Astrophotography 101 - Critical Aspects of Nightscape Photography - Lenses

When considering what is and is not important for executing a nightscape composition one has to take into account a huge number of factors. Ultimately what will dictate the final image quality of any given composition will come down to a number of factors and how the photographer accounts for those factors. An image, when distilled to its basic elements, is the accumulation of signal (light) over a given exposure time. The things that degrade image quality, such as noise or lens artifacts, can be mitigated via a number of methods. So, to maximize image quality you need to maximize signal and minimize noise/artifacts. Signal to noise ratio (SNR) is an important factor for producing a high quality Nightscape, signal (light) is collected by the lens and delivered to the sensor where the sensor converts it to a digital element. That signal can be altered by the lens based upon lens design, artifacts such as coma and longitudinal chromatic aberration (LoCA) as well as softness, decentering and other lens faults will degrade image quality. Things like increased noise will be relevant in areas where light pollution is prevalent, it will also become a factor when the sensor collects the signal and converts it digitally. Sensor induced sources of noise abound when images become heavily processed, noise increases due to temperature, increase in gain (ISO), as well as conversion/quantization noise/dark current. Let’s explore the basics of what camera and lens settings choices do to image quality and what lenses settings help maximize image quality while reducing unwanted artifacts/noise, let’s start with the lens:

A lens choice for Nightscape imaging is the single biggest factor for final image quality, a lens that delivers a large amount of light (has a large entrance pupil and aperture area) while having the fewest defects (such as coma, LoCA, softness, and decentering just to name a few) will make the biggest impact in terms overall image quality. Your camera sensor cannot deliver a high SNR exposure without the sensor collecting high amounts of signal, your lens is the only thing capable of delivering signal to your sensor. Step 1 for creating a very high quality Nightscape image is find a lens that delivers a large amount of signal, f2.8 is the absolute narrowest aperture that should be used for Nightscape imaging without a tracking mount, I recommend a lens in the f1.4 to f1.8 range (and options are now plentiful so there is no reason not to use one in this range). Also keep in mind the aperture area and resulting f-ratio will be proportional to the focal length of the lens, wider is NOT better despite repeated misnomers from people online. An ideal nightscape lens will be in the 20mm to 35mm range (full frame focal length equivalent) with an aperture in the f1.4 to f1.8 range, this focal length range and aperture range will deliver maximum amounts of signal, allowing exposure lengths in the 8-20 second range (use NPF rule for untracked exposures, virtually “unlimited” exposure length for tracked). Multiple manufacturers now have lenses in this focal length range that are mostly free of aberrations/artifacts so choices are not limited, budget is the only concern. Feel free to contact me for lens suggestions for any camera manufacturer, I’ve used A LOT of lenses from Sony, Canon, Nikon, Sigma, Voigtlander, and others in this focal range.

For tracked Nightscape images one has greater leeway on their lens choice, tracking mounts provide the freedom for an individual to stop down their aperture while still collecting large amounts of signal to deliver to the sensor. Stopping down a lens reduces aberrations and artifacts in many (if not most) cases, the most obvious of aberration/artifacts corrections that will be corrected are Coma and LoCA, which can be dramatically improved by even just a third or half-stop down of a given lens. Sharpness, especially towards the edge of the frame, can also be dramatically improved in some cases (although not all lenses respond equally, or even at all, in this area when it comes to stopping down a lens). Tracking mounts can allow lenses that are otherwise mediocre and full of artifacts when shot wide open to become solid value for cost performers when stopped down 1 or 2 stops. A cheap lens like the Rokinon 24mm or 35mm f1.4 may be weak in certain aspects, such as sharpness or coma, at f1.4, but used on a tracking mount and stopped down to f2.2 to f2.8 those lenses because much better performers capable of delivering high quality results.

Continuing the discussion of critical aspects of Nightscape Photography, next we'll discuss camera bodies:

Critical Aspects of Nightscape Photography - Cameras