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The AI lens in the foreground has a speed post, the AI modified lens behind doesn't have such a post. |
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Here you can see the lever which reads the speed post. |
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An AI modified Nikkor 85mm f/1.8 I have used for many years. Because I often had it on my F4 I added a speed post by myself. It's a piece of plastic, filed to the right size and just glued at the correct position. |
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I guess the DIY method is difficult for high speed lenses with a large rear lens element. Look at the speed post of the Nikkor 50mm f/1.2 shown here! |
If you think there is no difference between an AI and an AI-modified lens, you're wrong! But in conjunction with modern cameras and especially with all DSLRs there is no difference between an AI and an AI-modified lens.
An AI lens has a lens speed post inside the bayonet mount. The camera is able to read the lens speed of the lens through this post. AI-modified lenses do not have this post!
After the Introduction of the AI system in 1977 there was no camera body that had a need for this post. The aperture value was visible in the view finder due to the 'ADR' (aperture direct readout) and the only integrated automatic mode was the aperture priority mode of the EL2 and FE.
In 1979 the Nikon EM - targeted to the amateur market - was launched. This camera was the first one that needed the lens speed value (for its automatic flash mode).
Over the years Nikon built only seven cameras at all which read the speed post. Two of them were milestones with respect to exposure metering:
Both cameras offered matrix metering. And for that the camera must know the absolute brightness of the subject. In order to get an absolute brightness value, the lens speed has to be calculated into the brightness metered by the camera through the lens. Thus with an AI lens you have matrix metering with these cameras but with an AI-modified lens only centre weighted metering (and spot metering in the F4) is available.
The FA uses the speed post to evaluate and display the aperture value when in shutter priority mode, too. With an AI-modified lens only 'F--' is displayed (but nevertheless the S mode works).
If you are using a F4 or may be a FA in conjunction with AI-modified lenses, you should have a closer look at the mount of your lens. Many lenses allow an additional DIY modification, to make them to real AI lenses! You just need a piece of plastic, a file and good glue to build a DIY lens speed post. But you will also need a model to determine the size of the post, and a lens with the same speed for getting the proper position of the post.
If you plan to make such a modification and you don't have a model (meaning an AI lens of the same speed), I can maybe help you by mailing to you a photo of a matching lens mount. But I don't have lenses with every possible speed. Just ask me! Another helpful source is the website of Lars Holst Hansen, see my 'links' page.
