F-Numbers and F-Stops Explained: Mastering Focal Ratios for Better Astronomy and Astrophotography

Have you ever wondered why some astrophotography shots look amazing while others fall flat? It all comes down to light—light is everything in both astronomy and photography! In astronomy, capturing more light means your telescope or astro camera can reveal faint celestial wonders. In photography, getting the right amount of light is crucial for perfect exposure and image quality.

That's where the focal ratio—often called the f-number or f-stop—comes in. It’s your tool for controlling how much light enters your telescope or camera, and mastering it is key to achieving those stunning results. A lower f-number means more light enters the system, while a higher f-number means less light enters the system. Low f-number systems are often called 'fast' because they gather more light in less time, whereas high f-number systems are referred to as 'slow.'

To calculate focal ratio, divide an optic's focal length by its aperture. For example, a Celestron 8" Schmidt-Cassegrain telescope has an aperture of 8 inches and a focal length of 80 inches. Dividing the focal length by the aperture gives an f-number of f/10.

Focal ratio affects telescopes and cameras differently. Let's dive into each system to learn more.

F-Numbers and Telescopes

A telescope's f-number is fixed because it is based on the size of the scope's primary lens or mirror. However, adding an accessory like a Barlow lens or focal reducer alters the system's focal length, changing the f-number.

Using a Barlow Lens

Going back to our earlier example of the 8" Schmidt-Cassegrain telescope, adding a 2X Barlow lens doubles its focal length from 80 to 160 inches. The result is a slower system with an f-number of f/20. The f/20 system produces an image four times dimmer than an f/10 system—but with much greater magnification. Slow systems with Barlows are ideal for lunar and planetary photography. The Moon and planets are small, so you need high magnification for increased image scale. But they are also extremely bright, so the dimmer optical system cuts glare and reveals detail.

For more information about Barlow lenses, click here.

Using a Focal Reducer

On the other hand, a 0.63X focal reducer decreases the focal length of this same telescope to 50.4 inches, resulting in an f-number of f/6.3. The reducer makes the image about four times brighter compared to f/10, which is great for capturing faint deep-sky objects that cover a wide field of view, such as nebulae.

F-Numbers and Camera Lenses

With photographic camera lenses, the aperture is adjustable, which adds another layer of control over light exposure. A camera's f-stop refers to the setting of a diaphragm inside the lens that determines how much light enters.

Small f-numbers (Wide Apertures): Let in more light but can introduce image aberrations, especially around the edges. RASA and Origin are a special case. Their optics have corrector lenses that eliminate these aberrations allowing a flat field.

Higher f-numbers (Narrower Apertures): Reduce the light but increase image sharpness and depth of field, making them ideal for terrestrial scenes where both the foreground and background need to be in focus.

In astrophotography, stopping the lens to a smaller aperture can be counterintuitive, but it often results in sharper stars and overall improved image quality by reducing aberrations. For example, a 50mm f/1.4 camera lens may produce blurry stars at its widest aperture, while stopping down to f/4 reveals more stars in sharper focus, even if it requires a longer exposure.

In terrestrial photography, a smaller camera lens aperture also increases depth of field, which means more of the scene is in sharp focus. For instance, if you focus on a subject 10 feet away at f/1.4, only objects close to that point will be sharp. However, if you stop down to f/8 or f/11 and adjust the exposure time, the entire range from a few feet away to infinity can appear sharp.

The Progression of F-Numbers

The amount of light that reaches a telescope eyepiece or camera sensor decreases by the square of the f-number. For example, the light at f/4 is four times dimmer than at f/2. The progression of f-stop numbers follows a specific pattern, with each higher f-stop passing half as much light as the previous one. These standard f-stop numbers are:

1.4, 2, 2.8, 4, 5.6, 8, 11, and 16.

F-Numbers and the Human Eye

The human eye also works like a camera lens. The iris can expand or contract to vary the aperture of the eye's pupil. In low light, the iris opens to around f/2.1, while in bright light, it contracts to about f/8.3.

Dark Adaptation: When fully dark-adapted and opened as wide as possible, the aperture of the human eye typically ranges from 6 to 7 millimeters in young individuals. It decreases with age to between 4 and 6 millimeters.

Vignetting: In astronomy, the diameter of the eye’s iris is significant because the exit pupil (the diameter of the light cone exiting the eyepiece) should be smaller than the iris. If the exit pupil is larger, some light will be blocked, causing vignetting—a dimming of the edges of the image.

Increased Sensitivity: The human retina also adapts to darkness, increasing sensitivity equivalent to about 20 f-stops after approximately 30 minutes in darkness.

Aging Effects: As we age, the lens of the eye stiffens, reducing its ability to adjust focus, particularly in low light conditions when the iris is fully open. This is why corrective lenses are often needed for focusing in dim conditions, especially for those who have undergone cataract surgery and have a fixed artificial lens.

Key Takeaways: F-Numbers in Astronomy and Photography

Lower F-Numbers: A smaller f-number (wider aperture) is generally desirable for gathering as much light as possible from faint celestial objects. This is ideal for deep-sky astronomy.

Higher F-Numbers: A higher f-number (narrower aperture) is useful for increased magnification, such as in lunar or planetary observations, or for improving image sharpness and depth of field in photography.

Understanding F-Numbers: Knowing how to manipulate the f-number helps you choose the best optical system for your current viewing or imaging needs, whether you're exploring the cosmos visually or capturing a perfect photograph.

Now that you know the ins and outs of f-numbers, it's time to put that knowledge to use! Experiment with different settings, try using a focal reducer or a Barlow lens, and see how it changes your results. Clear skies!