3 ms·

> The fastest autofocus primes I see are f/2.8 and f/4, which are roughly equivalent in speed to an f/1.6 or f/2.3 respectively. < Can you elaborate on that? W

> The fastest autofocus primes I see are f/2.8 and f/4, which are roughly equivalent in speed to an f/1.6 or f/2.3 respectively. < Can you elaborate on that? What do you mean by "equivalent in speed?"

inferiorhuman 7y ago
Equivalent to ƒ/1.6 and ƒ/2.3 on a "full frame" sensor.
falcrist 7y ago
Similar depth of field and total light gathered. I'm aware that sensor size doesn't change depth of field, however if you compensate the focal length to produce the same field of view on both sensors, you then need to compensate the aperture to arrive at the same depth of field. So a 50mm f1.4 on a full frame sensor would be roughly equivalent to an 85mm f/2.4 on one of these digital medium format sensors.
- CydeWeys 7y ago
  The best way to look at total light gathered is to talk about the aperture diameter, not f-ratio. The f-ratio is what it is; it doesn't make sense to multiply it or divide it to 'compensate' for different aperture diameters. You should just use the aperture diameter at that point. And an f/4 for any size format will be gathering the same amount of light per sensor area. It's just that the sensor area is larger on larger formats, so the total amount of light (but not light per pixel!) is more. This is why telescopes have such wide apertures; they're limited by total amount of light.
  - falcrist 7y ago
    The amount of light per area at the sensor is directly proportional to the area of the aperture and inversely proportional to the focal length. That's why we use the f-number, which is a ratio of focal length to aperture diameter. Please note that the f-number doesn't indicate total light hitting the sensor. It indicates total light per unit area at the sensor. Thus, for the same f-number, a larger sensor will gather more light. All of this info can be found in the wikipedia page: https://en.wikipedia.org/wiki/F-number https://en.wikipedia.org/wiki/F-number

inferiorhuman 7y ago

Equivalent to ƒ/1.6 and ƒ/2.3 on a "full frame" sensor.

falcrist 7y ago

Similar depth of field and total light gathered. I'm aware that sensor size doesn't change depth of field, however if you compensate the focal length to produce the same field of view on both sensors, you then need to compensate the aperture to arrive at the same depth of field. So a 50mm f1.4 on a full frame sensor would be roughly equivalent to an 85mm f/2.4 on one of these digital medium format sensors.

CydeWeys 7y ago
The best way to look at total light gathered is to talk about the aperture diameter, not f-ratio. The f-ratio is what it is; it doesn't make sense to multiply it or divide it to 'compensate' for different aperture diameters. You should just use the aperture diameter at that point. And an f/4 for any size format will be gathering the same amount of light per sensor area. It's just that the sensor area is larger on larger formats, so the total amount of light (but not light per pixel!) is more. This is why telescopes have such wide apertures; they're limited by total amount of light.
- falcrist 7y ago
  The amount of light per area at the sensor is directly proportional to the area of the aperture and inversely proportional to the focal length. That's why we use the f-number, which is a ratio of focal length to aperture diameter. Please note that the f-number doesn't indicate total light hitting the sensor. It indicates total light per unit area at the sensor. Thus, for the same f-number, a larger sensor will gather more light. All of this info can be found in the wikipedia page: https://en.wikipedia.org/wiki/F-number https://en.wikipedia.org/wiki/F-number