Photography stuff, explained simply

It took me a long time and a lot of research to learn the intricacies and facets involved in photography, particularly star photography. This post will attempt to explain core photography concepts in simple terms, their context in star photography, and how they'll help you get better shots. Hopefully this will be an ever expanding list as I learn more and more.

TLDR means Too Long Didn't Read and will include a short summary

Terms and concepts:

  1. Focal length
  2. Full vs crop frame sensor
  3. Lens speed and aperture
  4. Rule of 500
  5. Exposure time
  6. ISO sensitivity

Focal Length

Focal length is arguably the most important concept to understand for star photography. At it's simplest focal length is the distance between the sensor and the lens while a subject is in focus. This is denoted on the lens, if you've heard of 35mm or 50mm lens you're already familiar with the basic concept. Focal length is extremely important in star photography because it affects the maximum time you can expose your shot. In short the longer the focal length, the less time you can leave the shutter open before the stars begin to blur from movement. (To find out how much please see Rule of 500).

To understand why check out this series of photos
Same photo different focal lengths Consider that all these shots have the same resolution, meaning they have the same amount of pixels. This means that a distance between the barns in the wider angle shots (smaller focal length) is represented by less pixels then that same distance with the longer lens. While this means that longer focal lengths allow for more detail it also means that movement, like that of the stars, is more pronounced and therefore will blur more quickly when taking long exposure shots.

TLDR: The shorter the focal length the longer you can expose the shot for without stars blurring. The longer you can expose the shot for the more light and detail you can gather.

Full vs crop frame sensor

DSLR cameras fall into 2 distinct categories, they are either full frame or crop frame. This label describes the size of the sensor, a full frame sensor is larger then a crop frame sensor. Because of the size difference a full frame sensor can go wider and capture more detail then a crop frame sensor. For star photography it's important to know that the mechanism in crop frame cameras actually increases the focal length by a factor of around ~1.5x depending on the make and model of the camera. This means that a 50mm lens would actually have a focal length of 75mm on a crop frame sensor. Make sure to take this fact into account when doing Rule of 500 calculations! It is also the main reason that a full frame camera is preferred (but not required!) for star photography.

TLDR: Full frame sensors are ideal because they are capable of shorter focal lengths, wider angles and longer exposures. This doesn't mean you need a full frame camera for star photography!

Lens speed and aperture

Lens speed refers to how fast a lens can shoot and achieve the same exposure relative to other lenses. If that's confusing it's much simpler to just think of the speed of a lens as it's maximum aperture size, aka lowest f-number available. To understand lens speed and aperture you need to understand what f-number (aka f-stop) means. An f-number is a simple ratio between the focal length and the width of the "pupil" (aperture) of the lens. F-numbers come in "stops" for example f/1.4, f/2, f/2.8 etc. These stops represent an exponential scale of the area of the pupil. This means that each "stop" has an aperture half the size of the previous stop. Check out this simple diagram for clarity:

F-number diagram

Prime lenses (lenses with fixed focal lengths) are the fastest and the best can achieve an f-number of f/1.4

Zoom lenses (lenses with adjustable focal lengths) are much slower by comparison and usually max out around f/3 or higher.

This is very important for star photography in particular, a faster lens (lower f-number) means a wider aperture, which means more light is gathered. A lens shooting at f/1.4 could get the same exposure as one shooting at f/2 in half the exposure time, or get twice as much light by exposing for the same amount of time!

TLDR: Lens speed = widest aperture size (how wide the lens can open). This is quantified by an f-number and a lower f-number is wider. The lower the f-number your lens can shoot at the more light you can get in the same amount of exposure.

Rule of 500

The rule of 500 is a simple and quick way to determine your maximum exposure time before stars begin to blur.

500 / (your focal length) = max exposure in seconds

For example if you have a focal length of 24mm you take 500 / 24 = 20.833 meaning you have a maximum exposure time of around 21 seconds. I tend to round down to be safe.

Exposure time (shutter speed)

Exposure time is simply how long you leave the shutter open. The longer the shutter is left open the more light can be gathered. In the context of star photography usually the longer you can expose the better, but factors like the rule of 500 will limit your maximum exposure time.

ISO sensitivity

ISO sensitivity is another important piece of the star photography puzzle. It's essentially a number to represent your sensor's sensitivity to light. Sounds great right? Crank that baby all the way up and let's shoot some stars! Unfortunately increased sensitivity comes with a caveat of increased grainyness or "noise" in the photos. This means that ideally you want to use the lowest ISO you can to still achieve the shot you want in order to keep the grain to a minimum. Check out this zoomed in comparison, the top shot is low ISO sensitivity and the bottom shot is a much higher ISO sensitivity:

ISO comparison

If you have any corrections or additions to make please let me know! You can hit me up at @philofadventure on Facebook or twitter.