DIY and focal length

[northeast16th]

I'm going to build my own large format camera with a fixed focus, but what I want to do is calculate how long I should make it based on a couple of things.

I want to shoot portraits from the waist up or a touch closer.
4x5 format.

So I'm wondering if I should build it to a normal focal lenghth of about 150mm, get a 150mm lens, and then experiment with distance to see where the focus falls. I suppose there is a more scientific way of doing this, but I'm not sure what calculations I would make.

And if I stopped down to 22 I suspect that I could get everything in focus anyway as long as I wasn't right up on the subject.

Anyone with insights on these calculations, please do pass it along, and thanks!

Scott

[northeast16th]

maybe i should word it like this.....

if i have a 150mm long (from lens to film) 4x5 camera with a 150mm lens, how far away from the front of the camera is the point at which the focus sits? so i could be at minimum aperture and as long as i was exactly this distance away, it would be in focus?

i just need to know how to calculate this before i build the camera.

[John Koehrer]

A 150mm lens is only 150mm at infinity. If you want to be closer, your 150mm lens has to be further than 150mm away from the film. Likely to be around 180mm.

[drew.saunders]

Start here http://www.ebonycamera.com/articles/lenses.html to get the "flange back" a.k.a. the flange focal distance of some lenses. This is the distance from the rear of the shutter to the film plane at infinity, and is often a few mm plus or minus the focal length. For example a Fuji 150 CMW is 144.3mm, while a Nikkor 150W is 146.8mm, so for either of those lenses, if you put the flange exactly at 150mm, you wouldn't be focused at infinity, but a bit closer.

The formula you're looking for is:
1/focal length - 1/bellows extension = 1/distance to subject.
or
1/focal length - 1/distance = 1/bellows extension

But a flange focal distance that isn't the same as the focal length acts as an extension (or recess) to your "bellows extension." For the Fuji above, it's as if you have a 5.7mm "top hat" extension. Remember that for the next step.

Let's say you want it fixed at 2m focus distance to the subject, the bellows you'd need is
1/(1/150 - 1/2000)=162.2mm of extension. If you're using the Fuji above, you get 5.7mm "extra" so put the flange 156.5mm from the film plane.

Using http://www.dofmaster.com/dofjs.html if you plug in the values for 4x5 film, 150mm lens, 2m focal distance, you have 1.69 to 2.46m in focus, so you have some forgiveness with that setup.

Or you could test with cardboard until you're happy and then build it, which might work better in the long run.

[ic-racer]

Even at f22 you will need a tape measure to focus a potriat unless you are going to contact print.

How are you going to make the film back? Will it use standard film holders. If so, your "T" distance will have a pronounced effect on where the camera is going to focus. What kind of view finder? You can easily pay as much for a nice optical view finder as a whole monorail view camera.

I'm sure you can get an old monorail veiw camera for the cost of your materials. Being able to focus and compose = less film waste = more economical in the long run.

[Jim Jones]

. . . And if I stopped down to 22 I suspect that I could get everything in focus anyway as long as I wasn't right up on the subject. . . .
Scott

From the data for the excellent Ektar 203mm f/7.7 lens: At a subject distance of 60 inches at f/22 the depth of field is from 56 to 66 inches. This is for "very critical definition", but depth of field is very subjective. A quick way of focusing an improvised camera is to have a string attached to the front of a camera and marked at the distance of precise focus. The subject moves to that distance, and the string is dropped out of the picture. It's cheap and accurate. I agree with ic-racer; the cost of adequate 4x5 cameras can be so low that building a comparable camera is a matter of personal satisfaction more than of economics.

[Edward (Halifax,NS)]

I have been thinking about making a similar camera which can be duel focus. It would be hyperfocal focused with a plain lens board and portrait focused with a tophat lens board. I would need one 10mm deep.

[Drew Bedo]

Why not get ahold of (borow, rent or buy) a 4x5 camera with bellows and set up the shot you want with your 150mm lens. You may use a real lens board or just mock-up something with mat board and gaffer's tape. Then mount the lens board with out a lens in it and measure from the front of the lensboard to the inner surface of the GG. Now you know the critical deminsion without reguard to nodes and such.

If borrowed or rented, the camera goes back to whereever it came from. If bought, you have to decide whether to sell it or just keep it and drop the DIY project.

[Edward (Halifax,NS)]

Drew, in my case I have a CC400,4X5 monorail. It is a big, heavy beast that doesn't get out much.

[sully75]

I think you are one of the few people who really needs a modded Polaroid 900.