Hey Karsten ! That’s an interesting observation, and I get where you’re coming from—seeing an obstruction behind the front element can make it look like the aperture is smaller than advertised. But the actual f-number isn’t based on a visual guess or the physical size of internal components. It’s a calculated value: the focal length divided by the diameter of the effective entrance pupil (as seen from the front).

In the case of the Lightdow 500mm f/6.3, the entrance pupil does indeed match the math—500 ÷ 6.3 ≈ 79mm. That’s what determines the f-stop, regardless of how things might appear through the lens.

As a side note, Lightdow also offers a 500mm f/8 version of this lens. It has a significantly smaller front element and weighs a lot less—so if we follow the same logic, shouldn’t that one be f/10 or more? But again, it’s all about the entrance pupil and how the optics are designed, not just the visible mechanics.

Optics can be deceiving, but the physics behind them is pretty consistent 🕵🏼‍♂️.

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Dear Tomomichi Nakamura,
I hope this message finds you well. Thank you for taking the time to visit my humble website.
I am familiar with the lens you reviewed on your beautiful website, and as you mentioned, it is a design that was highly regarded during the film era. What makes this connection even more meaningful to me is that a modern Chinese telephoto lens led you to my website and, in turn, introduced me to your wonderful work. For that, I am truly grateful.
I have subscribed to your website through the WordPress reader and look forward to exploring your content as soon as I can find some time.
Once again, thank you for your visit.
Best regards,
Marc

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Hey Ed, your message went to spam and I only found it today. Well, it’s certainly cheap, but big and manual. If you are looking for something over 200mm, the 75-300mm from Olympus and the 100-300mm from Panasonic are good choices. I have the 100-300 Pana myself, and I am very satisfied with the quality it delivers.Cheers !

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No thanks, Ted. I’m just really curious by nature, and my slightly obsessive perfectionist brain always wants to learn more 🙂 I just hope my articles can help someone. Have a great weekend !

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