Very confused by new lens or: Does f-stop = f-stop?

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Lichtspiel
Lichtspiel Veteran Member • Posts: 3,494
Very confused by new lens or: Does f-stop = f-stop?
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To celebrate Black Friday I bought myself a new lens - the Tamron 28-200. Despite being a Prime addict, I figured sometimes a bit of focal length flexibility may be beneficial, and this one got a lot of good reviews.

First thing I shoot is always my cat. He approves. Sharp where needed and no issues with rendering. There seems to be a tinge of chromatic aberrations, but hey, it's a zoom. Almost super too.

Mr Fluffy

Really like the ability to get up close and personal, especially at 28mm where the front element is almost touching whatever you're shooting. Colors are good. It's a bit heavier than I am used to, but hey, bag of primes and all, plus f2.8 is nothing to sneeze at.

Mrs Cherry vs Mr Amano

So then I decided to - just for fun, and completely unscientific - do a quick shootout at 50mm between the new super zoom, the lowly little kit zoom, and the highly appreciated VL APO Lanthar prime. Cause... why not.

Overall the results are pretty even, again, no scientific shooting of brickwalls. No issue with any of these lenses. However, there is one thing I am completely confuddled by...

Comparing the shots of the Tamron and Sony zooms at 5.6, the Tamron has significantly higher DOF, with the background at the top of the image much more in focus.

Likewise, when comparing the Tamron to the Voigt at 3.5, again the Tamron shows the background much sharper, indicating a smaller aperture.

This very much confuses me. Since DOF is driven by aperture and focal length (same distance was used for these shots) and the focal length is close (similar image captured)... what... the Tamron is cheating and reports a wider aperture than it is actually using? Weird.

 Lichtspiel's gear list:Lichtspiel's gear list
Sony a7C Voigtlander 15mm F4.5 Super Wide Heliar Sony FE 20mm F1.8G Tamron 70-300 F4.5-6.3 Di RXD III Samyang Reflex 300mm F6.3 +5 more
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