Halos at the Edge of the Moon
Halos are bad:
- Halos are ugly in closeup shots.
- Halos cause a loss of contrast in all types of pictures (especially at wide aperture and extremes of the zoom range).
Here are some crops of moonshots, taken with my SX30:
Upper Left: 150.5 mm-f/5.8-middle-sharp Upper Right: 150.5 mm-f/5.8-min-sharp
Lower Left: 150.5 mm-f/8.0-min-sharp Lower Right: 52.3 mm-f/5.0-min-sharp
- At full optical zoom (150.5 mm), f/5.8 is the widest aperture and f/8.0 is the narrowest.
- Middle-sharp is the camera's default setting, the middle of 5 steps.
- Min-sharp is the camera's minimum sharpness setting. ( I do not know whether or not this means no sharpening.)
The halos are not an atmospheric effect - they are produced by the camera.
Upper Left Shot with Default Sharpening:
There is a hazy white halo in the sky with a black band to the right of it that is often a little darker than the dark background. Then there is the overly-bright edge of the moon followed by a band that is slightly darker than the rest of the moon.
To summarize, the halos are most prevalent with:
- sharpening more than minimum
- large aperture (low f-number)
- extremes of the zoom range, but especially the high end.
Some web sites attribute the halos and other effects to sharpening:
Introduction to Sharpening, Norman Koren, Imatest: http://www.imatest.com/docs/sharpening/
Resolution and MTF, Oleg Kurtsev: http://www.quickmtf.com/about-resolution.html
I have recently been measuring the MTF curves of my SX30 for a variety of settings and found that the contrast drops off rapidly at low spatial frequency at extremes of the zoom range, especially the high end and when the aperture is wide. Usually, this loss of contrast is blamed on lens aberrations, rather than sharpening:
MTFs of Optical Imaging Systems Lecture 5, J. Verwerda (Sketch in Slide #17):
Norman Koren states that "lens aberrations tend to be worst at large apertures (small f-numbers)".
Sharpness: What is it and how is it measured?, Norman Koren: http://www.imatest.com/docs/sharpness/
In some of my tests, I removed the halos by setting their brightnesses to zero in my spreadsheet's Line Spread Function (LSF). This had the effect of correcting the problem of contrast drop-off at low frequency.
A small amount of halo is caused by diffraction, but the halos seen in these photos do not change location with change of aperture (as would be expected for diffraction) so diffraction does not appear to be the main cause of these halos.
So, the halos are worst at extremes of the zoom range and at the widest apertures, just as lens aberrations are worst under the same conditions. Also the halos appear to be directly responsible for a loss contrast at low frequency, an effect that is normally attributed to lens aberrations. These facts lead me to suspect that the halos may be caused, not just by sharpening, but also by aberrations.
I would be interested in your opinions about halos and aberrations. Also, are other bridge cameras better:
- because of their sharpening properties?
- because of less extreme zoom range?
- because of their sensors (SX30 is CCD rather than CMOS)?
- because of better lenses?