Focusing range of Goerz Dagor 305mm 6.8 for portraits?

[wallpaperviking]

Hi,
I have access to a friends 8 x 10 and a Goerz Dagor 305mm 6.8 and am looking to do some close up portraits with it. As I understand it, to focus at infinity, the lens is 305mm away from the film plane. For a close up portrait, just wondering how far the bellows is likely to be away from the film plane?

Would about 450mm be about right?

Do I need to factor in any bellows light loss when working out an exposure?

I have attached a reference to show how close I am talking about.

Thanks in advance!

[Corran]

Yes, infinity focus is achieved at roughly the focal length, while a 1:1 magnification is achieved at twice that. That headshot is probably close to 1:1 on an 8x10 camera, so 450mm will probably not be enough, and yes you definitely will have to contend with bellows light loss.

[wallpaperviking]

Yes, infinity focus is achieved at roughly the focal length, while a 1:1 magnification is achieved at twice that. That headshot is probably close to 1:1 on an 8x10 camera, so 450mm will probably not be enough, and yes you definitely will have to contend with bellows light loss.

Thanks so much! Much appreciated!

If you could take a guess, how much bellows draw is 1:1?

[ghostcount]

Thanks so much! Much appreciated!

If you could take a guess, how much bellows draw is 1:1?

From one of our esteemed members...

http://www.kennethleegallery.com/html/tech/bellows.php

[Bernice Loui]

1 : 1 or "life size" double the focal length. Example:

12" Dagor would need about 24" of bellows to image 1 : 1 or life size and there will be two f-stops of light loss.

Keep in mind if your portrait sitter is about 24" to the camera, consider how your sitter might feel about being this close to the camera and how would this portrait be lighted?

Typical 12" Dagor full aperture is f6.8, Optical performance happens at about f16, add two f-stops for light loss (1 : 1). This puts light at your portrait sitter at about f32 which will need to be achieved by light intensity or duration of exposure.

One more thing, sitter movement can be a problem. If duration of exposure is used any portrait sitter movement will result in blur. If a strobe is used, consider what happens to your sitters eyes and vision once that strobe goes flash.


Bernice

Thanks so much! Much appreciated!

If you could take a guess, how much bellows draw is 1:1?

[wallpaperviking]

Amazing! Thanks so much!

[Pere Casals]

Here you have the formulas http://www.kennethleegallery.com/html/tech/bellows.php

At 1:1 magnification, bellows draw = 2 x focal length and exposure compensation is 4x or 2 f/stops.

So for 1:1 you need 305mm x 2 = 610mm. Add the distance from the rear nodal point to the lens board, this Focal-FFL, (FFL is flange focal distance), it can be (a guess) 30mm more so around 635mm of bellows. You may substract the film plane to belows rear frame distance. So the 24" Bryan Said.


The most used formula is this:

1 / Focal = (1 / Distance) + (1 / Extension)


1/305 = 1/D + 1/420 (I take 420mm instead 450mm bacuse the nodal point can be displaced say 30mm inside)

1/D = 1/305 - 1/420 = 0.00089773614

D = 1/0.00089773614

D = 1114


With 450 bellows extension you may focus the Dagor at around 1114mm distance.

it can vary a bit depending on the rear nodal point displacement inside the camera.


You can focus a bit closer if you consider DOF if lens is stopped.


Subject distance used is the one from Front Nodal Point to subject. In dagor can be inside the front cell, I guess.

[Jim Galli]

Some folk enjoy doing the math and enjoyment is what we're after, but your sitter may roll her eyes if you produce a slide rule and start doing calcs after you've told her to not move a muscle. Another way, I do it all the time, is to bring along a 6 foot tape rule with a millimeter scale. Measure the aperture opening. (just hold it out front) Then measure approx half way between front and rear elements to the ground glass. Off to the side is plenty good enough. 20mm hole and 540mm bellows; divide 20 into 540 = 27. You're at f27 = f22 1/2 on your meter. Now, if you like doing calc's and your sitter is also married to you and very patient, ignore me. Most folk do.

[Pere Casals]

Jim, one question, what aprox focal has the front element of a Symmar convertible used alone ? (relative to the full lens focal), I'm to disassembly one to use it alone...

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Then measure approx half way between front and rear elements to the ground glass.

In a dagor perhaps we should take the midpoint on the rear cell

Measuring the aperture diameter as seen from the front is right, but for the distance we should take the rear node position, I ask...

[Jim Galli]

With a combined anastigmat I just measure from the aperture control ring to the ground glass. With a single anastigmat like a Single Turner Riech group I guess at halfway mid point of the lens glass. I don't know what the front is on a Symmar. Might be double. They weren't symmetrical like the G-Claron's.