Converting Pre-Ai Nikon Lenses

What is an Ai lens? How can I convert my pre-AI lens? This short article attempts to explain all!...

13 days ago

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Introduction

Nikon FE with Ai converted Nikkor-S.C Auto 50mm f/1.4 lens mounted.

After spending the last year buying, trying and then selling all manner of film cameras and lenses, I seem finally to have settled on Nikon as my favorite manufacturer of both. Why? Because the cameras are practically bomb-proof and the lenses are of the highest quality (Nikon started as an optics company after all!) and render beautifully, both on film and when adapted to my Sony Alpha7 Mk.II mirror-less digital camera. So, today I find myself to be the proud owner of both Nikon F-301 and Nikon FE camera bodies as well as Nikkor 24mm f/2.8, Nikkor 50mm f/1.8, Nikkor 85mm f/2 and Nikkor-S.C Auto 50mm f/1.4 lenses.

One of the joys of the Nikon system is that the lens bayonet mount, known as the "F-Mount" hasn't changed since Nikon introduced their first SLR camera body (the Nikon F, hence the name for the mount!) in 1959! This means that, with a few exceptions, every SLR lens Nikon has ever made will fit on any (D)SLR camera Nikon has ever made, right up to today's latest models. What has changed, and is often the source of much confusion, is the way in which the lens informs the camera body of the currently selected aperture. To understand why this matters, we need first to understand how cameras meter the light and calculate an appropriate shutter speed (skip the next bit if you already know this!).

Metering

Most SLR cameras measure light through their lenses. Most SLR cameras also hold the lenses aperture open to its maximum setting in order to get the brightest image in the viewfinder and make focusing as easy as possible the photographer. This results in the problem that the meter reading will be taken at a lenses largest aperture irrespective of the aperture desired by the photographer and will probably result in too fast a shutter speed and, thus, an underexposed photograph. There are two methods designed to solve this problem

Stop Down Metering

This is the simpler of the two methods. The user first composes their shot while the camera holds the aperture wide open. When they are ready, the photographer then uses the cameras stop down metering control to close the aperture down to the desired size, reads the meter value and transfers this to the shutter speed control. Job done... but it's clunky and inconvenient and also means a camera can't have an auto-exposure mode, it's all manual!

Aperture Coupled Metering

This method involves a connection or 'coupling' between the lenses aperture control and the camera. This connection tells the camera which aperture has been selected. The camera can then meter with the aperture wide open and then electronically apply a compensation for the selected aperture without the user having to stop down. This means we can now have auto-exposure mode, but the lenses are a little more complex.

Rabbit Ears & Ai

On the Nikon F and other early SLRs, Nikon used what are loving known as Rabbit Ears to tell the camera which aperture had been selected. Basically, the lens had a pair of prongs on the aperture ring. The camera had a pin that sat between these prongs and moved when the aperture ring moved and thus informed the camera. These lenses are often referred to as 'Pre-Ai' lenses.

In 1977, Nikon introduced a new aperture coupling system called "Automatic maximum aperture indexing" or "Ai" for short. Rather than use the rabbit ears, this system used cutouts in the aperture ring where the lens meets the mount. A reciprocal ring on the camera mount had a chock that sat in the cutout on the lens thus, when the aperture ring turned, the ring on the mount turned.

The upshot of this is that whilst all Nikon lenses should mount on all Nikon bodies, they may not be able to couple the aperture.

Conversion

Thankfully, it is simple to convert a non-Ai lens to work on Ai cameras. In fact, Nikon offered this as a service for a very long time. The factory conversion was as simple as swapping the aperture ring for a new one with the cutouts in the right place. Simple! Many people had their lenses converted this way, so it's not uncommon to see pre-Ai lenses for sale that have already been converted.

However, if you buy an unconverted lens these days, the only option available is to machine the cutout in the aperture ring either yourself or pay someone to do it for you. Thankfully, its really simple to do and all but the most cack-handed of people should be able to do it.

As I said above, I recently bought a Nikkor-S.C Auto 50mm f/1.4 unconverted pre-Ai lens. I wanted to use this with both my FE and my F-301 (which does not allow stop down metering), so I set about the process of converting it.

First things first, I removed the Rabbit Ears as these actually stopped it mounting on the FE. After that, I Googled a lot and found this guide which explains how to disassemble the lens and where to make the necessary cuts. The position of the start of the cut depends on the maximum aperture of the lens. As the maximum aperture of this lens is f/1.4, I needed to align the start of my cut with the f/8 mark on the aperture ring and extend clockwise (looking at the back of the lens) for ~40mm. The cut needs to be 1-2mm deep. So, I carefully disassembled the lens to remove the aperture ring, marked my cut with masking tape and then used a Dremel-like mini-drill fitted with a slitting disc to remove the necessary material, finishing with a small file to ensure the cut was smooth and clean and the ends were square. The result can be seen below

Nikon Nikkor-S.C Auto 50mm f/1.4 with DIY Ai Conversion.

N.B. The two brass screws just below the cut on the right originally held the Rabbit Ears in place. I replaced the screws a) so I didn't lose them and b) for cosmetic reasons.

I'm pleased to say the lens now works very well on both my FE and F-301 and happily recommends very sensible shutter speeds for the desired apertures (validated by comparing with other Ai lenses at the same apertures).

Disclaimer

If you chose to undertake a conversion on a lens and screw it up, don't blame me! Everything on this page represents only my personal experience and opinion and I cannot and will not be held responsible or liable for any damage or losses you may incur due to cack-handedness, lack of understanding, accident or for any other reason I am yet to think of.

Rob Kent

Published 13 days ago

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