Understanding Sensor Crop FactorsFebruary 24, 2014
The crop factor and size of a digital sensor affects numerous image characteristics, including depth of field, angle of view and cropping. In this article, we explore the relative trade-offs as they pertain to cinematic capture.
All standard lenses project an imaging circle onto a rectangular sensor. The crop factor describes how far a given sensor extends into this circle relative to another reference sensor. Higher values mean less of the imaging circle gets recorded, and vice versa:
In the stills world, the crop factor is almost always given relative to traditional 35 mm frames. Since stills use a variety of aspect ratios in both portrait and landscape orientation, the crop factor is calculated as the ratio between the diagonal dimensions of each sensor.
However, with motion, footage is almost always captured in landscape orientation, and wider aspect ratios are typically created by masking out the top and bottom of the on-screen image. For this reason, the crop factors in this article and our online tools are calculated based on the ratio between sensor widths. This is also a reason why cinematic resolution is specified by horizontal pixels (such as 4K), as opposed to megapixels or vertical resolution (such as 1080P).
ANGLE OF VIEW & DEPTH OF FIELD
Perhaps the most apparent consequence of a higher crop factor is that lenses appear as though they were a longer focal length. This is particularly useful for achieving more reach with an otherwise medium telephoto lens, such as when capturing wildlife, surfers on distant waves or sports action within a large playing field.
However, another less apparent consequence is also important. At the same focal length, a higher crop factor means one has to move further away in order to achieve the same subject framing. Alternatively, one could remain in the same position and use a wider angle lens. Either way, the depth of field increases as a result:
With consumer video cameras, this effect can be dramatic — and is often comparable to the difference between compact and digital SLR cameras. The combined effect is that smaller sensors can make video feel more like a television broadcast than cinema.
IMAGE QUALITY & IMAGING CIRCLE
The crop factor can also influence image quality. A larger sensor may mean correspondingly more light-gathering area, in which case many of the usual technical considerations apply (see Bayer sensor strategy). This is a primary difference between high-end digital cinema cameras and more consumer-oriented camcorders. However, the crop factor also affects image quality independent of underlying sensor technology.
The size of an imaging circle relative to the sensor is another key consideration. At one extreme, if the sensor diagonal is substantially larger than the imaging circle diameter, the edges of a frame could appear dark. At the other extreme, a much smaller sensor would mean light from only the central portion of an image is recorded, in which case image quality is also compromised since the remaining image has to be enlarged more for on-screen display. Although these scenarios are not representative of typical usage, they can help visualize the limits of crop factors:
Fortunately most camera and lens combinations lie between these two extremes. However, image quality is still influenced more subtly, in part because imaging circles do not end abruptly at their exterior. Instead, image quality declines from the center outwards. Then, at the very edges of an imaging circle, quality typically degrades rapidly and the image eventually disappears. Although the precise circle size and rate of degradation varies depending on the particular lens model and T-stop setting, all lenses follow this same trend.
In effect, the crop factor can be thought of as controlling the balance between image quality toward the center versus the corners of a frame. Higher crop factors mean the recorded image is primarily comprised of the central portion of the imaging circle, but at the expense of higher image magnification. Similarly, lower crop factors utilize more of the imaging circle, but at the expense of including the lower quality outer portions of the imaging circle. Optimal overall quality requires striking the right balance.
RED CROP FACTORS
The crop factor of RED camera sensors is designed to bridge both the traditional cinema and stills worlds. It fully utilizes the imaging circle projected by most Super 35 lenses, but also encompasses a substantial portion of the imaging circle from 35 mm stills lenses.
Another key characteristic of RED cameras is that the crop factor increases as resolution is reduced. This provides full crop factor control to suit a wide variety of lenses and imaging circles. Unlike cameras that utilize pixel skipping, this also minimizes aliasing by ensuring the OLPF remains equally effective regardless of the chosen resolution. Click on the screenshot image below to use our online tool and see how the crop factor changes depending on camera setting:
Note how the diagonal dimension of the newer RED DRAGON sensor extends further out into the imaging circle than MYSTERIUM-X (34.5 versus 31.3 mm). Depending on resolution setting, this can be used to match a wider variety of traditional formats. RED DRAGON at 5K has the same coverage as Super 35 3-perf, for example, and at 5.5K has the same coverage as the RED MYSTERIUM-X at 5K.
Size & Weight. Lenses with larger imaging circles are generally heavier and incorporate larger glass elements. This is a primary reason why consumer video cameras are typically smaller than high-end cinema cameras.
T-stop Setting. Higher T-stop (or f-stop) values typically decrease the rate at which image quality degrades toward the edges of the frame. Optimal results can usually be achieved by closing the lens aperture about two stops compared to wide open. Vignetting is often reduced or eliminated as long as this is not being caused by the rim of a filter physically blocking light in the corners.
Diffraction. Diffraction sets fundamental limits on any optical system based on f-stop and sensor size. A larger sensor means that a higher f-stop can be used before resolution is limited by diffraction, but larger sensors also have a shallower depth of field for a given f-stop. These competing factors end up cancelling out; therefore, for an equivalent image with the same depth of field, the diffraction-limited resolution of both the larger and smaller sensor will remain the same.
Anamorphic Lenses. These increase the effective sensor area by compressing the imaging circle horizontally, thereby also decreasing the effective crop factor. For a given focal length, the angle of view therefore increases, and at the same subject magnification the depth of field also decreases. See Understanding Anamorphic Lenses for more.
Aspect Ratio. Higher aspect ratios typically do not extend as far into the imaging circle despite a wide angle of view. RED DRAGON at 6K WS and 6K HD have imaging circles of 33.3 and 32.2 mm, for example, even though the full frame diagonal extends 34.5 mm. Corner vignetting may therefore become less of a concern with certain lens and filter combinations. See Video Aspect Ratios for a background on conventional options.
The optimal sensor size is one that makes full use of the imaging circle projected by commonly used lenses. Wide angle lenses will generate truly wide angle imagery, the optics and glass elements won’t go underutilized, and image quality toward the center of the frame will be maximized. Then, as long as the sensor resolves more than the intended output medium, the crop factor can be fully controlled along with all associated image effects. Any additional reach can be attained in post-production, image quality in the corners can be improved if needed, and depth of field can be controlled independent of aperture. Such sensors therefore become multipurpose tools that can better adapt to the unique needs of a wide variety of productions.