Why lens f-stops and front filter diameters matter for good signal
Sigma 28mm prime art lens showing the 77mm front lens element and filter area.
Light allows a photograph to be captured.
The three components that, when combined, affect the quality of light are collectively called “signal.”
The recipe for a strong signal is:
an f-stop of 2.0 or better, like f1.8 or even f1.4;
a front lens element filter size that is as large as possible like 77mm; and,
time (i.e. shutter speed).
Two of these three components are dependent upon lens choice: f-stop and the front glass element/filter size.
Meet my favorite Milky Way Dark Horse Nebula lens: a 28mm, f-1.4 Sigma art prime for Nikon F with a 77mm front lens element/filter.
Once I understood that signal is directly proportional to image quality, I invested in glass that met the criteria and my life was transformed for the better when it comes to low-light photography especially with regard to the Milk Way!
In addition, f-stops of 2.0 or better (i.e. f1.8 or f1.4) make it so much easier to SEE a bright star in a camera’s magnified LCD screen so I can achieve sharp focus without having to take a series of test images making minute lens focus adjustments between each one. To compare, my collection of lenses with f-stops of 2.8, 4 and smaller make it difficult or impossible for me to see to focus using the LCD screen because not enough light — i.e. signal — is being passed through from the lens to the camera.
It has taken me over 45 years, but now I finally get why lenses with larger apertures and larger front lens elements command a premium price, especially if they are primes and not zooms.
For my low-light exposure workflow especially when my objective is the Milky Way, I choose:
an f-stop of 1.8 or 1.4;
a lens with a large front element, like 77mm;
a camera shutter speed that is appropriate to the light source in my composition — more time means more light.
What I don’t want more of is ISO, which is the camera’s sensitivity to light. Why? Because a higher ISO means images will also be higher in pixelization or grain — normally, an unwanted result. For me, when photographing in low light, I strive to keep my ISO as low as possible given the other three components of signal that I can adjust in the field to achieve my creative vision.
I invite you to learn more about signal by tuning into episode #39 “The Power of Signal", which is part of the free audio (i.e. radio) show entitled, “After Dark Photography Podcast” — a series produced and narrated by Kristine Rose Richer of Kristine Rose Photography out of Nova Scotia, Canada.