That’s not an ass, that’s a horse.

I, too, enjoy the occasional ass photo.

Fast film (you can see how grainy it is when you zoom in a little) and shooting in full bright sun = you can shoot very short exposure and freeze motion. There were already cameras in the 1930s with mechanical shutters that could do 1/500th and even 1/1000th of a second exposure, which is plenty fast for this type of shot.

The lens looks pretty fast too - depth of field is very shallow, although part of that is also due to possible use of medium or large format - faster lens (lower f stop) and larger film both allow more light capture, and therefore faster exposure as well, but at the cost of less depth of field.

Edit: here is a good print of the full frame - looks like ~1.50 ratio, so probably 35mm film (not medium or large format) - I can’t find a lot of information about what cameras Max Alpert used in the 30s, although he did use a 35mm Leica on at least some photos from that era. A Leica III could do 1/1000 in 1935 for example. The early Soviet cameras from the 1930s were also basically direct copies of Leicas. The frame also looks a bit underexposed, which was a common technique to get faster exposure as well (called pushing) - you could then correct for it during development.

from the '30s*