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On my prime lense the barrel shows the "minimum" focus distance (MFD). According to the definition you find on internet you cannot focus an object closer to this distance (measure from the sensor plane).

I have a problem with the word "minimum" as in practice an object in foreground (also background) can be really sharp (i.e.,in focus) according to the depth of field of the optical system. This is actually what is indicated by the hyperfocal scale displayed on older lenses.

If an object in the foreground is also in focus then the MFD is not minimum anymore So the nomenclature should be replaced by something like mean focus distance , right ? .

Best

  • What would be the purpose of labeling it in that way? – mattdm Aug 6 at 14:18
  • What do you mean ? – Alexis Aug 6 at 14:23
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    Minimum focus distance is a useful, concrete value. What would the purpose be of giving a vague one instead? Is there a particular problem you are trying to solve? – mattdm Aug 6 at 14:25
  • My question was more about the word minimum. Considering that foreground seems to be sharp when you focus an object at the minimum distance the word minimum made no sense to me. I was wrong though as we cannot say the foreground is in focus even if it appears sharp ( see next reply) – Alexis Aug 6 at 20:00
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Minimum focus distance means minimum focus distance of the lens (unless additional accessories are added). Lenses focus at only ONE precise distance, period. Depth of Field adds to that (plus and minus, as an acceptable zone) only providing an approximate "good enough" sharpness zone as defined by CoC (Circle of Confusion).

Depth of Field numbers of say 2 to 10 feet or meters absolutely does NOT mean 2 to 10 distance is in focus. Any lens can be focused at only one distance. DOF 2 to 10 only means the out of focus blur does not exceed the CoC value until either 2 or 10 distance is reached. "Good Enough" maybe, but not critically sharp focus except at one distance.

  • please see complementary question below – Alexis Aug 6 at 14:22
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Many camera lens lash-up have a mechanical stop that prevents the lens from racking forward beyond a certain distance. This sets the minimum focus distance to about 2 feet (600mm). General purpose camera lenses are optimized for nearby and far subject distances and slightly compromised when tasked to work close-in.

Additionally, the f-numbers engraved on the lens barrel become invalid when tasked to work in close. The f-number is derived by dividing the focal length of the lens by the working aperture diameter. The engraved values are only correct when the camera is imaging objects at infinity (as far as the eye can see ∞). As you task the camera to work closer and closer, the physical distance, lens to image plane is elongated. At unity (magnification 1 often stated as 1:1), the lens is racked out 2X the focal length. Thus a 50mm tasked to work at unity performs as a 100mm lens. The engraved f-numbers thus are two f-stops in error. This is called “bellows factor”. The formula to correct is (M+1)^2 (magnification + 1 squared). Thus at unity (magnification 1) the math is (1 + 1) ^ 2 = 4. This value 4, is a multiplier, you correct for underexposure by multiplying the exposure time by 4 or opening up the aperture 2 f-stops.

For this reason, it is industry practice to stop the forward racking of the lens when the error reaches 1/3 f-stop. All this reasoning is moot if the camera has through-the-lens metering. Such a design reads the exposure under the influence of the bellows factor error. Thus the need for the photographer to make a manual compensation is avoided. This is why many modern cameras do not impose a close focus limitation.

One more thing: General lenses are designed to work a curved world (3 dimensional) and project this image on a flat image sensor or film. When working extremely close, subjects are generally shallow and this fact often results in some sharpness compromise. The countermeasure is to invert the lens as the rear elements are optimized to work a flat surface.

A macro lens is optimized to work in close, compromised when tasked to work afar. Its design is a seamless countermeasure that eliminates the bellows factor.

You can work in close with a general lens by spacing it further than its mechanical limiting stops. We use spacers called rings or tubes. If you do, you must manually take into account bellows factor unless the camera meter reads through the lens. You can also use close-up supplemental lens to work in moderately close (no bellows factor for this lash-up).

  • very interesting – Alexis Aug 6 at 16:25
  • Very interesting, so if i understand the message you are saying is that constructors limit the minimum focus distance (i.e reached when the lense group are moved forward , you cannot turn the focus ring past that, max magnification) based on an acceptable threshold for the “bellows factor" effect ? – Alexis Aug 6 at 16:37
  • @ Alexis -- You understand -- Add the degradation induced by the fact that a standard lens has curvature of field that will be noticeable when doing close-ups which are generally flat subjects. – Alan Marcus Aug 6 at 17:55

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