Focus Distance Calculation

<p>How does one calculate focus distance with LF cameras? For example, using a 150mm lens the lensboard is 150mm from the film plane for infinity focus. If I extend by 50mm for a distance of 200mm between film and lens, what would be the distance between in-focus object and camera (lens or film plane?). Couldn't find that equation anywhere... THANKS!</p>

The simple equation is:<br>1/focal length = 1/subject distance + 1/image distance.<br><br>It works, as long as you remember that the distances should be measured from the front and rear principal planes respectively. Which themselves are a distance apart, and usually not halfway the physical length of the lens. If you focus a 150 mm lens on infinity, you may well find that the distance from lensboard and film plane is not 150 mm. That sort of thing.<br>But the equation works well enough to get an idea.

<p>Thanks for such a fast response! The difference for infinity extension you mention, is that what's called "flange focal length"? For example, Rodenstock lists on their website a flange focal length of 147mm for their Apo-Sironar 150mm lens. So I should pluck 147mm instead of 150mm into this equation?<br>

Subject distance and image distance would both be measured from film plan, right?</p>

<p>That means that the rear principal plane location (or nodal point?) of the lens is 3mm forward of the flange. The focal length is 150mm, but you want the flange 147mm from the film to focus at infinity.</p>

<blockquote>

<p>For example, Rodenstock lists on their website a flange focal length of 147mm for their Apo-Sironar 150mm lens. So I should pluck 147mm instead of 150mm into this equation? Subject distance and image distance would both be measured from film plan, right?</p>

</blockquote>

<p>No, if the focal length is 150mm, that is what you should use. The flange distance is just a somewhat arbitrary mounting point.</p>

<p>With this formula, you ideally measure from the "rear principle plane" of the lens to the image, and from the "front principle plane" of the lens to the subject. </p>

<p>Unfortunately, you don't generally know where these "principle planes" are, except by means such as john Shriver says. You can usually find the approximate locations by focusing the lens at infinity, then measure 1 focal length from the image (this gives the rear principle plane). For the front principle plane, reverse the lens and repeat the procedure. Sometimes you may find that the planes are separated, other times they overlap, depending on the lens design. (If it's a strong wide-angle-type lens, the infnity image from the rear MIGHT BE internal to the lens, so you can't measure it so easily.)</p>

<p>Often, you don't need to know where these planes are. If you focus the lens at infinity, you can mark the lens standard position, then measure offsets from that.</p>

<p>By the way, the formulas are in David Jacobson's "Lens FAQ" or "Lens Tutorial" on this site. I don't see a menu link to them anymore, but here's the FAQ: <a href="/learn/optics/lensFAQ">http://www.photo.net/learn/optics/lensFAQ</a> (see Q4).</p>

<p>The data sheets for large format lenses usually show target focal length, location of the principle planes, flange focal length, etc. As already answered, you use the focal length in the optical calculations. The flange focal length is a mechanical figure, telling you where the lens will be located for infinity focus. Since you can't see the principle planes, you need something on the lens to reference to -- the flange, or rear of the shutter. For most LF lenses, the difference is only a few mm. For true telephoto designs, the difference can be large. The flange focal length can be useful in determining whether a camera has enough extension to use a lens. I wouldn't focus a camera by measurement -- ground glass focusing is more accurate.</p>

<p> Why not just focus with the ground glass and then measure— why measure at all?</p>

<p>Am I missing something?</p>

<p>Thanks everyone, I learned a lot!<br>

PS: I needed that info to make an informed decision about rail length purchase.</p>