For years I've used an old 3.5 60mm macro - now using it on my D700. Recently tested it agains the current AF 60 - greatly superior! But what I don't understand is that the AF lens seems to need less exposure for a properly exposed file? Like, f8 = f8, doesn't it?

I'm no scientist and don't claim to be an expert on optics, but I believe what you're seeing is a difference in the efficiency of the lens at transmitting light. F-stop is a simple mechanical ratio but not every lens with the same f-stop transmits the same amount of light to the sensor. I'm guessing that the newer lens is more efficient, thus requires less exposure at a given f-stop.

If you'd like to delve into a much deeper explanation, here's one web page on it - http://www.cctv-information.co.uk/i/Light_Transmission_Through_Lenses

Cheers -
John

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There are many possible explanations. One of them could be that the new 60/f2.8 AFS Micro has a T-stop closer to its actual f/stop than the older lens. In other words, it may be more efficient at transmitting light. However, I would not guess this first, because the 60 AFS has only 12 elements, and most older formulas such as the original 55/f3.5 Micro-Nikkor from 1963 has as few as five elements. It generally takes a pretty wild optical formula such as the 24-element 200-400/f4 AFS for t/stop to deviate much from f/stop.

A different explanation might be that the aperture blades on the f/3.5 lens (really a 55/f3.5, not a 60/f3.5, right?) might be slightly sticky. If so, they might be a bit slow to close all the way down to taking aperture, with the result that it may yield a slight (or perhaps even significant) underexposure compared to another lens that operates properly. And if my memory serves, the 55/f3.5 AI may be one of the ones that is more subject to a lubrication problem than some others...? Can't remember for sure, but surely one of the early short Micro-Nikkors is in this category.

There are other possibilities as well.

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Brian... a bicoastal Nikonian and Team Member

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Remember when I said "there are other possibilities?" Well, for some reason this morning it hit me: yeah, there is one other possibility that is REALLY likely. So likely that it's probably a "Doh!" moment for those in the know, but which I somehow missed.

See this thread. It talks about AF Nikkors, because we primarily get questions about seemingly broken AF/AFD/AFS Micro-Nikkors that "refuse" to open up to full aperture. But it also applies equally to the old manual focus lenses - it's just that they did not report their effective aperture, so nobody noticed. What happens is that as you approach minimum focus (and of course this is a common thing with a macro lens!) the effective aperture decreases. So at minimum focus and wide open, some of the older macro lenses may be a full stop or potentially even more from the marking on the aperture ring. The amount of differential varies with lens design. The 60/f2.8 AFS wide open is f/2.8 at infinity and f/4.8 at minimum focus. I've run into another macro lens that's f/2.8 at infinity and f/5.6 at minimum focus - I think that was the Tamron 90/f2.8 but I'm not sure. There are some that operate purely by extension so are not affected at all.

I suspect that if you compare your old and new lenses focused at infinity, you'll find that they have very similar exposure requirements. But at minimum focus, the differential could easily be a full stop or more.

I kind of wish the new lenses didn't bother reporting this. It's a needless complication, because if we just ignore it, we get perfectly good exposures.

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Brian... a bicoastal Nikonian and Team Member

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This could well explain my conundrum. My test to the old/new lenses was at 1:1 with the f-stop set manually at f16, my normal setting, using MF in both cases and electronic flash. Thus, if there is that much of a diference in effective lens openings, that would explain the approx 1-stop difference in exposure.

How old is your old 55mm macro? The very early one that needed an extension tube to go to 1:1 had a compensating diaphram. If you weren't at maximum aperature, the diaphram had a cam that adjusted the opening to be wider as the lens was focused closer to compensate for the effective aperature becoming smaller as the lens moved farther from the focal plane. I have one of those lenses, and before the days of thru the lens flash adjusting cameras, it was very useful for flash illuminated macro work. It was less useful for thru the lens meteing cameras which would automatically adjust the exposur as you focused closer. With this lens you would get overexposure as both the lens and camera meter tried to adjust the exposure.

Matthew

>I kind of wish the new lenses didn't bother reporting this.
>It's a needless complication, because if we just ignore it, we
>get perfectly good exposures.

...but only because, for the most part, we rely on the TTL metering systems in our cameras rather than using hand-held meters. If you were (perversely, maybe!) using a hand-held meter, knowledge of the actual effective aperture for the focus distance chosen would be very helpful indeed. I recall, back in the day when AF wasn't even a tremble in the kneecaps of the camera designers, my father using tables of correction factors to figure out the effective aperture of his macro lens depending on the focus distance set and the particular combination of extension tubes fitted, none of which is necessary anymore if you are using TTL metering.

Regards,
Tony

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