A reminder for some memebers as to what the F stop is all about.

Started Jan 26, 2014 | Discussions thread
pwilly Senior Member • Posts: 1,175
Re: Um...

bobn2 wrote:

Great Bustard wrote:

A 300mm f/2.8 m43 lens which creates an image circle with half the diameter of a 300mm f/2.8 FF lens will deliver the same amount of total light to the sensor.

No. A 300 / 2.8 will put the same density of light (same light per area) on the sensor regardless of the format, but not the same total amount of light.

And also, the image circle isn't very tightly coupled to the format in use. I would guess that if you removed the baffles, most SHG lenses would produce pretty much a FF image circle. It's one of the things that is done to give 'edge to edge' sharpness, make the image circle large and just use the centre of it. Long FL lenses in particular tend to have big image circles.

If you removed the baffles you would need to call the ghost and flair busters!

While the area will be smaller the density of light will be higher.

No. The same f-ratio results in the same density, regardless of format, but not the same total amount of light on the sensor.

Isn't this what the speed boosters do?

Speedboosters (focal reducers) compress the larger image circle into a smaller image circle, and thus put the same total amount of light on the sensor. For example, a 0.5x FR will result in half the focal length, but the aperture (entrance pupil) diameter remains the same, so the relative aperture (f-ratio) is halved since the focal length is halved.

They reduce the whole image, and the image circle as a result. Their operation without vignetting depends on a large enough image circle.

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Just an old dos guy

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