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Does T-stop exposure change across different sensor sizes?

· asked on YouTube

Does a lens T-stop rating convert or change when used on different sensor sizes, such as Super 35 compared to full frame?

Asked on YouTube,

A lens T-stop rating remains constant regardless of the sensor size behind it. A T2.0 lens transmits the exact same light intensity per unit area on a Super 35 sensor as it does on a full-frame sensor. Light output only changes if optical expanders or adapters are added to extend lens coverage. However, matching the depth of field and field of view of a smaller sensor on a larger sensor requires stopping down, which reduces exposure.

Exposure remains constant across formats

The thread agreed that T-stop and F-stop ratings do not change simply because a lens is used on a different sensor size. An aperture setting of T2.0 delivers the same intensity of light per unit area whether mounted on a Micro Four Thirds, Super 35, or full-frame camera.

Key factors governing T-stops and sensor size

  • Light transmission per unit area remains identical across all sensor sizes at a given T-stop.
  • Physical light loss occurs only when optical expanders or adapters alter the distance between the rear lens element and the sensor.
  • To achieve equivalent field of view and depth of field across different sensor sizes, focal length and aperture must be adjusted.

Matching depth of field versus exposure

One answer noted the distinction between exposure and depth of field equivalence. While a T2.0 lens yields identical exposure across formats, matching the background blur of a smaller sensor on a larger frame requires using a longer focal length and stopping down the lens. That act of stopping down to match depth of field is what reduces exposure, rather than any inherent light loss from the sensor crop itself.