Front/back focus, does that apply to manual focus lens?


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whoelse

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Oct 17, 2003
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#1
Hi, As per title, what's your experience?
 

scorpioh

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Jul 17, 2007
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#2
Yes. It does. Basically it's when what seemed to be in focus through your view-finder does not turn out to be in focus when projected on your sensor.
 

creampuff

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Jul 11, 2006
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#3
Yes. It does. Basically it's when what seemed to be in focus through your view-finder does not turn out to be in focus when projected on your sensor.
That can happen if you change the stock focusing screen to a third party one. The replacement screen may be too thick or too thin in cross section or if an incorrect shim is used.

But if you're using the stock focusing screen and it coincides with what the sensor records as sharp, then there really can't be such a thing as back or front focusing because any focus error lies with the operator.
Anyway the first thing I suggest is to make sure the viewfinder diopter correction is set to your eyesight first before you do anything.
 

whoelse

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Oct 17, 2003
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#4
"But if you're using the stock focusing screen and it coincides with what the sensor records as sharp, then there really can't be such a thing as back or front focusing because any focus error lies with the operator."

I'm not sure about this. I have K10D, I use the hack firmware to calibrate but it seems like different lens give different results. So the calibration is not 1 calibration fits all solution. So why when the composition appear in focus but the end result is not? Does that mean the CCD/CMOS sensor distance and viewfinder is not accurate? If this is the case then it should be consider manufactoring problems because it is out of specification. If the AF is result to some kind of chips in the lens, why does the image appear to be in focus in the composition? Maybe the chip should be in the camera body instead of the lens and manual focus lens should not have focusing error since there's no chip in the lens.

But easy, I do not have clear understanding of the focusing technology just some doubt and qns.
 

darrrrrrrrrr

Senior Member
Sep 19, 2006
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#5
That can happen if you change the stock focusing screen to a third party one. The replacement screen may be too thick or too thin in cross section or if an incorrect shim is used.

But if you're using the stock focusing screen and it coincides with what the sensor records as sharp, then there really can't be such a thing as back or front focusing because any focus error lies with the operator.
Anyway the first thing I suggest is to make sure the viewfinder diopter correction is set to your eyesight first before you do anything.
hmmm is TS referring to the AF confirmation beep being miscalibrated? like if you rely on the beep to determine accurate focus (regardless of stock or third-party screen), could the phase detection AF module thing be slightly off for that lens and beep when the subject is actually back/front focussed? in this case both the screen and sensor are OOF, just the AF module reporting sharp focus.
 

FIR

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Jan 3, 2005
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#6
Good info :D

Actually when I mean in focus, I mean the image in the composition viewfinder looks in focus include AF comfirmation light but image output have back/front focus.
It is not Pentax alone, u see the same complain (not every slr) for N & C too. So there must be a better way to do AF. I remember in LX film slr they have TTL OTF (Through the lens, Off the film plane metering) metering SPC cell embedded underneath the main mirror box, image stabilizer has also evolve from lens to body it would be a good advancement to move the phase detection AF module into the body too for good :)
 

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