Wednesday, 13 April 2011

The Man. that is older than AI

I don't shoot your traditional macros: I tend to shoot people although I do shoot products occasionally, but never enough to justify a dedicated macro lens. Not until a recent discovery, a Micro Nikkor 55mm f/3.5 AI.

These are manual focus lenses and according to their serial number a large number of them were built in around 1977, making these lenses older than me. The lens construction is solid with real engravings on the lens barrel and the silky smooth focus ring is an utter joy to use, even for an-autofocus-educated fool like myself.

More can be said about this particular model at MIR.

Whilst not being an avid macro shooter, I still shoot enough products that made me consider looking for a cheap way to do macro. I had considered close up rings, reversing rings and then the older AF-D Nikkors; all of these options were discounted except the AF-Ds but even these used/second hand lenses were starting around the 185GBP range - that was the starting range for 60mm AF-D, and the more conventional Micro-Nikkor 105mm f/2.8D and newer were even more expensive.

However, it was then pointed out that for macro/close up shooting, manual focusing is always the most accurate technique - this meant a whole new world of micro Nikkors would meet the search criteria: the AI, AI-S lens which, whilst are from the late 70s and early 80s, are compatible with all modern F-mount Nikon bodies.

It is very worth noting that the pre-AI (now also called NAI) lenses will cause significant damage to post 1977 F-mount bodies. Articles on MIR here and here illustrate the notable physical differences between the pre-AI, AI and AI-S lenses - most visible were the solid metering prong of the pre-AI lenses and the blue max aperture engraving on the aperture ring for AI lens (AI-S have this number in orange).

Now, to choosing between a 1977 AI or a 1979 AI-S (all made in Japan). The optical formula between the AI and AI-S equivalents may not have changed (although Nikon literature states this was not the case between the 55mm f/3.5 AI and f/2.8 AI-S) but the mechanical build quality certainly did. The introduction of the AI-S lenses also came with cheaper materials being used resulting in lighter equivalents - whilst not a drastic difference, some may prefer the heavy build of the AI sibling. Other than this, AI and AI-S lenses can be viewed as being functionally identical on modern Nikon bodies.

The amazing thing is that even with a lens made in the late 1970s, the higher end modern Nikon bodies (eg D300 series and above) will actually provide TTL metering; the lower end Nikons (D40,D3000 etc) won't provide metering but using a hand held incident meter and some exposure factor compensations (see later) will allow the means for dialing in correct exposures.

Why This

So, why the 55mm f/3.5, with such a small working distance (24.1cm from subject to film plane/~10cm from subject to front of barrel for the max 1:2 reproduction ratio)?

Firstly, the reputation for sharpness and optical quality remains.

Secondly, the 55mm micros were cheap when compared against the well sought after Micro-Nikkor 105mm f/2.8 AI/AI-S iterations and certainly much cheaper than the Micro-Nikkor 200mm f/4 AI-S.

Thirdly, I don't shoot bugs so the working distance isn't a problem, and most of my product lighting will be artificial (strobe) so I'll have to work around any limitation if any but I don't expect so (I almost never have to light products from front and if so, the apparent size of the light source will be so large in relation to the product that it won't be an issue).

The 55mm micro has had a rich history with numerous revisions: pre-AI versions dating back to the early 60s, AI f/3.5, AI-S f/2.8 (still manufactured and sold today), AF f/2.8.

However, of the MF 55mm lenses it has been noted that some of the f/2.8 versions did suffer from oily aperture blades which moved me towards the older AI f/3.5.

The reported sharpness certainly held up in the initial images - the crop of the British penny (tripod'd at f/4) below demonstrates this:

Other quick samples (converted from RAW to jpeg in CNX2, no sharpening):

Exposure Factor

Whilst the higher end Nikon bodies will provide TTL metering, there is still a potential problem.

Macro lenses achieve max reproduction ratios at it's shortest focus distance. As the focus distance decreases there is a loss of light that reaches the film plane as the 55mm's internal elements move further away from the film plane.

So as f/4 is dialed in on the aperture ring (aperture for AI lenses must be set on the ring itself and not via the command dial) and focused at max reproduction ratio, the TTL capable bodies will compensate for the loss of light and the metering system will show the exposure variables: these capable bodies will report the 'effective' aperture being used for exposure at the given focus distance.

Internally, such high end bodies are calculating and compensating for the exposure factor which is approximated as:
EF = (1 + R)^2
where EF is exposure factor and R is the reproduction ratio.

A 1:2 ratio (1/2 of real life size) gives us EF = 1.5^2 = 2.25. This means that for a 1:2 reproduction ratio, there is a 2.25x loss of light. Translating this to f/stops (F = log2(EF)) means an exposure factor of 1.17 stops is required to the base exposure.

The TTL bodies take care of this automatically but for non-TTL metering bodies OR for hand held incident metered (ie for flash lit) subjects, exposure factor compensation will be require.

Therefore, for an incident meter reading with a required working aperture of f/4 (dialled on the camera) and a 1:2 reproduction ratio, our incident meter must read f/4 + 1.17 stops = f/5.6 2/10 for the expected exposure at such a focusing distance.

The two different apertures are known as the effective aperture and nominal aperture - f/5.6 2/10 and f/4 respectively in the example above.

There is another documented, but crude, formula to obtain the effective aperture directly from the reproduction ratio:
EA = EN (1 + R)
where EA is the effective aperture and EN is the nominal aperture.

Again to our example, EA = 4 * (1.5) = f/6.0. Problematically, most metering systems don't give metered readings like this: certainly my Minolta IVF gives me f/x + y/10.

This table tries to summise the exposure factors for the different reproduction ratios
Repro RatioExp FactorStop Adj

So What?

And after all of this, what does it all this mean? For the high end Nikons, the 55mm f/3.5 can be used as a (slow) walkaround lens as it will meter correctly irrespective of the focus distance. For the low end Nikons, this lens can be quite cumbersome as an incident meter is required and also the calculation and adjustment of the exposure factor which is dependent on the required reproduction ratio.

The implication for shooting strobes with the 55mm f/3.5 AI will be that I will be constrained to staying in and around the 'hard' reproduction ratios (1:2, 1:10 etc) to ensure that we know the effective aperture that needs to be dialed into the camera. The steps would be:
  • determine desired reproduction ratio and set focus on lens
  • adjust camera position for sharp focus of subject
  • determine effective aperture for given reproduction ratio
  • select aperture on camera
  • meter and adjust strobes for effective aperture

One advantage of the new AFS/G lenses, such as the 105mm f/2.8G, are that as you find your final focus, the effective aperture is displayed on the body viewfinder/displays. Therefore, using the effective (not the nominal/user dialed in) aperture we can meter and adjust our lights for this aperture and no need to mental maths as noted above.

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