subject distance

[Rick Heitman]

I have a distance to subject question. Is there a formula that says ,if I have say a 150 lens on my 4x5, and I want to do a portrait ( and say I want head and shoulders shot which is about 16 inches), other than moving the camera around and keep checking, I am sure you all know of a better way. I might be asking this wrong so forgive me. but it seems like I should be able to put something in a calculator whether I would want a 16 inch head shot, a 5 foot full body or a 100 foot bridge and atleast come close. I use a little adjustable linhoff viewer right now, and maybe that is the way for me to keep doing it but I was just wandering if you all knew a formula. thanks for all of your help and this is a great website. Rick

[mark blackman]

lets see If I understand you - you want to know the distance from a 150mm lens that a 100 foot high object needs to be to produce an image 5 inches high?

h is the height of the object, and h' is the height of the image, o is the distance from the lens to the object, and i is the distance from the lens to the image.

The common gaussian lens equation gives us:

Magnification = i/o = h'/h

h' = 5 h = 1200 (in inches) i = 6 (150mm) so o = 1440 inches, or 120 feet.

This works fine, so long as the lens is focused at infinity ( i = 6 inches).

As you get near, such as for your portrait, you will be moving the lens out and I'm afraid I simply do not remember the maths to work that out.

(PS convention has it that in the above formula h' is negative as the image is upside down - but we can ignore that as I assume you would have noticed that for yourself on the ground glass!)

[David A. Goldfarb]

Do a google search for a shareware program called "f/calc", which does a number of useful photographic calculations, including figuring the field of view for a certain focal length, format, and subject distance. It includes formulas and explanations in the help screens.

[Ernest Purdum]

I hate math and have found out that calculators work correctly only if the right buttons are pushed. Even so, the following can be useful.



At two focal lengths extension, the distance to the subject will also be two focal lengths and the subject/image ratio will be 1:1. Increasing either dimension to three focal lengths will give us either a 2:1 or a 1:2 ratio. From there on, each focal length we add will give us one more whole number of magnification or reduction.



If you used the above with your head and shoulders example you'd find yourself awfully close to your subject. Big nose. I'm inclined to think that either your adjustable viewfinder or "moving the camera around and checking" might be a better idea.

[Michael S. Briggs]

The equations that you want are given in the Lens Tutorial at http://www.photo.net/learn/optics/lensTutorial. These equations relate focal length f, object distance So and image distance Si and magnification M. Just be sure to measure f, So and Si in the same units, such as mm. For most purposes it is accurace enough to measure So and Si from the center of the lens -- for very precise work you have to know where the principal planes of the lens are located.





For your example head shot, your goal is a magnification on the film of about 5 inchs / 16 inches, which is 0.313. The equations give, for f = 150 mm, an image distance Si = 197 mm and an object (subject) distance of So = 630 mm.

[Michael S. Briggs]

As Ernest noted and the calculation shows, for your hypothetical head shot you will end up very close to the subject: 630 mm = 25 inches. This will be intimidating to the subject and cause closer parts of the subject, such as the nose, to loom larger. This is why longer focal lengths are normally used for portrait head photos.

[Ralph Barker]

The magnification formula mentioned by others is probably the best way to do it "on paper", and I agree that a lens considerably longer than 150mm for 4x5 will produce more pleasing results. I use a Nikkor 360T on my (bellows-challenged) Toyo 45AX, for example, but others might prefer something like a 210mm.

If one doesn't like to do the math, for non-tele lenses a framing aid like that pictured below will get you close. The opening on the scrap of matte board is the same size as the image area on a 4x5 neg, and the knots are sequentially tied at the focal lengths of available lenses. The knot for the selected focal length is held against the cheek, then look through the opening with one eye.

[jantman]

I've used a viewing frame on occasion, and they're quite useful, and you can even make them in the field.

Sure, I occasionally use a caulculator to do this type of stuff, especially when in the studio and I take lots of time with things. I even have a program (Bob Wheeler's VadeMecum, linked from this site) on my scientific calculator and my palm pilot/cell phone which does this stuff for me.

But the best solution is still to learn your equipment. When in the field, I only use two lenses for 8x10, and I know them well enough that I can "see" as the lens, and rarely have to move the camera around more than a few feet once I select a position.

Furthermore, as to studio stuff, it's quite difficult to keep moving a camera around to find a set distance. I have a horizontal enlarger on a free-rolling cart (no tracks) and trust me, it's easier to setup and focus again than to try and get a movable object exactly 26" from another.