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11-05-2007, 01:19 PM   #1
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SMC-M 50/1.4: focus in viewfinder and focus on frame don't match

I recently acquired an SMC-M 50/1.4, and as I'm new to manual focus lenses, I don't know what to make of a focusing issue I'm having with it on a K10D.

I set the focus to MF, the dial to M, set the aperture on the lens, stop-down using the green button, focus until a) it looks in focus or b) hear the focus indicator beep (virtually always the same thing), then release the shutter. At open apertures, particularly f/1.4-2.8, the resulting frames are consistently focused further away from me than expected. For example, looking down at a fence post and focusing on its very top (as indicated both by the focus indicator AND my contact-lens-corrected vision), with the ground beyond clearly out of focus and an aperture of 2.8, I release the shutter. Reviewing the shot both on the LCD and in Lightroom, I'm getting the ground beyond in focus and the fence top out of focus. But I focused on the fence top!

The same shot with the kit lens doesn't show this, although the DOF from that lens certainly makes the comparison more difficult. The M lens at 5.6 still exhibits a shift in focus towards the background. What's causing this? Poor technique? I'm pretty steady with the camera, and I certainly don't consistently lunge toward the subject as I press the shutter. I don't even know how this might be occurring optically. Any other users noticing this, or do I just need more practice with manual focusing?

Kerey

11-05-2007, 01:26 PM   #2
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PS I should mention that although I may be new to manual focus lenses on an SLR, they're not foreign to me. My previous lens was a Rodenstock APO-Sironar N 210mm/5.6 mounted on a Cambo monorail. Where did my ground glass go? Of course, f/1.4 wasn't much of an option in 4x5, but it isn't much use now either if I can't get the plane of focus positioned properly while wielding my K10D.
11-05-2007, 01:37 PM   #3
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I have same issue. My understanding is that the K10D simply doesn't have a very accurate focus indicator (designed for slow zooms) -- especially under incandescent light -- this issue is documented all over the web. I sold my smcp-fa 50mm 1.4 because it would not focus correctly. I replaced it with a Pentax-m 50mm 1.4 so I could at least manually focus. The autofocus focus ring simply moved too fast to be useful.

Secondly, at f/1.4 the DOF is incredibly tight. I was fooling around and I noted that the difference between getting the tip of the nose in focus or the eyes from a distance of about 10 feet is an incredibly small movement of the focus ring -- sort of like gently stroking the focus ring without even really turning it.

My technique goes like this. from the prospective of holding the camera and taking a shot, I turn the focus ring to the left until focus indicator comes on. I then move it past that until the focus conformation goes off. Then I hold down the shutter release (I use the focus trap method) and slowly turn the focus ring to the right until the shutter fires.

That "side" of the focus indicator seems to give the the best results. If I go from the other "side" I get consistent out of focus images.
11-15-2007, 07:48 PM   #4
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Thanks for the suggestion re. technique. 'Coming in from the left' helps quite a bit but doesn't resolve the problem completely. The back focus ceases to be an issue around f/8, but that defeats the purpose of having a fast prime.

My question is this: ignoring for a moment the inaccurate focus indicator, How is it that a manual focus lens, optically focused through a clear viewfinder by a user with corrected vision, consistently back focuses? My impression was that SLRs showed ~exactly the view to be captured (not counting % coverage, etc.). I'm doubtful something in the camera is out of alignment, as there's a visible difference between the focus point of the kit lens and the 50/1.4 at f/5.6.

Any ideas on what/who could be the culprit?

Kerey

11-15-2007, 07:56 PM   #5
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Have you adjusted the diopter on your viewfinder? Maybe it got knocked off a bit, and you didn't know, it has happened to me a time or two !
11-15-2007, 08:11 PM   #6
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QuoteOriginally posted by Stratman Quote
Have you adjusted the diopter on your viewfinder? Maybe it got knocked off a bit, and you didn't know, it has happened to me a time or two !
Greeting ALL

Does not focus indicator come on regardless of diopter setting ?

andy
11-15-2007, 08:22 PM   #7
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QuoteOriginally posted by kerey Quote
The same shot with the kit lens doesn't show this, although the DOF from that lens certainly makes the comparison more difficult. The M lens at 5.6 still exhibits a shift in focus towards the background. What's causing this? Poor technique? I'm pretty steady with the camera, and I certainly don't consistently lunge toward the subject as I press the shutter. I don't even know how this might be occurring optically. Any other users noticing this, or do I just need more practice with manual focusing?
This is one of the biggest failures of modern cameras. It isn't just Pentax - Canon and Olympus are notoriously worse in this regard.

The focus screens are made to focus and meter with the kit lens, which is a very slow lens indeed compared to your 50. It is even worse with my 85/1.4.

Unfortunately the only remedy is a dedicated manual focus screen (Pentax does not make one) or a split-prism screen which you can get from third-party vendors. The problem with the split-prism screens is that they mess up exposure. Pick your poison, I suppose.

QuoteOriginally posted by Stratman Quote
Have you adjusted the diopter on your viewfinder? Maybe it got knocked off a bit, and you didn't know, it has happened to me a time or two !
If the diopter wasn't adjusted correctly, nothing would be in focus. It is the inherent DOF of the screen itself that can confound manual focusing with fast lenses.
11-15-2007, 09:07 PM   #8
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QuoteOriginally posted by carpents Quote
This is one of the biggest failures of modern cameras. It isn't just Pentax - Canon and Olympus are notoriously worse in this regard.

The focus screens are made to focus and meter with the kit lens, which is a very slow lens indeed compared to your 50. It is even worse with my 85/1.4.

Unfortunately the only remedy is a dedicated manual focus screen (Pentax does not make one) or a split-prism screen which you can get from third-party vendors. The problem with the split-prism screens is that they mess up exposure. Pick your poison, I suppose.


If the diopter wasn't adjusted correctly, nothing would be in focus. It is the inherent DOF of the screen itself that can confound manual focusing with fast lenses.
Well this is sad news indeed, and seems to fly in the face of the enthusiasm for manual focus lenses of some digital users. Is this not an issue for others? I certainly never ran across reference to it while researching the purchase. As a former large format user, the idea that the focus on one's viewing screen doesn't match that in the image reminds me of... P&S cameras. I think I may have to sell the lens and save for either the FA50 or maybe that beautiful 50-135.

Kerey

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