Tue, 23 Jan 2024

The Moon and a Lens

— SjG @ 6:59 pm

From my desk, I could see the the waxing gibbous moon rising. The geometry was such that the moon filled the gap between the open venetian blinds almost exactly. I took a picture with a 300mm lens, focused on the moon.

The moon behind venetian blinds

I was a little surprised how much the venetian blinds in the foreground blurred out over the moon when I did that. When looking through the viewfinder, the image seemed to match what my eye saw: the moon filling the gap between two horizontal lines.

Thinking about it, it made sense. I had the aperture at f/8, so the limits to the depth of field would cause the much closer objects to blur.

I took a few steps closer to the window, so there would be more of a gap between the blinds, and took another picture.


Although to my eye, the moon was only about 75% of the gap between the venetian blinds, the depth-of-field problem continued to obscure it. So I went even closer.


Even though the moon was taking up half the gap (to my eye), the venetian blinds still obscure it. So I went right up to the window, where the view through the lens made it seem like there was no obstruction whatsoever, just a moon floating there.


Well, even at this point, the venetian blinds intruded upon the image. Thinking about it more, I realize that the diameter of the lens is bigger than the gap between the venetian blinds. The lens gathers light from all across its diameter, so there’s no way it can see “between” the slats — the light is blocked. Also, as I got closer to the blinds, I was asking the lens for more depth of field (which it could not give me). The slats went further out of the focus and are relatively more blurred. In all likelihood, I didn’t see the effect through the viewfinder because I wasn’t paying sufficient attention.

Back in the day, I probably could have found the right equations to explain this phenomenon. Today, I’m content to notice it and say “Hm. Interesting.”

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