Definition Of Minimum Focus Distance

Here comes another dumb question. I have been playing around with my 18-55mm DX

lens as a close up lens. The spec says that the minimum focus distance is 0.9

feet throughout the zoom range. But I can get much closer than that (4-5

inches), while still in focus, if the distance is measured from the front of the

lens. So I assume that the minimum focus distance is measured from the

film/sensor plane, not the front of the lens. But I can't find that stated

anywhere.

You are correct, its from the film/sensor plane!

"Focus Distance" is offically supposed to be measured from the Nodel Point of the lens, which is where the optical path of the light flips or reverses. This is usually inside of the lens around where the shutter is located, but can actually vary with the lens design. For the average consumer the manufacture should use the front of the lens, and maybe some of them do, that's why the "minimum focus distance" can seem to vary from the official distance. But what won't vary is the max. magnification figure given, as that's actually a measurement made at the "film" plane itself.

Sorry, that's spelled Nodal.

Ken is correct. The question is "focal distance", not "focal length".

Focal length, is the distance from the film plane (or sensor) to the lens' optical center, when focused at infinity.

Steve, that is correct but magnification is also figured from the nodal point.

Maybe I need to clarify. I asked about the definition of minimum focus distance, not focal length. Based on the responses here I understand that minimum focus distance is measured from the film plane to the plane containing the object being focused upon.

 

Now the 18-55mm DX focuses to 10.8 inches at 55mm focal length. The 60mm Macro focuses to 8.75 inches at 60mm focal length. So it appears that the 60mm should have a 35% (or so) greater magnification factor. That's significant but not exactly dramatic.

 

I would appreciate an input as to why a serious (?) amateur should invest in the 60mm for close up work instead of just using the 18-55mm DX.

Maybe I've answered my own question. The 60mm is f2.8 whereas the others are in the f4.5-5.6 range.

Curtis,

 

A real macro (micro to Nikon) lens is designed to have a flat field and minimal spherical and chromatic aberation at close distances (< 1:15 reproduction ratio). A "macro" zoom lens like you describe is apt to be fuzzy around the edges. It's not a macro lens, it just focuses close.

 

Nikon Micro lenses have moving elements that keep the lens sharp over all distances (CLC design). After years of disuse, I find that an old 55/2.8 AIS Micro lens is outstanding for landscapes and closeups alike, using a D2x, with no discernible chromatic aberation.

 

FYI, the distance from the front element to the subject is called the "working distance", and is a useful parameter to keep in mind.

You need to look for attainable maghnification, not closest focused distance. The Micro-Nikkor 60/2.8 goes all the way to 1:1 (life-size) with outstanding flat-field correction and virtually no geometric distortion. A zoom lens, even those labelled "Macro" by the maker's marketing department, rarely can do better than 1:4, and image quality up close is at best a compromise.

 

Factoid: magnification is calculated from distance subject-front nodal point, divided by focal length (alternatively, added extension dived by focal length). The focal length itself can and in modern lenses often do change when the lens is brought to a closer focus. The working distance (from subject to front of the lens) is rarely the relevant parameter for estimating magnification, since the nodal point typically isn't coinciding with the front element, it can be inside the lens or be positioned well in front of the lens barrel.

Thanks for the good info Edward and Bjorn.

<p>I think I'm right in saying that the MFD does not include any DOF in front of the focal plane. According to <em>www.dofmaster.com/dofjs</em> , your 18-55 DX focused at (0.9ft) 10.8" should give a DOF near limit of 7.95" (at 18mm f/22) - almost 3" closer than the MFD! Under less optimal conditions - 55mm f/3.5 - the near limit is just 10.7".</p>

<p>Is the minimum focus distance the same as the point where your camera stops focusing because you're so close?</p>

<p>I just wanted to correct small misconception about focal length. It's not measured from the center of the lens. Focal length is measured from the back nodal point to the back principal focal point (where image comes to focus/light rays cross). Furthermore, you can measure lens' front focal point by reversing light through the lens and measuring it from front nodal point.<br>

This misconception comes from single element lens, which has its nodal point in the center. All lenses today have many elements, their nodal points are all over the place... they can be inside lens, in front of lens, and even behind.<br>

Just my 2 cents. =)</p>

<p>The engraved ie marked focus distance on a lens is almost always computed from the film/sensor plane.<br>

<br /> On Cine lens that go macro or close they actually on some say "measured from the film plane" thus on can use a steel tape and measure from the film to the subject to focus; if one doesnt have a reflex focus or a focus position on the lens turret.<br>

<br /> The REASON the engraved focus distance on a lens is from the film/plane is that is a PHYSICAL reference that a shooter can measure from. That is why there is a film plane symbol on better cameras; on some it is the top of the serial number; or the center or edge of a button./feature on the camera body.<br>

<br /> The marked/engraved focus distance is purposely NOT from a nodal point; since one CANNOT measure from some inside virtual/mythical :) feature that one cannot see with the eye.<br>

<br /> Thus from a real world; ie practical reason; the engraved distance is fom the film plane on most all lenses made; on a rare few cameras it is the front element.<br>

<br /> Think of it from a practical reason; on has to have an actual practical reference; ie the film plane. In some some medical lenses I have seen the "measure to the front element scheme used".<br>

<br /> On scientific cine lenses like my 16mm Fastax lenses; each cince lens has its actual measured focal length engraved on the barrel to a thousand of an inch. This and the focus distance allows one to figure the scale of the object on the film for computing how a machine busts apart.<br>

Most lenses are not marked with an actual measured focal length; it is just the marketing guys 5 percent number. On Leica rangefinder there is a groupong by focal lengths; and there is a focal length number(s) for that group. Here the focal length of the helix is matched to the focal length of the lens block.</p>

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