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

Started Jul 24, 2014 | Discussions thread
Tim Tucker Senior Member • Posts: 1,337
Re: Whoops!

Great Bustard wrote:

Wheatfield7 wrote:

Allan Olesen wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

The equivalent f-stop also describes the light gathering ability.

You need effective lens area to gather light. A 50 mm f/2 on full frame has much more area than a 12.5 mm f/2 on 4x crop. So the small lens cannot gather as much light.

This is what is known as wrong.

Um, it is absolutely correct.

Any lens will gather the same amount of light at any given aperture. A 50mm f2 lens on a 35mm format will gather exactly the same amount of light as a 12.5mm lens on a micro four thirds lens if both are set to the same f/2 aperture.

Incorrect.

Peoiple really need to learn what they are talking about before they start to spread the sort of misinformation that is quoted above..

Yes, they do. The fact of the matter is that 16x as much light will fall on the sensor for a given perspective, framing, and shutter speed with 50mm f/2 on FF as will fall on the sensor at 12.5mm f/2 on 4x.

- SNIP -

WHOA!!! Hang on there. I don't quite understand the point here, is this different with digital than it was with film?

Let's say I have one lens and I put it on my FF camera and take a picture at a shutter and aperture combination X1.

If I take the same lens and put it on a camera with a smaller sensor then it still transmits the same amount of light and the correct exposure would still be X1 or an equivelant of X1. However the smaller sensor would collect less of this light. If I used film it would be a simple crop to obtain the equivalent photo. The exposure would be the same. Yes?

What I'm getting at is that if you put film behind each of the lenses you discuss then with the exposure of X1 you will get correct exposure for all the film, just the coverage of the different lenses would change, yes? (the total light hitting the film would be the exposure - X1 times the total area exposed).

Are we just talking about noise generation in digital sensors here?

(Not forgetting that the aperture marking system is not limited to digital camera lenses, and it measures transmittance and not DoF. DoF is not even an absolute science as certain assumptions are made about enlargement and viewing distance.)

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