Speaking of the physical aperture.
When looking at the 18-200 mm, f/3.5-5.6, I wonder how this works. With diameter D = f/N I get:
- 18 mm / 3.5 = 5 mm
- 200 mm / 5.6 = 35 mm
So, does the aperture diameter change when zoomed in? I assume not, so how does this work?
Edit: I now took a picture of the visible size of the iris, or entrance pupil (which was what I meant when speaking of the physical aperture). I focused at the point where I took the picture from. The lens is a Sigma 70-300 f/4-5.6. The tape I used is gaffer tape. Ok, all important details mentioned ;) The results are closer to the lense's f-number (now calculating the other way round than above):
- 23 mm / 70 mm = 1/3.0
- 51 mm / 300 mm = 1/5.9
Is this method of measurement more or less valid?
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