Of all the many parts of your kit, the lens is your most powerful creative tool. Lenses, more than any other part of the camera, create the look and feel of your photographs. And it's not just about focal-length and aperture, either: the character of the glass and construction of your lens creates a certain visual quality. Some lenses are tack sharp, others silky smooth. Some lenses are warm and bright, while others are cool and moody. Photographers go to great lengths to seek out coveted lenses for the special way they draw an image.
In this tutorial we'll look at different kinds of lenses - zooms versus primes - for night photography. We'll consider the features and drawbacks of each, and how to choose your lenses for photographing at night.
Before we launch into choosing zooms versus fixed focal length lenses for night photography, we had better set some baselines. Any discussion about lenses, focal length, and aperture begins with terms of reference.
The 135 format is also referred to as "full frame," the modern term for digital sensors with the same image area as 35mm film. When the focal length for a lens and camera combination is given it is usually described in terms of a 35mm-format "focal length equivalent."
The focal length of a lens is normally given in millimetres, with shorter focal lengths providing wider angles of view. This angle of view can be split into three distinct categories: wide angle, normal and telephoto.
The "normal" lens for a camera and lens combination is equivalent to the image diagonal of the camera format. The image area dimensions for the 135 format are 24mm x 36mm: a diagonal of the image area of about 43mm.
Therefore a 50mm lens on a full frame camera approximately matches the diagonal of 43mm and is considered "normal." A normal lens gives approximately the same perspective as the human eye. Focal lengths for lenses of less than 40mm are referred to as wide angle and focal lengths above 60mm are called telephoto.
Cameras with a different size sensor to full frame use a "crop factor" to express the 135mm counterparts to a given focal length. This gives you the full frame "focal length equivalent" for any lens and camera combination. APS-C cameras have a crop factor about 1.5 times that of full frame cameras. For Micro Four Thirds you need to multiply focal length by a factor of two. This means that on a Micro Four Thirds camera a 50mm normal lens now becomes a 100mm telephoto lens once you multiply by the two times crop factor.
Crop factor also helps to determine the depth-of-field or zone of focus at any given aperture. As film format or sensor size decreases depth of field at the same aperture increases. You gain about an extra f-stop-worth of apparent depth-of-field for APS-C cameras and two stops worth for Micro Four Thirds systems.
For night photography, crop-sensor cameras can produce just as good results as full frame cameras. They don't have the same image quality at very high ISOs, but used in the 200 ISO to 1600 ISO range many APS-C and Micro Four Thirds cameras are very capable of holding their own when making night exposures. These cameras also have the advantage of being smaller, using smaller lenses, and requiring less power. The also typically cost less than their larger counterparts.
Aperture refers to the diameter of the opening through which light enters the lens. It has a numerical scale measured in f-stops. Fast lenses have a large aperture and allow more light into the camera and onto the film plane or image sensor.
By adjusting this aperture the amount of light entering the camera can be controlled in the same way our pupils control the light entering our eyes. When a bright light source is present the pupil contracts allowing less light into the eye. This has the effect of increasing the depth of field meaning more of the scene will be in focus.
The same rule applies when you decrease (stop down) or increase (open up) the aperture of a lens. As aperture size increases more light enters the camera and depth of field decreases and vice versa: as aperture size decreases less light enters the camera and depth of field increases.
Understanding the role of focal length and aperture is critical for controlling the "zone of focus" in your images. You can use this focus control to isolate a specific subject within the frame or to ensure the whole scene is in sharp focus.
A Matter Of Perspective
Lens choice also influences another important composition tool: perspective.
Subjects shot with a 50mm lens exhibit no discernible geometric distortion and appear... normal. Funny about that! However, If you substitute the 50mm lens for a 20mm extreme wide angle lens a completely different picture emerges.
Objects appear to be smaller and further away from the camera as the viewing angle increases. The distance between separate objects also seems to increase. Straight lines may seem to curve especially towards the edges of the frame and objects can appear distorted.
If you again substitute the 20mm wide angle lens for a 100mm telephoto lens it has the effect of zooming in on your subject by making the viewing angle narrower. Telephoto lenses also compress the distance between subjects making them appear closer together.
You can also increase or decrease subject size within the frame by moving the camera closer to or further away from your subject. Moving the camera changes perspective and you can control the scale of objects in the scene with the placement of the camera.
Controlling perspective is an important tool in every photographer's arsenal. It can be used in a variety of creative ways to compose your scenes.
The Sweet Spot
Very few lenses perform optimally at the extremes of aperture. In other words, you should avoid shooting at maximum and minimum apertures if possible. At their wide-open and closed-down settings lenses exhibit a number of optical flaws. This can degrade image quality considerably.
