The DSLR Filmmaker's Handbook. Andersson Barry

more light (or, put another way, you can use a faster shutter speed). A lens with a large minimum f-stop number is considered a slow lens.

      The sequence of f-stops follows an unusual pattern; this is a typical sequence:

      f/1.0, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64

      Each stop represents a change of half the size greater or lesser than the adjacent aperture; so, f/2.8 lets in twice as much light as f/4.

      The reason for these apparently strange numbers is that the aperture opening is essentially a circle, so each successive f-stop is calculated by dividing by the square root of 2.

      There are also half-stop and third-stop calculations, which allow for even greater control of the amount of light passing through the aperture opening and the exposure. If you see other numbers between the standard full stops, these represent half or third stops.

      An f-stop is a geometric calculation between focal length and aperture, but light can be lost within the lens or optics. This loss will usually be more apparent with zoom lenses or when shooting with multiple lenses. The determination of a t-stop is used to handle this loss of light within a camera system. A t-stop is a “true” stop or “transmission” stop and measures exactly how much light is making it through the lens to the sensor. A t-stop is a simple measurement of actual light and deals only with exposure, not depth of field. In general, t-stops will not be an issue, but certain lenses are calibrated in both f-stops and t-stops.

       Cine-Style Lenses vs. Photo Lenses

Cine lenses were lenses originally designed with the goal of the recorded image being projected in a theater. A good cine lens is designed to be incredibly sharp and has amazing glass that transmits light nearly flawlessly with high resolution and often low contrast. Cine lenses were also designed to fit a 35 mm movie film print, so if these lenses are used on a DLSR camera (Figure 1-10) – where the sensor is larger than 35 mm motion-picture film – vignetting will occur. Also, cine lenses may be marked with t-stops instead of f-stops.

Figure 1-10: A cinema-style lens on a DSLR camera

      One of the main differences of cine lenses from traditional still photography lenses is that cine lenses are matched sets optically: they have matching t-stops, barrel size, focal length, and back focus throughout the set of lenses. This means there is no change from lens to lens when switching from one cine prime lens to the next. Still lenses are not always matched, and the f-stops, barrel sizes, focal lengths, and so on can change from lens to lens. Another feature of a cine lens is properly calibrated marks for distance, and on cine lenses there are usually more than on a still lens.

      A cine lens is designed with focus in mind and has focus gearing. The lens has a wider and smoother range of movement from one focus point to another than a still lens. This increased turning distance is necessary for various focus changes within a shot. A cine lens is designed so that it can be manipulated by a second person and can be done with extreme accuracy because focus is even more crucial when the image is going to be projected in a theater.

      A cine lens also has internal focus. As you obtain focus, the lens front does not move forward because cine lenses utilize internal focusing. Cine lenses do not “breathe” much, or at all, as you focus.

      “Breathing” Lenses

      This phenomenon happens in some lenses (both prime and zoom lenses): when you focus the image, it temporarily appears to expand and reduce. If you are doing a rack focus from a foreground image to an image in the background and you have a lens that breathes, you will not get a smooth rack focus from the foreground to the background. The only way to find out whether your lens breathes is to do a rack focus and see if it does it.

      Some cine lenses may still have slight breathing problems, and some photo lenses won’t breathe. Testing is always critical because breathing is more apparent when the image is projected.

      Cine lenses also do not have “hard clicks” and have a step-less aperture for f-stops. This feature means that you can set the f-stop exactly at the level where it needs to be with no need to move to the next level in order to get close.

      “Hard Click” for f-stop

      Most still-camera lenses have spring-type mechanisms that click as the aperture setting of the lens is turned. The click indicates that the next f-stop setting or step has been reached, and the f-stops are generally accurately set. Importantly for moviemakers, this also means that only the preset f-stops can be used.

      In Chapter 4, “Cameras and Lenses on Location,” we’ll tell you how to “de-click” a lens.

Figure 1-11: Zeiss CP.2 lenses are modified still lenses that are housed in a cinema-style body that allows for measurements on both sides of the lens and a longer focus throw for better focus pulling.

Lenses can be put into cine-style casings (Figure 1-11), or you can get a cine-style lens that is easier to use when shooting video. A real cine lens is more expensive than a comparable still lens. Cine lenses are also quite heavy and can feel cumbersome to use, especially if used with added matte boxes or other accessories.

      Photo or still lenses are and should be used for DSLR video because they give you great results. The advent of high-quality DSLR cameras and options for the filmmaker has opened the floodgates on lenses that can be used. Traditional still lenses are now being used to shoot movies. There are some noteworthy differences between a cine-style lens and a still lens.

      In general, the focus and zoom on photo lenses are designed for quick adjustments for stills. The movement is not going to be incorporated into the shot either with a zoom or with a focus change, so the focus and zoom on a still camera can sometimes be too sensitive for easy motion on video. This means the focus on a still lens may change drastically by moving the focus ring only a small amount. This small amount of movement can make pulling focus and smooth zooming difficult but not impossible. Some photo lenses also have the focus, zoom, and aperture ring set in the opposite direction of a cine lens and so shifting between lenses forces you to shift more gear around or mentally take note of the shift. Some still lenses rely on the camera settings to make aperture changes, which limits the aperture range that can used for the shot. So just like with cameras you need to use trial and error to find the exact lens or lenses you want to use for the long run.

      “Hard Stop” for Focus

      A hard stop occurs when the lens will not spin any further. Cinema lenses or high-quality still lenses like Zeiss ZE or ZF lenses will not spin around forever. They have a hard stop at infinity and the other end of their focus ring. This provides the ability to accurately predict your lens when pulling focus.

       Types of Lenses

      Most films are shot using a combination of lenses. The combination of lenses you choose will depend on a number of factors. These include availability, desired look and effect, budget, and location parameters. Some combination of primes and zooms will be used on most DSLR shoots.

       Prime Lenses

      Prime lenses are lenses with a fixed focal length; this means each lens has a single angle of view. It is this angle of view that categorizes what type of prime lens it is.

You will often hear the phrase “a set of primes,” and that just means a multitude of prime lenses. There is no standardized set of prime lenses; rather, a set of primes is a collection of lenses of various highly used and versatile focal lengths (Figure 1-12). The goal in choosing a set of primes is to allow for desired shots in a variety of locations and situations.

Figure 1-12: A set of Leica R prime lenses

      If