# How to choose the right focal length for your astrophotos

To perfectly frame objects in your field of view, you’ll need to know your telescope’s focal length — and a bit of math.
| Published: August 22, 2024

On the first night out with my brand-new Celestron NexStar 8SE back in July 2015, I knew little about the relative sizes of astronomical objects. I was bowled over by my first view of Saturn — an impressive sight through an 8-inch Schmidt-Cassegrain telescope (SCT) — and I was eager to see more.

I knew the view through the scope would not be like images I had seen online, but when I slewed to the Lagoon Nebula (M8), all I could see were stars, with no hint of nebulosity. On my third night out, with a newly acquired T-adapter to connect my DSLR to the telescope, I took a 30-second exposure, and could finally see the colorful nebula! But it filled the entire frame and didn’t look anything like pictures I’d seen. “Why?” I wondered.

## Focal length and field of view

One important number associated with every telescope is its focal length, or the distance the light travels after passing through the objective to your eyepiece or camera. This figure is commonly given in millimeters (mm). For example, my 8-inch SCT has a focal length of 2,032 mm. Imagine it like an enormous 2,000mm camera lens!

This number can also be expressed via focal ratio, or f-number. Unlike camera lenses, telescopes have no iris to reduce the effective aperture, so the focal ratio is simply the focal length divided by the aperture. For my 8-inch, whose aperture in metric units is 203 mm, the focal ratio is f/10. Another example is my 106mm f/5 refractor, whose focal length is 106 × 5 = 530 mm.

People at stargazing events often ask me what my magnification is. But when framing a particular object in your eyepiece or image, the more useful value is the field of view (FOV), or how large an area of sky can be seen with your telescope and a given camera or eyepiece.

The quick and dirty calculation for a camera is FOV = 3,436 × (sensor dimension) ÷ (focal length), where the sensor dimension is the length or width of your camera sensor in millimeters, and the focal length is also in millimeters. The result is the FOV in arcminutes; divide by 60 to get degrees. For a Nikon DSLR with an APS-C (crop) sensor, with dimensions of 23.6 mm by 15.7 mm, the FOV of my 8-inch SCT is then 39.9′ by 26.5′, or 0.67° by 0.44°. For an eyepiece, FOV = (eyepiece apparent FOV) × (eyepiece focal length) ÷ (telescope focal length).

There are also online calculators; my favorite is https://astronomy.tools/calculators/field_of_view.