.66 meters from the target. At this distance and magnification ratio, and assuming (aside from the errors already noted above), that the lens was at the correct 45 degree angle, every 2mm of indicated focusing distance error represents an error in the lens extension (i.e. how far the element moves front to back relitive to the sensor plane) of 13 microns or just over 1/100th of a mm. (for US folks who still think in inches, every 2mm mark on the target represents 5 tenthousandths of an inch focusing error)

Maybe someone can answer just exactly what they expect from a consumer product that gets moved all over the place, bumped, banged, occasionally dropped, frozen in winter, baked in side a car in summer, and making an adjustment in much less than a second.

Honestly I think, as jfdavis58 has suggested, put the test target where it belongs and go out to take pictures. I have yet to see a photograph of anything approaching a lens test target win a photo award!

Last edited by Lowell Goudge; 11-17-2007 at 09:07 AM.
11-17-2007, 08:16 AM   #8
Veteran Member
jfdavis58's Avatar

Join Date: Nov 2006
Location: 13 S 0357397-3884316
Photos: Gallery
Posts: 876
Here's a novel idea: put the test chart in the circular file, grab a couple SD cards and go out and take some photographs. You might be surprised at how quickly you can develop an intuitive feel for exposure, focus and composition if you just practice the art form instead of searching for mechanical perfection in a camera.

QuoteOriginally posted by kerey Quote
OK, deep breath. I've been having issues with manual focus on a recently acquired SMC-M 50/1.4, used on a K10D. In short, the point I've been focusing on (based on both visual appearance and the green focus indicator) is consistently blurry, with the resulting plane of focus well behind the subject. I shot the below test targets to make sure I wasn't going nuts. Aperture of f/1.4, OEM focus screen, diopter adjusted correctly, manual focus mode, M exposure mode, shake reduction off, and mounted on a tripod. Oh, and corrected vision (contacts) for my 27 year old eyes. Based on recommendations from others on this forum, I've attached results showing differences in focus based on the direction from which you approach the focus point.

Focused 'from the left', from near to far until green focus indicator lights:
Attachment 5986

Focused 'from the right', from far to near until green focus indicator lights:
Attachment 5987

In both cases, focus looked SPOT ON to the line of text, which actually reads "This text should be perfectly in focus." How is this happening??? Some have mentioned the need for a better screen, but these results are WAY off. Are all users of manual focus lenses using a split focus screen etc., or is this an equipment problem? Lens, or camera?

My previous experience with manual focus lenses was in 4x5 with a ground glass, where everything was at it seemed... so this problem really has me stumped. As it is, this lens is unusable at wide apertures, which is of course it's intended use. I can see how there might be a range of focus, resulting in the differences seen based on direction of approach, but NEITHER of them contain the actual point of focus!!! Any ideas what might be causing this?

Kerey


11-17-2007, 11:48 AM   #9
Inactive Account




Join Date: Oct 2007
Location: Moscow, ID
Posts: 21
Original Poster
I appreciate everyone's feedback, and certainly acknowledge this wasn't a real 'test' of anything other than an attempt to show people the problem I was having, using something other than a photo of a tree. The reason for the effort was an entire day's worth of impromptu portraits rendered useless because all were back-focused (though not apparently so on the LCD), so I don't consider my problem to be a minor one or due to unreasonable expectations. To answer Lowell directly, I expect my consumer manual focus lens to focus where I tell it to focus, not (as in the portrait situation) ~1 foot to the rear. This is not an unreasonable expectation, in my opinion, of any product manufactured explicitly for use in the field and in otherwise excellent condition.

I wasn't able to find any reference to this problem prior to my post, but thanks to the link from Fritz I've been able to understand the problem better. As it's still under warranty, I think I'll be sending the camera to Pentax to have the sensor adjusted (rather than attempting the process myself). Hopefully others facing this problem are able to manage something similar.

Kerey
11-17-2007, 12:33 PM   #10
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by kerey Quote
I appreciate everyone's feedback, and certainly acknowledge this wasn't a real 'test' of anything other than an attempt to show people the problem I was having, using something other than a photo of a tree. The reason for the effort was an entire day's worth of impromptu portraits rendered useless because all were back-focused (though not apparently so on the LCD), so I don't consider my problem to be a minor one or due to unreasonable expectations. To answer Lowell directly, I expect my consumer manual focus lens to focus where I tell it to focus, not (as in the portrait situation) ~1 foot to the rear. This is not an unreasonable expectation, in my opinion, of any product manufactured explicitly for use in the field and in otherwise excellent condition.

I wasn't able to find any reference to this problem prior to my post, but thanks to the link from Fritz I've been able to understand the problem better. As it's still under warranty, I think I'll be sending the camera to Pentax to have the sensor adjusted (rather than attempting the process myself). Hopefully others facing this problem are able to manage something similar.

Kerey

All I can comment on is that your test shows an error of millimeters not feet. I think the problem is elsewhere
11-17-2007, 02:51 PM   #11
Veteran Member




Join Date: Aug 2007
Location: England
Posts: 309
QuoteOriginally posted by kerey Quote
To answer Lowell directly, I expect my consumer manual focus lens to focus where I tell it to focus, not (as in the portrait situation) ~1 foot to the rear. This is not an unreasonable expectation, in my opinion, of any product manufactured explicitly for use in the field and in otherwise excellent condition.
I have the same lens (SMC-M 50/1.4) but I have a K100D. I tried the same focus test a couple of weeks ago (I was curious, and anyway it was raining so I could not go out and shoot some pictures). My tests were spot on, no BF at all.

Some people have reported that the red dot may not be in the same place as the place where the AF system is detecting focus. Perhaps you can test this by printing just one horizontal line on a piece of paper and repeat the test with that. The AF focus confirm can only work with this line. So when the green hexagon shows, note where the red dot flashes. If it is not on the line then the red dot is misaligned.

I think Sean's answer about the DOF explains why you are focusing on something that isn't actually in focus. When I first used the M50/1.4 on my camera and looked through the viewfinder I was surprised that the DOF was not shallower. However, when I looked at the final photo the DOF was noticeably shallower.

Richard
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!
focus, indicator, k-mount, lens, lenses, lights, mode, pentax lens, results, screen, slr lens, text
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Focusing technique with fast manual lens Corvairfan Troubleshooting and Beginner Help 14 10-28-2010 09:09 PM
Focusing manual lens with OEM focusing screen, is a split screen really needed? skid2964 Pentax Camera and Field Accessories 16 09-17-2010 02:54 PM
What might be causing this??? Pentax K10 with manual lens 8540tomg Pentax DSLR Discussion 10 09-19-2009 04:56 PM
Focusing and manual lens... schmikey Pentax DSLR Discussion 22 08-24-2007 06:13 AM
12-24 manual focusing taveren Pentax SLR Lens Discussion 1 06-29-2007 05:31 PM



All times are GMT -7. The time now is 08:29 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