Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
Lee Jay Forum Pro • Posts: 52,718
Why worship exposure?

EinsteinsGhost wrote:

bobn2 wrote:

EinsteinsGhost wrote:

bobn2 wrote:

Albert Silver wrote:

tko wrote:

Remember that F4.0 is considered kind of slow on FF, but is equal to F2.0 on M43rds, which is considered "fast."

That's not entirely accurate. You are describing the depth of field equivalence, from one sensor to the next, not the light. f/2 on a m43rds may have the depth of field of f/4 on a full-frame, but the light will still be f/2.

The 'light' of a FF f/2 and a FT f/4 will be the same, which is the point he is making. In the end, given equally efficient sensors, you can achieve the same result at the same shutter speed using an f/4 on FF as you can on FT. The density of the light of the f/4 is one quarter but there is a sensor four times the area to collect it, so it ends up the same.

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The point tko is making is wrong. DOF equivalence applies, exposure equivalence does not (for the reason you state above).

Whether the point tko is making is wrong or not depends entirely on what you think is the definition of 'fast'. If you think that 'fast' is to do with exposure when comparing between formats, then he is wrong. However, that's not a very sensible point of view, so if you assume that he thinks that 'fast' means 'puts more light on the sensor' then he is right.

Fast has only one meaning: Shutter Speed, as in exposure time, the time value part of exposure. And that makes you just as much wrong as tko.

Photographic exposure is independent of media size.

The fastest speed at which I can set my shutter speed is limited by the amount of noise I'll accept in the final image.

The noise is driven by two things - the technology used in the sensor and the rest of the camera pipeline, and the TOTAL amount of light that falls of the sensor.

It makes no difference if that light is thinly spread out (slow f-stop, large sensor) or concentrated down (fast f-stop, small sensor). This is why f-stop equivalence holds across sensor sizes and why, say, ISO 400 and f/2 on 4/3 is equivalent to ISO 1600 and f/4 on full frame - they both produce the same image with the same noise from the same shutter speed given the same technology.

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Lee Jay

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