[disord3r]

Does the orientation of the aperture blades have any effect on its performance? When browsing lenses, I see some have the tips of the blades visible when looking at the mount side while others have the tips visible on the reverse. I've come across models that even seem to have reversed somewhere along their life span.

[Na Horuk]

Pretty sure the following things matter:
a) material of blades. You want them to be durable, light, not shiny, fairly quiet. Some materials require lubricants, others are too shiny, some are loud, others bend too easily.
b) shape of the aperture they form. The shape affects the shape of the bokeh and starbursts on highlights. They can be shaped as squares, stars, hexagons, or nearly round. Pretty much any lens can get completely round bokeh if you open the aperture all the way, so the blades dont interfere
c) position of aperture - it matters whether the blades are in front, in the middle, in the back. I don't know the details, but hopefully the lens designers do. This is far from simple because modern lens designs are so complex. Some lenses even have two sets of aperture blades (Think they are called STF? Made by Minolta, Sony, and now possibly third parties)

Whether they come in at an angle shouldn't matter, as long as they only obstruct the correct amount of light, in the right position. It can matter if they are reflective, I guess. Similarly to how the inside of the lens hood can cause problems if it is shiny?