Star Focusing and Circular Blobs

Last updated 2008-01-01 - Send Feedback
Related: how-to, focus
  

Updated: March 29, 2008

See also: Focus Accuracy.

Achieving accurate focus for night-time star shots is a tricky business. This article shows the effects of varying amounts of defocus (inaccurate focus).

Examples

Using the Nikon D3’s Live View feature, a very bright star was chosen. In Death Valley in late February, there was a tremendous amount of twinkling, which made the Live View display jump, almost like a moving video! Such things drive nocturnal astronomers “batty”.

What at first appears to be superior focus on a very bright star might be deceiving! The table of image pairs below show the optimal focus at top, with increasingly defocused results proceeding down. Note that the 2nd or 3rd rows might be judged superior if only the bright star were considered; in reality the other stars throughout the frame show that focus is actually off significantly.

Near-center Star (target of focus) Off-center Star

Aperture matters

In terms of optical formulas, an f/2 lens is considerably easier to correct for aberrations than an f/1.4 lens (and an f/2.8 lens considerably easier than f/2), so not too much criticism should be leveled at f/1.4 lenses using wide open, but see Canon EOS Meets Leica: 1D Mark III mated to APO-Summicron-R 90/2 ASPH.

The exampes above were taken with the Zeiss ZF 85mm f/1.4 Planar @ f/1.4. The ZF 85/1.4 is a superb optic. But like every “fast” lens design there is a “haloing” effect at wide apertures which leads to ambiguous focus and decreased image contrast. Some of the halo may in fact be an Airy’s Disc effect, but your author is not an expert on such things and is simply speculating. The differing color thoughout the image circle (spot) might or might not also be influence by uncorrected chromatic errors (eg lateral and/or axial chromatic aberrations).

Rather than guess, I consulted optical engineer J. Brian Caldwell, Ph.D. of Caldwell Photographic Inc, the optical designer for the Coastal Optics 60mm f/4 UV-VIS-IR APO macro about the effects seen here. He states:

Part of the discoloration is due to ordinary secondary chromatic aberration, but some of it may also be due to spherochromatism (variation of spherical aberration with color). I would be surprised if any attempt at all has been made to reduce the secondary color in your lens. The basic problem is that fast double Gauss lenses need high index glass for their positive elements. The only reasonable way to achieve this is with lanthanum borate based glass formulas. But these glasses give a negative partial dispersion, which is the opposite of what you need for good secondary color correction (i.e. "apo" correction). So fast double Gauss lenses always have a bit more than normal chromatic aberration, and this becomes especially bothersome for longer focal lengths like 85mm.

The bright ring effect is most definitely due to spherical aberration - not just simple third order spherical but also also fifth, seventh, and possibly ninth order as well. This is very common with fast double Gauss lenses like your 85mm/1.4.

Well, that’s a mouthful, isn’t it? I translate it to “it’s darn hard to correct this stuff in an f/1.4 lens”.

Bokeh and depth of field

Note the bright outer ring, a less bright inner band, and the darker core—all varying in color and intensity! This is the actual nature of an out-of-focus image spot and why one can’t just calculate depth of field and understand perceived image quality; lenses don’t image perfect circles with a uniform brightness and color; they produce non-uniform “blobs”—and how they interact with a digital sensor means that perceived image quality can include a strongly subjective component, not just something that can be calculated or measured.

Conclusions

Even with modern “Live View”, it can be tricky to achieve optimal focus when shooting stars. With experience, understanding optimal focus can be done realiably, but bracketing is never a bad idea.

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