The D400, will it come at all?

Started Aug 11, 2012 | Discussions thread
Leonard Migliore
Leonard Migliore Forum Pro • Posts: 14,770
An unreasonable statement

redhed17 wrote:

MichaelEchos wrote:

Equivalence at the same angle of view:

A FX 75mm f/2.0 has larger entrance pupil than a DX 50mm f/2.0. And therefore, there is more light going into FX.

f2 is f2 regardless of the lens, the same amount of light should pass through.

That is clearly not the case. In Michael's example, he's comparing a 75mm f/2 to a 50mm f/2. The f/number is the ratio of the focal length to the aperture. So a 75mm f/2 has an aperture of 37.5mm while a 50mm f/2 has an aperture of 25mm. Since the 75mm's aperture has twice the area of the 50mm's aperture, twice the light will go through it.

The reason both lenses give the same exposure is that the sensor is further from the 75mm lens than it is from the 50mm lens. The angular diameter of the aperture as seen from the sensor is equal for both lenses, which is what creates the equivalance in exposure.

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Leonard Migliore

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