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Why Aperture is so Important in Astrophotography

Aperture is king…but WHY?

If you ask any seasoned astrophotographer what they want the most out of a lens, they’ll usually say fast glass. Aperture is one of the most important things to consider when choosing a lens for astrophotography.

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In normal daytime photography, the focal ratio is most often more important for depth of field reasons, but in astrophotography, we care more about the amount of light we can get onto the sensor.

Everything in the sky is far enough away that we don’t have the ability to do anything but focus on it all at the same time, so astrophotographers could care less about depth of field — we just want as much light to enter the camera as we can get, as quickly as possible!

Why do we need fast glass?

Because the nature of astrophotography means taking long-exposure photos of something we can hardly see with our eyes, we want to maximize the amount of light that enters the lens. We want to make sure as much light as possible hits the camera sensor as fast as possible.

The focal ratio is called such because aperture is a ratio of the focal length of the lens to the size of the opening that allows light to enter the camera and illuminate the sensor.

You may think that getting a fast f/2.8 lens is not much of a step up from your f/3.5 kit lens, it’s only 0.7 f-stop units different! However, that is huge in the aperture world.

Take the following example:

A photograph shot at, say, 18mm and f/3.5 has captured only 64% of the light available to the same lens at f/2.8!

So that means the same photo shot at f/2.8 is 156% as illuminated, or requires a much shorter exposure duration to get the same illumination at f/3.5. A shot of around 16 seconds at f/2.8 is just as bright as a shot of 25 seconds at f/3.5!

Differences in aperture settings aren’t what you may think

You may think that an image captured with a lens at f/2 is twice as bright as the same image captured at f/4, but it’s not. The relationship between the amount of light available to the sensor and the focal ratio is not linear! An image captured at f/2 vs. f/4 on the same lens is far “brighter” than double!

As a more graphic representation, each bold f-stop value below lets in approximately twice the amount of light as the next bolded f-stop.

f/1.4

~200% of the light at f/2
f/1.8 Common consumer lens
~123% of the light at f/2
~378% of the light at f/3.5

f/2

~200% of the light at f/2.8

f/2.8

Common fast lens
~156% of the light at f/3.5
~200% of the light at f/4
f/3.5 Common kit lens
~131% of the light at f/4

f/4

Common kit lens
~200% of the light at f/5.6

f/5.6

~200% of the light at f/8

f/8

~200% of the light at f/11

The math

The focal ratio for a particular lens is just that: a measure of the focal length of the lens divided by the f-stop. It doesn’t stop there, though. That particular number (focal length / f-stop) indicates the physical diameter of the hole in the lens that lets in the light. Knowing the diameter of the aperture opening allows us to calculate the approximate area of the hole in the lens that allows light in to hit the camera sensor.

Area of aperture opening = π * ((focal length / focal ratio)/2)2

We know that the area of a circle is calculated with the equation π*r2, which allows us to compare the sizes of the aperture openings for all focal ratios available for a given lens.

Why does that matter?

Think of yourself in a dark room with the door closed, with daylight on the other side of the door. The wider you open the door, the more light is let into the room, and the brighter it gets.

apertures

The door, in this case, represents the aperture of the lens, with a larger or smaller opening to allow more or less light to illuminate the camera sensor. Viola! – turn up the lights, you get a brighter image.

For a longer explanation of aperture regarding general photography, see this article that provided me with research data about the topic: A Tedious Explanation of the f/stop, by Matthew Cole

The photographic proof

Here we compare the same frame on the same night, minutes apart. Both have identical camera settings and lens with the exception of aperture. First we have f/4 and second, the same shot at f/2.8.

Milky Way and Histogram f/4 ISO3200 20s, Photo: Cory Schmitz
Milky Way and Histogram f/4 ISO3200 20s, Photo: Cory Schmitz
Milky Way and Histogram f/2.8 ISO3200 20s, Photo: Cory Schmitz
Milky Way and Histogram f/2.8 ISO3200 20s, Photo: Cory Schmitz

 

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About the author

Cory Schmitz

Co-founder of PhotographingSpace.com, co-owner of several telescopes and mounts, too many cameras, and not enough hard drives, Cory is an American expat living in South Africa with his wife, Tanja Schmitz.

An avid astrophotographer for timelapse, deep-space imaging, lunar, planetary, and star trail imagery, he is an all-around jack-of-most-trades for night-sky photography.

He is also an internationally published and commissioned astrophotographer, where his photos have been used in multiple online and print publications.

11 Comments

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  • One would ask, why don’t we all use F 1.0 apertures? In the Netherlands we have “Cruijffs law”: “Elk voordeel hep se nadeel” (Every advantage has it’s disadvantage). In this case price, quality (chromatic aberration, coma) and critical focus depth.

  • Awesome stuff, happy to have F2.8.

    The question is this though,

    if you are shooting static with no tracking, it makes sense to shoot wide open and the longest exposure stime possible.

