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

Len Shepherd

Yorkshire, UK
12722 posts

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"RE: Macro Lens Apertures (Micro-Nikkors)/ further imp"

Len Shepherd Gold Member Nikonian since 09th Mar 2003
Wed 13-Aug-03 09:02 PM

> Does the lens
>rating (such as 60mm) for a prime lens only refer to the
>focal length when focused on infinity?

Step 1 - Yes

Step 2 - if it is a symetrical lens (the aperture looks the same size from the front of the lens as it does from the back) - it needs its own focal length (60mm) in extension to get to 1:1 and the film plane to subject distance is 4 times focal length (240mm).

With a symetrical lens exposure time at 1:1 is 2 stops longer than at infinity and the camera screen looks 2 stops darker.

Step 3 - (a digresion but an important one) - increasing from 60mm by 60mm to 120mm to get 1:1 halves depth of field, but the aperture is still the same same number of mm. in size so at the new combination length f4 becomes f8 (that is why the screen can look darker) which doubles dof.
One cancels the other out so you get dof as in a dof table.

Step 4 - Sven's point - if the lens is not symetrical (the aperture is a different apparent size when viewed from the front and back) you have to allow for thew P factor.

As others have pointed out the 60mm is nearly symetrical and is equivalent at 1:1 to a symetrical 55mm lens so differences between practice and theory are insignificant - except on command dial bodies.

However the 105D is far from symetrical and at 1:1 is a equivalent to a symetrical 78.5mm f2.1.
The 200D is equivalent to a 125mm f2.5.

These differences have an influence on dof and exposure adjustment if using a hand held meter.

To the best of my knowledge the P factor formulae was first published by Kodak in "Close up Photography and Photomacrophotogrhy" over a quarter of a century ago in 1977.

***

Step 5 - all this is irelevant if you use a Nikon AF macro and command dial body to set the aperture.

The magnification factor and the P factor are of no consequence.

The body knows what the extension factor and what the P factor are for each macro lens (I do not know how) and physicaly changes the size of the aperture to get back to any infinity exposure time i.e. the total change in effective aperture is 2 stops and the total time change is the equivalent of 2 f stops.

If you have a 60, 105 and 200 AF lens set the aperture 2 stops down you can compare the aperture sizes using dof preview at infinity and 1:1.
In each case there is a change in the physical aperture size equal to 2 stops less the P factor.

I believe the reason for command dial converting the extension factor and P factor by physically changing the aperture size (no other camera system does this) is something to do with matrix flash.

Step 7 - if you are confused by all this (whoever wrote the explanation of what is going on in the lens instruction leaflets was)close down a couple of stops at 1:1 and everything is exactly as it was in the days before modern technology.



Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.

Len Shepherd

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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