PK-13 with AI 50/f2

[charles_swanson1]

<p>I am considering another 55 micro-Nikkor. I have had both the 3.5 and the 2.8 in the past. I have too many 50mm lenses, so I was considering getting a PK-13 and using it with my AI 50/f2, but I can't find, from any source, what the results would be. Does anybody know what reproduction ratio I could get with this combination? And, what would the resulting minimum focus distance be? Also, this is a second question... what results would be achieved with a reversing ring BR-2? I also have an AI 28/f3.5 and an AI 20/f4.<br>

I have called Nikon, but they eliminated the film support people that I used to rely on regularly. Now you are prompted to leave a message for a call back. But I can even get a call back. :(</p>

[SCL]

<p>Nikon issued a magnification guide with their PK-13. I seem to have lost mine over the years...but perhaps you can find one with a web search.</p>

[Michael R Freeman]

<p>I have the instruction sheet for the PK-13, but alas not the (apparently) separate sheet of reproduction tables alluded to in the instructions. But I do have the Ai 50/2 and a PK-13.</p>

<p>A quick and dirty test shows that (if my math is right) the reproduction range available with the PK-13 and Ai 50/2 is from 1:2 (lens at infinity) to approximately 1:1.44 (lens at min. focus of 0.45m). That means you will not be able to cover the range between 1:2 and 1:6.7 (the native maximum reproduction ratio of the 50/2) unless you also have access to shorter tubes. That could be a fairly significant gap depending on your intended macro subject.</p>

<p>Minimum focus distance from the subject to the image plane with the lens at minimum focus is approximately 8.25in (21cm) and the free working distance is approximately 3.5in (9cm).</p>

<p>One thing to be aware of is that the 50/2 is not a flat field lens at minimum focus, so some curvature of field will be evident with flat subjects (as in the test shot below). This is where the Micro-Nikkor will shine as it is much better corrected for flat subjects. On a positive note the 50/2 appears to show very little barrel distortion at minimum focus unlike it's faster brethren, the 50/1.4, which shows very obvious barrel distortion.</p><div></div>

[Michael R Freeman]

<p>With the Ai 50/2 lens reversed on a BR-2, the (fixed) reproduction ratio is approximately 1:1.21, minimum focus is ~8in, working distance is ~4in.</p><div></div>

[charles_swanson1]

<p>Great info. Thanks so much M. Freeman</p>

[Michael R Freeman]

<blockquote>

<p><em>"...the reproduction range available with the PK-13 and Ai 50/2 is <strong>from 1:2</strong> (lens at infinity) to approximately 1:1.44."</em></p>

</blockquote>

<p>Just to be a little more (less?) precise, that should be <em>"<strong>from approximately 1:2</strong> (lens at infinity) to..."</em>, since the 27.5mm of extension from the PK-13 is a bit more (2.5mm more to be precise) than the 25mm extension needed for 1:2 magnification. :-)</p>

[rodeo_joe1]

<p>The Conjugate Focii formula should give you the answer, as long as the nominal focal length of the lens is close to its actual focal length.<br /> 1/F=1/v+1/u; where F is the focal length, u is the subject distance from the front node of the lens and v is the distance of the rear node from the focal plane. In this instance we can assume that F=v when the lens is focused at infinity (even if that's physically not the case).</p>

<p>We need to transpose the formula to find u, so u=1/(1/F-1/v).<br /> With the extension ring in place v = 50 + 27.5 making 1/v = 0.0138. Take that figure from 1/F = 0.0062, and invert = 161mm. That's the theoretical <em>maximum</em> distance from the front node that you can focus with the extension ring in place.</p>

<p>From knowing v and u, the magnification (m) is given by v/u, which is now 77.5/161, or 1:2.07. This bears out the figure just given by Michael.</p>

<p>With the unreversed lens; for the minimum distance we need to recalculate u for the extra extension of the lens focusing helicoid. Looking at my own 50mm f/2, this is about 5mm by guesstimation - sorry, mislaid my vernier caliper - and would make the minimum focusing distance 126.9 mm from the front node. Therefore magnification at minimum focusing distance = 82.5/126.9 = 1:1.54.</p>

<p>BTW. By far the easiest way to find the helicoid extension is to measure it. The distance marked on a lens barrel is the total distance from subject to focal-plane, and comprises v+u +/- the lens's internodal distance. Therefore calculation of u and/or v from only the focusing distance and focal length requires knowing the internodal distance of the lens and solving a quadratic equation. The solution to the quadratic equation is the least difficult bit!</p>

<p>PS. As you can see fom Michael's examples, the 50mm f/2 suffers badly from curvature of field at macro distances. IME any of the 55mm Micro-Nikkors is far superior in this and nearly every other respect.</p>

[charles_swanson1]

<p>Thanks to all... I will probably get another 55 Micro-Nikkor AND and ext tube.</p>