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Topics - noisejammer

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Lenses / Zeiss Lenses for Astrophotography [Long]
« on: May 16, 2013, 11:59:41 PM »
A few days back, there was a discussion regarding whether the ZE 2/25 was a reasonable lens for astrophotography. I've been clouded out since then but caught a break under a dark sky. I decided to try put this to rest - at least for my lenses. The usual caveat applies - I tested one sample of each. Take this for what it's worth.

From inspection of their published performance, I believe the 2/25 Distagon, 2/50 MP, 2/100 MP and 2/135 Sonnar should be the sharpest lenses Zeiss offers. I own copies of the first three, so this these will have to do. I used a 5D Mk II with about 11k clicks. it had just come back from a sensor cleaning at Canon.

Very few flat-field astrographs are faster than f/4. Further, these lenses all exhibit 2 stops vignetting at large apertures which would make them unsuitable for astrophotography at f/2. To keep things reasonable then, I decided to stop the lenses down to f/2.8 and f/4.

I used a 3x loupe and 10x magnified live view to achieve critical focus on Spica, a bright, blue-white star, about halfway to the edge of the image. After focusing, Spica was relocated to the centre of the image. I did not use stop-down focusing - I think the camera stops itself down when it's in live view mode anyway. Note that critical focus was not achieved when the lenses were turned to the hard stop corresponding to focus at infinity. It is reasonably close but the spot size from stars was bloated by about 50%.

I used mirror lock up and the camera is piggy-backed on my heavy duty observatory mount. White balance was set to sunlight. I released the shutter using a timer and a cable. If I missed something, too bad...

The exposure was 30 seconds with long exposure noise cancellation enabled and ISO was adjusted to correctly expose the dark sky. This records lots of stars (some of which will be saturated.) This was to allow faint aberrations to be observed from a point source.

Here are my results -

ZE 2/25 @ f/3.2
Centre - faint stars form images of 1 pixel. Bright stars are round but exhibit spherochromatism of about 1 pixel.

Midfield - Faint stars form images of 1 pixel. Bright stars remain round with slight radial CA (perhaps 1 pixel.)

Extreme corner - Faint stars - < 2 pixels. Bright stars form images about 2x2 pixels with some sagittal astigmatism. This extends over about 6 pixels. There is some radial CA - perhaps 1 pixel wide.

ZE 2/25 @ f/4
Centre - faint stars illuminate a single pixel. Bright stars form perfectly round discs with barely visible spherochromatism.

Midfield - faint stars illuminate a single pixel. Bright stars form perfectly round discs with barely visible radial CA.

Extreme corner - faint stars are close to single pixels. Bright stars exhibit slight sagittal astigmatism (4 pixels). Some radial CA (perhaps 2 pixels, green to blue.)

ZE 2/50 MP @ f/2.8
Centre - faint stars are near single pixels. Bright stars are perfectly round with some violet fringing on Spica.

Midfield - faint stars are near single pixels. Bright stars are nearly perfectly round. Radially reversed coma-like pattern visible on extremely bright stars - probable indicator of field curvature.

Extreme corner - faint stars appear round. Bright stars exhibit sagittal astigmatism with a small fraction of their light spread out over 6 pixels.

ZE 2/50 MP @ f/4
Centre - all stars appear round. CA visible on Spica (but it is a special case.)

Midfield - all stars appear round. CA only visible on the brightest stars.

Extreme corners - faint stars are single pixels (these were not visible in f/2.8 image - perhaps because of vignetting.) Bright stars are slightly distorted from round but have minimal astigmatism.

ZE 2/100 MP @ f/2.8
Centre - faint stars are single pixels. Spica is a perfectly round disc with clear diffraction rings around it. Some CA is visible - but Spica is a special case of a very hot blue star.

Midfield - all stars appear to be round. Satellite tracks are easily visible as single pixel lines.

Extreme corner - all stars are round. There is no evidence of lateral CA.

ZE 2/100 MP @ f/4
Centre - all stars are perfectly round with their apparent diameter being dictated by their brightness. Spica exhibits some CA but less than was visible at f/2.8

Midfield -  all stars are perfectly round with their apparent diameter being dictated by their brightness.

Extreme corner -  all stars except the brightest recorded are perfectly round with their apparent diameter being dictated by their brightness. Bright stars have slight sagittal stretch but it's barely visible.

1. When focused accurately at f/4, each lens can produce superb results.

2. The ZE 2/100 MP and ZE 2/50 MP are quite usable at f/2.8 however CA and slight astigmatism may hinder things.

3. The ZE 2/25 should not really be used for astrophotography with a faster aperture than f/4.

4. The user should be aware that the each lens has about 1-stop illumination fall off at f/2.8 which is probably undesirable for astrophotography.

5. The user should be aware that critical focus cannot be expected if the lens is simply focused against the hard stop.

6. It would be reasonable to expect similar performance from the ZE 2.8/21 and 2.8/15 if they are stopped down to f/5.6. I can't think of many good reasons to use such wide lenses for astrophotography (except making star trails or images from a static mount.)

If ppl really want the raw data, I'll make that available on the Canon DSLR Digital Astro group

Clearest skies
73 DE TO.

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