Focusing Accuracy

[pfogle]

I just tested my lenses for accuracy of focus. I only tried one thing - a ruler on the floor 6 feet from the camera, at 45 degrees. Lens wide open, on a tripod.
Results:
35/1.7 Ultron 3.3 inches front
21/2.8 Avenon 8.4 inches front
28/1.9 Ultron 0.6 inches back
50/2.0 Hexanon 2.2 inches back
35/2.5 C-S PI 2.8 inches front
21/4.0 C-S 6.1 inches front

These are line-of-sight distances, as you would see on the scale on the lens barrel, not the distance along the floor.

interesting, no? I haven't really got a clue what to make of this. The focus seems repeatable, so it's not random; it's both front and back, so it's not a systematic R/F bias. And they all show coincident rangefinder images at infinity. All the adapters are CV, and seem to be identical.

Anyone got any thoughts? Any input welcome
Phil

[pfogle]

I re-ran the test, with the following result:

35/1.7 Ultron 1.7 inches front
21/2.8 Avenon 7.8 inches front
28/1.9 Ultron 0.6 inches back
50/2.0 Hexanon 1.1 inches back
35/2.5 C-S PI 1.1 inches front
21/4.0 C-S 2.2 inches front
I also included a Summar which focused 2.2 inches in front.

Two things... I seem to be getting better at focusing! And the 21/4 did much better the second time, perhaps because wides are more sensitive to accurate focusing (since this is about depth of focus, not depth of field). There does seem to be a tendency to front focus, but not much, and probably focusing errors are just as significant. The Avenar is still off, though, and I'll have to think of some way round that. Seems to me that changing the thickness of the adapter is going to put the infinity setting off for the R/F. Maybe I could split the difference and live with it, since I'll probably never use it wide open.

Phil

[Sukiari]

Hi. I just got my R-D1 last week. It's my first interchangeable lens rangefinder camera. Previously I had an Olympus XA.

Since I didn't have any screw or M mount lenses when I bought the camera, I got some Cosina Voigtlander lenses, namely the 15, 21, 28 f/3.5, 40, and 90 lenses. I checked the ability of the rangefinder to accurately focus them at a variety of distances. Luckily, my R-D1 is dead on with every one (except the 15, of course).

I only have one screw mount adapter as the rest of the gear cleaned out the camera fund. It may be that some of your adapter rings are off, or more likely your R-D1 has a misaligned RF.

There are a couple of other issues to consider as well. The moving frame lines sometimes encourage one to move the eye a bit as well, moving the RF patch out of the center of the eye's field of view. Also, sometimes there is a bit of focusing throw between where RF images converge and diverge again. I try to be closer to the point of divergence than convergence, because at typical distances the depth of field is twice as deep behind the plane of focus than it is in front.

[jlw]

Any chance you could snaffle a Nikon focusing magnifier, put it on your R-D 1 eyepiece, and try a couple more rounds? I've found that my own focusing repeatability isn't always as good as I'd like!

There does seem to be a distinct statistical pattern in that lenses generally are "off" the same way (front or back) and that some are always off more than others, but there's quite a bit of scatter in your data set. There are some potential mechanical sources for this, but I'd guess that the biggest one is the little ol' wet human eyeball!

(If it makes anyone feel any better, years ago I did a bunch of similar tests on film cameras, and found that they all also showed some variability in focus accuracy -- even though ALL of them produced results that looked in focus! I suspect that part of the reason we're all discussing this more with the R-D 1 is that having digital files makes it so EASY to conduct focus-accuracy tests!)

As to what to do about the Avenon, I've been thinking about that this morning and think that you may in fact be able to deal with it by using a thinner adapter, without much (if any) penalty in infinity-focus accuracy. I'll need to develop my idea at more length than I can do here (I'm at work and surfing RFF between job requests) but will try to write up my thoughts and post them this evening.

Basically, the gist of the idea is that changing the adapter thickness by amount X doesn't change focusing accuracy by the same amount, because it displaces the RF cam by the same amount as it displaces the lens; in effect, the RF will compensate for some of the change in adapter thickness. The amount of actual focus shift you get will depend on the ratio between the focusing movement of a 50mm lens and that of the particular lens you're using.

I'm hoping I can actually document this by measurements with a dial indicator; will take some time to do it, though.

[pfogle]

jlw...

I have rubbed the adapter down a bit with fine grit... more to make sure any burrs were taken off. I'll never be able to thin it down this way! It seems to have marginally improved things.

you're right about the variability of the ol' eyeball there. I did a few ad hoc things for fun... took a wide open shot of some books, and found very little difference in visual sharpness when focusing 6in in front and behind the rangefinder optimum. This is partly because the aberrations mask the focus shift.

Then I put the CV and the Avenon on the legs and did a shot of my room at all the apertures. Surprisingly, there's very little difference visible between them at the same apertures, at 100% on the screen. There's a touch more bite to the CV down to 5.6-8 but they're very close. I was surprised to see that the CV was a bit for prone to flare. There's a bit less vignetting on the Avenon, but not as big a difference as I expected.

So, all in all, I feel happy with the new lens. Tomorrow I'll be off to find a decent lens shade for it!

Phil