Originally posted by dtiberio my question is: if I were to buy a 645D camera, which lens would I buy to be able to shoot the same photo, and what settings would I use?
The camera used for that photo has an APS-C sensor, which has a crop factor of approximate 1.9x compared to the Pentax 645D's "medium format" sensor.
The focal length used on the original shot was 300mm (quite long for portraiture, I'd suggest), so the equivalent focal length on the 645D would be (approximately) 300 x 1.9 = 570mm, so let's say 600mm rounded up.
The aperture for the original shot was f/5.6, so the equivalent aperture on the 645D (using the focal length above) is 5.6 x 1.9 = 10.64, so around f/10 - f/11.
So, you'd be looking at a 600mm lens at f/11 on the 645D to get a shot with a similar field of view, perspective and background blur when taken at the same distance.
But there isn't a 600mm lens in production for the 645D. The longest is 400mm, and that's quite a beast of a lens. There is an older manual focus model available, however.
I believe the photo you've linked to was taken by you, given your user name here and the profile name on Shutterstock?
On that basis, I'd ask a question in return:
Is there a reason why you chose 300mm as the focal length for that shot? It's entirely your creative choice to do so, but that's rather a long focal length for portraiture that will lead to a significant flattening of features. That can be flattering and appealing in some cases, but I'd suggest that most portraiture on APS-C cameras is usually taken within the 50 - 135mm range. That would be equivalent to 95 - 256mm on the 645D, and there are several lenses within and near to that range currently available.