The non-linearity problem.

This is of interest to those who are contemplating the addition of a CPU to a pre-Ai-S Nikkor lens, meaning an Ai or a pre-Ai lens which has been modified to Ai specifications.

Because I have found that pre-Ai lenses are the ones I enjoy using most – the quality, fit and finish are in a different league from what followed – I have accumulated no fewer than nine. A related advantage is that these lenses are often seen as orphans in a digital world and sold as “use on film cameras only” owing to the absence of the Ai conversion. That keeps their prices low and makes them outstanding bargains in the process. The least I paid was $39 for the 200/4, the most was $252 for the 28/2. Most were around $125 and are all close to mint condition.

Of my collection, two came already converted to Ai specifications, five were converted by the estimable John White, and two I converted myself. As you can see from the picture, I have installed CPUs in each at $30 per lens, with the 28/3.5 and 50/1.4 being a challenge, all the others being a simple glue-on job.

Top: 24/2.8. Middle: 28/3.5, 28/2, 35/2, 50/2. Bottom: 50/1.4, 105/2.5, 135/3.5, 200/4.
A CPU has been installed on each. All are pre-Ai, with scalloped, metal focus collars.

When using auto exposure, I always use ‘A’ mode, meaning aperture priority. I set the aperture and the camera chooses the shutter speed. Because these pre-Ai-S lenses have non-linear aperture stop down geometry, I pass control from the camera to the lens for aperture setting by appropriately programming the CPU and setting the camera controls. Not only is this my preferred way of setting aperture, as I find the large ring on the lens far easier to use than the fiddly control wheel on the camera, it also avoids exposure issues. Plus it’s very much the way I have been using cameras for 40+ years and that’s how I like it.

When Nikon introduced the Ai-S lens design, the optics generally remained unchanged. What did change is that the mechanics were redesigned so that the aperture lever in the lens moved a constant amount from stop to stop. This allowed the use of Ai-S lenses with modern digital bodies which did not have the ability to pass control to the lens – like the D40 for example. The aperture could be accurately controlled with the body figuring out the extent to which the stop down lever had to be moved rather than by the user definitively dialing it in on the lens. Clever.

On the costlier DSLRs (D200, D300, D300s, D700, D7000, D800, D800E, D1-D4) Nikon made it possible to pass control to the aperture ring on the lens together with a ‘non-CPU lens’ option in the body for better EXIF data recording. That’s not as good as using a CPU, and I spell out the advantages of a CPU here, but it does result in proper exposure in ‘A’ and ‘M’ (but not in ‘P’ or ‘S’) modes when the aperture ring on the lens is used to set aperture. A fine way of extending the life of lenses many of which are approaching 50 years in age.

The upshot of all this is that if you let the camera set the aperture on pre-Ai-S lenses then you will likely run into exposure errors. To determine the extent of these errors, I set the camera to take control of aperture, setting each lens to its minimum aperture (just as you would with an AF or AF D lens) and setting the aperture using the control dial on the body. Then I used the simple expedient of running through the aperture range, using auto exposure in ‘A’ mode, determining which (if any) was correctly exposed, then adjusted the others in LR4 to match.

The result is not pretty, and the variations are all over the map. A ‘0.00’ reading means exposure was correct. Negative numbers show the number of stops of underexposure, positive ones the number of stops of overexposure.

Exposure errors with pre-Ai-S lenses when aperture control is passed to the camera.

Some of these variations have no discernible pattern whatsoever. Take a look at the 28/2 or 35/2 columns above, as an example.

With recent sensors, 1 or even 2 stops of underexposure is not a deal killer. With the outstanding processing controls in Lightroom 4, a significant improvement over Lightroom 3, you would be hard pressed to tell the difference between a RAW file which has been correctly exposed compared with one fixed after two stops under exposure. You might see a little more noise in the shadows, but that too can be fixed. But overexposure is a different kettle of fish. One stop over is close to being a deal killer, even with RAW, when it comes to bright highlight details. Two stops is a no-no on any but the lowest contrast, low key subjects. So that absolutely rules out the 200/4 which overexposes through most of its aperture range. The 28/2 is pushing the underexposure limit at f/16 and the 105/2.5 likewise at f/22. The 135mm is not a happy camper at full or minimum aperture and the 50/1.4 is not good at its two smallest apertures. You can decide whether these are debilitating issues for your method of use. The other lenses you can pretty much get away with, but you will constantly be tweaking exposure adjustments in post processing.

