Читать книгу The DSLR Filmmaker's Handbook - Andersson Barry - Страница 10

The lens is the “eye” of your camera; what is captured in the lens is what will end up on the screen. Shooting with a DSLR opens up the world for most videographers who were limited by a single lens or complicated adaptations, and filmmakers are curious to see how a familiar lens interacts with the new system.

Because the choice of lens is the single most important decision you will have to make for every single shot, it is important to start with the relevant basics. Here we will talk about the lens choices and how they affect your final footage.

Aperture, f-stops, and t-stops

Aperture (the measure of the space that light passes through in the lens) is measured in f-stops. An f-stop is a ratio or fraction, so smaller numbers mean more light. Often lenses are considered fast or slow: a lens with a low-numbered minimum f-stop is considered a fast lens because it allows in 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 you have no prime lenses and you want to start with three lenses, it would behoove you to grab a wide-angle lens, a “normal” lens, and a portrait or telephoto lens. This gives you a range to work with. With this said, you should move your focal lengths farther away from each other. For instance, don’t buy a 35 mm for your wide, a 50 mm for your normal, and an 85 mm for your portrait lens. These focal lengths are so close together you don’t get much variety. You would be better off with something like a 24 mm for your wide, a 50 mm for your normal, and a 100 or 135 for your portrait/telephoto lens. Again, these are guidelines and not hard and fast focal lengths you should buy. Some of what you need is dependent on what you are shooting.

Zoom Lenses

Zoom lenses (Figure 1-13) are lenses that have a range of focal lengths and angles of view available in a single lens. The zoom range refers to these lengths; for example, an 18–70 mm lens will cover the focal lengths between 18 mm and 70 mm. Zoom lenses are named based on the ratio of their longest to shortest focal lengths or their magnification factor.

Figure 1-13: Canon 70-200 2.8 IS zoom lens on the set of The Shamus in Burano, Italy

The other major factor on a zoom lens is aperture and whether the lens has a variable aperture or a fixed aperture. A variable-aperture lens means that the f-stop will change depending on the focal length the lens is set at or zoomed to. If you have a lens that is a 28–135 mm f/3.5–5.6, the range of f-stops indicates that it is a variable-aperture lens: at 28 mm wide, the f-stop can be 3.5, but when the lens is zoomed to 135 mm, it can be opened up only to f/5.6.

A fixed-aperture lens allows you to set the aperture or shoot at the lowest f-stop the lens will allow at any focal length.

One drawback of zoom lenses is that they can be large and heavy. If size and weight are of the utmost importance to you, then you might want to not use or at least minimize the use of zoom lenses on your shoot.

Categorizing Lenses Based on Angle of View

Lenses are often broken down into categories based on the focal length’s interaction with the angle of view. The angle of view, or field of view, is determined by the focal length and the dimension of the image format, which in a 35 mm film shoot is the size of a frame; for a DSLR, this correlates with the sensor size.

Extreme wide-angle lenses are typically in the 8 mm to 16 mm range.

Wide-angle lenses are typically in the 16 mm to 35 mm range.

Normal lenses are typically in the 35 mm to 80 mm range.

Telephoto lenses are typically in the 80 mm to 200 mm range.

Super telephoto lenses are typically in the 200 mm to 800 mm range.

Specialty Lenses

Specialty lenses are increasingly being used in DSLR productions. Often they were limited to big-budget feature films that had the luxury of time and budget to include elaborate shots. Now DSLR filmmakers can easily rent, buy, or borrow these lenses to achieve unique looks in their productions.

Macro Lenses These are lenses designed for close focusing and for getting close to the subjects they are recording. They are typically used for small items or items where tiny details are crucial. The depth of field is limited, which allows the subject that is the focus of the shot to be prominent.

Fish-Eye Lenses These are ultra-wide-angle lenses that result in an extremely wide hemispherical image. They are deliberately distorted and have a view up to 180 degrees.

Tilt/Shift Lenses These are distinctive because they move side to side (laterally) and up and down (vertically) while allowing for a coherent image to be captured with the sensor.

The Tilt

With a normal lens, the sensor plane and the plane of focus are parallel to each other (Figure 1-14).

Figure 1-14: Angle of standard lens to a building that cannot fit entirely in the frame

Tilting the lens and moving it to the side allows the plane of focus to be at an angle to the camera instead of perpendicular to it. This means you can change the plane of focus in relation to the sensor and you can have more control of what is in focus in a given shot (Figure 1-15).

Figure 1-15: The angle of a tilted lens to the same building; notice how the entire building is in the frame with the lens correction.

The Shift

By shifting, or moving laterally, the lens, you have control of the relationship between the image plane and the subject plane. Shift lenses allow you to move the centerline without moving the camera or changing the angle of the camera or perspective of the image (Figure 1-16).

The lens elements are still parallel to the sensor/film, but they are no longer directly in front of the sensor/film (Figure 1-17).

Figure 1-16: The plane in relation to the camera and the shifted lens

Figure 1-17: The lens is to the side of the center of the camera.

In addition to allowing for special-effect shots such as miniaturization, tilt shift lenses have a utilitarian purpose. Tilt movements allow you to obtain a wide depth of field even at the maximum aperture and still keep the entire subject in focus. Shift movements correct the trapezoidal effect seen in pictures taken of tall objects so as not to distort the subject. This can be helpful when using your DSLR camera in low-light situations.

Lensbaby, Subjectiv, and Loreo Lenses

These kinds of lenses are used for various special effects. The effects include pinhole, multi-element anachromatic, some unique tilt functions, and selective focus. These special-effect lenses are used for shots where a distinctive look is the goal, and they have specialized components and construction.

Anamorphic Lenses

Anamorphic lenses are often used with DSLR filmmaking to achieve unique lens flare and bokeh. An anamorphic lens is designed to “squeeze” a widescreen image onto the sensor and must be “unsqueezed” in post to be viewed correctly. When you use an anamorphic lens, your horizontal image will be up to two times wider than a standard 16×9 frame so you will have letterboxing on the top and bottom of your image. This process creates an interesting look and effect on the footage.