Why Do Anamorphics Have Shallower DoF?

[Peter Dmitriyev]

Just curious technically speaking why do anamorphic lenses have shallower depth of field? Just wondering if anyone who knows how anamorphics are constructed, what it is about them that makes their DoF shallower?

And second question is there any where a resource that can calculate roughly what the equivalent shallowness is? i.e. for instance a 50mm anamorphic prime at f/5.0 is equivalent to a 50mm spherical at f2.8 or something like that?

[Nick Gardner]

Depth of field is a function of image magnification, and f stop. The more you magnify the image, the shallower the dof. If you look at the unsqueezed image area of an anamorphic image, it is larger than the sensor, 2x larger as a matter of fact. You are magnifying the image more. It's akin to shooting on medium format versus 35mm stills film.

Nick

[JustinGD]

Depth of field is a function of image magnification, and f stop. The more you magnify the image, the shallower the dof. If you look at the unsqueezed image area of an anamorphic image, it is larger than the sensor, 2x larger as a matter of fact. You are magnifying the image more. It's akin to shooting on medium format versus 35mm stills film.

Nick

So you are magnifying the image more with an anamorphic lens than with a standard lens for the same framing. Does that mean that a 50mm anamorphic lens will deliver a wider field of view than a 50mm standard lens?

[David Mullen ASC]

Anamorphic lenses do not have less depth of field. A 50mm anamorphic has the same depth of field more or less as any other 50mm lens.

The reason why practically-speaking anamorphic photography has less depth of field is that you choose longer focal lengths to get the same view as a spherical lens image cropped to 2.40 -- a 2X anamorphic "sees" twice as much horizontally as a spherical lens, so on the same sized sensor area, you'd use a 40mm anamorphic to get the same view as a 20mm spherical lens image cropped vertically to create 2.40, and a 40mm lens has less depth of field than a 20mm lens. Think of an anamorphic lens as a spherical lens with a wide-angle adaptor in front, only the wide-angle effect is only applied horizontally.

Now one factor is that anamorphic lenses may not use the same width of the sensor for the final image as would a spherical lens. In 4-perf 35mm photography, the anamorphic format uses the sound aperture area which is 22mm wide rather than the 24mm-wide area of Super-35 spherical, so the equivalent lens for Super-35 is not exactly half the focal length because it's a 1.8X difference rather than a 2X difference. Using 2X anamorphics on a 16x9 sensor means that you end up using even less width of the sensor for a final unsqueezed 2.40 image and therefore need even shorter focal lengths than if the sensor were 4x3 to get the same view, so the net effect is that the differences in focal lengths between anamorphic and spherical become even less extreme and thus the depth of field differences are less obvious.

Keep in mind for a final 2.40 image, a 2X anamorphic only needs to use a 1.20 : 1 area of the sensor or film format.

[Peter Mosiman]

^^ Thank you for the explanation David. Was having an "argument" about this very topic the other day.

Cheers to your wisdom and knowledge!
PM

[Marc-André Gray]

I've heard that the focus distance on anamorphic varies from the center to the sides. Not sure if this is true maybe someone knows more about this. So if someone is in focus at 5 feet in the center of the lens and walks to the edge but still at 5 feet from the lens they could be out.

[BRANDON JAMESON]

David Mullen, you are extraordinary.
Thank you, Sir!

[Marc Wielage]

I've heard that the focus distance on anamorphic varies from the center to the sides.

This was a terrible problem with the original Bausch & Lomb CinemaScope lenses of the 1950s. Those lenses also had barrel distortion and pincushioning problems, where objects would change size depending on where they were in the frame. If a person got up and walked from extreme left to extreme right, they literally would go through a "funhouse mirror" effect as they moved.

I've been through the Panavision lens-testing department when they go through the alignment process on their anamorphic lenses, and it's pretty comprehensive. There are dozens and dozens of adjustments and tests they have to make to ensure that not only is the lens uniform in terms of focus and exposure, but also that the focus markings on the barrel measure out perfectly. I had no idea so much went into lenses until I had that little tour a few years ago.

The American Cinematographer Manual goes into a lot of this theory in detail:

http://www.ascmag.com/store/product....cat=294&page=1

Also covered on Wikipedia:

http://en.wikipedia.org/wiki/Depth_of_field

My feeling in general, judging by film projects I've worked on in post, is that anamorphics to me feel a little softer and have a shallower depth of field, plus there's more of a vignetting effect when the lens is wide open. I know DPs that like this look and feel it's a quality they want for the particular project. My gut feeling is that anamorphic lenses are a detriment, and that we can get that look in the D.I., but I concede it's a creative choice like anything else.

[Christopher Probst]

Here is a pop quiz I use on 1st Assistants all of the time... Which has more depth of filed, a 50mm Spherical or a 50mm Anamorphic. And, yes, it IS a trick question. Who will get it right?

[Björn Benckert]

Another trick question..... 2x Anamorphics gets a DOF that is 2x in vertical or is .5 horizontal... ? what number is equal to the same Spherical lens?