How to calculate distance from shutter to film ?

Falkenberg · Oct 10, 2007

In my planning of making a 8x10 portrait camera I have run into some roadblocks. I have found several webpages that can calculate my front and rear focus distance at any given aperture. My question is how to calculate the distance from the lens/shutter to the film/ground glass plane. I was thinking about making a cardbox camera to have something to test the distance, but since I dont have a ground glass yet, I will not be able to see any changes.

TIA.

Best regards
Mikkel

Ty G · Oct 10, 2007

You would first need to know the focal length of the lens you are using and build the camera to that. You can figure it out without the ground glass; take a cardboard box, cut a hole in the front bigger than the lens, and cut a hole at the top so you can look inside the box. Aim to a bright object and while looking inside your box, move the lens back and forth 'til focused. Then measure that distance. You can do the same for far objects and very close objects to get the whole range.

Ty Guillory
www.tystintypes.com

Falkenberg · Oct 10, 2007

this lens and shutter

The Lens is a 240mm/f5.6 with a separate shutter. How would I calculate the distance with this information ?

freygr · Oct 10, 2007

Focus at infinity is 240mm from the nodal point to the film. But portrait's the person is not at infinity so the lens will be farther away from the film plain. If you have a 4x5 view camera just mount it and focus at the distance your subjects will be at and measure. Other wise a high school or first year college physics text book should have all the formulas in it.

BradS · Oct 10, 2007

for lenses of conventional design, the formula is:

1/f = 1/a + 1/b

where:
f is the focal length of the lens
a is the distance from the film plane to the front surface of the lens board (roughly and subject to some assumptions about lens design) and,
b is the distance from the front of the lens board to the plane of sharp focus (the subject).

Falkenberg · Oct 11, 2007

Math

Does this mean that with the 240 mm lens and a person at 3 meters (3000 mm) it would give me a distance 260,86mm from lens to film. Is this correct ? If this is true why are the dedicated portrait 8x10 cameras so huge ?

Dan Fromm · Oct 11, 2007

They don't use lenses that are wide angle on the format, they use lenses that are long on the format.

BradS · Oct 11, 2007

Falkenberg said:
Does this mean that with the 240 mm lens and a person at 3 meters (3000 mm) it would give me a distance 260,86mm from lens to film. Is this correct ? If this is true why are the dedicated portrait 8x10 cameras so huge ?

Yes. Substituting the values in your example, I get...

distance from lens to film = 1 / ((1/240) - (1/3000)) == 260.87 mm

and as Dan has already pointed out, the 240mm focal length would not normally be considered for portraits with an 8x10 camera. It would likely be considered much too short a focal length for such a task. Also, most LF portaitist that I've encountered seem to prefer to position the camera about a meter from the subject.

Falkenberg · Oct 11, 2007

wich lens

For my project the 240mm will have to do, but wich lens should I be looking for to get the best portraits with my portrait camera ? I guess that the distance from camera to the person will be something like 1,5 to 3 meters. I want to fill the frame with the head and a little of the shoulders and neck.

argus · Oct 11, 2007

Falkenberg said:
For my project the 240mm will have to do, but wich lens should I be looking for to get the best portraits with my portrait camera ? I guess that the distance from camera to the person will be something like 1,5 to 3 meters. I want to fill the frame with the head and a little of the shoulders and neck.

To fill the frame with head and some shoulders with my appr. 440mm portrait lenses on 8x10", the lens sits less than 1 metre from the nose of my subject and have about 600mm bellows draw. (all estimations, I don't actually measure that when doing)

I'd advise a slightly longer lens.

larsco2002 · Apr 8, 2008

The 240 is not a short telephoto on 8X10

If you insist on using a short wide angle on 8X10 AND FURTHERMORE, if you plan on filling the frame with head and shoulders, you will suffer the "big nose" effect. This is considered very unflattering.

I would consider 240 a bit more of a portrait lens on 4X5, since it is longer than the normal focal length for that form (slightly)

Short telephotos are normally considered to render a more flattering face on all film (and digital now) formats. For instance, for 35mm, most photographers prefer for portraits, 85 to 105mm, where 50 is considered normal.

To get back to your question, however, there was mention of using Nodal point as the measurement in the lens. I have a basic understanding of this, but I would also like to think that the aperture of any large format lens would be placed at the Nodal point (the point at which light beams cross over in the lens on the way to the ground glass). If my presumption is true, the appropriate distance for a 240mm lens would be 240mm, or 24cm from the aperture blades to the ground glass. All else would be handled by the movements of focusing. If, however, you are building or designing a camera that has a fixed distance from (no focus movement) lens to ground glass, then you are going to have to set up a measured distance for your portraits which will always be constant, and use a mock up of the lens, box, and ground glass and focus it on the chosen distance before nailing things together.

Dan Fromm · Apr 9, 2008

larsco2002;614533<snip> I have a basic understanding of this said:
Eh? Wot? Your understanding of nodal point is incorrect. Every lens has two of them, front and rear. Focal length = distance from rear nodal point to film when the lens is focused at infinity.

Now think about a telephoto lens, whose rear nodal point is in front of the front nodal point. With teles, flange-to-film distance at infinity is considerably less than focal length.

There are books on optics for photographers, e.g., R. Kingslake's Lenses in Photography, and there are online tutorials, e.g., http://photo.net/learn/optics/lensTutorial . Read and learn.

Ole · Apr 9, 2008

BradS said:
for lenses of conventional design, the formula is:

1/f = 1/a + 1/b

where:
f is the focal length of the lens
a is the distance from the film plane to the front surface of the lens board (roughly and subject to some assumptions about lens design) and,
b is the distance from the front of the lens board to the plane of sharp focus (the subject).

Also the reproduction ratio will be M = a/b

So with a working distance (lens to subject) of 1.5m and a scale of 1:3 - about right for a tight head-and-shoulders portrait on 8x10", we will nees an extension of 1.5/3 = 50cm of bellows.

Putting that into the first equation, we get f = (1/ ( 1/1500 + 1/500)) , or 375mm focal length.

So that will be a good portrait focal length on 8x10". A 360mm or 420mm will do nicely, depending on how tight you want the portrait, how much distance you want, and how long bellows/extension your camera has.

How to calculate distance from shutter to film ?

Ty G

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