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It appears that neither the Sony 55-210 mm f/4.5-6.3 nor the Canon EF-S 55–250mm f/4–5.6 reports the actual focal length to the camera: both cameras' metadata show the lens but report the focal length as the minimum zoom.

I took pictures of the full moon and got some odd results, looking at the exposures closely. In particular, the size of the subject changed in the frame, which makes me think the zoom was not staying where I put it.

Could someone help me reverse-engineer the focal lengths actually used? Besides better comparing the two systems, I wonder if the longest zoom on the Canon in giving more chromatic aberration and less contrast, since even after reconciling the exposure difference I have effects I can't explain. So knowing the settings used will be helpful.

Exposure 1: Sony α-6000 which has a crop factor of 1.5 (15.6mm tall tensor) and a vertical resolution of 4000 pixels. Image of full moon is 137 pixels. I had intended to zoom the lens to 210mm.

Exposure 2: Canon 70D which has a crop factor of 1.6 (15mm tall sensor) and vertical resolution of 3648 pixels. The full moon is 135 or 136 pixels.

Exposure 3: Canon 70D, full moon is only 125 pixels tall. This makes me think that the setting changed between exposures, obviously.

The shots were taken on 2015-August-1 just after 1:30 AM local time, just north of Dallas Texas.

Could someone give me a hand with this photometry problem?

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    It might help to know exactly when the photo was taken, because the moon varies in apparent size from perigee to apogee. – mattdm Aug 4 '15 at 14:26
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    The EF-S 55-250 certainly does report focal length; see eg my photo of the moon. For avoidance of doubt, that was the EF-S 55-250 IS (ie not IS II and not STM) on a 550D running firmware 1.9.8, shot in RAW. – Philip Kendall Aug 4 '15 at 14:27
  • @PhilipKendall lens reporting of focal length isn't always that accurate as there aren't usually that many sensors on the lens barrel... – Matt Grum Aug 4 '15 at 15:45
  • My DNG files are reporting 55mm on all the exposures in Lightroom. Using another file properties tool to look at EXIF, it's the same. I'll take a look later to see if it's a glitch in Adobe DNG Converter. – JDługosz Aug 4 '15 at 15:52
  • Interesting, @PhilipKendall that your shot is 579 pixels square (the moon is slightly smaller than that). I wonder if I really did use a completely different setting than I intended? Note that images with different sized moon disks all show 55mm, so it's not the case that the lens really was set to 55. – JDługosz Aug 4 '15 at 15:59
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A back-of-the-envelope calculation based on field-of-view calculators and the known size of the moon, and the ratio of 125:4000 vertical portion of the frame, indicate that 55mm is the right ballpark.

Testing (CR2 processed by Adobe DNG Conveeter) shows that the lens indicates specific marks only, e.g. 55 then 60, not a continuous measurement. I verified that the field of view can change visibly in the frame whilst both exposures report the same focal length.

Thanks all for the hints in the comments.

  • You might want to accept your own answer here to stop the Community bot prodding this question :-) – Philip Kendall Jan 6 '16 at 13:38

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