Lens Question

[filmbase]

With large format lenses, is there a way to find out what the

minimum distance my subject has to be from the lens to be in

focus when the lens is focused at infinity? I�m looking for a

mathematical formula not a tape measure. :-)

 

In other words�

 

I'm looking to find out (approximately) how many feet away my

subject has to be, to be in focus when my lens is at infinity and

wide open. Does that make sense?

 

I have a Schneider APO-Symmar 210 f5.6, Nikon SW 120 f8 and

a Nikon M 300 f9.

[leonard_evens]

Welcome to Optics 101.

 

The distance you want is called the hyperfocal distance. It depends on several factors: the focal length of the lens, the f-number, and your criterion for "being in focus". A formula for the hyperfocal distance (which is accurate enough for most practical purposes) is

 

H = f^2/(N*c)

 

where f is the focal length, N is the f-number, and c is the diameter of the so-called maximal allowable circle of confusion. c, f, and H should all be measured in the same units.

 

A common value for c for 4 x 5 photography is 0.1 mm. So say you have a 210 mm lens, and your f-number is 5.6. Then

 

H = (210)^2/(5.6*0.1) = 78,750 mm or about 79 meters (or 258 feet).

 

I'll let you practice by doing the calculations for the other lenses.

 

Note that the answer depends critically on the value of c chosen. That value is based on assumptions about how much the negative will be enlarged, at what distance the resulting enlargement will be viewed, and the visual acuity of the viewer. Some demanding people will insist that c be smaller, perhaps even as small as 0.05 mm for 4 x 5 photography.

 

If you want as much in focus as possible, then you shouldn't focus at infinity but instead at the hyperfocal distance. Then everything from infinity down to half the hyperfocal distance should be in focus.

 

I'm still unclear about why you would want to be doing this wide open instead at a smaller aperture.

[leonard_evens]

P.S. Since you asked for the answer in feet, here is the formula with a conversion from mm to feet included

 

H in feet = (f in mm)^2/(N*(c in mm)*304.8)

 

For your information, 25.4 mm equals one inch and of course there are 12 inches in a foot. 25.4*12 = 304.8. The f-number N is a dimensionless number.

[filmbase]

Ouch! Something just popped in my brain.

 

Thank you very much, Leonard, that was exactly what I was

looking for, maybe that was a little bit more than I bargained for.

:-)

 

I wanted to know with a wide-open lens for a couple of reasons:

 

1) If I have to quickly set up to do an impromptu portrait I would

like to know "roughly" how far I should have my subject stand

from my camera. (5 feet, 10 feet, 15 feet, etc.)

 

2) Lets say I�ve determined that with my 210 lens my subject

needs to be 8 feet from my camera to be in focus with the lens

wide open at infinity, I now can utilize a faster shutter to stop any

motion I have them perform. Or if I stop down and I'll know for

sure the depth of field will cover the focus.

 

I do many person-on-street portraits and I�m trying not to spend a

lot of time with a lupe on the ground glass, for they get very

uncomfortable very quickly.

 

Thanks again, Leonard.

 

-- Jeff

[ralph_barker]

It sounds like you might be using the wrong kind of camera, Jeff. You might be far better off with a press-style 4x5 with a coupled rangefinder for the type of work you are doing.

[scott_fleming1]

What does this mean, ^, in a math equation? Did they change things since last I did any real math or is this a computer thing?

 

Jeff, I wanted the same thing you did for different reasons. Hating math I just set up my camera with markers at all pertinent distances and and went through the drill with all my lenses. I then made up a simple chart I keep handy in the camera bag. It references to a scale I glued on the side-rail of my Toyo 45.

 

Hyper-focal distance: If you want the horizon sharp in BIG photos this may not be the best focus point.

[paul_frank]

in math, as it's always been,

 

x^y is x to the y power.

[steve_hamley]

Scott,

 

^2 means squared. There's not any convenient way to do superscripts, so there's a few common ways to abbreviate. People also use "E" to indicate "exponent", so 2x10E+2 would be 200.

 

Thanks!

 

Steve

[michael_briggs2]

The equation that Leonard gave is for the hyperfocal distance -- as Leonard says, when you focus at this distance, all objects from one half of this distance to infinity will be in focus. This is different from asking the distance to the closest object that will be in focus when the lens is focused on infinity. If you want both a close object and infinity in focus, you will be able to do so at a wider aperture by focusing on the hyperfocal distance instead of on infinity.

 

For the example that Leonard gave, if you focused on 79 m with your 210 mm lens, objects from 40 m to infinity should be in focus. Even so, the result suggests that taking portraits by focusing on the hyperfocal distance of a wide-open LF lens will result in extreme environmental portraits, with the person being a small fraction of the area of the photo.

 

A LF camera with ground glass focusing isn't the easiest tool for speedy portraits. What you could do is pre-focus the lens at some distance such as 5 m and have a string of that length tied to the camera to use in positioning the subject. Or maybe use one of the ultrasonic distance measuring devices sold in do-it-yourself stores to adjust the camera-to-subject distance to match the pre-focused distances. Or as Ralph says, switch to an LF camera with range-finder focusing. Also, the wide-open aperture of most LF lenses is intended by the manufacturer for focusing and not taking the photo -- most LF

lenses have poor sharpness wide-open -- which is ok if you want this effect, but not otherwise. If speed is of the essence, might consider using a MF format camera.

[jason_greenberg_motamedi]

For those who, like myself, are a bit damaged when it comes to math, I might recommened the use of Bob Wheeler's "Vade Mecum", which is a great program for your PDA, including Palm and Windows CE machines. It will provide all the LF number crunching you can dream up... It might be worthwhile to buy a cheap palm just to use his software! Ok, it ain't pretty, but it works flawlessly.

 

http://www.bobwheeler.com/photo/Software/software.html

 

Best of all it is free. There are also shareware programs for Palm and Windows CE. Bob has great reviews of them:

 

http://www.bobwheeler.com/photo/Surveys/surveys.html

[wieslaw1]

The above numerical answers regarding focusing and hyperfocal distances are correct, providing that you do not tilt the lens. If the lens is tilted - that�s another story.

 

One extra comment regarding the c value: Without going into complex explanations, if you really need acute sharpness extending over foreground and horizon, just use more restrictive distance settings, i.e. the range of sharp focus associated with aperture one stop wider than you actually use. For example focus at 8 and stop down to 11 - easy to do on lenses with distance with calibrated scale.

[philip_sweeney]

check this link for a convenient depth of field calculator:

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<a href="http://www.tangentsoft.net/fcalc/">fcalc</a>

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