So why not just make Adobe lens correction profiles to compensate for these exposure errors? I suspect that is possible from my extensive work with Adobe’s excellent application, and you can download any number of profiles I have created here, each of which can be automatically recognized by LR or PS on import of files taken with a CPU-equipped lens. But profiles are applied upon import of a file into LR or PS, by which time the bird has flown, meaning that highlights which were burned out at the taking stage will remain burned out no matter how accurately the profile corrects the exposure error.

A range of exposures with the 28mm, f/3.5 Ai’d. Lens aperture control in top row,
camera control in bottom row. Camera in ‘A’ auto exposure mode.

The Bottom Line:

First, you must be comfortable using Manual Focus lenses, because every Ai’d, Ai or Ai-S lens is MF.

Then if, like me, you appreciate great engineering and execution, and further conclude that it makes you a better photographer (it works for me), and if you like a bargain and are not averse to a bit of tinkering, the Ai and Ai’d lenses are the optics of choice. Most will accept a CPU with the application of epoxy only and they are pure joy to use – touch, feel, optics are one, and as good as it ever was. I have demonstrated optical quality here time and again, and my standard for judgment is not an 800 pixel LCD display or test chart, but is rather a 24″ x 18″ glossy print. These lenses have fabulous optics.

Destroying classics:

As for ‘destroying’ classics, well that’s easy. Unless it’s something really costly and rare, these are so cheap that you can always find another. Millions were made and remain in circulation. If the installation of a CPU makes you a better snapper, forget the ‘classic’ issue and have at it. No one except you will ever see the CPU! Type “Nikon Nikkor lens Ai” into US eBay and you will routinely get over 1,000 listings, two-thirds pre-Ai-S, the remainder Ai-S. KEH lists hundreds more, generally costing a little more, but with no risk about returning a dud. Assuming that eBay items transact no more than once a year, there are some 50,000 new listings coming to the eBay market alone annually.

If, however, you like to snap in ‘S’ or ‘P’ modes and/or your camera body does not permit transfer of aperture control to the aperture ring on the lens, then you should avoid Ai and Ai’d lenses and search out Ai-S versions instead, if you propose to add a CPU. They typically run a little more, far fewer were made, and the lovely scalloped metal focus rings of the pre-Ai versions are replaced by plastic surfaces, but they will deliver correct exposure when the camera controls aperture selection and they remain far better made than any AF, AF-D or AF-S plastic monstrosity.

And for those interested, the optics you will most often find on my D700 are the 50/1.4, because it is so awfully good, and the 24/2.8, because that focal length largely accords with how I see. But then I must confess the 28/2 and 35/2 are real corkers, I love the 105 and 135 for what they can do and the 200 is a sheer delight. Well, you get the point.

A fine wide angle optic.

The aperture claw has been removed here, as it serves no purpose on modern digital bodies.

The 28mm f/2 Nikkor had a very long life, starting in 1970 and finally yielding to the Ai-S version, likely of similar nine elements in seven groups construction, in 1981. The Ai-S version continued through 2005, finally replaced by the AF D, which was recomputed.

Mine is the N.C. version, meaning multicoated, from 1974 and is in absolutely mint condition, claimed to have had but one prior owner, and ran me $252. It is a pre-Ai lens so I removed the rear flange – five Philips rather than slotted screws – pulled off the aperture ring and conferred the appropriate Ai relief on the rear of the aperture ring using a small flat file. Those less courageous can send any pre-Ai lens to John White who can convert it for use on the D700 and like bodies for a modest sum.

The floating element design first used in the 24mm f/2.8 is used in this optic, helping with corrections at closer distances.

In practice the lens’s definition falls in the extreme corners on full frame cameras through f/4 by when they almost match the very sharp center. By f/5.6 full sharpness is secured across the frame, with f/2 being perfectly useable for large prints and sharp as a tack in the center of the image, well toward the corners. Contrast is very high at all apertures (mine is the multicoated version, which probably helps) and I see no diffraction even stopped down to f/22 (but I do see all the crud on my sensor at that aperture!). Minor vignetting gradually disappears by f/5.6.

Quality of construction and the sheer tactile pleasure in use have to be experienced to be believed. Superb in every way. I have the HN-2 hood on mine though that’s actually intended for the 28/3.5. The HN-1 is the correct model, but the HN-2 works fine, with no vignetting fully open.

I installed a CPU in mine – a simple glue-on operation.

Lens correction profile:

Owing to the gradual drop in vignetting from f/2 through f/4, I have made the lens correction profile at four apertures – f/2, f/2.8, f/4 and f/5.6. You can download it here.

Some snaps from this optic appear here.

The final touch.

