How much draw for 480 lens.

[ColinRH]

I have an Agfa Ansco 10x8 and am wanting to buy a 480 lens for portraits.
Although the bellows are in good condition and they extend to about 600mm happily, I am reluctant to extend them fully. With a 480 lens fitted and the bellows at 600mm, would that be a decent draw for a head and shoulders portrait?
There is no doubt a formula for such a calculation somewhere but I can't find it. Thanks.

Colin

 

[Alan9940]

Why don't you sit somebody down and try it? Since the 480 is a 19" lens and you have about 24" of total bellows draw, I'm thinking that maximum extension may get you pretty close but I've never tried it myself. That said, however, what is the rigidity of your camera at full extension?

Best regards,
AlanH

 

[BradS]

the formula is:

1/f = 1/do + 1/di

where: f is the focal length, do is the object distance and di is the image distance.

For your case,
do = 1 / (1/480 - 1/600)
do = 2400mm = 2.4 meters.

So, a 480mm lens with 600mm of bellows will focus on a subject about 2.4 meters in front of the lens.
If I remember correctly, that gives a magnification of 600 / 2400 = 0.25


EDIT: You also need to give about 2/3 of a stop more exposure to compensate for the bellows extension.

 

[Ian C]

Suppose that you want a portrait that encompasses 600mm from left to right at the subject plane to be mapped onto the roughly 195.5mm minor dimension of an 8” x 10” film with a 480mm lens.

That requires a subject distance (subject to first nodal point) of about p = 1953mm. This is a Thin-Lens-Equation estimate and could be made accurate if we knew both the nodal distance and flange distance of the lens.

Disregarding the unknown nodal distance and flange distance of your lens and assuming that the nodal points are likely relatively close to each other and that the flange distance is similar to the nodal distance, the lens board would need to be roughly 636mm forward of the film in this case. This is about 25” and is close to the estimate in post #2.

If the lens board is large enough you might solve the problem by building a box-type extension lens board to place the lens 200mm to 300mm forward of the lens board receiver. Then you should have sufficient bellows in reserve for close focusing when wanted without overstretching the bellows.

 

[removed account4]

colin

just to add a little bit more to your confusion

if your lens is a "convertible" design ( like a symmar convertible )
it might take more [bellows] than a regular lens... a lot more ! [ nodal points mentioned above ]
to give an example ...
i have a 210/370 convertible ... converted to 370 ( removing the front element )
it takes about 450mm to focus at infinity and quite a bit more at closer distances..

have fun!

 

[wiltw]

Is your 480mm a 'telephoto' design, in which its FL is NOT the same as (but FL is longer than) the distance from film plane to rear node? Long FL lenses are not always conventional optics due to the very long bellows draw needed to support their use at that distance,

 

[ColinRH]

ALAN9940 - 'Why don't you sit somebody down and try it?' I have yet to buy the lens hence my question

BRADS and IAN C - thank for the formula and opinions. From what you say I'll take a chance. The lens is a 480 f9 Rodenstock Apo-Ronar.

WILTW thanks for the warning but I don't think its a true tele. And the same to jnanian - not a convertible.

I do at least have the option of a return to the seller if not suitable. Thanks for your input.

 

[Ian Grant]

The Apo Ronar is a Process lens so nor a telephoto. Shouldn't be a problem for head and shoulders.

You could make a top-hat lens board for the Agfa Asco it's something I've thought of doing for my Universal View, but I rarely use it preferring my Commercial View which has an extension rail, more movements and 36" bellows extension.

Ian

 

[Dan Fromm]

Hmm. With 600 mm of extension a 480 mm lens will reach 1:4. Colin, will your 8x10 portraits need more magnification than 1:4?

 

[RobC]

If you grab this free software you can plug in all your values and see whats what

http://www.opticalsoftware.net/index.php/how_to/lens_design_software/predesigner

http://www.winlens.de/index.php?id=70

 

[Lou Baleur]

Suppose that you want a portrait that encompasses 600mm from left to right at the subject plane to be mapped onto the roughly 195.5mm minor dimension of an 8” x 10” film with a 480mm lens.

That requires a subject distance (subject to first nodal point) of about p = 1953mm. This is a Thin-Lens-Equation estimate and could be made accurate if we knew both the nodal distance and flange distance of the lens.

Disregarding the unknown nodal distance and flange distance of your lens and assuming that the nodal points are likely relatively close to each other and that the flange distance is similar to the nodal distance, the lens board would need to be roughly 636mm forward of the film in this case. This is about 25” and is close to the estimate in post #2.

If the lens board is large enough you might solve the problem by building a box-type extension lens board to place the lens 200mm to 300mm forward of the lens board receiver. Then you should have sufficient bellows in reserve for close focusing when wanted without overstretching the bellows.

Just out of curiousity, say that I knew the nodal points and the flange focus. How would these factor into the calculation? I've never see these accounted for in any calculation.

 

[BradS]

Just out of curiousity, say that I knew the nodal points and the flange focus. How would these factor into the calculation? I've never see these accounted for in any calculation.

It is a little elaborate for a discussion forum...try to google "thin lens equation" and look at some of the images to get a feel for the physics.

 

[ColinRH]

Thank you all for your replies.
I have now bought the lens - on return if necessary - and having fitted it to the camera and with max. extension I dare - about 605mm - it does not quite do as I had hoped. However, it's a lovely lens having tested it in the garden with paper negs and I look forward to using it for some landscape stuff and slightly longer portraits.

Thank again all.