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I'm looking at 14mm Rokinon f1.8 lenses for astrophotography. I keep seeing the Built-in AE Chip for Nikon. What does the chip do? I realize I have to use manual focus and actually everything manual.
I tried to research but all I get is lenses on sale.
It probably uses the electrical contacts in post-AI bodies to tell the camera the maximum aperture of the lens so the camera can calculate correct exposure.
light levels are measured with the lens at max aperture, the camera needs to know what that is so it can work out how many stops to reduce aperture. Or to set speed in aperture-priority mode.
The B&H description of that lens says
It includes a special chip that allows the lens to work with the camera's focus confirm, auto exposure, auto metering and auto white balance functions.
The Rokinon catalogue says
AE (Auto Exposure/"Chipped"): Capabilities include focus confirmation with fine tuning, exposure modes Auto/P/A/S/M and EXIF data.
The Nikon electrical contacts are proprietary - meaning Nikon publish no information about it and you probably have to be a lens-maker under NDA to get details. SOme information is available from patents and other sources
You might like to see the answers to What are the electrical contacts in a Nikon F-mount?
Aperture rings on F-mount lenses are redundant on most Nikon bodies produced this millenium. Early Nikon AF lenses with aperture rings typically have a lock so that the ring could be locked at the lenses smallest aperture. The camera controls the aperture. Later lenses (G designation) omit any aperture ring.
This chip can accomplish three tasks:
Aperture control from the camera Instead of setting the aperture via the aperture ring, the chip communicates enough information (maximum aperture) for the camera to accurately meter. The D3x00 and D5x00 bodies, in particular, cannot perform stop-down metering without this information and would not meter accurately without it.
Autofocus confirmation Even though the lens is manual focus only, with the chip communicating information to the camera body, the green dot in the viewfinder will light up when focus is achieved at a designated AF point. This can be very useful without the viewfinder feedback we used to have in old manual focus film SLRs, such as the prism collar and split circle, but is not as accurate.
Partial EXIF information The chip can also supply some of the lens's EXIF information that would otherwise be missing without any electronic communication: such as maximum aperture and focal length. I do not believe it reports the aperture setting used, though.
You should also be aware of the many names of Samyang, of which Rokinon is merely one.
As far as I know...there is NO control of aperture on these lenses. The response above is incorrect to say that you don't have to manually dial in the aperture, "instead of setting the aperture via the aperture ring." With these lenses you still always do need to set aperture with the ring manually. At the most...the chip will communicate the aperture dialed in for exposure calculations.......but these lenses have NO mechanical electrical connection to change aperture from the camera's electronic aperture controls.
Regarding Nikon F mount, CPU chip is installed in it. Turn the aperture adjustment ring until minimum aperture so the lens can communicate with the camera accordingly.
Nikon users have focus confirmation and can control aperture from the camera instead.
Both answers are correct as to camera lens communications. Samyang Optics, the manufacturers of Rokinon report automatic communications on Samyang AF 14 mm f/2.8 FE, Samyang AF 35 mm f/2.8 FE, Samyang AF 50 mm f/1.4 FE. Of course, these are translated to Rokinon. The AE chip on any other lens simply communicates the aperture chosen and assists in determining proper focus and exposure.
