ELI5: why does the moon look big to our eyes but when we take a picture it’s really small

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Others are right about your brain’s ability to focus, but also many cameras (including until very recently basically all mobile phone cameras) are very wide-angle. This is great for fitting in all of your friends from just arm’s length. It’s also absolutely awful at capturing things at a distance, since such a tremendously large area is in-frame.

FreshEclairs

It’s an illusion of perspective. When you look up at the moon, your eyes focus on it and your brain decides that everything around it is background. When you take a photo, the device decides that the entire image is important and doesn’t seperate the moon from the sky.

Kordiel

Focal length of eyes and cameras are different. As are the aperture and the ability of the brain to process the image compared to a cameras processing. The human brain tends to stretch an image that is small and far away to make it more useful for us too!

AlexWheeldon

A lot of good answers here, most of which are right. I’ll just add the term ‘size constancy’, which is a thing our brain does with things we see. When something big like a truck is close to us we see it as a certain size. If that truck was instead far away *we can still tell* that it’s a big truck, even though it looks tiny because of the distance. Our brain takes into account the distance of the truck when figuring out the truck’s size. It does this mostly using binocular cues (how much our eyes need to move to join each eye’s image together) plus some other visual cues. For the moon, we can tell it is very far away and we can also sense its size *in spite of its distance.* The moon is indeed real big, so our brains go all “damn that’s a huge bitch” which kinda warps what we feel about the thing we are seeing, which in reality is a rather small circle. Photos don’t let our brains do this size constancy thing as much, so they show us the moon more as it really is. Luckily you can trick your brain into not doing size constancy. Try holding out your hand and ‘grabbing’ the moon between your finger and thumb. Then look at how much stuff is not in that little gap. You should suddenly be aware of the tinyness of the moon.

nyqu

A few others have mentioned it, but not given any particular examples. u/kordiel mentioned the mental processing, which is accurate, but isn’t the primary reason. The human eye sees at a focal length of give or take 50mm, which translates to a (estimated by me) roughly 90 degree viewing angle. The pictures you refer to taking are most likely from a phone, and phone focal lengths are something like 14-25mm. I’m not sure of the viewing angle on these, but I’d hazard a guess of around 110-125 degrees. This makes for a far larger amount of “stuff” you can see in the same image size (I know, I’m comparing an image to eyes, just pretend the picture is held close enough to your eyes to take exactly your field of view, no more no less, and it’ll work). Because of the different viewing angles, and having more space in the same image size, the moon will take a much smaller percentage of the space, making it smaller. If anybody wants actual numbers, I can do the math when I get home in the morning. Edit: source – am photographer :p

perpterds