Bell extension factor vs. effective aperture

[lipi]

Hi there!

I'm a bit confused...

The aperture size is f=F/d where F is the focal length, and d is the lens diamater. Right?
The bell extension factor is used when you extend the bell far away from the focal length (at infinity). There is a rule: (extension/focal length) on square. Then log base 2 that. The result is a multiplier for the metered exposure. Right?

Am I right if I say when I calculate the effective aperture on that way: actual extension/actual aperture diamater will give me the effective aperture size? It is equal with the gived aperture size multiplied by the bell extension factor?

Why I ask this?
I made a home made slide box camera, and my aperture solution is that: different diameter holes (mask) are in front of the lens for the different apertures. To me is more simple way to divide the extension by the holes (mask) diameter (like it's a "very big holed pinhole camera") and metering exposure time for that aperture instead of the calculate the f number by the hole(mask) and then use a table with the extension factors and then multipling the exposure time with the bell extension factor...

[Vaidotas]

If your aperture is out of lens center it will induce vigneting not to mention additional factor calculating exposure. It might be slim deviation but it exist.

[gypsydog]

Didn't Realize a Bell had an extension factor!

[Mark J]

If you are sure that the aperture in front of the lens defines the beam diameter, then your simpler method should work OK.

[xkaes]

There can be a BIG difference between the optical focal length of a lens and the flange focal length of the lens -- messing up your approach.

[maltfalc]

distance between film plane and entrance pupil divided by diameter of entrance pupil. there are youtube videos on how to measure both. it's a bit of a pain in the ass since the entrance pupil isn't a physical object.

[Jim Jones]

Maltfalc - Thanks for accurate and concise information.

[lipi]

distance between film plane and entrance pupil divided by diameter of entrance pupil. there are youtube videos on how to measure both. it's a bit of a pain in the ass since the entrance pupil isn't a physical object.

Thanks!

In my case the aperture diameter can be determined.

[Drew Bedo]

As you extend the bellows, the aperture stays the same. The aperture is related to the focal length and the dimeter of the lens. These do not change.

What changes is the amount of light that falls on the area of the format. As the lens moves away from the focal plane, the image circle expands, which spreads the available light out. The intensity of the light falling on the area of the format (the film) drops off inversely with the square of the distance.

See https://en.wikipedia.org/wiki/Inverse-square_law for an explanation of the inverse square law.

Look at https://en.wikipedia.org/wiki/F-number for an discussion of f-stops.

[maltfalc]

As you extend the bellows, the aperture stays the same. The aperture is related to the focal length and the dimeter of the lens. These do not change.

What changes is the amount of light that falls on the area of the format. As the lens moves away from the focal plane, the image circle expands, which spreads the available light out. The intensity of the light falling on the area of the format (the film) drops off inversely with the square of the distance.

See https://en.wikipedia.org/wiki/Inverse-square_law for an explanation of the inverse square law.

Look at https://en.wikipedia.org/wiki/F-number for an discussion of f-stops.

focal length changes with focus. the focal length printed on a lens is the focal length specifically at infinity focus. same goes for the f-number.