Читать книгу The DSLR Filmmaker's Handbook - Andersson Barry - Страница 11

One of the major options for DSLR cameras is sensor size, and one of the factors to take note of is how sensor size affects lens choice or usage. Here we will split the sensors into two rough categories: full-frame sensors and crop sensors. The format – that is, the sensor – changes either in relation to 35 mm or because various cameras have different size sensors. The sensor size affects how a lens will work with that particular format, so lenses that you are accustomed to working with in a different format may behave differently than you think. The change can be compared to moving from a standard 35 mm still camera to a medium-format camera or moving from 35 mm film to 16 mm but still using the same lenses on each camera body.

Film movie cameras have always had lenses designed specifically for their format, but with DSLRs, the lenses can be used on different formats and in many cases interchangeably. Additionally, a 35 mm movie frame is a different size than the DSLR formats, most dramatically with a full-frame sensor camera, so the lenses may not behave as you envision. When comparing the size of a DSLR sensor to 35 mm, most people mean in the context of the 35 mm being the size of a 35 mm film frame taken with a still camera, not a movie camera, because that is where most of the lenses for these cameras originated; ultimately, the cameras are both still cameras and video cameras. The great news is that camera manufacturers are meeting the demand for lenses, and lenses are manufactured with sensor size/format type in mind.

Canon makes an EF-S series of lenses specifically for the APS-C sensor size. EF-S lenses are designed to provide a narrower field of view (sometimes referred to as a light cone) to match the smaller sensor, allowing the actual focal length to be achieved on crop-sensor cameras. The main benefit is that wide-angle lenses actually stay wide angle instead of the 1.6× that a regular lens would have. EF-S lenses are a bit more expensive, and you might have a hard time finding used lenses of the EF-S series. Nikon makes a DX lens for its APS-C sensor; however, they make only a limited number and are mostly consumer zoom lenses.

Focal Length Multiplication Factors

In an effort to figure out how the different lenses work with different size sensors, you can look to focal length multiplication factors. The focal length multiplier (FLM), or sometimes format factor, is stated as a ratio between the size of the sensor and the size of 35 mm still film.

A full-frame sensor is the same size as a 35 mm film still, so its FLM is 1. A larger FLM indicates a smaller sensor and vice versa. Table 1-2 shows some of the most common FLMs.

Lenses are designed with a minimum FLM. A full-frame lens is designed for an FLM of 1 but can be used with smaller sensors, such as those with an FLM of 1.5. However, a lens with an FLM of 1.5 cannot be used on a full-frame camera.

Table 1-2: Common focal length multiplication factors

Focal Length and Field of View

What do you do with these multiplication factors? You multiply them by the given focal length of the lens to determine the field of view (see Figure 1-18).

Figure 1-18: Common video sensor sizes and how they visually compare to the other available sensors

Focal length is a measurement of the distance from the lens, specifically a point of the lens called the nodal point, to the focal plane when the lens is focused on an object set at infinity. The focal plane is inside the camera at the point where the light rays are brought back into the lens to form a point. Practically speaking, this measurement is gauging the capacity of the lens to bend the light back to a point where it will hit the sensor. This measurement is made inside the lens casing, and lenses are labeled by their focal length measurements (Figure 1-19).

Figure 1-19: Focal plane of full-frame and crop-sensor camera

The focal length is an indicator of the angle of view of the lens and the perspective of the shot. In simple terms, it measures how much the lens can see in a given area or how much of the scene will be in the shot. For a given sensor size, a longer focal length will generally provide a narrower field of view.

For instance, looking through a 50 mm lens on a 1.6 crop-sensor camera yields a field of view that is equivalent to 80 mm on a full-frame body or on 35 mm film (50 × 1.6 = 80). With a 1.6 FLM, in order to achieve the “normal” field of view traditionally seen through a 50 mm lens on a full-frame/35 mm film, you must use a 30 mm lens. Multiply the 1.6 crop factor by the focal length 30 mm, and you get the equivalent 48 mm field of view.. close enough.

Because of the explosion of digital photography and the popularity of smaller camera sensors, manufacturers have begun to develop lenses that are exclusively designed for specific sensor sizes of digital SLRs. In general, a DSLR with a smaller sensor size can use a lens that was designed for a larger sensor size but not vice versa.

Figure 1-20: Field of view circle with sensor sizes

The size of the camera’s sensor determines the field of view in the recorded image. Most lenses used on DSLR cameras cast a light circle that is intended to cover a full-frame sensor/35 mm negative. Since crop sensors are actually smaller than a 35 mm negative, the sensor captures a smaller portion of the light circle that is covered by a full-frame sensor. Figure 1-20 is a rough illustration. By capturing a smaller area of light, the field of view that is recorded in the picture has the appearance of having been cropped from the center of a 35 mm photograph.

It is important to note that sensor sizes do not change the actual focal length of a lens. A 50 mm lens will always be 50 mm no matter what the sensor size is of the camera being used. A crop sensor simply changes the field of view in an image taken at a given focal length.

Angle of View and Magnification Factor

Angle of view is defined as the angle that fills the frame (of any format) of a still or motion-picture camera. The actual angle of view is determined by the film format (35 mm, 16 mm, full-frame sensor, crop sensor, and so on) in relation to the focal length of the lens (50 mm, 28 mm, 200 mm, and so on), not the focal length on its own.

As previously stated, the size of a DSLR’s sensor affects the viewing angle of any given lens. The angle of view is thus magnified on most DSLR cameras since the majority of DSLR cameras use sensors that are smaller than the 35 mm format.

Compare a standard 35 mm film camera equipped with a 35 mm lens to a digital SLR with a sensor size of 22.5×15.0 mm. The DSLR’s image sensor is smaller than the full-frame 35 mm camera’s film area by a focal length multiplier of 1.6. This means the 35 mm lens translates essentially to a lens of 50 mm (35 mm × 1.6 = 56 mm) when used on this particular DSLR.

The angle of view of a lens is proportional to the sensor size. The smaller the format, the shorter the focal length needs to be for any given angle of view.

The magnification factor affects wide-angle lenses as well; thus, it’s challenging for a digital photographer or cinematographer to find a wide-angle lens that is truly a wide angle when dealing with a smaller sensor size. When translated by the multiplier, most wide-angle lenses offer only a standard angle of view. For example, an 18 mm lens would be extremely wide on a full-frame camera but has an effective focal length of 28 mm, or moderate wide angle, on a DSLR.

The term crop is commonly used to describe the alteration of angle of view: the imaging area is physically smaller. If you look at the image circle projected by the lens, less of the total area is used. However, crop is more than a simple angle-of-view change.

The image remains the same size at the film plane for a given lens and subject distance; it is not magnified. However, it does take up a larger proportion of the (smaller) frame. It is easy to see why some people refer to it as a magnifying effect.

Small sensor sizes have other big advantages when it comes to telephoto lenses. You can achieve a 300 mm telephoto angle of view, using a camera with a 1.5 FLM, by using a smaller-focal-length lens. You can use a 200 mm telephoto lens and get in effect a 300 mm image. Therefore, you can buy telephoto lenses that are less expensive and lighter weight.

Tele-extenders

You can use a tele-extender to increase the focal length of your lens. To use a tele-extender, you must have a lot of light available because you lose from one stop to two stops of light when using one. By using a crop sensor and lens, you don’t lose any stops to achieve the same focal length image.