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Photographing Stationary Stars

February 10, 2014

I have photographed stars in the past but usually star trails like this:

650xW GI_101117_ST3 V1

But lately I have been enjoying some nighttime photography and in particular, photographing stationary stars like this: 

1200x GI_140118_5065 V1

For stationary stars, the best results are produced using a very fast lens (e.g. f1.2 or f1.4) because you want to collect as much light as possible in a relatively short exposure. Due to the rotation of the earth, the stars will appear to move in the sky with longer exposures. That is what makes star trail photographs possible.  But for stationary star photographs the idea is to limit the exposure time such that the stars don’t blur (don’t appear to move). The general rule for determining the longest exposure possible before star “movement” can be noticed is as follows:

500/focal length = seconds of exposure

So, for example, using my 24mm lens, 500/24 = 20.83 seconds. That means that exposures of 20 seconds or less should have stars that are not noticeably blurred due to the movement of the earth. It makes sense that the wider the lens, the less apparent the movement is and likewise, the more telephoto the lens (the closer stars appear) the more apparent the movement will be.

So, as you can see, with the shutter time pre-determined in order to avoid star blur (in my example, no longer than 20 seconds), the amount of light gathered is controlled by aperture and/or ISO.

But besides collecting as much light as possible during the exposure, the biggest problem facing photographers in these situations is having enough light during pre-exposure focusing. Most modern auto focus lenses focus past infinity (for the most part this is so the lens motor can “search” for focus without hard stopping at infinity). So it is not possible to just set the lens to infinity and shoot. Focusing is required to find infinity. Looking for stars and focusing through the viewfinder is just not possible. Instead I use Live View and zoom. But even with dark nights and lots of stars in the sky, finding a star in Live View can be difficult.  One key to success is a fast lens. Using an f1.4 lens at ISO 1600 makes finding stars possible and enables manual focusing using Live View. A slower lens of f2.8 or f3.5 etc. makes the task of seeing a star on the LCD and manually focusing it virtually impossible.

During the limited exposure time (e.g. 20 seconds) a fast f1.2 or f1.4 lens makes all the difference in gathering as much light as possible. Remember that each stop is a doubling or halving of the amount of light. So an f1.4 lens will “collect” four times as much light as a f2.8 lens. This is really one time where equipment really does make a difference. Try to achieve similar results with slower lenses is just not possible.

462px-Aperture_diagram.svg

When it comes to those little specs of light in a black sky, this makes all the difference.

It is really quite an amazing experience to setup the camera, see just a few stars on the Live View display while focusing and then being completely blown away 20 seconds later when the exposure is displayed on the screen. The camera with such a fast lens is capable of “seeing” even more stars than the human eye. It can be very exciting.

I also find that beyond a certain point, increasing the ISO has no benefit and only impacts negatively. I usually shoot at ISO 1600. If I shoot at 3200 or 6400, the image is brighter but more noise is introduced. Taking an ISO 1600 image and adjusting the levels in post to a point similar to the 3200 or 6400 exposures, creates an image of similar illumination but with much less noise.

1350x GI_140202_6940 V1

I have recently been experimenting with attaching an external smartphone to my camera via USB in order to use the larger display to assist in manually focusing on stars. I’ll tell you all about that in an upcoming post.

Cheers, Scott

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