Re: Voigtlander Bessa RF 6x9
Posted: Mon Jan 22, 2018 10:23 pm
A reversed element would give you center sharp, with the focus falling off towards the edges, especially at wide open aperture. And it usually involves flipping the middle element on a triplet, or the second element (could be a cemented doublet) on a four element lens.
If it's too difficult to make a mark on the front element side and lens barrel, I just note what letter or symbol on the name plate of these older front focusing lenses winds up at Top Dead Center (TDC) when it separates from the barrel. Usually it's not necessary to do the same with the rear element, just insure it doesn't get flipped.
For checking focus, I use a piece of clear plastic from something like a CD case, cut to fit the frame space over the inside rails (usually, there will be two sets of rails, the inner ones are for holding the film at the focus plane, and the outer two are for keeping the film in register). I put some frosted tape on the side that will face the lens. This acts as a ground glass surface, and gives you something to focus on. Mount the camera on a tripod, and open the lens at maximum aperture on T or locked on B. Look at something high contrast, like a wooden fence line or trees at least 30 meters (100 feet) away (a bit farther doesn't hurt, it's not a hard number), and start turning the front element until it is in focus. As Julio has suggested, whomever did the cleaning of the shutter may have not checked the focus after putting the lens back together, so you may have to try different landings until you hit on the right one. Then put the focus ring back on at the Infinity mark, and clamp it down with the set screws, trying not to move the front element from it's Infinity focus position which you have just determined. Then check the near focus to make sure it is good. If not, then refocus the Infinity setting until you get a decent close image (that's physics for you). Maybe choose a target that is farther away for the Infinity setting. You'll get there eventually.
Then of course you need to see if the Infinity setting agrees with the rangefinder, and also the close focus setting. It can be a long, drawn out affair, or you might hit it right on the first try, Alex.
PF
If it's too difficult to make a mark on the front element side and lens barrel, I just note what letter or symbol on the name plate of these older front focusing lenses winds up at Top Dead Center (TDC) when it separates from the barrel. Usually it's not necessary to do the same with the rear element, just insure it doesn't get flipped.
For checking focus, I use a piece of clear plastic from something like a CD case, cut to fit the frame space over the inside rails (usually, there will be two sets of rails, the inner ones are for holding the film at the focus plane, and the outer two are for keeping the film in register). I put some frosted tape on the side that will face the lens. This acts as a ground glass surface, and gives you something to focus on. Mount the camera on a tripod, and open the lens at maximum aperture on T or locked on B. Look at something high contrast, like a wooden fence line or trees at least 30 meters (100 feet) away (a bit farther doesn't hurt, it's not a hard number), and start turning the front element until it is in focus. As Julio has suggested, whomever did the cleaning of the shutter may have not checked the focus after putting the lens back together, so you may have to try different landings until you hit on the right one. Then put the focus ring back on at the Infinity mark, and clamp it down with the set screws, trying not to move the front element from it's Infinity focus position which you have just determined. Then check the near focus to make sure it is good. If not, then refocus the Infinity setting until you get a decent close image (that's physics for you). Maybe choose a target that is farther away for the Infinity setting. You'll get there eventually.
Then of course you need to see if the Infinity setting agrees with the rangefinder, and also the close focus setting. It can be a long, drawn out affair, or you might hit it right on the first try, Alex.
PF