PHOTOOG Photography writings by Olivier Giroux

2Jun/080

There’s always a catch

[Here I'm continuing the series of sightings (1 , 2 , 3) of the Zeiss 2/100 Makro-Planar T* lens.]

Alas... this is a bit bad of news: the lens is definitely not apochromatic. Do check out this last link, it'll be relevant for the rest of this discussion.

There seem to be two definitions in use for the fancy term Apochromat (wikipedia). Here are the competing definitions:

  1. Theoretical, Superchromat, Apochromat, Achromat: lenses that are corrected to project images of the same size (i.e. that come into simultaneous focus) for 4, 3 or 2 wavelengths of the light spectrum (respectively). All other lenses, “ordinary” lenses that make up 99% of the market, project images of different size for each wavelength – the symptom of which being chromatic aberrations.

  2. Operational, Apochromat: a lens corrected to reduce chromatic aberrations of the secondary spectrum (all wavelengths other than the reference or references) to within a percentage of the focal length of the lens. So if you can’t tell the difference (but maybe someone else can) then it’s an apochromat, put an APO sticker on it.

Although the theoretical definition appeals to perfection in the alignment of key wavelengths (e.g. red & blue) which is nice, it says nothing about the deviation of other wavelengths. One would expect green to fall in line between red and blue, but the definition doesn’t prevent it from being creatively misplaced. In contrast the operational definition does promise high imaging quality, but leaves each manufacturer to make this call for themselves (apparently there’s an ISO standard in play here).

Most importantly however, note that the operational definition says nothing of the number of wavelengths to be corrected for – it says all of them will be within a small tolerance. I much prefer this definition personally.

What’s so special about an apochromat?

First off, even small chromatic aberrations chew up your MTF. Although many (incl. people making lenses now) believe that chromatic aberrations can simply be fixed by software, it’s not clear that this holds true to the same precision that an apochromat can achieve. It’s likely that the chromatic aberrations can be corrected to imperceptible levels (of separation), but the impact on resolution is almost guaranteed to be general softening.

Most importantly though while everyone is well aware of lateral chromatic aberrations (blue/yellow), causing their correction to be a popular concern, the longitudinal aberrations (magenta/green) are less well known. The former can be easily seen at the edges of the frame on many lenses with great depth-of-field (particularly consumer lenses on consumer cameras) but the latter can only be revealed in the defocused area of images shot by macro lenses or super-telephotos.

Who makes Apochromats?

Many manufacturers do to some extent, though not all apochromats are labeled "APO". To muddle things, Sigma claims “APO” performance on just about every telephoto they make with low-dispersion glass (even the 200$ ones, I'm skeptical). Now the Canon and Nikon super-telephotos are probably all very close to apochromats – these would probably be unbearable to use if they weren’t (esp. at that price...) – but none of their macro lenses (that I know of) are apochromats even though they'd benefit from it as much.

The only system that offers apochromats for a variety of focal lengths is Leica’s, as par for the course on most of their lenses 75mm or longer. Carl Zeiss currently do not sell APO lenses in SLR or Rangefinder mounts, though they did have several designs in the past and may bring them back in the future.

Wrapping up

It’s a bit surprising that Carl Zeiss didn’t see the 2/100 as a product deserving the extra APO touch. It would have made the perfect answer to the 2/90 APO Summicron-R. This won’t stop me lusting for the 2/100 Planar lens, mind you.


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