I just unloaded a 28mm Nikkor lens and thought it had a flat front surface, under the UV filter I put on it. Might that be a front-located field flattener, usually seen at the back, analogous to reversed Tessar formulae - just designed differently?
Of course this would come up when I'm here in Tucson and the 28 is back in NYC.
But I keep grabbing Nikkor and 3rd party Nikon-compatible lenses I have here (quite a few) and still cannot find one with a flat front element.
Now, here in Tucson I have for my aging D-100 a AF-S Nikkor 18-70mm 1:3.5-4.5G ED.
I pulled off the filter and the front element "looks" like it's flat by a quick "eyeball" - but I ran a cloth across it - and it's curved, as one would expect.
Now, this is a digi lens but I did once mistakenly use it on my F-100. So I know it will vignette all the pics. Thus, it's unlikely that the OP is using this kind of lens on his F-100.
Might make more sense to this discussion if the OP would tell us what the lens is on his F-100?