Estimating Effective Aperture, need help please.

[xpatUSA]

I've been able to ignore the subject for years.

But now I need to know how to estimate Ae because I'm doing some research into the ISO of my Sigma cameras.

Not needing super accuracy - and reading about entrance and exit pupils, etc. is hurting my brain.

I've read that Ae = A x (1+m) - is that fairly close for my purpose (estimating ISO)?

. . where m = the magnification factor = size on the sensor / size in the scene.

For example, my Sigma 17-50mm is specified as 1:5 (in Sigma-speak) at 50mm . . so m = 0.2 and 1+m = 1.2 thus f/2.8 becomes about f/3.4.

I know that some cameras/lens report effective aperture - my Nikon D50/60mm macro did. Sigmas do not, AFAIK.

[miatab]

You lost me on line one. I am so old it was f8 or f11 , bright blue sky, 125 th of second with ASA 100 positive film!

[dje]

I've been able to ignore the subject for years.

But now I need to know how to estimate Ae because I'm doing some research into the ISO of my Sigma cameras.

Not needing super accuracy - and reading about entrance and exit pupils, etc. is hurting my brain.

I've read that Ae = A x (1+m) - is that fairly close for my purpose (estimating ISO)?

. . where m = the magnification factor = size on the sensor / size in the scene.

For example, my Sigma 17-50mm is specified as 1:5 (in Sigma-speak) at 50mm . . so m = 0.2 and 1+m = 1.2 thus f/2.8 becomes about f/3.4.

I know that some cameras/lens report effective aperture - my Nikon D50/60mm macro did. Sigmas do not, AFAIK.

Hi Ted

This article by David Jacobson gives good detail on the subject. He brings pupil magnification into it too.

Dave

[xpatUSA]

Hi Ted

This article by David Jacobson gives good detail on the subject. He brings pupil magnification into it too.

Dave

[edit] Reading that article, Dave, I think I've asked the wrong question. Since the "marked" f-numbers of lenses already include effective aperture and effective focal length, I should have used a different phrase like, for example, "working aperture".

So now I'm comfortable with the (1+m) close-up factor which I can get off the lens barrel (thank you, Sigma) or from the lens spec or by just plain eyeballing through the viewfinder. [/edit]

Thanks for your help!

[xpatUSA]

You lost me on line one. I am so old it was f8 or f11 , bright blue sky, 125 th of second with ASA 100 positive film!

I must be even older, because we used to say "100 ASA" and write "f/8" and "1/125" . .

[rpcrowe]

Ted, I wonder if we don't micro worry about things

[miatab]

Ah, Ted, you just think you are older. I say it differently because back in the glory days I was living down under.

[xpatUSA]

Ted, I wonder if we don't micro worry about things

Nah, just a bit of subtle back-and-forthing, i.e. no worries.