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# Thread: How to calculate distance from shutter to film ?

1. ## 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.

2. [QUOTE=larsco2002;614533<snip>
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.<snip>[/QUOTE]

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.

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.

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