HDSLR Guide Chapter 3: The Lens
The ability to choose from literally hundreds of lenses opens up a world of possibilities once impossible for small-budget productions. Being able to capture a wide view of a small room with a super-wide-angle lens, or getting a close-up of a subject from a distance, are just two of many powerful options at the photographer’s disposal.
HDSLRs aren't limited to using a fixed lens, or limited to just a few interchangeable lenses, like some prosumer video cameras. If high resolution is needed for detailed scenes, there's a lens for that. If the project calls for ultra-low-light capabilities, there's a lens for that, too. On lower-budget productions, cheaper still lenses can be uitilized as well.
With so many choices, finding the right lens can become quite complicated. So before leaping to a decision, it’s best to look at—and understand—the features, characteristics and limitations of the different types of lenses available. Lens quality and accuracy are the most important features for getting the best images from an HDSLR camera, so it may even be preferable to own a few high-quality lenses than a large selection of lower-end ones.
Cine versus Still Lenses
Aside from the wide assortment of still lenses available for use on HDSLRs, there are adapters that allow the use of high-end cinema PL lenses. A good cinema lens is one of the best options for shooting scripted productions. While most shooters won't be purchasing cine lenses anytime soon (the prices range from $4,000 to over $100,000), renting a set of prime lenses for specific projects can open the door to a whole new world of available glass. Although many still lenses hold their own in image quality when compared to cinema lenses, there are many features that make cinema lenses a better option for certain productions.
Cinema lenses are designed to stand up to years of production use. Still lenses are designed to be small, light and inexpensive—three things that limit quality. For the most part, optical aberration and distortion are far reduced on cinema lenses.
Pulling accurate focus is very difficult with most autofocus still lenses because there is no fixed relationship between the ring position and the focus distance. Additionally, the focus marks are condensed, hard to read and not placed on the side of the lens, where it can be seen by a focus puller. Some still lenses appear to zoom in and out when the focus is adjusted, a phenomenon known as “focus breathing.”
In the world of film production, working with lenses that are closely matched in color, size and maximum aperture is crucial. These qualities aren’t required for still photography; hence still lenses don't do well in these areas, either. And to use a follow focus with a still lens, a focus ring needs to be added because still lenses are not geared the way cinema lenses are.
Lastly, some shots require the aperture to be changed during filming. Most still lenses have hard transitions from one aperture setting to the next, so the recorded image will jump in brightness as it is changed. For this reason, cinema and video lenses have smooth aperture steps.
With all of these limitations, one might get the impression that still lenses can't be used for filmmaking. However, in terms of image quality, still lenses compare very well to most cinema lenses. Still lenses have been used effectively on many feature films for years, but their drawbacks need to be addressed to make the most of their capabilities. Some companies, such as Century Optics, have even been re-housing still lenses to make them more suitable for filmmaking purposes.
Some still lenses are better suited for filmmaking than others, so it’s important to consider the features to look for in a still lens. Choosing the right lens depends on the situation at hand. A high quality "normal" (50mm on full-frame sensor) lens can be had for a few hundred dollars, but as the focal lengths get more exotic, finding a good balance between quality and functionality can be difficult. The criteria listed below focus on the most important characteristics of a lens for video. However, each scenario requires a different balance of priorities. For example, close-up shots of people might be best reproduced with a lens that has good contrast and color but less resolution. On the other hand, for a wide shot—like a detailed cityscape—minimal distortion and higher resolution are more important. The great thing about still lenses is that they are much cheaper than cinema lenses, making a wide variety of specialty lenses available to more people.
Autofocus versus Manual
Autofocus lenses are great for still photography, especially with today's advanced camera autofocus systems. Most still shooters use some form of autofocus mode because it's very effective. Autofocus for video, however, is another story. Most professional filmmakers don't use autofocus in video because it's not as accurate and doesn't always "know" where the intended point of focus is. This limiation is exacerbated in low-light situations where the camera has a harder time knowing what is or isn't in focus. Regardless, most HDSLRs do not have usable continuous autofocus. This means manual focus is the only option for the majority of users. For this reason, it's important that the mechanics of a lens be optimized for manual focusing.
