Nature Scenes journal
What 2.39 takes away
The wide crop is not free. Thirty-seven per cent of the frame goes, and which thirty-seven per cent decides everything.
Almost nothing is photographed at 2.39:1. Nearly every plate you use in that ratio started as something else and lost a third of itself on the way.
The arithmetic
A 3:2 frame cropped to 2.39:1 keeps about 63 per cent of its height. A 16:9 frame cropped the same way keeps about 75 per cent.
Those are large numbers. It is not a trim; it is a substantial redesign of the composition, performed by whoever sets the crop.
Where it comes from
You have three options and they are not equivalent.
All from the top. Works when the sky is empty and the interest is in the land. Frequently improves the image, because a big empty sky was doing nothing.
All from the bottom. Works when the foreground is uninteresting and the sky is the subject. Rarer, because foregrounds usually carry the depth cues.
Split evenly. The default in most software and almost never the right answer. It takes half your sky and half your foreground and centres a composition that was not built to be centred.
Set the crop deliberately. The default is a decision made by nobody.
What the crop does to the horizon
Whatever the horizon was doing, the crop intensifies it. A low horizon becomes lower relative to the frame; a centred horizon in a 3:2 frame remains centred and now looks more deliberate, for better or worse.
If the frame will be used at more than one ratio, check the horizon position in every one. A composition that works at 16:9 and looks wrong at 2.39 is common and is discovered late.
Resolution after the crop
A 6000 by 4000 frame at 2.39:1 becomes 6000 by 2510. Comfortably above 4K, well below 8K in the vertical.
Then subtract whatever your camera move consumes. This is where a plate that looked enormous turns out to be marginal, and the moment to find that out is before the shot is approved.
Choosing for the ratio
The practical conclusion is to select plates knowing the crop.
Look for low horizons, empty skies and interest concentrated in a horizontal band. Frames with strong vertical elements - a single tree, a waterfall, a tall cliff - are the ones that suffer most, and they are worth keeping for a different format rather than forcing.