Extension tubes

[piterengel]

Hi, I've bought an used set of Hasselblad extension tubes: 21, 32 and 55 mm. I'll use them on a 500 C/M body with 80 mm lens. What is the light compensation I have to apply? Is there a tabel on internet riassuming these data?

Thanks all

P.

 

[qnu]

Yep.

After some clicking through menus you can find a table on Hasselblad's website: http://www.hasselblad.se/

With 80 mm lens, the compensation ranges are:

21 mm ring: 0.7 - 0.8 stop
32 mm ring: 0.8 - 1.1 stop
55 mm ring: 1.3 - 1.5 stop

55 + 21 combined: 1.7 - 1.9 stop
55 + 32 combined: 1.9 - 2.0 stop

All three: 2.2 - 2.3 stop

 

[piterengel]

Very useful and synthetic! Thanks a lot.

P.

 

[dnsoho]

Hi, I brought an used Hasselblad H-1 AF camera kit without the user manual. Can anybody help me to get this manual so I can make use of this camera. Thank you.

 

[qnu]

David,

You could try Hasselblad. Ask at info@hasselblad.se .

 

[marc_peeters]

>Try this link, here you will find what you are looking for

www.clearsightusa.com/extubecomp.html

Bye Marc

 

[simonpg]

Marc, that's a handy link - thank you. I actually find the Hasselblad web site tables hard work as a "ready reckoner". But that's just me I suppose.

 

[qnu]

Here are the few formulae you need to know it all.
Put them in a spreadsheet, or write a simple program, together with lens data (can be found at Zeiss' website).

Magnification = Extra extension / Focal length

Aperture correction factor = 1 / (Magnification + 1)

Shutterspeed correction factor = (Magnification + 1)^2

Correction in stops (EV) = log(Shutterspeed correction) / log(2)

or,

Correction in stops (EV) = log(Aperture correction) / log(sqr(2))

These formulae are correct for symmetric lenses. Asymmetric lenses require a little more (telephoto), or less (retrofocus) compensation.

---
To calculate the compensation (and that only) for asymmetric lenses, multiply the focal length by the pupillary magnification of the lens:

Pupillary magnification = diameter exit pupil / diameter entry pupil

so

As.Focal length = Focal length * Pupillary magnification

and use As.Focal length instead of the true focal length in the above mentioned formulae.

The clearsight tables do not do this, though the difference between these 'plain' and 'asymmetry corrected' values can be surprisingly large.

---
Remember that the focussing helicoid is providing extension too.
The clearsight tables give one compensation value for each lens-tube combo, but since you can use the focussing helicoid too, they should show the upper and lower limit of a range.

You can deduce the amount of built-in extension using the above formulae and the minimum focussing distance.
But you can also measure the differences in physical length of the lens set to infinity focus and minimum focussing distance.