Question about aperture and depth of field

[squirrel$$$bandit]

Just out of curiosity. I know, of course, that depth of field is connected to aperture, that it narrows as aperture decreases. But does one, say, 50mm lens have the same DOF at 1.4 as another 50mm lens at 1.4? Or does depth of field vary among different lens designs, even at the same aperture?

 

[ferider]

They do vary - I cann't explain why though.

I had empirical evidence when comparing Canon 50/1.2 to 50/1.4 (both LTM), and Summilux ASPH 35/1.4 to Nokton 35/1.4.

Best,

Roland.

 

[FrozenInTime]

There is a small factor in the depth of field equations that relates to lens construction/symmetry:

see the bottom of this page: http://toothwalker.org/optics/dofderivation.html

 

[ferider]

Thanks for the link, FIT.

I think the way the background is rendered plays a role, too. Here are, from left to right, 3 wide open shots of Nokton 35/1.4, Lux pre-asph and Lux asph, that illustrate the difference:

The Lux ASPH has the thinnest DOF.

FOV on all three lenses is very similar.

Roland.

 

[ferider]

Here is a corner crop of an identical picture taken with Canon 50/1.2 and 1.4, both at 1.4.

Again, I would be surprised if the focal lengths of the 2 lenses differ by more than a couple of mm.

The 1.2 has clearly thinner DOF.

Roland.

 

[squirrel$$$bandit]

I ask this because I was testing out a CZJ for Contax Sonnar 50/1.5 wide open, and found the DOF to appear to be nearly as thin as the Canon 50/1.2 wide open. Results are below.

This lens is weird, though, focus is way off at greater distances, and I'm sending it out for adjustment. but still--that picture does not look any different, with regard to DOF, than anything I took with the Canon.

 

[ferider]

With a Sonnar there is also field curvature and higher center resolution that can give the impression of reduced DOF. Part of a Sonnar's charm ....

But your lens could be de-centered, too (NW corner sharper than SW corner).

 

[squirrel$$$bandit]

SW corner was also farther away--I was above the hens, pointing down. But you might well be right.

 

[payasam]

I was about to speak of curvature of field, Roland, but you got there first.