The 51.6% solution

This is just a quick note on a technical problem that plagues digital Leica cameras when used with older Nikkors: back focus. It is gratifying to know that Leica has finally recognized that many of its lenses don’t work so well on digital Ms due to “focus errors” that allegedly compound over the years. The real reason is probably more that film planes are actually and unintentionally curved, and a lens that makes the grade at the center there back-focuses elsewhere.

I was struggling a bit with a 10.5cm f/2.5 Nikkor, which though absolutely lovely aesthetically is one of the worst-engineered Leica lenses ever from a mechanical standpoint. And it back-focused. It back focused more with some Leica M adapters than others, but still.

Strike one with this lens is that the aperture unit rotates along with the entire optical unit. This means that if you adjust the collimation washer (for reasons I don’t fully understand, it’s always 0.05mm needed with any lens – just about the same thickness as Scotch tape), you also then have to reset the aperture ring to read properly. Also not 100% sure that infinity optical focus was really the problem.

Strike two is that the amount of front cell movement needed to compensate for back focus is absurdly great. So here, you’re messing around with focal length, but this the same way the MS-Optical Sonnetar gets calibrated…

Strike 3 is that the RF cam is not adjustable at all, with the tab pushed by a plunger running on a wheel that fits in a spiral track in the helicoid. Guess how this tab was adjusted for infinity at the factory? With a file. It makes sense, in a way. Calibrate the fixed infinity point on the focal plane by shimming the optical unit, calibrate focus at infinity by grinding the RF tab, and fix close focus by shimming the front cell. But it utterly sucks when you find out, 60 years later, that the tolerances that looked good on film with a Leica IIIc look like holy hell on digital.

So when you are dealing with focus errors, you have to imagine that the standard is a 51.6mm lens. At that focal length, if the RF matches the film-plane focus, the focus will always be correct, even if the infinity stop of the lens is beyond “infinity” on the scale.

For a telephoto lens, the rear cam still pretends it moves like a 51.6mm lens, but the actual optical unit moves much further. Hence, in a lot of cases, you can simply use a thinner LTM adapter (I think I’ve written about this before… somewhere). Most cheapo ones are thinner than the 1.0mm they are supposed to be.

But there is a different way to hack this with the 135mm, 105mm, and 85mm Nikkors: simply apply a thin and even coat of clear nail polish to the RF tab on the lens. This is a trick that you could theoretically do with lenses that have a rotating RF coupling ring (not tab), but it works exceptionally well with the Nikkors because the camera’s RF roller simply rests on the tab and doesn’t roll along it. This means that you only need to get the coating thickness right over a very short distance. Materials needed:

• Sally Hansen clear top coat (not “nail nourishing,” just the hard kind).
• CVS Beauty360 brand Nail Polish Corrector Pen (essentially a marker full of acetone that you can use to thin or remove extra nail polish).
• LensAlign focusing target (if you own a Leica, you really want one of these anyway, just to figure out what the devil all your lenses are doing as you stop down).
• Reading glasses.

So basically all you need to do is put a very thin coat of polish on the polished surface of the tab. Let it dry for 20 minutes. Here is the goal:

• At f/2.5, your focus should be such that the 0 point is barely focused, with most of the DOF in front.
• At f/2.8, your focus should be dead-centered around 0. The lens is actually way sharper here than at f/2.5. Doesn’t seem like much of an aperture change, but it is.
• At f/4, your focus will be such that 0 will barely be in focus, with most of the DOF to the rear.
• From f/5.6 down, the DOF will grow so that 0 is always in focus.

If it works, you’re done. The focusing errors this might induce further out are subsumed by depth of field increasing. If you need another coat, add one. If you are now front-focusing too much, use the Corrector Pen to remove some of the extra (or use a very fine nail buffer to remove some).

Never file or try to grind down the tab if your lens is front-focusing. Unless you can do it totally square, your lens will behave differently on different cameras. Leave this situation to a pro.