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08-20-2009, 01:58 PM   #1
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Pentax-M f1.4 50mm on a K20D focusing problems

Hey everyone, I need some help.

I just got my 50mm manual lens. It's a SMC Pentax-M 1:1.4 50mm. I put it on my K20D, but it's having focusing problems.

I knoticed that the first few shots were out of focus and I thought that I messed up. However, I tried another 50mm manual lens on the same camera and both 50mm lenses are out of focus by about 20mm on a Focus Test Chart even though it was in focus in the view finder. I used the same lens on a different camera (K100) and the lens worked fine. On the K100 they're right on the money.

I tried my 600mm manual lens on the K20D and did the same focus test, and it did just fine.

So, it seems to me that this K20D is out of adjustment somehow with these two 50mm lenses. Is there any corrective measures I can do to get this camera to accept this 50mm lens? Is there a setting I need to change?

Any help would be great.

08-20-2009, 02:21 PM   #2
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couple of points,

first , the lens could be a little soft wide open also leading to focusing errors wide open

second, the K10D and K20D screen show more apparent DOF than exists, and the screen design, if I recall correctly, was a trade off between brightness and focusing ability (brightness won). Have you checked with the focus indicator in the camera for focus confirmation

I can't speak for the K20 but on my K10D i have a split image finder and it is right on the money with both actual focus and the focus confirmation for both my SMC 50mm F1.4 and my vivitar Series 1 85mm F1.4. You might have made teh mistake yourself focusing.
08-20-2009, 02:38 PM   #3
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Thanks for the reply.

Even the focusing sensor says that it's in focus. It's off by about 20mm or so. It doesn't seem to matter what the f. stop is. I've tried it with a flash and without.

On the focusing test chart, I actually have to focus 20mm past the 0 mark to get that line in focus. It makes me think it's a defective lens, except that it works just fine on a K100 camera body.
08-20-2009, 03:14 PM   #4
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might be interesting to post photos, and also check your AF points, make certaiin that the AF is on center spot.

ALso how about trying the different AF points deliberately.

08-20-2009, 03:41 PM   #5
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QuoteOriginally posted by CraneGuy1 Quote
Thanks for the reply.

Even the focusing sensor says that it's in focus. It's off by about 20mm or so. It doesn't seem to matter what the f. stop is. I've tried it with a flash and without.

On the focusing test chart, I actually have to focus 20mm past the 0 mark to get that line in focus. It makes me think it's a defective lens, except that it works just fine on a K100 camera body.
You might try the "trap focus" or "catch in focus" methods to test the focusing ability of your lens. I have a SMC Pentax-M 50mm f1.4 lens that I've used on my K10. I tried and tried to get it to come into proper focus with the stock focus screen. I tried the Chinese split prism focus screen and it would seem in focus with both screens but would come out - out of focus.

I've tried the "catch in focus" method and consistently had in focus shots. To do that set your camera to manual focus, and Manual operation. Start out with the lens out of focus and use the green button to establish proper exposure and with your camera pointed at your subject with the lens out of focus, slowly sneak up on proper focus with the shutter release button depressed.

When the lens is in perfect focus the shutter release will trigger and a photo will be taken. One caution is to "sneak up on proper focus" or you might go beyond the point of perfect focus. The camera has a better eye on focus than I do so when I use my M lens I let the camera do the focusing.

Your camera will choose the spot metering and center focus point when using a manual lens.

Last edited by jimH; 08-20-2009 at 03:42 PM. Reason: Add thought.
08-20-2009, 08:24 PM   #6
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QuoteOriginally posted by jimH Quote
You might try the "trap focus" or "catch in focus" methods to test the focusing ability of your lens. I have a SMC Pentax-M 50mm f1.4 lens that I've used on my K10. I tried and tried to get it to come into proper focus with the stock focus screen. I tried the Chinese split prism focus screen and it would seem in focus with both screens but would come out - out of focus.

I've tried the "catch in focus" method and consistently had in focus shots. To do that set your camera to manual focus, and Manual operation. Start out with the lens out of focus and use the green button to establish proper exposure and with your camera pointed at your subject with the lens out of focus, slowly sneak up on proper focus with the shutter release button depressed.

