Overcoming Coma Aberration - Part 2

Stars and coma: Capturing stars as points of light is a challenge in optical design. Without the addition of special aspherical elements, stars and other points of light are often reproduced with great distortion, particularly near the edges of a photograph. This lens aberration is called coma. Stopping down the lens to a smaller aperture can overcome some of this distortion, but not all of it. Surprisingly, very few general photography lenses come with coma correction!

Canon 24mm f/1.4L II lens

A disappointing experience: Since the typical photographer rarely photographs subjects that require coma correction, most general photography lenses do not incorporate aspherical elements in their lens designs. For this reason, I was quite surprised when I spent $1,800 in 2011 for one of Canon's best 24mm lenses, only to be extremely disappointed with it's wide-open performance when photographing stars (the lens works great for everything else).

The Canon 24mm f/1.4L II lens seemed like the perfect answer for my star photography. All the reviews I read said that it performed well, even wide open, with only moderate vignetting (which is important when you want to stitch several images together into a panorama). By being able to shoot at f/1.4, I could lower my ISO and greatly reduce the noise I had been getting in my starry night skies.

None of the reviews mentioned the problem of coma. To my horror, I noticed stars in my photographs that were shaped more like white doves in flight! Returning to the Canon dealer in distress, I was told the only solution was to stop down my new 24mm lens. (Note: Not all aspherical lens designs are created equal. This expensive Canon lens has two high-precision aspherical elements, and 2-UD elements, but exhibits much more coma than another prime lens costing about one-third the price!)

Canon 24mm @ f/1.4: enlarged area near the edge shows stars with severe coma distortion (click image to enlarge).

My tests soon revealed that at f/2.8 most of the coma did indeed go away, but I was now back to using the same aperture as my other less expensive lenses! It seemed like there was no way to avoid coma without shooting at f/2.8 or using an even smaller aperture. This meant using longer exposures, but that produces star movement or trails. I could use higher ISO's to the prevent longer exposures, but that meant increased digital noise. (Click image below to enlarge.)

Aspherical elements in Samyang Optics 24mm f/1.4

Samyang Optics to the rescue: Canon and Nikon have chosen to make only a few prime lenses with aspherical elements that control coma. Over the past year I have discovered there is a manufacturer that does make fast prime lenses with aspherical elements, and at inexpensive prices: The South Korean optical company, Samyang Optics. Samyang produces three wide-aperture aspherical lenses under a variety of brand names (Rokinon, Bower, Samyang, and ProOptic). These lenses are often one-half to one-fourth the price of Canon and Nikon lenses with similar focal lengths and maximum apertures specifications. The Samyang / Rokinon / Bower lenses that I recommend for starscape photography are the 14mm Ultra Wide-Angle f/2.8 IF ED UMC, the 24mm f/1.4 ED AS UMC Wide-Angle, and the 35mm f/1.4 Wide-Angle US UM. All three have the largest apertures in their class, and all three have aspherical elements.

COMPARE the same stars near the edge of the photograph (shown at 100% enlargement) with the $600 Samyang/Rokinon lens vs. the $1,800+ Canon lens. Coma is almost completely controlled on the left image; however, light fall off or vignetting is more noticeable. Vignetting can be corrected with software. Coma cannot! [Click or touch image to enlarge] ~ © Royce Bair

The Samyang / Rokinon / Bower 24mm lens also includes ED elements that are usually reserved for more expensive lenses. (ED means extra-low dispersion, referring to a type of glass that disperses light less than ordinary glass. Dispersion is the breaking up light into its original colors. Because dispersion can cause chromatic aberration, ED glass elements help reduce purple fringing and other chromatic aberrations.)

Aspherical lenses by Samyang Opics: 14mm f/2.8 - 24mm f/1.4 - 35mm f/1.4

Manual everything reduces their cost: What makes the Samyang lenses so inexpensive is that they lack all of the automatic features we've come to rely on, i.e. auto focus and electronic coupling with our digital cameras. However, none of these features matter much to an astro-landscape photographer. Auto focus does not work on a night sky, so we must resort to manual focus, anyway. Lack of auto aperture (where the camera stops down the lens' aperture to the preset aperture value) is of little concern when you plan to use the lens wide open.

Recommendations and Reviews: Several of my night photography friends have purchased these lenses and recommend them. For example, Masahiro Miyasaka (Astrononomy Photographer of the Year 2012) uses the 14mm; Mike Berenson uses the 24mm; and David Kingham uses the 35mm for awesome panoramas. I have the 24mm and the 14mm.

Grand Canyon with the Milky Way - taken with Rokinon 14mm f/2.8 ~ © Royce Bair (click to enlarge)

The Samyang / Rokinon 14mm has a 114º angle of coverage, and produces awesome vistas of the Milky Way. It has so much depth of field, even at f/2.8, that some have difficulty focusing it, but its worth the extra effort! It has a little more barrel distortion than than the $1700 Nikon 14mm f/2.8D ED, and more barrel distortion than the $2400 Canon EF 14mm f/2.8 L II USM. Both the Canon and the Nikon are excellent choices for architectural photography, where it is important to maintain straight lines, but you're probably not going to notice this too much on landscapes and starscapes. Here is a Digital-Picture review on the Samyang 14mm.

Grand Canyon - Nankoweep area - taken with Rokinon 24mm f/1.4 ~ © Royce Bair (click to enlarge)

The Samyang / Rokinon 24mm is a favorite lens because of its 84º angle of view, its fast, f/1.4 aperture, and its excellent correction for coma and fringe color. For starscapes, I would actually prefer the wider coverage of the 14mm, but its slower f/2.8 aperture gives overall preference to the 24mm. A Flickr contact of mine, Rick Whitacre, did this coma comparison between the Canon 24mm f/1.4 and the Rokinon 24mm f/1.4 (here's a 260% larger view). Rick did other tests on sharpness, brightness, and vignetting starting here (click image to view all sizes). Nikon users will want to see this LensTip coma comparison between the Nikon Nikkor AF-S 24 mm f/1.4G ED and the Samyang 24mm. Here's a Digital-Picture review on the Samyang 24mm.

The Samyang / Rokinon 35mm is probably the least purchased starscape lens because of its narrower 63º angle of view. However, because it has a less distorted view (closer to that of a normal lens) than most wider angle lenses, it is preferred by many photographers for panoramas, where several vertical images are stitched together (like this one by David Kingham). Here's a Digital-Picture review on the Samyang 35mm.

NOTE: All of these lenses are designed for full-frame camera sensors (i.e. a Nikon FX). They can be used on a Nikon DX camera or a Canon camera with an APS-C size sensor, but the angles of view are less. The angle of view with the 14mm becomes 94º using the Nikon APS-C sensor, and 90º using the Canon APS-C sensor. The angle of view with the 24mm becomes 62º using the Nikon APS-C sensor, and 58º using the Canon APS-C sensor. The angle of view with the 35mm becomes 43º using the Nikon APS-C sensor, and 41º using the Canon APS-C sensor.

Pricing and ordering: (the links are to B&H, which often has lower than list pricing)
Rokinon 14mm f/2.8 Nikon mount - Canon mount - Sony mount ~ List: $399.00
Rokinon 24mm f/1.4 Nikon mount - Canon mount - Sony mount ~ List: $599.00
Rokinon 35mm f/1.4 Nikon mount - Canon mount - Sony mount ~ List: $499.00
(Disclosure: using these links give me a 2% referral commission, but add nothing to your cost. Thanks for your support!)

More Low Coma Lens Recommendation

Here is my first article on Overcoming Coma.

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