7-10x enlargement from 5x7 questions..

[chris77]

hello apug.
i am wondering, if i were to enlarge 5x7 to approx. 35x50 using a process lens (apo germinar 240mm, a symmetrical dialyte (45°)), what size of projection space would be needed (vertical).
what is the proper math behind it? my guess is that 7xenlargement would need 7x the focal length on both sides of the negative stage if focus were on infinity, correct? i know the image circle is getting bigger when stopping down. at 1:1 image circle should be double, no? how to calculate for 1:7?
thanks for helping me out here..
chris

 

[Ian Grant]

Well at 1:1 the lens is going to be 48cm from the negative, and 48mm from the print so aready around a metre. A 240mm enlarger lens is designed for a 10x8 negative.

The formulae is: Easel to carrier distance = Focal length * (2 + 1/M + M)

M is the negative to print magnification. In your case 1:1 is 98cm, 1:7 is 2.194 metres, and 1:10 is 2.904 metres

Use a 180mm lens and that comes down to 1.62 metres for x7 ans 2.18 metres for x10, I'd use a 210 lens on my De Vere 5108 but it's floor standing and can cope with that degree of enlargement but the base board isn't large enough to go more than about x6 off a 7x5 negative.

Ian

 

[ic-racer]

Ian has posted a good formula. I might suggest another practical formula. (50x35x4)/144 = square feet of table space needed to process the print in processing trays. (almost 50!)

 

[mshchem]

I used a nice Wollensak Graphic Raptar ,161mm for 5x7 . I had a nice old Elwood enlarger that you could fold in the middle and project on a wall.

 

[chris77]

hello ian.
if i understand you correctly, for a 7x from 5x7inch neg. (using the 240 lens i have)the distance between neg. and lens would have to be about 120cm? well.
apart from a powerful lightsource, would there be any other disadvantage?
i am about to build a camera that can be modified to serve enlarger...
but maybe i should start to look for a shorter lens soon, as space is limited...
thanks for your detailed reply! very helpful..

 

[Ian Grant]

No with the 240mm the lens to negative distance at 1:1 magnification is 480mm, the greater the enlargement the closer it gets towards its Infinity focus which is 240mm.

Cameras are typically double or triple extension, that's bellows extension 2x or 3x the FL of a standard lens for the chosen format, with 7x5 that's typically a 210mm lens so you'd want an extension of 42cm - 63cm so a lot less than 120cm. Most enlargers focus to approx 1:1.

Ian

 

[chris77]

No with the 240mm the lens to negative distance at 1:1 magnification is 480mm, the greater the enlargement the closer it gets towards its Infinity focus which is 240mm.

Cameras are typically double or triple extension, that's bellows extension 2x or 3x the FL of a standard lens for the chosen format, with 7x5 that's typically a 210mm lens so you'd want an extension of 42cm - 63cm so a lot less than 120cm. Most enlargers focus to approx 1:1.

Ian

oh, of course. silly me, the longer the bellows in enlarging, the smaller the projection.. well...