When the lens is in perfect focus the shutter release will trigger and a photo will be taken. One caution is to "sneak up on proper focus" or you might go beyond the point of perfect focus. The camera has a better eye on focus than I do so when I use my M lens I let the camera do the focusing.

Your camera will choose the spot metering and center focus point when using a manual lens.


Well, I did the test again several times, this time using catch manual focus. I focused in very slowly so that the camera's sensor would decide when it was in focus. It gave me the same results. It's still 18mm to 20mm out of focus on the focus test chart. All of my other lenses focus just fine, but not this 50mm, and not the other 50mm I borrowed. Weird.
08-20-2009, 10:34 PM   #7
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I don't know what to say, other than it sounds like a weird problem.
In my opinion there's no way that you'd mistakenly focus that far off, so I won't try blaming that. Or camera shake.

If I was you I'd try a M series 50mm f2.0, and if you can get your hands on one a 50mm f1.2.
See if they both act the same.
If you lived in my city I'd loan you my 35mm f2, and 55mm f1.8 to see if they acted the same.

The other thing I might try, although it's a long shot. If your using the SR turn it off. There is a chance it's somehow causing the problem, when it's set to 50mm.

I know that both my suggestions sound like long shots, however in my opinion it can't be the lenses if they're fine on other cameras... plus if it was a simple in-body calibration all lenses would have the same problems.
08-21-2009, 01:13 AM   #8
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Craneguy

Under what lighting conditions are you doing the test ?

08-21-2009, 05:03 AM   #9
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QuoteOriginally posted by pschlute Quote
Craneguy

Under what lighting conditions are you doing the test ?
I have the focus test sheet on my kitchen countertop and two halogen spot lights pointing at it. My tripod is set up so that I'm about 2' away, shooting down at about a 45* angle. With the f1.4, it is a very shallow DOF, so I can see clearly that the target line on the scale is out of focus, but the 14mm line and the 20mm line are in focus. I''ve tryed it with a flash and without. I've tryed my f1.4 and my f1.7. I've tryed my sister-in-law's f1.7. These 50mm lenses have the same results.

However, my 18-250, my 28-80 with macro, my 50-500, and my 600mm all focus perfectly on my K20D.

The crazy thing that I can't figure out is why these three 50mm lenses won't focus on my K20D, but they work fine on my sister-in-law's K100D.
08-21-2009, 08:01 AM   #10
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If it's still under warranty I think that I'd talk to Pentax and see if I could send it in. Along with one or 2 of those 50mm lenses.
There has to be something wrong with it, although I haven't the faintest idea what.
Although I am kinda wondering what an auto focus 50mm would do on there. I can't picture it being any different.
08-21-2009, 08:41 AM   #11
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Let me give a stab at this. This are the facts as far as I can tell:
  • Inaccurate focus with several 50mm lenses on a K20D when assessed with a focus target
  • These same lenses work fine on a K100D
  • Same problem with "trap focus"
  • Other manual focus lenses seem to work fine on the K20D
Here is what is not clear:
  • What is being used to determine when the subject is in focus? Are you using image sharpness on the focus screen or are you relying on the focus confirm from the camera?
  • If using focus confirm, where is the focus point set to?
  • If using the focus screen, have you calibrated the viewfinder diopter adjustment to your eyes?
  • What focus target are you using? Front/back focus strip?
If you are using the front/back focus target, I would suggest that you not bother. I may be wrong, but I don't believe that you can do a front/back focus adjust for any non-AF lenses on your K20D. (Other members please correct me if this is wrong.)

Here is what I would suggest to test your focus:
  • Calibrate the viewfinder diopter to your eye using a blank white wall. When the grid lines appear sharp, you are in business.
  • Flat vertical target with printed lettering of varying size (cereal box?)
  • Camera 1 meter from target (20X the focal length)
  • Sensor plane parallel to target
  • AF point set to center
  • Adequate natural light
  • Lens at maximum aperture
  • Tripod
  • No SR, 2 second shutter delay
  • Do a best of 5 test using the viewfinder
  • Repeat using the focus confirm (green hexagon lit)
  • Evaluate the images primarily for center sharpness
My expectation are that the better results with the M 50/1.4 and the stock focus screen would be from the AF system. The stock focus screen sucks for fine focus at wider apertures. I would also expect somewhat soft results wide open regardless of focus method.

