Focal Length at
Closest Focus - 105mm f2.8G IF-ED AF-S VR Micro-Nikkor
Prepared 2017-07-27 by Bill Claff
I applied my Measuring Focal Length technique to determine the focal length and inter-nodal distance at closest focus.
The focal length is 73.5mm and the inter-nodal distance is 15.3mm
Here's break down of the measurement and cross-check.
|
|
Measured |
|
Computed |
||||
extension |
|
m |
S |
WD |
|
m |
S |
WD |
mm |
|
x |
mm |
mm |
|
x |
mm |
mm |
|
|
|
|
|
|
|
|
|
0 |
|
1.07 |
310 |
146 |
|
1.07 |
310 |
146 |
13.5 |
|
1.25 |
313 |
136 |
|
1.25 |
313 |
135 |
19.1 |
|
1.32 |
316 |
133 |
|
1.33 |
315 |
132 |
32.6 |
|
1.51 |
322 |
126 |
|
1.51 |
322 |
126 |
36.0 |
|
1.56 |
323 |
124 |
|
1.56 |
324 |
124 |
49.5 |
|
1.74 |
332 |
119 |
|
1.74 |
332 |
119 |
55.1 |
|
1.82 |
336 |
117 |
|
1.82 |
336 |
117 |
68.6 |
|
2.00 |
347 |
114 |
|
2.00 |
346 |
114 |
Notes:
m is magnification.
S is Subject distance. S = f * ( 1 + m ) + i + f * ( 1 + 1 / m) where i is the
inter-nodal distance.
WD is Working Distance.
The actual extensions of my Kenko tubes differ slightly from the nominal
values.
At closest focus the magnification was 1.07x which is slightly more than the
advertised 1x.
I also observed the change in Effective F-Number.
I compute the Effective F-Number at
1.07x and 73.5mm closest focus and 102.9mm infinity focus as f/4.8, which is
what the camera reports.
The formula I used is Effective F-Number = F-Number * 2 ^ (((1 + 1.07) * 73.5 /
102.9) / 2)
Infinity focus was measured using the same technique as closest focus.