Astrophotography with the Sigma 14mm F1.8 Art lens
As an astronomer and science filmmaker, I have been photographing the night sky and astronomical observatories professionally for more than ten years. I have photo-documented more than thirty science research sites around the globe in remote places such as the South Pole, the Atacama desert, the South African Karoo, and a lab almost one mile underground in South Dakota.
For wide-field night-sky photography my main lens has been the Nikkor 14-24mm F2.8G ED. Its wide, sharp, and rectilinear field of view (FOV) of 114 degrees makes it the ideal lens to capture as much of the sky as possible in the context of a natural landscape — or as much of an astronomical telescope as possible when shooting inside an observatory. Its wide aperture of F2.8, combined with good DSLR performance at high ISO, lets one capture features such as The Milky Way in a short amount of time, while avoiding or minimizing star trails due to the Earth’s rotation.
When the new Sigma 14mm F1.8 DG HSM Art was announced, it got my immediate attention because of its obvious applications in astrophotography. It's 1.3 EV faster than the Nikkor lens, which means an image that needs a 25 second exposure at F2.8 requires just 10 seconds at F1.8. I was able to test this lens on a Canon EOS 5D Mark IV earlier this month in Badlands National Park, South Dakota. Here are my first impressions of using it in the field.
I'll give you my impressions of using this lens in the field, but for those who really want to see the type of images it produces, I won't make you wait.
Image Brightness at F1.8
First, let's see how much brighter a scene looks at F1.8 than at F2.8 in these straight-out-of-the-camera (SOOC) images (no vignetting or lens correction applied). Naturally, the increase of 1.3 EV is highly appreciated.
F1.8, 10 sec, ISO 1000 (SOOC)
F2.8, 10 sec, ISO 1000 (SOOC)
Next, let's take a look at another scene shot at F1.8 and stop down the lens to F2.8 in 1/3 f-stop increments while increasing the exposure times accordingly in order to keep the same EV.
Vignetting and Lens Flares
As one would expect, vignetting is stronger at wider apertures but I like how soft the vignetting gradient is. On site, it was apparent how much brighter the image at F2.8 appeared on the LCD screen compared to the one shot at F1.8. Subsequent analysis in Photoshop demonstrated that although the central portions of the images are equally bright, overall, the image at F2.8 is 27% brighter than the one at F1.8 due to the effect of vignetting. This was easily corrected in Lightroom's Lens Correction panel, albeit more aggressively than what I would normally do.
You can see a lens flare caused by the off-frame Moon. Although it is small and faint, I noticed (anecdotally) that the Sigma lens was more susceptible to the Moon's glare (when located at very shallow off-frame angles) than the Nikkor lens. You can see the flares in the 100% crops below. Also, notice how much you can reduce star trails by cutting the exposure time to 10 seconds. This makes stars appear much closer to pinpoints of lights and makes other astronomical objects like the M4 globular cluster less smudged. Of course, the latter point is relevant depending upon the final size at which the image will be displayed and the distance at which the image will be seen.
Now, let's take a look at the presence of coma, an optical aberration which causes point sources such as stars appear distorted and display a tail (coma) like a comet. This kind of aberration is worst at the widest apertures and, just like vignetting, can be reduced by stopping down the lens, which might go against the motivation of investing in a fast lens! Therefore, it is very important for a fast lens to exhibit very low coma.
Below are 100% crops of the boardwalk scene shown above shot at F1.8 and F2.8. For such a fast lens, the amount of coma is very small. In the upper row, notice that at F1.8 coma is only apparent in the bright stars and when the lens is stopped down to F2.8 the coma disappears altogether. That is, unless the "star" is extremely bright as it is in the case of planet Jupiter in the lower row. (Click to enlarge.) The upper row shows the top-left corners of the F1.8 and F2.8 images, while the lower row shows the top-right corners. Both sets of SOOC images were exposed for 10 sec at ISO 1000.
|100% crops of the upper corners of the two images shot at F1.8 and F2.8. Both sets of SOOC images were exposed for 10 sec at ISO 1000.|
Even at F2.8 Jupiter exhibits a significant amount of coma, but this does not concern me much since the odds of having multiple very bright stars or planets near the edge of the FOV are not high. Have in mind, the amount of coma decreases quickly as you move away from the edges of the FOV. You can see this behavior in the following full-resolution images.
