There is always a lot of interest in getting closer to a subject, particularly in nature photography. One reason is that getting closer reveals more detail and it is amazing how complex some seemingly simple things are. For example, there is often a confusion over the difference between WORKING DISTANCE and MINIMUM FOCUS DISTANCE .
The Working Distance is the distance between the front element of the lens and the subject. Other things can change this, For example, if you are using accessories like the Nikon R1 closeup lighting kit or a ringlight, the lights will be forward of the front element and will intrude into and reduce the working distance. The working distance will also vary by magnification ratio. In the case of the picture below, the lens is set to a 1:1 ratio and the working distance is about 6 inches.
Minimum Focus Distance is always marked as one of the specifications for a lens. What it means is the distance from the subject to the focal plane of the camera. The focal plane of a camera is the location that any lens is focused on. On film cameras, it was often called the film plane on digital, it’s sometimes called the sensor location or plane. The plane is marked on most cameras. The standard symbol is a circle with a line through it where the line is the location of the plane. Here is the focal plane mark on a D300. You can see it on the left below and behind the top LCD. On the D90, the focal plane mark is on the right side between the function dial and the prism. It’s black on black so it’s difficult to see. The minimum focus distance is always calculated or measured at the maximum magnification of the lens. In this case, A 105mm f2.8 Micro-Nikkor that is 1:1 and the Minimum Focus Distance is about one foot.
The Magnification Ratio is a comparison of the image on the sensor or film plane and the actual subject. If a lens has a magnification ratio of 1:1 it means that the image of the subject will be the same size as the original subject. If the ratio is shown as 1:2 then the image of the subject will be one half the size of the original subject. So, it’s easy to think of the colon as a slash and see that 1:2 is the same as 1/2 when reading specifications. So at 1:4 the image on the sensor would be 1/4 of the size of the original subject.
And, then there is Bellows Extension Factor. With in camera metering the bellows extension factor does not need to be calculated and the aperture adjusted to accomodate it. We now get our light measurements at or near the focal plane so the loss of light from extending the lens forward is accomodated automatically. That doesn’t mean it isn’t a factor.
I often see posts on photography forums about lenses that don’t appear to be the proper aperture. A typical complaint might be “I bought an f2.8 lens but I can only set it to f4.0 when I’m shooting macro.” This usually occurs with an internal focus lens which doesn’t seem to be extending to focus. The effect is the same though.
In order to focus close, you have to move the lens closer to the subject than the focal plane. As that happens, the amount of light at the plane is reduced by the inverse of the square of the distance (the inverse square law). That means that if you are twice as far from the focal plane, you get only one quarter of the light energy on the sensor. This actually happens at every focal length and magnification but becomes most apparent only in situations where the lens is extended far from the focal plane as it is in closeup work. If you look at the figure below, the left rectangle represents 1 unit of light, in a straight line at twice the distance, the same amount of light energy is spread over 4 times the area.
Because of this, the effective aperture of a lens is reduced. Some lenses report this as a part of the cameras data included in pictures and some only record the nominal f-stop of the lens.