I agree about designing around a sliding box with short bellows for focusing.
A while ago I sketched up a telescopung box with bellows scrounged from a junk 8x10 camera.
In This case the bellows were to be out front, focusing the lens (smaller, lighter, easier to make).
The only downside is you need looooong arms to reach the focus knobs...
After I created a camera obscura in the wife's sewing room, I set this idea aside.
Here's my original sketch...
Given that it's conceivable that a portrait taken on 20x24 might be close to life size, then the question becomes much simpler. Since the perspective is determined by the distance between the lens and the sitter, then half that distance will give you the focal length. If the distance between the lens and the sitter is 1.6m, then the bellows draw at 1:1 will be equal to that, and the focal length will be 800mm.
Originally Posted by Dan Fromm
I'm not used thinking like this and want to understand the theoretics of this logic. Reading your posts leads me to this conclusion. Am I correct?
Originally Posted by jb7
Size 1:1 means that the object on the negative is the same size as the original. To do so the distance between the object and the lens must be the same as the distance between the negative and the lens.
An other factor is the lens size. But what is the relation here? Why 800 mm lens for 1.6 m bellow draw? This is a factor of 1/2 for an aspect ratio of 1:1. Is this a constant? So a bellow draw of 400 mm would need a lens of 200 mm (besides any image circle problems)?
And what if I use a 1000 mm lens on a distance of 1.6 m? Will this make the ratio 1000:16000 = 1/1.6 giving an aspect ratio of 1:0.8? This would mean a linear relation in the formula. And using a 500 mm lens on 1.6 m will give an aspect ratio of 1:2?
How does this works?
Bert from Holland
BTW: I ordered the 790mm f:5.4 meniscus lens lens from Reinhold this week. My first real ULF lens I want to use for building a large camera. My only experience up to now is only with an old Russian FKD field camera (negative size 18x24 cm, lens 300 mm). See also: http://thetoadmen.blogspot.nl/2012/0...ives-with.html
I also have a Kodak Commercial Ektar 10" (like Ansel Adams used) but I haven't used it yet since I don't have a 8x10" camera yet to put it on.
I think you might want to replace 'Aspect Ratio' with 'Magnification'. A search will provide simple formulae for magnification and focal length- try http://www.physicsclassroom.com/Class/refrn/u14l5f.cfm
Regarding your 1000mm lens at 1.6m, your magnification will be less than the 800mm example, but your bellows draw will be longer.
You have some nice lenses there, you'll get an idea about magnification by playing with them- a small window in a darkened room will tell you a lot about magnification and coverage...
Also, due to the inverse square law, which is very strict, you'll lose two stops at 1:1 magnification, so an f/8 lens becomes f/16... Nice portraits on your blog...