Why is aperture usually quoted without a crop-factor equivalent?
Asked 6/8/2012
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We often use 35mm-equivalent focal length to compare field of view across sensor sizes. But lens aperture is usually still quoted as f/2, f/2.8, etc., without a sensor-size equivalent.
Why is that? For example, a compact camera may advertise an f/2 lens, but it won’t give the same depth of field as f/2 on full frame. Does it make sense to compare aperture differently depending on sensor size, and if so, in what way?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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The problem is there are two factors determined by the aperture - depth of field and exposure. Sensor size affects depth of field but not exposure.
With focal length it makes sense to talk equivalents as there is exactly one factor involved, angle of view.
To be correct you'd have to say that a 30mm f/1.4 mounted on a 1.5x crop camera had a 45mm equivalent field of view, an f/2.1 equivalent DOF-aperture and an f/1.4 equivalent exposure-aperture. Which would just wind up confusing every body.
Also focal length is much more important to most amateur photographers who might not understand the influence of aperture but do understand zoom.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
14y ago
0
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Because aperture affects more than one thing, and sensor size does not change them all in the same way.
The f-number itself describes exposure at the sensor/film plane, so f/2 on one format is still f/2 for exposure. A smaller sensor does not make that become a different exposure aperture.
What does change with sensor size is the resulting field of view and, for equivalent framing, the depth of field. So a crop-sensor camera can give a depth-of-field result similar to a larger format at a different f-number, even though the actual lens is still, say, f/2.
That’s why focal length is often converted to a 35mm equivalent: it maps cleanly to one comparison, angle of view. Aperture does not, because you’d need to separate at least:
- actual aperture for exposure
- “equivalent” aperture for depth of field
So an “equivalent aperture” can be useful for apples-to-apples depth-of-field comparisons, but using it as the main aperture spec would be confusing, since the real lens still transmits light according to its actual f-number.
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