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

Started Jan 26, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 43,011
Re: Um...

DonSC wrote:

Great Bustard wrote:The total light is the total amount of light that falls on the portion of the sensor used to for the photo during the exposure: Total Light = Exposure · Effective Sensor Area. The same total amount of light will fall on the sensor for equivalent photos but, for different formats, this will necessarily result in a different exposure on each format, since the same total light distributed over sensors with different areas will result in a lower density of light on the larger sensor.

I've always wondered if this were really true.

It is.

For sure it's true if you mount the SAME lens on cameras with different sized sensors. But what if the lens creates a smaller image circle? A lens isn't a tube. A 300mm f/2.8 lens doesn't have to create a FF image circle. It can create a larger image circle or a smaller one.


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.

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.

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