What lens to get for most shallow depth of field on 4x5?

[blue4130]

I honestly don't know why you are all talking about macro shots? The pictures I shared from flickr are not macro shots but environmental portraits! And that is the use-case where those fast lenses shine - the background is melted away nicely and you get nice subject isolation (full body portrait range and further).
The faster the aperture the more environment you can include in your shot without loosing the nice shallow DoF.
Like this:
https://www.flickr.com/photos/anasbenj/6414433027/

( I finally found the flickr page for the kodak aero-ektar 178mm 2.5 )

Edit: Or this shot: https://www.flickr.com/photos/kiddokiddo/20709538908/
That's just unbelievable beautiful. Seems like all the people are using the Aero Ektar on a Speed Graphic?

So basically the Aero Ektar on 4x5 will give you the same "look" in terms of focal length and amount of DoF as the famous kubrik 50mm f/0.7 on 35mm does. Pretty amazing.

To get that look, you need sufficient DOF to have the person sharp from front to back and nothing else. So about 12 " of "sharpness" at whatever distance will get you full body using the focal length of your choice. It really isn't about having the largest aperture, its about understanding how to get the amount of DOF that you need for the shot. If you like the aero ektar look, then just get an aero ektar. But LEARN how to use it effectively. Otherwise, you won't get shots like that.

[Kerosene Hat]

No, it depends on your lens, your aperture and the magnification. Not with film size.

Also, subject to lens distance (hand in hand with magnification). OP isn't seeing the whole picture about longer lenses with larger fields of view, and the ability to get closer with the longer lens.

Also, gong for razor thin DoF out of the gate is entering the learning curve from the side and further up...the first trick is actually learning to use the view camer, then working with more advanced topics, like DoF, movements, etc. Recommend to OP to learn view camera fundamentals as he works toward razor thin DoF. Chances are very high he doesn't need such a fast aperture, but that depends on the variables Bob S mentions, as well as subject matte. This was shot at f8... https://flic.kr/p/2gPdZaU