In these F1.8 SOOC images you can see comatic aberration performance across the entire FOV. In order to show point-like stars as much as possible I am including two 10-sec exposures (minimizing star trails), a 13-sec, and a 20-sec exposure. To show lens performance as best as possible none of the Raw images were processed or corrected in any way. In terms of coma, these images can be considered worst-case scenarios since they were all shot at the widest aperture. The results are impressive and, for those interested in seeing a full-frame star field image, I am including one below. (Click to enlarge)
|F1.8, 10 sec, ISO 200, SOOC||F1.8, 10 sec, ISO 2500, SOOC|
|F1.8, 13 sec, ISO 1000, SOOC||F1.8, 20 sec, ISO 3200, SOOC|
As I mentioned earlier, I enjoy wide-field astrophotography because it lets you present the night sky in the context of beautiful and dark locations around the planet. Badlands National Park provides very dark skies over a rugged and fascinating terrain.
My favorite time for doing this kind of photography is near the First and Third Quarter Moon, because you get moonlight during roughly half of the night. This lets you capture the beauty of the landscape bathed in moonlight during half of the night and the glorious starlight and The Milky Way during the other half.
Of course, the beauty of the Badlands doesn't end at dawn and the combination of daylight and clouds give another dimension to the landscape. The wide FOV of this lens helps you easily capture the vast grass prairies intermingled with sharply eroded buttes and pinnacles. And when these are covered by afternoon stormy clouds you are in for a treat.
You can see some examples that illustrate these things in the gallery at the top of the article.
Another advantage of photographing during the First and Quarter Moon is that you can capture the changing light of a moonset or moonrise during a time-lapse sequence. Below are three different sequences captured with the Sigma 14mm F1.8 DG HSM Art, processed in Lightroom, and edited in After Effects.
As I mentioned, my go-to lens for wide-field astrophotography is the Nikkor 14-24mm F2.8G ED. If I needed to pick just one lens to photograph the night sky, would I trade it for the Sigma 14mm F1.8 DG HSM Art? The answer is yes. Although I often use the Nikkor lens at different focal lengths to photograph buildings and monuments (it's the perfect lens for cathedral interiors), I rarely use it to photograph the sky at focal lengths others than 14mm. When taking single shots of the night sky (something I often do while other cameras are shooting time-lapse sequences) the extra 1.3 f-stop would allow me to take shorter exposures and use that gained time to creatively experiment with different compositions and angles.
When it comes to taking time-lapse sequences of the rotating night sky, you wouldn't necessarily gain much by shortening the exposure time (and acquiring more frames per minute) since you could end up oversampling the rate of motion of the sky. After all, the idea of time-lapse photography is to speed up time and show things that happen too slowly for us to appreciate. But it would let you use a lower ISO and get cleaner images — or a combination of slightly lower ISO and shorter exposure times.
Now, I can see one phenomenon where the wide aperture of F1.8 would let me shorten the exposure time and increase the temporal sampling of my time-lapse sequence: the Northern Lights! When auroras quickly increase in brightness they also move faster, and the extra 1.3 f-stop will let me shorten the exposure time by a factor of 2.5 without the need to push the ISO. Or, alternatively, to shoot video of the Northern Lights. I can't wait to try this lens for aurora photography in September. Stay tuned!
The Sigma 14mm F1.8 DG HSM Art lens is available in Canon, Nikon and Sigma mounts. I'm looking forward to hearing your thoughts in the comments section.
José Francisco Salgado, PhD is an Emmy-nominated astronomer, science photographer, visual artist, and public speaker who creates multimedia works that communicate science in engaging ways. His Science & Symphony films through KV 265 have been presented in 175 concerts and lectures in 15 countries.
José Francisco is a seasoned night sky and aurora photographer and filmmaker. If you would like to view, photograph, and learn about the Northern Lights then you can inquire about his Borealis Science & Photo Tours in Yellowknife, Canada.
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