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Understanding Macro Lens Apertures (Micro-Nikkors)...

SzennyBoy

London, UK
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"Understanding Macro Lens Apertures (Micro-Nikkors)..."

SzennyBoy Awarded for his shared hands-on experience in macro photography within the community forums and the Resources Registered since 28th Jan 2002
Sun 29-Dec-02 11:02 AM | edited Thu 15-Jan-15 01:49 AM by jrp

The question about why or how the aperture values on a Micro-Nikkor (or any macro lens for that matter) keeps changing during focusing, especially at high magnifications, keeps cropping up...
So I decided to post an explanation to this "confusing" issue to all macro lens owners & would-be owners.
(Actually this is a slightly modified re-post of my answer to a similar query on a different forum)

The answer to this query about the baffling aperture change with the macro lens is actually very simple.
What you're seeing in your viewfinder is the effective aperture as opposed to the actual aperture that you set on the lens.
In theory, there is a requirement for some compensation the moment you are not focusing at infinity.
The amount of compensation required is dependent on the film plane/aperture distance to the focal length.

In most normal lenses in normal photography, this amount of compensation is very small and is often ignored because of the magnification scales involved... i.e. Effective Aperture = Actual Aperture.

However, the moment you delve into macrophotography, all these compensation issues becomes very critical... i.e. Effective Aperture does not equal to Actual Aperture.

The following mathematical formula is the actual definition of Effective Aperture:

Effective Aperture = A x ( 1 + M / p )

A = aperture
M = subject magnification
p = pupil magnification (exit pupil diameter/entrance pupil diameter)


Looking at the mathematical definition for the effective aperture, you can see that the effective aperture is a function of both the subject magnification (M) as well as the pupil magnification (p).

To be more precise:
The subject magnification is set based on the actual reproduction magnification as indicated on the lens... i.e. the 1:M ratio.
For a true life-size magnification between the actual subject and the film plane, the reproduction magnification would be 1:1. Half life-size would be 1:2. With me so far...

At whatever magnification setting, each lens will have an associated pupil magnification value.
This is the ratio between the exit pupil (the inner most objective diameter or the lens element you see when you look at the back of your lens) and the entrance pupil (the outer most objective diameter or the lens element you see when the lens is mounted on your camera).
The value of p is given as:

p=1 - for a normal lens.
p>1 - for a wide-angle lens.
p<1 - for a telephoto lens.

A lot of people tend to approximate p=1.
This is fine if you're using say the 60mm micro-nikkor or 55mm micro-nikkor as it is effectively a normal lens, but in the case of the 105mm AF micro-nikkor, it actually falls under the p<1 case.

So, back to our equation...

Effective Aperture = A x ( 1 + M / p )

If you substitute the appropriate numbers into the above equation, you'll find that the effective aperture for the 105mm at life-size 1:1 magnification is approximately equal to 5... i.e. f/A = f/5 (i.e. the f/stop number).
If you try this equation out at the various magnification settings, you will find that it will give you the appropriate effective aperture conversion.

Note that most modern Nikon AF body CPUs do all these calculations for you because it knows what Nikkor lens is mounted via the electronic coupling between the lens mount and the lens.
MF body and/or MF Micro-Nikkor users will need to know this to work out the appropriate effective aperture for metering purposes.

Szen ARPS... A London Nikonian

Visit my Nikonians gallery.

Pinned topic Understanding Macro Lens Apertures (Micro-Nikkors)... [View all] , SzennyBoy Awarded for his shared hands-on experience in macro photography within the community forums and the Resources , Sun 29-Dec-02 10:02 AM
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