Distance measurement for focusing

[alan_rockwood]

<p>OK, here's a real basic question. I should know the answer, but I don't.</p>

<p>How do you measure distance for focusing? Do you measure it from subject to film plane?</p>

<p>Thanks.</p>

<p>Actually the question might not be a stupid as it seems. In optics you normally measure distances from the subject (called the "object" in optical-talk) to the lens, and from the lens to the image. Measurement from the subject to the image (i.e. film plane) is not a measurement that works as nicely into the optical formulas.</p>

[richard_oleson]

<p>The standard generally is to measure from the film plane to the subject, although I'm not sure if this was universally adhered to. </p>

<p>An exception is where you are using a diopter close-up lens on the front. In this case, you set your camera lens at infinity and measure from the close-up lens to the subject - the distance should be the focal length of the close-up lens (1M for a +1, 500mm for a +2).</p>

<p>For the purposes of focusing, however, measuring distance is not as reliable as a groundglass or a rangefinder .... if you have to do it that way (ie, you are shooting close up with a scale focusing camera) I would advise stopping well down to allow for measurement errors.</p>

[User_4525289]

<p>In the instructions to the Soviet cameras like Moskva the distance on the scale corresponds to the distance from the front lens to subject.</p>

[Rob_the_waste]

<p>A number of the cameras that I own have a marking on the top of the camera indicating the location of the film plane. I would imagine as the distance to the subject increases, the distance between the film plane and the front of the lens becomes insignificant.</p>

[SCL]

<p>Leica cameras and many others have a mark on the top of the body indicating the film plane so that measurement to the film plane shouldn't really be an issue.</p>

[SCL]

<p>I should have been more specific...SOME Leica cameras have the mark, some don't (my SLRs do, my M doesn't), my Canon T90 does.</p>

[alex_lofquist]

<p>Focussing distance is always from the subject to the film plane. If measured from subject to lens it is called working distance.</p>

[bernard_lazareff]

<blockquote>

<p><em>always</em> from the subject to the film plane</p>

</blockquote>

<p>Not so sure. Need not be the same for all cameras. On several folders with front element focusing I noticed that the 2-m mark seems to be exactly halfway between infinity and 1-m marks. If true, that would mean (not the proper place to give a detailed explanation why) that distances are from lens (for those cameras).</p>

[mark_brown14]

<p>Alan,</p>

<p>From the 'Pictorial Cyclopedia of Photography', Focal Press, 1968 (1974):</p>

<p>'In most cases the lens-to-film distance is so small in comparison with the subject distance, that it does not matter whether the subject distance is measured from the front or the back of the camera. But for accurate work, particularly for close-ups, the subject distance should be measured from the front of the lens, or still more exactly from the forward nodal point (approximately one-third of the thickness of the lens from its front surface).</p>

<p>'A few cameras, however, carry a warning that all distances must be measured from the plane of the film.'</p>

<p>The mark on the camera body indicating film-plane position is helpful when working out magnification in close-up photography.</p>

<p>Sounds like you can keep using the optical formulas you have.</p>

[kelly_flanigan1]

<p>Bernard; the Vigilant 620 my dad bought new in 1947 has its focus distance measured from the film plane; it has the unscrew front element focus scheme. In this scheme ONLY the front element moves; thus one has a compromise type optical design; one dodn for mechanical simplicity. In this type of design the Spherical Aberration varies with focus distance; its often forced in the optical design to be a minimum at say 5 meters; and infinity and super close are worse; but still OK. The spacing of the TICK marks is abit different on this type of lens design; because only the front element or group is moved; NOT the entire lens.<br>

<br /> In about every camera made on the planet the focus distances are taken from the film plane.<br>

<br /> Its even marked as "distance from the film plane " on many Kodak Cine lenses on the barrel too.<br>

The film plane mark on many cameras is often just one side of the serial number; a feature on the camera; it does not have to be just the infinity mark if it is not on the camera. <br>

The reason the film plane is used is it is the thing that doesnt move with respect to the subject. +</p>

[paul ron]

<p>Measure to the film plane... it's how the camera is adjusted when calibrating it.<br>

Guestimates are fine as long as you are using DOF to zone the shot. If it's critical, use a tape measure or a rangefinder that has been calibrated to your camera's film plane focus. Also you may want to see the focusing of our camera as accurate so try using a ground glass on the film plane to see exactly the distances coincide with the markings on your lens.</p>