Most modern still lenses are designed to be used primarily in autofocus mode. Although they can be manually focused this is not their primary design, so they don’t offer the same level of control for manual focusing. For example, the focus ring does not stop turning when the focus has reached its limits. This makes pulling an accurate and repeatable focus very challenging. Because of the way the focus ring engages the lens elements indirectly, it may turn slightly and not change the focus until it's turned with more force. This can be referred to as "play," the level of which varies from one lens to the next (higher-end autofocus lenses have less of this issue). The lack of a tactile and responsive control over the focus will be more frustrating to those experienced at pulling focus because they're used to a certain degree of accuracy. Entry-level users, however, may be more forgiving.
Pulling focus requires that the distance, or focus markings, on the lens be very clearly visible to the person pulling the focus. Because autofocus lenses are mostly designed to be used in autofocus mode, the markings are small, in hard to see areas, and don't display the full range of distance markings .
Most modern still lenses also do not have a way to control the aperture from the lens, but only from the camera. For HDSLRs that don't have manual exposure control, a lens with a manual aperture ring is essential and should be a priority when choosing lenses.
Manual lenses, on the other hand, fair better in all of these areas. Manual still lenses such as the Nikon AI and Zeiss ZF/ZE are widely available and do a much better job than autofocus lenses. These provide a much more accurate focus control and also have manual aperture rings. However, if plans call for using them for still work as well, they won't allow the use of autofocus, which may be an issue for some photographers.
External versus Internal Focus
Lenses focus on different distances by moving optical elements within the lens. This can either be done within the body of the lens (internal focusing), which does not result in a shift in the length of the lens, or it can focus by extending the front element of the lens outward (external focusing)—which effectively changes the size of the lens as the focus is being pulled. These lenses can be quite tricky to work with, especially when using a matte box where the front of the lens would collide with the back of the matte box. Additionally, the change in lens size can throw a stabilizer system out of balance. Higher-quality lenses will typically have an internal focusing design.
Internal versus External Zoom
As with external and internal focusing lenses, the zoom elements can either be internal (i.e. the size of the lens does not change when zooming) or external (i.e. the front element of the lens expands as the focal length is changed). An internal zoom lens is more ideal, for the same reasons as having an internally focused lens. Some external zoom lenses also have an issue of "zoom creep." When the lens is tipped forward, the force of gravity pulls on the front element and changes the zoom position. Additionally, if the external zoom lens has its focus ring in front of the zoom ring, zooming will disrupt the alignment with a follow-focus unit if the focal length is changed.
Barrel distortion is an optical effect where the image is spherically distorted, making straight lines bend inward. This effect appears mostly in wide-angle and fisheye lenses. With both moving-image and still photography, software can be used to correct this aberration after the image has been captured (although there is always a resulting resolution penalty). This effect can be very distracting in videography, especially if the camera pans across a scene with vertical lines and structures. Despite this, some genres—such as surf and skate documentaries—commonly use the fisheye effect intentionally.
Sensor Size Determinants
The size of the sensor in the camera is also a determining factor when choosing a lens. Certain lenses are designed for the smaller APS-C-size lenses and cannot be used with full-frame cameras. They are, however, less expensive and can be a good entry-level option for APS-C cameras.
When using 35mm/full-frame-compatible lenses (like Canon's EF line) on a "cropped" sensor (APS-H, APS-C, etc.), the field of view will change. This essentially has the effect of cropping an area of the lens, so rather than having the field of view of a 50mm lens, it will be more like that of an 80mm. This means that with APS-C-, APS-H- and Micro-Four-Thirds-sized sensors, wider lenses need to be used, making it more expensive to get the same field of view as on a full-frame camera. The use of super-wide lenses as regular wide lenses also means having more distortion characteristics in a narrower field of view. So although a 50mm will have the same FOV as an 80mm on a full frame, the 50mm will not have the other optical characteristics of an 80mm lens.
In this respect, full-frame cameras are better for wider shots. On the other hand, a cropped sensor means the shooter can get closer to the action with a shorter lens. For example, a 200mm lens becomes a 320mm lens on an APS-C camera, which is cheaper than buying a 300mm lens (of the same quality).
