Thursday, August 9, 2018

Focus Confirmation

Checked and Double Checked


As mentioned in an earlier post, I've been working through the steps to properly measure infinity focus with the Pentax 67 400 mm EDIF lens.

I've been able to properly measure and confirm that the lens will reach infinity focus and that the Stiletto device is properly calibrated.


I knew that I'd need to find third method to check focus without having to go through the process of developing film.  What I opted to do was to use the lens as an intermediary device.  I estimated that if the lens was focused at infinity using the Stiletto device and then I checked that setting on the digital camera with using a bahtinov mask, then recentered focus with that bahtinov mask, and then confirmed how the Stiletto appeared, I should be able to better understand how close the system is to being true.

I was also considering taking a picture of the projected star image with the digital camera via a long exposure since I can't see the dim stars.  While this seemed like a great thing to do, it was difficult to accomplish because of the inability to see an image on the focus screen of the 67 with the dslr.  I'll have to check during the daytime if the image on the ground glass is seen at infinity or at the surface of the glass.

What I found was that using the Stiletto got the focus point very close.  The bahtinov mask showed a very slight offset.  When the mask showed ideal conditions, I checked again with the Stiletto and found that there is an undetectable middle spot where the screen does not provide resolution.  The focus was actually centered, but my eyes don't perceive the difference.  Maybe if I jiggled the telescope to observe a shimmer effect, but at this point, I know the technique works and is repeatable.

What this means to me is that I can use the Stiletto on any lens to get close focus, then get the last bit with the bahtinov mask.  The speed at which this is done means that I can be confident to refocus in the middle of the night as necessary.

To better check infinity on lenses that do not have their own tripod support, I'll need to carry another camera mount (like a ball-head) to do the check with the dslr. 

Another curious thing was found by doing this task.  In the past, when focusing via star images on the ground glass, I had been pleased with seeing star images projected in the 67.  However, it seems that my eyes were not able to see the small star sizes of a properly focused image.  Thus, the brightest apparent star images were actually defocussed.  Maybe they needed to be spread out across the grains of the ground glass? 

I'm happy that this procedure has worked so well.