Focal length 35mm equivalent, but not F-stop?

Started Jul 24, 2014 | Discussions thread
Tim Tucker Senior Member • Posts: 1,337
Re: Let's correct it, then.

bobn2 wrote:

Lee Jay wrote:

Tim Tucker wrote:

Erik Magnuson wrote:

Tim Tucker wrote:

Erik Magnuson wrote:

Tim Tucker wrote: -

Is this the same as saying all f2 lenses transmit the same amount of light, but different sensor sizes collect different total amounts for the same exposure?

Amount is not a precise term. A 55gal drum and a 20oz red solo cup left out in the rain might both collect 1" of water. Is the same "amount" of water collected in each?

Eh? Total amount IS a precise term. Measure your volume in litres and you have a precise answer. But that is not what I asked.

It was the answer to your first question and use of "amount." All f/2 lenses do not transmit the same amount of light or my f/2 cell phone lens would transmit as much light as a 200mm f/2 lens. Even if I managed to put a 24x36mm sensor behind the call phone cam lens, it would not help much. What the lens transmits and what the sensor collects may be related, but they are not the same thing.

Sematics, as corrected by Lee Jay (my original statement above). Lenses for different formats do not have the same coverage therefore the total amount of light will not be the same but the light/square metre of sensor size (transmitted?) is the same at f2. A Super Angulon 90mm at f8 will produce the correct exposure regardless of whether I put 4"x5" film or 35mm film behind it.

More semantics...

"A Super Angulon 90mm at f8 will produce the same exposure regardless of whether I put 4"x5" film or 35mm film behind it."

It may or may not be "correct", whatever that means.

Here's a thought on this 'correct' exposure nonsense. Let's go back to film, since it is film based methodology which has confused people. Suppose we added to the equivalence requirements the condition 'equal granularity', and were looking at equivalence between 110 (roughly Four Thirds) and 135 (what we'd now call 'full frame'). If we wanted equal granularity in the image enlarged to the same output size, we'd want the same number of grains in each shot, which means that the 135 grains would need to be four times the area. Since each grain requires two photons to reduce it, both shots would need the same number of photons to expose. Thus the exposure of the 135 would need to be one quarter of that for the 110, or in other words, the ISO of the 135 film would need to be four times higher (which it would be in any case, since the grains were four times larger in area, so it would produce the same density for one quarter the number of photons).

Ah, but grain is a function of the film emulsion and development (development produces the grain not exposure), but is also affected by focus and over/underexposure. If you scan you have artefacts of noise, nyquist considerations, sharpening artefacts, if you print then you have development and over developed/under exposed/temperature etc. considerations. Noise in film is not translated into the final image in the same way that digital noise is.

The point being that it as a pointless argument because it doesn't apply to grain size. I know this is supposed to be an analogy but this is getting out of hand. Stick to the facts.

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