D7 Focal length

Actual focal length of the D7 lens appears to be smaller than claimed.

Summary: I measured the focal length of the D7 lens using several techniques. In all cases, the ratio of the smallest to the largest focal length was about 6.5, not "7X" as claimed. This implies that either the wide angle setting is not really 28mm, or the telephoto setting is not really 200mm or both.

Method #1. Counting pixels.

I took a shot of a distant window at wide angle and at telephoto. I counted the number of pixels subtended by the window.

Wide angle - 45 pixels

Telephoto - 293 pixels

Ratio - 6.51

Method #2. Photograph of a yardstick

I photographed a horizontal yardstick from a constant distance.

Wide angle - 35.0 inches

Telephoto - 5.38 inches

Ratio - 6.51

 

 Method #3. Photograph of a diagonal yardstick.

I thought that perhaps barrel distortion might compress the far edges of the wide angle shot resulting in a wide angle shot being "wider" just at the corners. I photographed a yardstick diagonally so that it went from corner to corner in the shot.

Wide angle - 31.25 inches

Telephoto - 5 inches

Ratio - 6.25

For some reason, this was actually a worse ratio!

 

Method #4. Comparison of EXIF data.

Assuming that the data in the EXIF is correct....

Wide angle - 7.325mm

Telephoto - 49.125mm

Ratio - 6.71

Note: The exact focal lengths at the ends seem to depend on the focus position.

Conclusion:

In every case, the ratio of the longest to shortest focal length does not meet the advertised specs of 200mm/28mm = 7.1X.

Unfortunately, I am finding that this is quite common, the CP990 and CP950 were not "3.0X" as advertised either, for example.