If you are able to reliably attain accurate focus using the viewfinder, but focus confirm fails using the same setup, there might be an issue with the default bias in the AF system. (Reset?)

Steve

(Seldom use focus confirm myself...have an aftermarket focus screen instead...)

Last edited by stevebrot; 08-21-2009 at 08:59 AM.
08-21-2009, 10:34 AM   #12
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It seems like you have a different optical distances in your k20d. There are two optical path distances to consider:

1) the distance to your AF sensors, which is the distance from the bayonet of the camera body (where light from lens enters), passes through the (semi-transparent) mirror, hits a secondary mirror and is directed downward then is split and uses phase detection algorithms to assess proper focus.

2) the next distance is the one from bayonet to the sensor itself, which is only active during the exposure.

NOTE: There is a third optical path to the viewfinder, which is the remaining light reflected by the mirror, but the viewfinder cannot distinguish focus for such a large aperture, so we will ignore this... especially since you mention that you are relying on the green light 'confirm focus' function on the camera.

So, returning to the point... these two distances must be calibrated accurately to one another. Any difference will manifest itself as front or back focus. This effect is probably undetectable to you for slower zooms, or even the longer manual prime, since I'm guessing that the aperture opening is smaller than f3.5 or f4 for that lens. It is a problem that is most likely only visible on *fast* primes - around f2 or less.

I had a similar problem on my k20d.

So, you have 2 options. You can udjust the AF under the custom menu for All lenses until the fast primes indicate focus corrently. This is what I did. All my other slower zooms seemed fine after that, but if yours don't focus okay after, you can return the AF bias to 0 for those specific lenses (only for AF lenses though... the M lenses will all be seen by the camera as the same lens). My guess is that any zooms and/or longer, slower primes are experiencing the same problem, but the larger depth of field is hiding the problem.

The second choice is to send the camera to Pentax for calibration.

Last edited by firefly; 08-21-2009 at 10:42 AM. Reason: my dreadful spelling...
08-21-2009, 10:58 AM   #13
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Guys, awesome, awesome info.

Here is the latest developement. After trying pretty much all of the recomendations, except for sending it in, I figured something out.

I tried the M-50mm again. This time I used Live View, got it as focused as I could through the lcd screen and took a shot. It worked. : )

The settings were 1/60th of a second, f1.4, 100 ISO, SR off, center focus sensor, natural and halogen lighting indoors.

This tells me two things. 1) the view finder needs to be adjusted to match the image sensor. 2) the focus sensor is matching the view finder.

I'll start doing some adjustments to the auto focus settings and see what happens.

Thanks again for the great input.
08-21-2009, 12:25 PM   #14
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I'm happy for you. By using live view, the sensor image is drawn to the screen, so you are eliminating the distance calibration problems from lens to AF sensors and lens to viewfinder. I hope you are well satisfied with your lenses
08-21-2009, 01:54 PM   #15
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QuoteOriginally posted by CraneGuy1 Quote
Guys, awesome, awesome info.

Here is the latest developement. After trying pretty much all of the recomendations, except for sending it in, I figured something out.

I tried the M-50mm again. This time I used Live View, got it as focused as I could through the lcd screen and took a shot. It worked. : )

The settings were 1/60th of a second, f1.4, 100 ISO, SR off, center focus sensor, natural and halogen lighting indoors.

This tells me two things. 1) the view finder needs to be adjusted to match the image sensor. 2) the focus sensor is matching the view finder.

I'll start doing some adjustments to the auto focus settings and see what happens.

Thanks again for the great input.
So what you are saying is that both the focusing screen and the AF sensors are not correctly alligned to the image sensor, but that the focusing screen agrees with the AF sensor.

To me, this would cause every lens to be focusing incorrectly on the body.
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