    But with a tacker, its a game changer, but i have noticed that with trackers some shots are stopped down to f3.2 or even F4 and ofcourse exposure time much much longer, i assume to increase sharpness? Since exposure over time isnt an issue with a tracker, it makes sense to me.

    BUT

    Would it ever be beneficial to do it at F8.0 with a tracked shot? or would that require a huge amount more of exposure over time?

    Keen to her your response.

    • Hi Alex,

      The main reason those tracked shots are often stopped down is for a few reasons:

      * Sharpness
      * Chromatic Aberration
      * Coma
      * Misc. lens issues when wide open

      Even the best of the best lenses create better looking images when they are not wide open. There is a sweet spot when stopped down a step or two for almost every lens, so that is the main reason.

      f/8 is a little much, just a stop or two to f/4 or f/3.2-3.5 is fine in most cases, because you still want light-gathering power. Many telescopes are f5-f11 without an adjustable aperture, but they have a much larger objective, so the light gathering area is huge, unlike the small camera lenses. And remember — the step down in light gathering from f/2.8 to f/8 is far more than linear! So yes, it takes a lot more time.

      Cheers,
      Cory

      • I got an Opteka fisheye lens (~$150 USD) as a gift and immediately went out that night to shoot “wide open” (f/3.5) since we were vacationing away from typical suburban light pollution. I was very excited to capture so much of the sky. I attributed the fuzziness of stars to the quality of lens and decided to live with it. But only once I got home, I thought to step down the aperture by one click (f/5.6) to reveal beautiful, crisp, clear stars.

        Next time I’m away from light pollution, I’ll try some long, tracked (iOptron SkyTracker) exposures to see how much better it can be.

        That coma can really ruin a photo!

    • Hi Alex,

      Another drawbacks for long exposure are “not perfect tracker” and “not perfect sensor”.

      The first one may smudge the stars due to some instability in mechanical parts or bad pole alignment.

      The second one brings too much noise and you have to deal with it using stacking techniques which requires much more long exposed light and dark frames (time grows very significant and you can’t remove the noise completely).

  • Hi, I`m a newbie on this issues, but I want to improve my skills. I bought a Canon FD f/1.4 lens and the FD-EOS adapter to use on my Canon T1i. This lens 1.4 is better than f/2.8 for astrophotography? How to focus the stars if this lens is 100 % manual? Please, any help/info will help me a lot.

    • Hello Elizaul,

      Yes, to answer your question as far as aperture is concerned, f/1.4 is better than f/2.8. Especially because you can drop it back to f/2 or so which will sharpen things up a lot, and still be some extremely fast glass!

      Focusing when the lens is manual is the way to go with stars anyway. So, to start out, set the focus at infinity and take some test exposures. Move the focus ring each direction *slightly* until you find the sweet spot for the focus of the stars, then you’re good to go.

      Cheers,
      Cory

  • I was out last night shooting a very faint Milky (at rise plus light pollution). I began shooting at F2.8, 24mm 20 seconds. The stars looked like lima beans.

    After several long minutes in the dark adjusting exposure length, ISO and mm at F2.8, but could’n’t dial in a sharp star. Finally finding F4.0, 30MM at 20 secs ISO 6400 and a few with ISO 3400 I got a couple I can feel comfortable playing with in editing. I was thinking my fastest glass at F2.8 would be the way to go, but since F4 gave the better result, I’m thinking my slower lenses should tag along for the evening.

    My buddy told me that sometimes it’s just the glass you are using and no fiddling is going to make it better.

    Your thoughts?

    My gear last night:
    Canon 5DM3,
    Tamron 24-70 mm F2.8.

    All manual settings using your advice in the “How To Milky” video. So glad that you emphasize the “test test test” prior to shooting all night. Learned that last year after many unsuccessful adventures.

    • Hi Chilly,

      The lens quality makes a huge difference, and I’m not sure about your lens specifically.

      But, what I CAN tell you is that the reason f/4 on that f/2.8 lens gave a good result is because it wasn’t wide open. Stopping down the aperture a bit on some lenses is often required to make imperfections in the glass go away.

      Next time, try this: Stop the lens down to f/3.5, ISO6400 (on the 5DmkIII, depending on temperature), 24mm, and 20s exposure. Depending on your location, the light pollution may require a lower ISO, but if it’s dark enough, you should be able to shoot at ISO6400 with that sensor and clean it up in post.

      Also, curved stars could easily be movement in the tripod or camera during the exposure. Make sure everything is EXTREMELY stable, especially if the stars don’t look the same from one exposure to the next.

      Best of luck!

      Cheers,
      Cory

      • Thanks for the feedback Corey. Really appreciate all you and T share with us. I’m going to rent a lens and see what happens. Thinking the Canon 16-35mm F2.8 might be the one to play with.

        Chilly

        • Hi Chilly,

          I have the 16-35 f/2.8 L2 Canon lens, it’s what I shot almost everything in my new timelapse short film, SUTHERLAND, with. For just AP, though, I prefer the 24-70 f/2.8 L2…it’s near perfect!

          Cheers,
          Cory

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