Hasselblad & Macro 120 exposure

[Willie Jan]

Hi,
i just bought a macro 120 for my hasselblad 500 and am thinking about how to calculate the amount of overexposure to compensate.

At 1:1 the exposure is +2 stops, but what for example at 1:4?


thanks

 

[Willie Jan]

I think i found something...

m = magnification desired
F = the focal length of the lens
d = length of the extension tube

m = d/f

Example

8mm Extension Tube and 80mm lens
---------------------------------------------------

Using the formulae above m = d/f = 8/80 = 0.1

Therefore the magnification = 1: 10

 

[EmilGil]

The formula for calculating the exposure compensation is slightly more complicated but it does take your magnification formula into account.

Number of stops=ln[(1+magnification)^2] / ln(2)

I made a table for it a long time ago, I have attached it to this post. Unfortunately, it's only in Swedish but "Oändlighet" means infinity, "Närgräns" means closest focusing distance and "Brännvidd" means focal length.

Using the 120mm at 1:4.5 (closest focusing distance) requires 0.5 stops of compensation.

 

[Willie Jan]

The formula for calculating the exposure compensation is slightly more complicated but it does take your magnification formula into account.

Number of stops=ln[(1+magnification)^2] / ln(2)

I made a table for it a long time ago, I have attached it to this post. Unfortunately, it's only in Swedish but "Oändlighet" means infinity, "Närgräns" means closest focusing distance and "Brännvidd" means focal length.

Using the 120mm at 1:4.5 (closest focusing distance) requires 0.5 stops of compensation.

Thanks.
Never to old to learn Swedisch...

 

[Q.G.]

The mathematics.

A calculator that takes everything into account.

 

[Dan Fromm]

Um, QG, y'r calculator is cute and it shows a lot but the internals are invisible. I think the answer to my question is yes but asking is cheap ... Does it compensate for pupillary magnification?

 

[Q.G.]

Dan,

Yes, of course it does. Would be of little use if it didn't.


The only thing it is not good at (but still does that well enough) is dealing with the focal length shift present in lenses with internal focussing.

 

[Willie Jan]

The mathematics.

A calculator that takes everything into account.

Looks nice.

if I enter 1:1 for the 120mm I get 1.84 EV adjustment. Shouldn't that be 2 EV? or is the 2 EV rounded.

 

[Q.G.]

Willie Jan,

No. That's the result of the very same pupillary magnification Dan was talking about.
The "classic", simplified formulae assume that the distance between the center of projection and the film at infinity is equal to the focal length.
That may be true for a simple (1 element) lens, but not for the lenses we are actually using.

So we need to use the true distance between film and center of projection, i.e between film and exit pupil of the lens. Depending on how asymmetrical a lens is (the Makro-Planar is only a bit), the fault produced by not doing so varies, and can be quite considerable.
You can see how asymmetrical a lens is by comparing the diameters of the exit and entry pupils: the pupillary magnification.

You can read about all that, by the way, on the page the first link i posted links to: http://www.hasselbladhistorical.eu/HT/HTComp.aspx

 

[Q.G.]

Willie Jan,

Just so it is clear: my answer "no" was to "Shouldn't that be 2 EV?"
The result the calculator produces is correct.

And by the way, Dan: that (entering 1:1 and looking at the compensation figure the calculator produces) is exactly the way you could have checked if the thingy indeed did take asymmetry into account or not.