A principal characteristic of a lens is its maximum aperture—the maximum size of the hole through which all light must pass. The wider the aperture opens, the "faster" the lens, because in still photography, a wide aperture allows the use of faster shutter speeds to minimize motion blur (although this is not something most videographers would do).
Some zoom lenses employ a variable-aperture mechanism that can use its widest aperture only within a certain focal range. As a rule of thumb, the faster a lens is, the more expensive it will be (assuming that other factors, such as optical quality, are equal). In photography, a fast lens is great for taking pictures in low light, but when shooting video, a very wide aperture impedes the ability to pull accurate focus due to the extremely shallow depth of field it creates. This is more of an issue with full-frame cameras than with a cropped sensor. A 2.8 aperture is typically as wide as the shooter will want to go (on an APS-C camera) in order to control depth of field. That means a 1.4 lens won’t be used at its maximum aperture unless absolutely necessary.
Some zoom lenses cannot maintain their maximum aperture at all focal lengths. Zooming with most variable aperture lenses is not recommended because the aperture will change while the focal length changes. The resulting footage will have noticeable jumps in exposure when zooming.
Focus breathing is when the lens appears to zoom in and out as the focus is changed. All still lenses have a certain amount of breathing, but to varying degrees—some of which can be very distracting to watch. If this presents a problem in post production, the image can be digitally zoomed to counteract the zoom effect of the breathing. However, on HDSLR footage, this may cause more trouble because of high compression and loss of resolution.
The focus throw is the distance the focus ring on a lens travels from the closest point of focus to infinity. A longer focus throw means it takes more turning to get through the full range of focus. Longer throws are preferable when there is a dedicated focus puller because this increases the chances of hitting focus marks more accurately. Short focus throws can lead to more errors because a small turn will change the focus drastically. In situations where the camera operator is pulling focus alone, there may be a benefit to shorter focus throws because the operator doesn't have to turn the lens as much—keeping the camera more stable.
Using a small follow focus gear drive together with a larger lens gear to expand the lens's diameter will result in the focus throw being longer/slower and more precise.
Some lenses, such as those made for Nikon mounts, have focus rings that turn in the opposite direction of cinema lenses and even most still lenses (like those made by Canon). An experienced focus puller will have difficulty getting used to this reversal. Using different sets of lenses with different focus directions will cause confusion and be difficult to work with. One solution is to use an electronic remote focus, which often provides a reversing option at the flick of a switch. A mechanical follow focus system with a reversing gear can also be employed to correct this issue.
Some of the higher-end lenses, such as Canon's USM lenses, use an ultrasonic motor to move the elements inside the lens. These motors create movement through ultrasonic vibrations that are much quieter and faster than regular motorized lenses. They also consume less power and generate less operating noise. Additionally, the lenses don't need to be switched to manual mode in order to manually focus, which is not the case with regular autofocus lenses.
Contrast can be a confusing term when discussing lens qualities. In a lens, the level of contrast is its ability to distinguish between differences in brightness levels. In images, however, contrast refers to the rate at which the brights turn to darks (and vice versa). The final image can have high contrast (with a rapid progression of brightness to dark levels), but still be missing many of the in-between tones because it was created using a low-contrast lens. The resulting image will be muddy, dull and flat in appearance. A high-contrast lens will produce an image with a more expanded range of tonality.
The ability of a particular lens to resolve finer detail is referred to as “resolution.” The higher the resolution a lens can resolve, the better it will be able to capture finer details in an image. Of course, the final result is dependent on the camera's sensor as well. Ideally, the resolution of the lens should outperform the sensor's native resolution in order to take full advantage of its capabilities. Resolution is mostly noticed when a video is displayed on a larger screen. If the footage doesn't have enough fine detail, it will look soft at larger sizes.
A lens can also have varying degrees of sharpness across its surface. There are also other factors that can affect the differences in resolution, such as the distortion found on the edges of wide-angle lenses. Most lenses aren't perfect in this sense, but higher-quality optics usually have a more consistent degree of resolution across the frame. There are also lenses with different degrees of contrast and resolution. Unless the footage is bound for the big screen, contrast is more of a priority than resolution is for most video productions.
