I hate to be a spoil sport, but wouldn’t the correct answer be 42?
Perfect for photographing whales dropping from the sky
I think I would go for 16 indoors and 50 outdoors. (aps-c)
600mm. Anything less isn’t worth talking about
1000mm. I wonder what strap would be a good fit for this lens. It’s only 15kg
Or maybe canon’s 5200mm?

The Sigmonster! Truly a lens for all applications!
(Not to be confused with the Bigma, which is merely a Sigma 150-600. Gotta love those nicknames.)
This very scientific analyses was not objective enough - failed to consider sensor size.
given that they spoke the whole time about FF equivalent focal length the sensor size is irrelevant.
Now we’re talking! It’d be best to strap that baby to a satellite for some portrait shots.
What you really need is a superzoom!
This is a 40-400mm f4 Taylor-Hobson TV camera lens. I think 70s. Weighs about 10kg! I don’t know how it came to be fitted with a Nikon F mount - it’s on loan from a friend of mine to try it out. Haven’t yet!
Sorry about the poor photo - bit too busy to do it properly

8mm, because “I’m sorry, Dave; I’m afraid I can’t do that.”
Let me see…
find . -name 'IMG_????.JPG' -exec exiftool -FocalLength -p '$FocalLength' {} + | sort -n | uniq -c | sort -n
While I’m at it, my use of the various focal lengths:
Neatly done.
Last month, 423 pics, 66% of them at 85mm (Sony APS-).
Of course, I know that my FE 85/1.8 was my favourite lens, and the one I’d take to a concert (Indian classical) if I could take only one.
That was an interesting exercise for me. I discovered that 45% of my shots are at the minimum or maximum focal length of the lens in use. That’s definitely not ideal for IQ, and indicates that I should switch lenses more often rather than trying to get by with what’s mounted.
As for the video, that was 8+ minutes of my life I will never get back…
Using zoom lens exif stats to understand ones focal length preferences is tail wagging dog thinking. Us humans are the tools of the lens not the other way around. Vast majority of people end up using the ends of the zoom range because that’s where the turning stops.
I’m ‘young’ enough to start out photography with zooms even on compacts. Now only use primes because I noticed that they make my photos better (not primarily in technical terms)
Picking a prime lens fl is a creative choice, you’re picking a way of seeing.
Only one lens? My pick would be 28mm of course.
Interesting. Had thought of limitations forcing creativity but not this. Seems odd/irrational we wouldn’t frame at appropriate focal length given freedom to do so but on other hand can see tools end up affecting the job. Will have to check exifs
I have a vague theory is that there is a misconception floating around. The idea of an appropriate focal lenght.
The misconception is that this appropriate focal length comes from the subject not the project of the photographer. That there is a photograph to be taken and that the photographer will find this correct view and adjust their zoom until it matches. This seems to be a common idea among zoom users. Their own statistics tend to show its not the case. Instead they are ruled by their gear.
Despite zoom lenses in practice being two primes in a barrel they differ from primes in how they make you see and how you find your frames. The prime user explicitly makes the choice a zoom user makes ( judging from comments I’ve heard over the years) unknowingly
yep, a 42mm lens would be the answer to everything…
I think it is very useful to think in terms of the working distance, which affects the perspective. Particularly crucial when photographing people.
A zoom gives one the liberty of first choosing suitable working distance, then adjusting the field of view by zooming, to select how much of the surroundings are included in the photo.
A prime doesn’t let you do these independently. One has to make certain compromises about the cropping and working distance. That’s not necessarily a bad thing and can lead to some creative decisions and a distinct style, but can also necessitate some cropping in post.
From what I’ve seen, the most common misconception about focal lengths is that the focal length itself somehow causes perspective distortions. As long as we’re talking about reasonably rectilinear lenses (with distortions corrected), it does not. It is the working distance that matters.
The question is. Does your exif stats, how you actually use your lens, support this idea?
I also have a niggle the working distance idea isn’t solid. Why maintain perspective going from headshot to full body? Is consistent working distance/perspective important when the image is so different. I understand that the face will “look the same” but does it matter when its so small in the latter frame?
If the zoom pull is smaller then conversely the change in perspective from “zooming with your feet” is so small that it will be imperceptible.
Then again I feel that using the same, or a small (3?), selection of focal lengths does bring a coherence to a set of photos that is valuable when putting them together. So maybe the opposite working distance model also brings something of value.