Back Focus Revised - New Info...

Started Dec 29, 2005 | Discussions thread
Rinus Borgsteede Veteran Member • Posts: 3,207
I am not wrong, just more cautious

What I propose is not a theory, it is fact! It is simple optics and geometry.

What I do recognize is the fact that the VF mirror moves in an arc so the position of the autofocus mirror is only out of position by a very small margin that stays well within the depth of field of even a 1.8 standard lens. No matter, the geometry shows clearly that the word "within" suggests a point away from "correct".

Adjusting the VF mirror will byond a shadow of a doubt afffect focus on points above and below the center of the adjusted focus unless it is exactly at 45 degrees to the groundglass. It simply must or my 4x5" tilts and swings adjustments were wrong at least for my entire carreer.
Geometry and optics are a funny thing, they adhere to laws.

One aspect that was never mentioned was obvious focus shift in any (yes any lens) when used wide open. Another variable that distorts accuracy in personal observations. Some lenses have very little of this focus shift and some lenses where tested for that by various magazines.

The Leica rangefinder always claimed better focus accuracy due to focus shift and some depth of field visible in SLR systems which the geometry based Leica focussing system did not have.

No, I do not think you have to revisit the methods in the adjustment web page but I do know that geometry and optical laws do not compromise. Based on this, the order is best to do Cam2 first or not at all.

If anyone has doubts about causing keystoning, a new test target could have + markings in a narrower area with a similar area at the top and bottom of the page. A level can be used to hang the target and a tape measure can be used to measure the center of the target so that is is at the same height as the center of the lens. Than tilting the camera and centering the target inside the viewfinder (and repeat) will align the camera correctly for checking the top and bottom of the focussing plane.

In lenses with a curved focussing field, the focus error at the top and bottom should be similar if not identical to each other but different from the center. If they are different from top to bottom, the mirror is no longer at an exact 45 degrees to the focussing groundglass.

I one cannot acertain these conditions it is better to refrain from making any adjustments to Cam2 at all for my previously stated reasons. Cam1 can always be adjusted to correct for autofocus errors as it is easy to change again and again.
Rinus of Calgary/Kelowna

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