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

Started Jan 26, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 62,183
Re: A reminder for some memebers as to what the F stop is all about.

tray48 wrote:

bobn2 wrote:

tray48 wrote:

Yes it is true that 4 times the amount of light will enter through a 50mm lens at f/2 than a 25 mm lens. But since area of the sensor of the of the camera with a 50mm lens is 4 times larger than the camera with the 25 mm lens, the amount of light falling on the sensor per unit area is the same for both cameras.

Indeed it is, that si where we started.

In other words the light intensity at the sensor is the same for both cameras even though the camera with the 50 mm lens is working with 4 times the amount of light as the camera with the 25 mm lens. I think the problem arises from confusion about the difference between light intensity and total light

Yes, and in terms of the noise in the image, and therefore how small and exposure you can stand, and therefore how big an f-number you can tolerate for a given shutter speed, what matters is the total light.

So what would be the outcome if instead of a digital cameras with electronic sensors, you had two cameras with film,

Exactly the same.

one say 35 mm and the other a medium format.

Of you could say 110, which is the same frame size as Four Thirds, and 135.

Both cameras would have exactly the same film with the exception of the size of the frame.

Let's go with that just for the present.

Now if you placed lenses on both camera such that you had equivalent angles of view, set them to same f-stop, photographed the same object from the same position and same light, would you expect a better image from the larger format camera?

Absolutely, that's why you bought a larger format camera in the first place. The four times as many photons reaching the 135 frame produce four times as many photoelectrons which reduce four times as many grains. So, enlarged to the same size, the 135 image contains four times as many grains as the 110 image so will look smoother and contain more information. If one really needed a certain fast shutter speed, and had a minimum quality threshold, you could load the 135 camera up with a four times faster film with four times the grain size and get the same result as the 110 and a two stop larger f-number.

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