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.