Understanding Anamorphic Lenses
Anamorphic lenses are specialty tools which affect how images get projected onto the camera sensor. They were primarily created so that a wider range of aspect ratios could fit within a standard film frame, but since then, cinematographers have become accustomed to their unique look. This article discusses the key considerations with anamorphic lenses in the digital era.
Two classes of lenses are typically used in production: spherical and anamorphic. Spherical are more common and are the assumed lens type unless specified otherwise. Spherical lenses project images onto the sensor without affecting their aspect ratio. Anamorphic lenses, on the other hand, project a version of the image that is compressed along the longer dimension (usually by a factor of two). Anamorphic lenses therefore require subsequent stretching, in post-production or at the projector, in order to be properly displayed.
Anamorphic lenses were originally designed so that wide format imagery would fully utilize the film area of standard 35 mm frames. Otherwise wide format imagery would have left the top and bottom of the frame unused, and required cropping these out using masks in the projector:
Anamorphic lenses therefore improved image quality by both enhancing vertical resolution and reducing the appearance of grain. For example, using a standard spherical lens to capture 2.40:1 imagery on 35 mm film only utilizes 50% of each frame’s area. With anamorphic, 100% of the frame area contributes to the final image.
However, the advent of the “Super 35” format lessened the difference between spherical and anamorphic lenses, because Super 35 provides more horizontal film area by not recording the audio next to each frame. Anamorphics are still better at utilizing film area though, especially with 2.39:1 aspect ratios.
Anamorphic lenses typically serve a very different purpose with digital. Since most digital sensors have a higher aspect ratio than 35 mm film, spherical lenses often record sufficiently wide images with minimal to no cropping. Using an anamorphic lens often produces an unnecessarily high aspect ratio, in which case the sides of the image aren’t utilized and horizontal resolution is reduced.
Anamorphic lenses therefore only improve image quality when a higher aspect ratio is needed than captured by the digital sensor. However, unless the required aspect ratio is unusually large, cropping the image vertically will often preserve more pixels.
For example, the Full Format RED DRAGON resolution is 6144×3160, which has an aspect ratio of 1.94:1. To produce 2.40:1 wide format imagery, one could crop the top and bottom of the frame using the “6K WS” setting, in which case 81% of the pixels would be preserved. The other route would be to use a 2X anamorphic lens, in which case the sides of the frame would need to be cropped, and only 61% of the pixels would be preserved. One could potentially utilize a less common 1.3X anamorphic lens and preserve 95% of the pixels, but the effect would likely be too subtle to justify any added cost or complexity.
The main reason to use anamorphic with digital is for its other effects. Lens flare and out of focus backgrounds (“bokeh”) will appear elongated as opposed to circular (unless the lens has a specially-designed ovular iris). Flare may also appear as bluish horizontal or vertical streaks which span the entire frame. Vignetting may appear as an oval, although this unique shape can be emulated in post-production.
Depth of field is also affected. Although anamorphic and spherical lenses technically have the same depth of field, in practice you have to use a longer focal length with anamorphic in order to achieve the same angle of view. Therefore, at the same subject magnification, anamorphic lenses produce a shallower, more cinematic depth of field. With film, this difference could be as pronounced as going from a 2X to a 1X crop factor, depending on whether area was reserved for audio. With digital, the difference is much subtler, and with RED® is equivalent to going from a 1.25X to a 1X crop factor:
The unique design of anamorphic lenses also has implications. Many anamorphic lenses are just ordinary spherical lenses with additional glass elements that compress or “anamorphize” the output. This makes the lens larger, reduces light transmission and can introduce additional distortion. The added complexity and larger glass elements almost always mean that anamorphic lenses will be more expensive and heavier than standard spherical lenses. Since spherical lenses are more common, the available combinations of focal length, T-stop, quality and cost are also more diverse.
However, the type of anamorphic lens can also be an important factor. Most of the characteristic look of anamorphic lenses is associated with the “front-mounted” type, where the aspect ratio compression is done by a front lens element. “Rear-mounted” anamorphic lenses are usually much less identifiable, and are therefore typically used with film when utilizing more film area is a primary goal. Bokeh, flare and vignetting will appear similar to spherical lenses, but depth of field will still become shallower. Rear-mounted anamorphic lenses often also reduce the maximum available aperture, and are much less common with wide angle lenses since they increase the effective focal length.
Anamorphic lenses typically aren’t as sharp as equivalent spherical lenses, in part because these have additional glass elements in the optical path, but also because images are often produced using an ultra-wide angle of view. With film, the end result was typically still sharper though, primarily because these images had to be magnified less during projection.
The signature anamorphic style has been a key motivation behind RED’s incorporation of various in-camera and post-production anamorphic tools. For easier on-set visualization, a de-squeeze feature is enabled by default in the viewfinder and LCD preview screens when an anamorphic resolution is selected:
Within REDCINE-X®, images can be defined as anamorphic within the viewer window to avoid having them appear squished. Exporting and transcoding also requires aspect ratio adjustments using the framing panel.
Since the final image has to maintain a standard aspect ratio, pixels from the anamorphic image are interpolated to produce an equivalent spherical image. If the finishing resolution is close to the capture resolution, this means that horizontal detail will be less resolved than vertical detail. For example, an anamorphic image captured at 5K widescreen and finished in 4K widescreen is undersampled from just 3.2K pixels horizontally, but is oversampled vertically from 2.7K down to 1.7K pixels.
Anamorphic lenses have played various roles as capture has transitioned from film to digital. With film, anamorphics were originally designed to utilize more film area with wide format imagery, which in turn increased resolution and reduced grain. However, digital sensors are typically better-suited to wide format cinema than standard 35 mm film. The benefits are instead primarily for imparting a unique look with a shallower depth of field and characteristic bokeh, flare and vignetting. This signature anamorphic style has been an important creative tool with several recent digital and film productions.
- See the Guide to Video Aspect Ratios to understand anamorphic widescreen in the context of other potential video formats.