G2 Focusing Accuracy

keoj

Active Member
Quick question on G2 autofocus. For soem on this forum, this will be an easy one. If I am trying to measure focusing accuracy, where does one measure from? As an ex&le, if I see in the DX LCD a measurement of 1.72 meters, where is plane in the camera that distance is measured to...the film plane(which is the same as the paralax that the AF sensors are set to) or is it something else? Thanks.

Joe
 

coodeville

Well-Known Member
I didn't look at my G2 yet since it's not here at work. My Nikon has a mark indicting the film plane. Does the G2 have the same thing?
 

coodeville

Well-Known Member
I was right! I never had a reason to know this about my G2 and I'm sure that I've looked at it a million times. But I'm glad our friend has had his question answered!
 
D

ddai

Joe,

If my memory right. The distance value on the LCD, according to the manual, is the distance between subject to the film plane. It's NOT necessarily the AF sensor plane coincides the film plane, as long as the perpendicular distance between two planes in known. The placement for the AF sensors is compromised with the body design.

The sensors detect the distance from the subject to the sensor plane (relatively easier 'cause only simple double-convex is used where principle points are fixed) based on contrast phase difference, then calculates the subject distance to the film plane.

Rgds, David
 
D

ddai

Colin, Yes G1 does have the mark for the film plane and the distance reading. Rgds, David
 
D

ddai

"I didn't look at my G2 yet since it's not here at work. My Nikon has a mark indicting the film plane. Does the G2 have the same thing?"

K Michael,

If your Nikon takes zoom lens, distance marks on the lens will be the distance from the subject to the lens front element.

Rgds,
DAvid
 

coodeville

Well-Known Member
I've measured from the film plane to the subect and then set the lens to the correct distance mark. The pictures have always been in focus.
 

afranklin

Well-Known Member
David,

> If your Nikon takes zoom lens, distance marks on the lens will be the > distance from the subject to the lens front element.

What do you base that claim on? Perhaps it's true, but it would be contrary to what most manufacturers/lenses do.

Regards,

Austin
 

wilsonlaidlaw

Well-Known Member
Logic would dictate that the focus distance must be to the film plane on a G2 otherwise what on earth is the point of marking the film plane on the top of the camera. A typical use could be data back, time interval exposure shots, where you could dial in a manual focus distance, which you would measure to the film plane. The read out would not necessarily be to the sensors, as I think someone else has explained before, it would be very simple in the distance computation firmware, to have an algorithm, which added a constant to the sensed figure. Wilson
 

coodeville

Well-Known Member
About three hours ago, I set up my F100 along with a 50mm lens. It used a flower pot as my subject. I measured the distance between the film plane indicator and the flower pot. I set the lens to the measurement. I never looked through the veiwfinder at all and I turned off the AF. The negative was in perfect focus.
 

coodeville

Well-Known Member
I wonder why my Fuji S2 Pro II has the CCD plane indicator marked in black? My other Nikons are white. You can barely see the Fuji's.
 
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ddai

Austin,

I apologize for not being accurate. What I mean here is that whether the distance reading is based on the front element or film plane depends on the manufacturer and the operatiing manual. For those sophisticated AF Zoom, more than often its front or rear nodal point or both are not outside the lens elements. To have a more attractive spec, they claim the closest focus distance based on the front element. Please image how can a lens focus at 25cm to the film plane while the lens itself is 28cm long. For some macro lens (Tamron SP90mm Macro f1/2.8), the second principle point is farther than the first one to the film plane (according to the diagram that comes with) to achieve a 1:1 magn. ratio.

Just a reminder for those users that will take that for granted. Always refer to the manual not personal judgement.

For the statement for Nikon zoom I had tested the 28-135 AF Zoom some years ago (now I don't own them any more). Accidental enough I picked up Contax was because I read the history of Nikon and found their fundamental designs were from CZ.

Regards, David
 
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