June 7, 2014

Using Nikon AI/AIS manual lenses on a DSLR

Unlike other camera manufacturers, Nikon has kept virtually unchanged the mechanical mount of its lenses throughout over 50 years. As a consequence, most old manual Nikon lenses (albeit with some limitations) can be used on modern DSLRs.
However, there are a few things you need to know before attempting to mount an old lens on a digital camera.
I will circumscribe these notes to AI and AIS lenses, introduced respectively in 1977 and 1981. They are manual focus lenses, renowned for their top-notch quality and sturdiness.
They were rather expensive when still in production, but today can be found very cheap on second-hand market.
They can be distinguished from the previous lens series (named non-AI or pre-AI) by the two rows of aperture numbers (where the smaller one was intended to be read by a small periscope, so that the aperture setting was visible in the viewfinder).

Lens coupling elements

The mechanical mount is the classic Nikon-F, introduced in 1959 and still in use today on DSLRs. However, looking at the back of the lens, other coupling elements can be noticed. Let’s have a closer look at them.

A coupling ridge (whose ends are marked A and B in the above image) has the function of communicating to the light metering system which aperture has been manually set on the lens. This is obtained by coupling this ridge (which rotates together with the aperture ring) with a lever on the camera body.

Consumer-targeted DSLRs (all DX cameras, except D200, D300, D7000/7100) do not carry this lever, so, although they can mount AI/AIS lenses, no metering is possible: the camera shall be put in manual mode, setting the shutter speed and the aperture by hand. Aperture-priority metering with AI/AIS lenses is only possible on professional DSLRs (FX cameras, D200, D300, D7000/7100).

Most AI/AIS lenses have automatic diaphragm.  This means that, in spite of the aperture which has been set, the diaphragm always remains open at its largest aperture, allowing to compose and focus the scene at the maximum brightness. The pin C is operated by the camera body at the moment of exposure to close the diaphragm at its actual aperture.  This mechanism has been kept unchanged on modern DSLRs and lenses, so no difference is noticed when mounting an old AI/AIS lens. 

The same pin had a second purpose on FA and FG cameras, allowing to achieve shutter-priority (and programmed) automatic exposure. The exposure was more precise on AIS lenses, owing to the linear relationship between the movement of the above pin and the aperture. Apart from this minor detail (affecting only Nikon FA and FG), there are no real grounds for preferring an AIS lens over an AI.

The small hole G receives the locking rod of the camera body.

The remaining coupling elements are simply ignored by modern DSLRs:
  • The prong D, named Focal Length Indexing Ridge, informs the body whether the focal length is >135mm.
  • The prong E, named Lens Speed Indexing Post, informs the body about the the maximum aperture available on the lens.
  • The notch F, named Lens Type Signal, informs the body that the type of the lens is AIS. It is not present on AI lenses.
  • The mythical rabbit-ears (H) are present for backward compatibility with old cameras without the aperture coupling ridge. 
  • The prong I, named EE Servo Coupling Post, was used by the so called "DS-12 EE Aperture Control Attachment", a rare accessory which enabled shutter-priority automatic exposures with the Nikon F2.

Compatibility of DSLRs with AI/AIS lenses

With the exception of a few fish-eye lenses, all AI/AIS lenses can be mounted on a DSLR. However, you should keep in mind that:
  • Focusing is only manual. This is quite obvious, being AI/AIS lenses non autofocus.
  • Focusing can be difficult, owing to the absence of micro-prisms and split-image rangefinder, so familiar on the old manual focus cameras. It’s true that there is the electronic rangefinder (a led which lights when the correct focus is setup), but (IMHO) is a much less precise tool.
  • As already said, no light metering is possible with consumer grade DSLRs (i.e. all DX cameras, except D200, D300, D7000/7100). That means that either an external light meter is required, or the exposure shall be estimated using the histogram.
  • On DX format DSLRs the equivalent focal length will be incremented by a factor 1.5x. That means that a 135mm will behave as a 200mm, a 200mm as a 300mm, and so on.
IMPORT WARNING: the back glass of a few Nikon fisheyes (notably 7.5mm and 8mm) protrudes inside the body. Mounting such lenses on a DSLR would break the mirror! So, never mount these fisheyes on a DSLR!

A few practical considerations

On FX cameras
Old AI/AIS lenses give their best with full frame DSLRs, for the simple reason that the full area of the image is captured and the lens work in the same conditions it was designed for.
Aperture priority automatic exposure is still available and the absence of any crop factor makes the full range of focal lengths perfectly usable.
The only issue could be the fact that focusing can be a bit critical in some situations.
On DX cameras
In general, AI/AIS lenses can be used on DX cameras. However:
  1. Unless you own a professional DX camera (D7000/7100, D200, D300), you will loose light metering.
  2. Owing to the crop factor, it will be difficult to cover short focal lengths.
  3. Some optical quality concern may arise from the fact that the crop factor involves using only a portion of the image. However, the optical resolution should still be owing to the high quality design of AI/AIS lenses.

DX format DSLRs are best combined with normal and telephoto AI/AIS lenses. For instance, a 50mm becomes a luminous 75mm, which is great for portraits.
Depth of field
Most AI/AIS lenses are prime lenses, that means that they have a maximum aperture larger than modern zoom lenses. So, it is possible to exploit the reduced depth of field to put the background out of focus.
       Nikon D40 + 135mm f2.8 AIS, 200ISO, 1/4000s, f2.8                                
Using tele-converters
A rather extreme case is combining an AI/AIS lens with a tele-converter and mounting it on a DSLR.
With a 80-200mm zoom, at its maximum focal length, with a 2X tele-converter on a DX format DSLR you will get the equivalent focal length of 600mm.
Carrying a beast like that on a small DSLR is definitely not handy. Moreover, the overall weight of the lenses put under considerable stress the camera mount.

Although I have not made enough tests on the overall image quality, I could notice a general drop of the contrast, which can be partly compensated in post-production.