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DOF is different than speed of the lens. F value for aperture is standard no matter the size of sensor and sensor will get the same amount of light if the shut

by adrr 6y ago

DOF is different than speed of the lens. F value for aperture is standard no matter the size of sensor and sensor will get the same amount of light if the shutter speed are the same.

sudosysgen 6y ago
I explained to you exactly why this is incorrect. The amount of light the sensor catches from a point source depends on the diameter of the aperture and the distance from the point. So, at the same FoV, the diameter of the aperture is what matters. The light intensity at the same f number is indeed the case, however, since the image is smaller, then amount of light is four times smaller for an M4/3 lens at the same FoV. Another way to understand this is to realize that the focal length is smaller, therefore the image for the same object is smaller, therefore despite an equal light intensity the total light is the same. DoF is not actually any different from lens speed. There is a physically indestructible link where increasing the amount of light from the same object reduce the depth of field. DoF and amount of light are two faces of the same coin. I highly suggest actually doing the math. It's a very counter-intuitive concept. Take a point source of light, take some distance, then calculate the angle of the triangle formed by the point and the opposite points on the aperture. This angle is directly proportional to the light. You will see that the angle is equal in both. Then, to explain why the ISO is going to be four times as high in the FF camera, calculate the intensity of the image by taking the source to now be a sphere, and calculating the image size using the thin lens equation. You will se that the image will be twice the size with the 50mm FF lens, and thus the light intensity will be half that, which is why ISO is going to be four times higher in the FF camera. This is also why you can get away with ISOs four times on full-frame than on M4/3. Again, if you don't believe me, you really have to do the math in your own, failing that you should do an experiment and look at the noise levels at the same shutter speed at f/2.8 vs f/5.6 in the same speed with equivalent focal lengths. You will see.
- deleted 6y ago
  [deleted]
- colanderman 6y ago
  Parent (sudosysgen) is correct, I don't know why they're being downvoted. Another way to look at it is, if you have two setups with different sensor sizes, but the same relative aperture (i.e. f number), same ISO, same sensor resolution, and equivalent focal lengths (i.e. scaled by crop factor), you will have more noise in the image from the smaller sensor. As the parent stated, this is purely due to physics: the lens in the setup with the smaller sensor has a smaller absolute aperture; thus less total light is entering the camera.
brudgers 6y ago
Less luminous flux arrives to the surface of the smaller sensor. However the illuminance is the same due to the photons being spread over less area. Measuring lux at each sensor would provide similar readings.
colanderman 6y ago
You get the same exposure, not the same amount of light. A larger sensor requires more total light for the same exposure (the light is spread out over more area, after all). This is true equally of film and digital.

by adrr 6y ago

DOF is different than speed of the lens. F value for aperture is standard no matter the size of sensor and sensor will get the same amount of light if the shutter speed are the same.

sudosysgen 6y ago

I explained to you exactly why this is incorrect. The amount of light the sensor catches from a point source depends on the diameter of the aperture and the distance from the point. So, at the same FoV, the diameter of the aperture is what matters. The light intensity at the same f number is indeed the case, however, since the image is smaller, then amount of light is four times smaller for an M4/3 lens at the same FoV. Another way to understand this is to realize that the focal length is smaller, therefore the image for the same object is smaller, therefore despite an equal light intensity the total light is the same. DoF is not actually any different from lens speed. There is a physically indestructible link where increasing the amount of light from the same object reduce the depth of field. DoF and amount of light are two faces of the same coin. I highly suggest actually doing the math. It's a very counter-intuitive concept. Take a point source of light, take some distance, then calculate the angle of the triangle formed by the point and the opposite points on the aperture. This angle is directly proportional to the light. You will see that the angle is equal in both. Then, to explain why the ISO is going to be four times as high in the FF camera, calculate the intensity of the image by taking the source to now be a sphere, and calculating the image size using the thin lens equation. You will se that the image will be twice the size with the 50mm FF lens, and thus the light intensity will be half that, which is why ISO is going to be four times higher in the FF camera. This is also why you can get away with ISOs four times on full-frame than on M4/3. Again, if you don't believe me, you really have to do the math in your own, failing that you should do an experiment and look at the noise levels at the same shutter speed at f/2.8 vs f/5.6 in the same speed with equivalent focal lengths. You will see.

deleted 6y ago
[deleted]
colanderman 6y ago
Parent (sudosysgen) is correct, I don't know why they're being downvoted. Another way to look at it is, if you have two setups with different sensor sizes, but the same relative aperture (i.e. f number), same ISO, same sensor resolution, and equivalent focal lengths (i.e. scaled by crop factor), you will have more noise in the image from the smaller sensor. As the parent stated, this is purely due to physics: the lens in the setup with the smaller sensor has a smaller absolute aperture; thus less total light is entering the camera.

brudgers 6y ago

Less luminous flux arrives to the surface of the smaller sensor. However the illuminance is the same due to the photons being spread over less area. Measuring lux at each sensor would provide similar readings.

colanderman 6y ago

You get the same exposure, not the same amount of light. A larger sensor requires more total light for the same exposure (the light is spread out over more area, after all). This is true equally of film and digital.