Measuring Distance

[sanyasi]

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In testing lens and rangefinder accuracy, do you measure the distance from the front of the lens, the rangefinder window, or some point inside the camera (the front of the sensor, for example)? I saw a reference in another post that suggested some point in the camera, but that suggestion was not seconded by others and seemed to be ignored in other posts.

 

Thanks

[digitalfx]

to the sensor

[adan]

As said - from the sensor's image plane (which is not the front surface of the cover glass, but the surface of the silicon behind the .8mm thick cover glass).

 

Some cameras have a focal plane index mark that looks like kind of like a Greek letter phi with the line straight instead of slanted. Visible on the right side of this Sony prism beside the LCD:

 

http://a.img-dpreview.com/reviews/SonyDSLRA900/images/inhand2.jpg

 

For the M9, my trusty micrometer puts the image plane level with the middle of the fifth number in the serial number on the hot shoe, e.g. 3822_2_22, or the back edge of the smallest round contact on the hot shoe.

[digitalfx]

unfortunatley M lenses are so small they dont have witness marks, which makes it difficult to accurately judge exact distances. The best way to check rangefinder accuracy is to shoot a ruler at a slight angle away from the lens wide open and then compare your focus point with the image recorded.

 

here is a fancy device that does the same thing:

LensAlign® Focus Calibration System

[Guest mc_k]

27.8mm from flange marks the spot???

[lars_bergquist]

27.8mm from flange marks the spot???

 

Right-o.

 

The old man from the Age of the Folding Rule

[rob_x2004]

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And when you arent measuring cubits you straigten it occasionally or leave it bent at the bar:confused:.