Let's say I have a micro-4/3rd camera and a full frame camera, both set to 1/60 at f/2.8, taking a picture of the same scene in the same lighting. Will the exposure be the same across both cameras despite the different sensor sizes?
The reason why I'm asking is because of the difference in depth of field between micro-4/3 and full frame sensors. I'm finding that, in order to take a picture of certain scenes with the full frame camera at the same depth of field as the micro-4/3rd camera, I have to increase the aperture, which in turn forces me to crank up the ISO.
Answer
Yes. Exposure is based on the amount of light that hits any given point on the sensor (or film), not the total amount of light for the whole area. (The light hitting the corners doesn't have any effect on the light hitting the center, or anywhere else.) Or to put it the other way around, a full-frame sensor records more overall light, but for the same exposure, it's exactly as much more light as there is more sensor area.
Think of it this way: if you take a full-frame image and cropped out a small rectangle from the middle, the exposure there (ignoring vignetting and light falloff) is the same as the exposure for the whole thing.
Now instead of cropping, imagine replacing the full-frame sensor with a smaller one. Same exposure, just less of the image recorded.
Of course, a cropped image does have less light overall. The secret is that we "cheat" when enlarging. We keep the brightness the same, even though the actual number photons recorded per area is "stretched". That is, if on the sensor, 200 million photons collected in a square represents a medium gray, if we print so that square is 10"×10", we don't spread the brightness out making it much dimmer — we instead keep the brightness so it's the same gray.
Also, yeah, you have to increase the ISO (or shutter speed) to get the same final image brightness with a smaller aperture for higher depth of field on a larger sensor. But, assuming roughly equal technology, the larger sensor should give about the same amount of noise at that higher ISO as the smaller one did at lower sensitivities.
In concession to the long comments thread below, I will add: if you're literally comparing two camera combinations in the real world, the exact exposure may vary for several reasons. One of these is the actual transmission of light for a given lens at a certain f-stop — the lens elements themselves aren't perfect and block some light. This differs from lens to lens. Second, the lens makers round to the nearest stop when stating aperture, and may not be perfectly accurate. Third, the accuracy of ISO varies from manufacturer to manufacturer — ISO 800 on one camera may give the same exposure as ISO 640 on another. All of these factors should be (even cumulatively) less than a stop. And most importantly, these factors are all independent of and unrelated to the sensor size, which is why I left them out of the original answer.
No comments:
Post a Comment