Focomat 1c - magnification scale & f-stops

[alkos]

My 1c (i was given free month ago :-D) has a wedge magnification scale attachment at the side of the column, just like this II:

The question is: what is the correct equation to calculate the f-stop correction from the magnification change?

Thanks in advance

Alkos
--
http://alkos.info

 

[Nicholas Lindan]

(m + 1)/(M + 1)

(there was a url link here which no longer exists)

 

[alkos]

Thanks! Strangely I've read that topic yesterday, but could not find direct translation between mag and fstops... only "time converters"... ;-)

So, changing from magnification 5 to 8 requires +1,5EV correction, and from 4 to 5 as much as 1.2?

Frankly, sounds bit too much for me.

 

[Nicholas Lindan]

Thanks! Strangely I've read that topic yesterday, but could not find direct translation between mag and fstops... only "time converters"... ;-)

To convert from time to f-stops try http://www.darkroomautomation.com/support/stopstable.pdf

You can use the 'seconds' column as the correction factor from the above equation (M + 1)/(m + 1). Say the correction is 1.75, then from the table the appropriate stops correction is 0.8 stops (opposite 1.74 'seconds').

Since enlarging lenses are hard to adjust accurately except at their click stops, and some are none too accurate even then, time correction provides a more accurate method. For large changes a combination of lens aperture and time may be convenient. Say the required change is +2.7 stops: open the lens 1 stop; then apply the remaining 1.7 stops by time correction - multiply the time by 3.25.

Of course, with an enlarging meter and f-stop timer the exercise becomes trivial.

A budget alternative is to use an Ilford EM-10 comparator and do all the adjustment with the lens aperture.

 

[Nicholas Lindan]

Errrm....Errata....The formula should be (M + 1)^2/(m + 1)^2....

Changing from 4x to 5x requires a 1.44 exposure correction, about 1/2 a stop. As a crude check on this point a 4x5 print is 20 square inches, a 5x6 print is 30 square inches, so a correction in the neighborhood of 1.5 is reasonable.

 

[alkos]

Thank you, tonight I'll try this in practice

 

[Ronald Moravec]

3.5x5 5x7 8x10 11x14 16x20 Are all one stop different in exposure if you are working in 35mm.

Just use 30 sec and 5.6 8 and 11 and you will have no problems.