Magnification Factor Calcs and Infinity Inschminity

[nick mulder]

Hello,

I suspect I've a little conceptual hurdle to get over when it comes to magnification - maybe its just terminology - we'll see ...

I like shooting at what I call '1:1' which to me means that the image caught on film is the same size as it is in reality - quite nice in portraiture in 8x10" and above.

This usually equates to a doubling or so of the focal length of any given lens I have in terms of the bellows extension.

Magnification factor from I think the Hasselblad website is defined as the extension beyond that of focus at infinity (0mm) divided by the focal length - which works nicely with my observations as a doubling of focal length equates to a extension equal to the focal length >>> focal length / focal length = 1

1:1

So its as simple as that ?

I can always know that to get 1:1 I need to double the focal length of any lens I'm ogling on feePay (more or less) ?

Maybe I answered my own question - heh heh

Its making me ponder - I guess it depends on what is important to you, but you could if you wanted define focal length that way right ? So all focal lengths would be doubled and they would be defined as the extension required to achieve 1:1 - infinity inschminity

[wiltw]

It's as simple as that! Reproduction ratio = extension length / lens focal length, assuming there is not internal focusing mechanism within the lens being used...focused at infinity. You can get closer (raise the repro ratio) via usage of any focus mechanism within the lens itself.

[nick mulder]

There's just something about infinity focus and 1:1 on the film that seems unrelated to me that something as simple as doubling the focal length in extension should work to get me from one to the other.

Mind you, I remember noting one day that to turn a car say left you had to turn the wheel both left and right to do that and it was the order (and timing) that you did this that determined the direction you'd end up in... Much days of furrowed brows in the passenger seat, trying to make connections withe the price of fish and the ultimate fate of the universe :rolleyes:

Maybe I need to look up the definition of focal length...

[TimFox]

At 1:1 magnification, the bellows extension = lens-to-object distance = 2 x focal length, but that is not the same as doubling the focal length.
(Of course, this assumes a non-telephoto lens if you treat the bellows extension literally.)
The bellows factor for exposure relates to the fact that the lens is now twice as far from the film as when the camera is focused at infinity, therefore the inverse-square law results in the light hitting the film to be 4x (2 squared) less than when focused at infinity.

[nick mulder]

Yes - I understand bellows factor calcs - that's a bit off topic.

I don't understand what you mean by "2 x focal length, but that is not the same as doubling the focal length" ...

My point is not so much understanding it all per se - not to say this isn't required, and I suspect it is - my point is that there is no intuitive link for me between magnification ratios and focus (at the moment).

There's nothing jumping out at me and making it obvious that at 2xfocal length extension that the projected image will be 1:1 on the film plane. A deduction would have to be made by playing with the math or by trail and error as I did - the math as easy as it is doesn't appear to have an intuitive or trivial derivation ...

Probably a brain fart