In earlier pieces dealing with the installation of a CPU in old Manual Focus Nikkors, I have explained most of the benefits of CPU use. Further, by making tailored lens profiles, I have made it possible to get the best possible results as regards freedom from vignetting, distortion and chromatic aberration.

The last frontier is optimal focus.

When using MF lenses on the D700, which is blessed with a large, bright finder and an LED confirmation dot, with arrows showing which way to turn the lens when it’s not in focus, things are pretty easy. The LED is fairly accurate and the addition of a magnifying eyepiece further simplifies regular old screen focus, if the dot is not to your liking.

However, get in really close at f/1.4 with something like the Nikkor-S 50mm, f/1.4 and any errors start to show themselves.

To check things out, I placed the camera on a tripod with a QR head (to permit accurate replacement) at a roughly 45 degree angle to a bookcase and four feet distant. Not surprisingly, the contents are solely photography books! One snap was made with the single center focus rectangle placed on the ‘O’ in ‘HORST’.

I then loaded the image in LR4 and looked at the enlarged version to see what was what.

Well, the LED dot was wrong. I have marked the focus point and the point of sharpest focus on the image below.

Red line is where the LED dot focus was made. Green dotted line shows where the image is sharpest.

The sharpest focus is closer to the camera than the point at which focus was aimed.

The CPU installed in the lens permits adjustment of the point at which the LED illuminates. You can move that point back or forward in 8 discrete steps in either direction, making a total of 17 settings including the default. The provided instructions use English poorly and it’s hard to divine which way to go.

Here’s the rule. After doing the 1-6-1 second programming setup:

• If you hit the shutter release when the top LED panel indicates F10, the LED will illuminate at a shorter focus point distance from the camera as indicated on the lens, meaning further from infinity.
• If you hit the shutter release when the LED panel indicates F20, the LED will illuminate at a longer focus distance from the camera as indicated on the lens, meaning closer to infinity.
• Each subsequent shutter release at F10 or F20 will move the LED illumination point further in the respective direction.

Now the LED is not that sensitive. In practice there’s a small range (“more or less right”) where it stays illuminated and I simply oscillate the lens’s focus collar either side until I am centered on the focus range for the LED illumination point.

In the above example the LED was illuminating on a point too far from the camera as optimal focus was closer than the point at which the LED was aimed. Thus I needed to move the LED illumination point away from the camera, meaning that I operated the F20 switch in the programming instructions once.

I then refocused the lens on the ‘O’ in ‘HORST’ and took a second picture. Here is the result:

Red line is where the LED dot focus was made. Green dotted line shows where the image is sharpest.

Bingo! Bang on. The LED illumination and best focus point are identical.

Just to confirm, I then cycled the F20 one more time, moving the LED illumination point yet further away from the camera, with the focus collar set closer too infinity to illuminate the LED at the same point in the identically framed image.

Red line is where the LED dot focus was made. Green dotted line shows where the image is sharpest.

In the above example the LED was illuminating on a point too close to the camera as optimal focus was closer than the point at which the LED was aimed.

So I went through one F10 cycle to revert the optimal LED illumination point, as in the second picture above.

In practice, it’s really only necessary to do this with wide aperture lenses used at close distances, or extremely long focus lenses which have very shallow depth of field, like the 500mm Reflex Nikkor. In these circumstances, you must remember to do your critical focusing using the LED light not the focusing screen if you have found it necessary to move the LED illumination point using CPU programming.

But this absolutely puts the icing on the cake for fast and/or long MF Nikkors. For AF Nikkors (AF D or AF-S) a like functionality exists in the body of the camera in the rear LCD menus. With AF Nikkors the camera ‘remembers’ the setting for each lens; with MF + CPU lenses, each lens stores its own setting in the glued-on CPU. This does not address the case where the photographer wants to use the same MF + CPU lens on different Nikon (or Fuji) bodies. In that case, if the bodies differ significantly, it’s best to have the factory fix the body rather than trying to fix the lens.

I have only found it necessary to move the focus point on three lenses – the 50/1.4, the 85/1.8 and the 105/4 Micro – of the dozen I own. All the others are spot on including the 500mm f/8 Reflex! Why these two lenses should differ, I do not know, but differ they do, as multiple tests have consistently confirmed. Indeed, I only really started to see what the 105/4 Micro-Nikkor could do after this focus fine tuning process. It is ‘scary sharp’!

Is this all worth the effort? If you use the lens at f/1.4 or f/2, just check this out.

By the way, to learn more about Horst, one of the greatest fashion photographers of the twentieth century, click here.