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In the article, Photographing Stars Using a Kit Lens, the author talks about keeping the widest aperture, e.g. ƒ/3.5. But it's my understanding that a wide aperture means a smaller area will be in focus. I want to capture the whole sky, so ideally it should be ƒ/16 or ƒ/22, correct?

I tried ƒ/3.5 and it worked like a charm.

So, can anybody please put a light on this? What exactly is going on here?

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    Aperture works similarly to squinting your eyes: try it. It doesn't change the field of view – cmason Apr 22 '15 at 20:01
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Even though the distance of various stars from your camera on Earth can vary by astronomical distances, they are all far enough away that the light from them enters your lens as collimated rays. This means you don't need much depth of field because the lens must be focused to precisely infinity for any and all of them to be in sharpest focus.

The reason the wider aperture can help is because it allows more light through and that in turn allows a faster shutter speed for the same exposure value/allows collecting more light in the same amount of time. If your camera is stationary this allows you to capture dimmer stars without creating star trails.

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    I wouldn't apply the "larger aperture allows for faster shutter speed" when it comes to astrophotography. That is daytime terrestrial photography thinking there, and it doesn't really apply to astrophotography. You need long exposures in astro, LONG exposures, so a faster aperture just means for whatever maximum exposure time you can handle, you get more light. You generally wouldn't want to reduce exposure when moving to a faster aperture, because you aren't getting enough light to start with. In astro, you work with around 0.002-0.005 lux, whereas most daytime photography is 500-100,000 lux! – jrista Apr 24 '15 at 17:25
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    That is a difference of 4-7 orders of magnitude difference in light levels. The reason you would use a faster aperture is to get more light, not use a shorter exposure. The only caveat to that might be if you are imaging in a heavily light polluted area, in which case you might be limited to exposing for 2-3 minutes at f/4, 30s-90s at f/2.8. However in a light polluted area, you should be using a filter like the IDAS LPS-P2 or -D1 to block some of it. – jrista Apr 24 '15 at 17:27
  • @jrista You are correct if you have thousands upon thousands to spend on exotic tracking mounts and can take enough time off from your work to place yourself in remote places where light pollution isn't the primary concern when doing astrophotography. For the rest of us, who don't have those luxuries, a wider aperture allows us to limit the shutter speed to avoid star trails when shooting from static mounts in moderately light polluted environments. To get "long exposures" we have to take many short exposures and align and then stack them. – Michael C Apr 25 '15 at 2:37
  • There is no need to spend thousands on exotic tracking mounts. You can spend about $400 on something like a Polarie or SkyTracker, which is actually what I would recommend for someone using fast primes on a DSLR. I then also VERY strongly recommend using a tighter aperture. I know quite a few milky way imagers who use the iOptron SkyTracker. They will track most of their shots, and usually landscape aspects will blur. Then they will grab a few static shots to stack for the foreground, and a bit of manual blending corrects the interface of ground and sky. Absolutely no need to spend thousands. – jrista Apr 27 '15 at 18:37
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    Again, you are correct for those who wish to not have star trails at the pixel level, who are willing to spend a little, and who are more than casually devoted to astrophotography. But the original question is in light of using a kit lens, which probably eliminates even a $400 tracking mount over a better lens for those whose primary objective isn't strictly astrophotography. – Michael C Apr 27 '15 at 23:51
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You don't need to use the widest aperture. In fact, in many cases, using the widest aperture for astrophotography can result in very poor quality stars. If you are doing wide field untracked imaging (i.e. milky way imaging), then you can usually get away with using maximum aperture, and the larger aperture allows you to use shorter exposures, which reduces star trailing.

If you are doing any kind of tracked astrophotography, it's usually better to stop your lens down a bit to sharpen stars up. With tracking, you can expose for many minutes, so being able to use the fastest aperture is not as important. The impact to star quality from CA and other aberrations can be quite extreme with many lenses, so stopping down for quality becomes critical.

For example, few astrophotographers using 50mm-100mm lenses will use them wide open when using some kind of tracking, as the stars get horrible blue or purple halos around them. Most of the time, a fast 50, even an f/1.4, will usually be stopped down to f/4 to get better, tighter stars and better vignetting. For astro, f/4 is still quite fast in the grand scheme of things, where telescopes at native focal length are often f/8-f/11.

As for what aperture to use. Focus with stars doesn't require deep DOF. The stars, for all intents and purposes, exist at the same focus distance, since they are effectively at "infinity." You can get away with focusing at f/3.5, or even f/2.8, if you have to. The thiner DOF makes it more challenging to acquire focus, but once focus is acquired, then there isn't anything else to worry about. Because of the extremely low light levels, you would not want to use f/16 or f/22...that would just be wasting photons. However, apertures between f/4 through f/8 are acceptable, although even f/8 is kind of pushing it. You shouldn't need to use apertures that low unless your slapping a 2x teleconverter on an f/5.6 lens. Usually stopping down a third of a stop is enough unless your using a very fast lens, (those faster than f/2.8), in which case stopping down to at least f/2.8 is better, and f/4 is usually more ideal.

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    Man am I glad you are around rista, you and your astro knowledge :) – dpollitt Apr 22 '15 at 21:54
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    Oh god ! Hats off to your Astro knowledge for photography... – Saurabh Bayani Apr 23 '15 at 7:24
  • Glad to be of service. :) Astrophotography has become my obsession these days...learned a lot from years ago when I was just poking around with it. – jrista Apr 24 '15 at 17:22
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Widest apertures allows you to capture more light, and using a wide angle lens, the depth of field is considerably bigger. And if you focus at infinity, basically nothing will be out of focus. ;)

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