At maximum aperture with the lens fully open you will get a softening of the image due to coma, also known as astigmatism. You may also observe more vignetting and colour fringing along with a loss of contrast when the lens is fully open.
When a lens is stopped down to its minimum aperture you will also observe a decrease in image quality from the effects of diffraction. Diffraction causes a loss of resolution, making images appear soft. This is more obvious on larger-sensor cameras.
So, image quality is best at middle apertures. When you combine these apertures with low ISO values and a stable camera platform you can increase the quality of your night photography images considerably.
As a rule of thumb you should use apertures at least two stops up from the maximum and two stops down from the minimum. I generally shoot using apertures between f/5.6 and f/11. Rules are made to be broken and with experience you will know when you can bend or break these rules.
Having a large maximum aperture - say f/1.4 or f/2 - makes the image in the viewfinder brighter. This assists in ensuring critical focus in low light conditions. It also means these lenses are performing optimally at apertures around f2.8 to f/4, which happens to be the max aperture of most professional-grade zoom lenses. Those fancy zooms still require stopping down to f4 or f5.6 before maximum image quality is assured.
Zoom Lenses or Prime Lenses?
Lenses come in two distinct categories: zooms and primes.
The Prime Directive: Quality and Character
Prime lenses have a fixed focal length. This means that the design of the lens is optimized. Prime lenses also generally have a faster maximum aperture. This allows more light into the viewfinder making it brighter and easier to focus, which is handy in low light scenarios.
My personal preference is to use full-frame, fast-aperture, manual-focus primes: a set of Nikon AI-S lenses, older now, but with wonderful optical qualities. For my style of night photography these lenses offer several advantages, including calibrated infinity stops, wider apertures, and versatility.
Prime lenses, especially older, manual-focus ones, have an infinity stop. An infinity stop is a calibrated physical limit on focus. This ensures focus on distant objects in low light. Subjects I like to shoot, like lightning and astrophotography, benefit greatly from having this feature and I use it all the time.
Older primes also have marked hyperfocal ranges. Knowing you can set focus and gain maximum depth of field without even looking through the viewfinder can come in very handy at times.
My prime lenses work on the three camera formats I shoot with. They are equally at home on an APS-C or Micro four thirds camera as they are on a full frame body. This versatility makes them a good long term investment and I can use them for video, time lapse and photography work.
On the other hand, depending on the subjects you primarily photograph, prime lenses can be less versatile than zooms because they may need to be changed frequently. This leaves you susceptible to getting dust in the camera or onto the digital sensor.
The Zoom: Flexibility for a Price
Zoom lenses have a variable focal length, making them more versatile then prime lenses. A general purpose zoom lens can fulfill a variety of roles.
Most entry level cameras come with a zoom lens that allows you to vary the viewing angle from wide angle through to telephoto. Some modern "superzooms" cover a very large range of focal lengths. They can replace a whole suite of prime lenses, meaning you could leave one lens attached to your camera indefinitely.
This versatility does have a cost. Zoom lenses generally have compromises in their optical design and image quality is usually - but not always - inferior. Due to design limitations they are also generally slower, allowing less light in through the viewfinder making it more challenging to gain critical focus in low light.
Zoom lenses have further disadvantages for night photography. Most zooms, and especially recent ones, do not have infinity stops or marked hyperfocal distance ranges. They're often also more optically bland and clinical, lacking the attractive character that older lenses can have. Beware that many zoom lenses today have an image circle that does not cover full frame sensors. When considering a zoom, double check to make sure it will fit your camera.
Recent advancements in design and manufacturing technology, however, have shrunk the quality gap between primes and zooms. I have added several zooms to my lens collection for added versatility and for when I don't want or need to carry a suite of prime lenses.
The Sigma 18-35mm f/1.8 zoom, for example, could replace several of my prime lenses and its fast too! Add a fast 50mm prime and a 70-200mm f/2.8 zoom and with these three lenses you would be covered for most night photography situations.
Your Lens is a Powerful Creative Choice
Now that you have some background information on lenses you can begin to make informed decisions about the type of lenses you might like to have for the type of work you wish to do.
Is the focal range versatility of a zoom more important to you than the ultimate image quality of a prime lens? Do you really need full frame lenses if you only intend to shoot on APS-C camera bodies? Only you can answer these questions. From my perspective prime lenses are generally a better investment for night photography: contrast and resolution is sharper, and they have fewer defects from spherical and chromatic aberrations. I encourage you to come to conclusions of your own.
Knowing what equipment will serve you best and understanding how to get the most from your gear takes time. Research your options. If you are curious about a certain lens, try renting it before you buy. Experiment with different lenses and observe how changing aperture and focal length effect image quality, field of view, perspective and depth of field.
See you next time where I will look at some of the basic equipment you will need for night photography and make suggestions regarding what gear to buy on specific budgets.