Digital Imaging Global Site

Lens mount and sensor formats

Sony A-mount and E-mount systems

Sony α series interchangeable-lens digital cameras are currently produced in two categories, each of which uses a different lens mount and different types of lenses: A-mount cameras with advanced translucent mirrors, and compact E-mount cameras that don’t use reflex mirrors at all. Despite their remarkable compactness and portability, E-mount cameras feature sensors that are capable of delivering image quality on a par with A-mount cameras.

In addition to overall size, the main difference between A-mount and E-mount lenses is their “flange back distance.” The flange back distance is the distance from the rear of the lens to the image (sensor) plane. Since many A-mount cameras are traditional SLR designs that have a reflex mirror between the rear of the lens and the sensor, they need to have a flange back distance that allows space for the mirror. E-mount cameras, on the other hand, do not have reflex mirrors and therefore can be designed with a much shorter flange back distance, and consequently smaller lenses overall.

Sensor formats: 35 mm full frame and APS-C

You may have heard the term “full-frame camera,” but did you know it refers to the frame size of 35 mm film? The image area of a frame of 35 mm film is approximately 36 mm x 24 mm (“35 mm” is the width of the strip of film), and that’s the size of the image sensor in a 35 mm full-frame format camera. Many interchangeable-lens digital cameras use slightly smaller “APS-C” format sensors that measure approximately 24 mm x 16 mm or less. There are a number of other sensor formats, including smaller sensors in digital point-and-shoot type cameras, but APS-C and 35 mm full-frame formats are the two most commonly used in interchangeable-lens cameras.

It is important to understand that there are two “formats” for A-mount interchangeable lenses as well: lenses with an image circle large enough to cover a 35 mm full-frame sensor, and lenses with a smaller image circle that is sufficient for APS-C format sensors. Sony lenses that have “DT” in the model name are compatible with APS-C format SLR cameras only, while all other lenses will work with both APS-C and 35 mm full-frame format cameras.

35mm full-frame


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