Sharpness is the combined qualities of contrast and resolution—each of which contributes to the overall apparent sharpness of an image. They both work in concert to create an image that is both high in small detail and vividly reproduces the smallest differences of brightness levels.
Generally, the higher a lens’s quality, the sharper it will be. This is an important factor to consider in a lens for HDSLRs because of the cameras’ already-limited color space and compressed video files. Although HDSLRs have more than enough native resolution in their sensors, the way in which the image is processed into the final HD video clip means it ends up losing a lot of the original information. Therefore, starting out with a lot more information in the image (i.e. with sharpness) is key to maximizing these cameras and overcoming their limited color space and compressed video.
If there’s already a collection of lenses available, test them to determine if they have acceptable sharpness when viewed on the same size screen on which the audience will view the production. For those just beginning to invest in a lens collection, obtaining the best lenses possible is the key to maximizing the image quality of current (and future) HDSLR cameras.
The quality of the optics in a lens affects the colors as recorded by the sensor. Ideally, a lens should faithfully reproduce the colors in a scene. However, it is more expensive to design and manufacture such lenses, so many still lenses will shift an image’s colors, usually tinting it magenta or blue. A small amount of tint may be desirable for its “feel,” or the image can be adjusted in post production. In digital photography, the color quality of a lens has become less important because of the RAW file format and digital processing capabilities. For HDSLRs that record video with high compression, however, it is ideal to limit the amount of post-processing and use a set of lenses that is either neutral in color or somewhat close to the final look of the intended picture. In-camera image processing that takes place before compression can also be used to adjust for lens tint.
Manufacturers have a distinct look and feel across their lenses, and even more so within a series of lenses, but cinema lenses will have the closest color-matched sets available. This can help keep color consistent from shot to shot and ease the post-processing workflow.
Bokeh is the way in which points in the image look when they are out of focus. Bokeh is most noticeable in small highlight areas, like out-of-focus lights and reflections in a background. The way in which these points are reproduced affects the overall look and feel of the out-of-focus parts of an image. The optics and aperture design of a lens determines the look, or quality, of the bokeh. Examining out-of-focus bright points is the best way to determine the quality of a lens's bokeh characteristics.
The shape of the bokeh is created by the shape of the aperture opening. The aperture is made of a group of blades converging toward the center of the lens to create the different size openings (i.e. f-stops). The way in which the aperture blades are designed, as well as how the lens corrects for aberrations, determines the different characteristics of bokeh. When more blades are used, the out-of-focus bright spots will be rendered as smoother circles. When fewer blades are used, the spots will be shaped more like a polygon or pentagon.
The ideal shape of the bokeh is a matter of preference. Some prefer more circular points, while others like the more distinct look of a polygon or pentagon. However, there are certain bokeh characteristics that are universally agreed upon. Good bokeh will render out-of-focus points as soft and even shapes. The area will be smooth and not distract attention with uneven jagged details and color shifts. A bad bokeh will be distracting, making out-of-focus parts look doubled or ghostlike. Although this effect will be more noticeable in highlights, bokeh affects all out-of-focus areas, even if the untrained eye notices only subtle differences.
The camera can be stabilized with a number of options, but lenses with built-in stabilizers do a very good job at smoothing out small, jittery motion typically caused when handholding the camera. Although this does not replace a shoulder mount or stabilizer system, it is a fantastic option to have when other options are unavailable. Image stabilization is harder to find on high quality, wider-angle lenses. Surprisingly, some lower priced consumer zoom lenses tend to have IS built in, but many higher-end models lack this feature.
One of the biggest advantages of shooting video with an HDSLR is the huge assortment of available lenses. While each brand offers a wide variety of lenses, some users own or prefer other branded lenses. Most lenses that need to be adapted to fit on another camera mount will not have any automatic control (i.e. focus and aperture). Manual control is the only option with these lenses, so fully manual lenses should be chosen for this application. Many adapted lenses may also disable the camera's metering function, so an alternative such as the histogram or light meter, must be used.