I have Hasselblad 500C camera body (not C/M) manufactured in 1965. The focus screen is held on place with four screws.
I ordered the Bright Screen focusing screen and will replace the original one on the weekend.
Needless to say, I don't have any kind of specialized service equipment such as collimator or other optical instruments to actually adjust the focus precision. But I do have my CFV 100C digital back which I will use for my advantage here.
Any tips how should I do it?
I was thinking about securing the camera to a tripod and make sure it is in level in all three dimensions. I have spirit level for that. Then, at a usual distance I would use the camera to photograph things, such as 3 meters, I would attach some kind of calibration sheet, be it a book cover, bills, something with fine detail. Then at the largest aperture of the lens (I'm thinking of using 150mm f/4 here) I adjust the focus with the new focusing screen, take a picture, and immediately review it on the back screen of CFV 100C. If the focus is off, I take it apart, adjust the height of the focusing screen and repeat, until I get properly focused image.
I know that 100C does not cover the whole 6x6 frame, but I assume it is critical to achieve the center focus?
I was also thinking to take a picture of something at far distance (if not at infinity) placing the subject at the edges of the 100C framelines. Focus with focusing screen and take four pictures, examine the achieved focus between them and possibly adjust individual screws to attain proper focus at each corner.
I had some spare time yesterday evening and did the change.
I ordered the screen with 44x33 rectangle to make it easier to compose with CFV 100C.
I lost one of the four screws inside the camera body, I don't mean in the mirror box, but somewhere between the outer housing and the mirror box.. Took me a while to get it out there, I had to search for service manual and figure out how to take it apart, but I managed to shake the screw out without completely separating the mirror box from the chassis.
Installing the new screen was easy. There was instructions included with the screen how to adjust the focus by focusing to infinity and they actually worked quite well. Digital back was a huge help here, though. I did the final fine-adjusting by reviewing the digital pictures and adjusted the screws little by little iteratively.
I'm not completely sure the focusing screen is perfectly aligned but I don't know if it does matter that much. I aligned it by eye to the edge of the waist level finder grooves where it slides but I don't know for sure that each corner are perfectly aligned at equivalent distance to the mirror. The microprism at the center of the screen works just fine.
It is so much brighter than the original that I can actually focus it precisely. The original one can be "ballpark focused" but with the resolving power of 100 mpx it was hard to tell at the field if the focus was "spot on" or "just ok". I still do need the magnifier of the waist level finder, though. But now I can focus it roughly without the magnifier and then just check and fine-adjust with the magnifier.
