Jump to content


Photo

DOF


  • Please log in to reply
2 replies to this topic

#1 Simpson Parker

Simpson Parker

    New

  • Basic Members
  • Pip
  • 1 posts
  • Cinematographer

Posted 18 March 2011 - 06:40 PM

Hello all,

I have a question regarding focus, DOF tables, and lenses. I am still a noob, so apologies in advance for something I should have understood before an elaborate post! I am reviewing a series of lens tests, shot in 35mm and scanned to 2k files. In testing each prime lens, I used a focus chart and pulled tape to an exact metre mark on the lens. I had an actor in front (slightly) of the chart itself, to have something for the focal plane to settle on as the lens was racked in and out of focus. But, the depth of field tables tell me I should have a given range of "acceptable" sharpness at a specific f-stop, lens, and distance. Most of the tests revealed a RAZOR thin DOF in setups where the near and far were two meters apart. Shouldn't this range be in focus? For example, not having to pull focus on a shot because the action is well inside?

Thanks,
Simpson Parker
  • 0

#2 David Mullen ASC

David Mullen ASC
  • Sustaining Members
  • 19765 posts
  • Cinematographer
  • Los Angeles

Posted 18 March 2011 - 07:07 PM

Hello all,

I have a question regarding focus, DOF tables, and lenses. I am still a noob, so apologies in advance for something I should have understood before an elaborate post! I am reviewing a series of lens tests, shot in 35mm and scanned to 2k files. In testing each prime lens, I used a focus chart and pulled tape to an exact metre mark on the lens. I had an actor in front (slightly) of the chart itself, to have something for the focal plane to settle on as the lens was racked in and out of focus. But, the depth of field tables tell me I should have a given range of "acceptable" sharpness at a specific f-stop, lens, and distance. Most of the tests revealed a RAZOR thin DOF in setups where the near and far were two meters apart. Shouldn't this range be in focus? For example, not having to pull focus on a shot because the action is well inside?

Thanks,
Simpson Parker


There's no hard demarkation where something is "acceptably but barely sharp" versus "slightly out-of-focus" -- my belief is that there is only one point of exact focus, anything near that is, well, not in focus but may look semi-sharp.

Which is one reason why I try to break my AC's of the habit of "splitting focus" between two people because half the time, it doesn't work even if a DOF chart tells them it should. Part of the problem is the higher MTF of modern lenses and digital cameras, so the point of focus is more clearly visible. The only time I would allow splitting the focus is in a "Citizen Kane" type deep-focus set-up where I'm shooting at f/11 or so and want as much of the frame to look in focus as possible.

But also, DOF charts are based on a Circle of Confusion figure, which in turn is based on certain assumptions about viewing distance, degree of enlargement. You may be looking at your scans more closely than the CoC figure allows.
  • 0

#3 John Sprung

John Sprung
  • Sustaining Members
  • 4635 posts
  • Other

Posted 20 March 2011 - 02:48 AM

The best way to get a handle on this is to shoot diagonal newspaper tests using the lenses and stops you're going to use for the shoot. That way, you can pick for yourself where "sharp enough" ends. You can even use them to derive your personal circle of confusion value to compute your own tables.




-- J.S.
  • 0


Glidecam

Broadcast Solutions Inc

rebotnix Technologies

Metropolis Post

CineLab

Media Blackout - Custom Cables and AKS

Gamma Ray Digital Inc

Paralinx LLC

Tai Audio

Willys Widgets

Opal

The Slider

Rig Wheels Passport

Visual Products

Wooden Camera

FJS International, LLC

Aerial Filmworks

Technodolly

Ritter Battery

Abel Cine

CineTape

Media Blackout - Custom Cables and AKS

Abel Cine

Opal

Wooden Camera

Gamma Ray Digital Inc

Willys Widgets

Visual Products

Metropolis Post

Broadcast Solutions Inc

Paralinx LLC

Rig Wheels Passport

CineTape

Tai Audio

The Slider

Ritter Battery

rebotnix Technologies

Glidecam

CineLab

Aerial Filmworks

FJS International, LLC

Technodolly