Question on Depth of Field Calculators...

[Mark Sawyer]

Disclaimer: I never use those tables or calculators myself, preferring inspection with a loupe after closing down the aperture instead. (As if those dof calculations could take into account the contortions of my focal plane anyways...) But I was curious...

When using a dof calculator, should one use the factory-indicated f/stop at infinity, or the actual f/stop being used?

Example: If doing a 1:1 close-up on 8x10 with a 300mm lens set at f/22 on the aperture scale, would you use f/22 in your calculation, or f/44 (okay, f/45, close enough...), which is the actual f/stop in that situation?

[Bob Salomon]

On the rodenstock calculator an indicator tells you howw much exposure correction is necessary. So you can keep f22 and just increase your exposure time as it indicates (1 second at 1:1 becomes 4 seconds) or you open the aperture as indicated so f22 becomes f11 or a combination of both.
The Rodenstock calculator has a scale to read off your mognification ratio being used and the very first step is to tell it what format is being used and the next step is to set the angle of the camera to the subject. Then you focus on the near and far points, tell the calculator how far apart the are in mm on the camera and the camera back is then set to the indicated point on the calculator.

So it will also do your DOF and Scheimpflug calculations. Regardless of your contortions.

[Dan Fromm]

Mark, you have to use effective aperture, that is, aperture adjusted for magnification. I have no idea whether any DoF calculator does this. I also don't know, and this is a fine point that isn't always relevant for LF work, whether any DoF calculator takes proper account of the lens' orientation (facing normally or reversed) and pupillary magnification.

Bob, it isn't a question of exposure correction, but of using the right aperture when calculating DoF.

[Bob Salomon]

Mark, you have to use effective aperture, that is, aperture adjusted for magnification. I have no idea whether any DoF calculator does this. I also don't know, and this is a fine point that isn't always relevant for LF work, whether any DoF calculator takes proper account of the lens' orientation (facing normally or reversed) and pupillary magnification.

Bob, it isn't a question of exposure correction, but of using the right aperture when calculating DoF.

I know, the Rodenstock calculator tells you what aperture is required for any given displacement between the near and far point and how much exposure correction is required for the magnification desired for that magnification ratio. Your choice is do you do it strictly by adding time, or not.

Here are the instruction sheets for the DOF side and the Scheimpflug sides of the calculator.

[Dan Fromm]

Thanks, Bob, for adding considerable clarity. What a cute device.

[Mark Sawyer]

Mark, you have to use effective aperture, that is, aperture adjusted for magnification. I have no idea whether any DoF calculator does this...

Thank you, Dan! I thought that would be the case, but wasn't sure so I had to ask. I wonder how many users of such calculations are aware of this? I suspect quite a few photographs have had unexpectedly out-of-focus areas because the photographers used the infinity-based scale rather than figuring out what f/stop they were really at.

Bob, that's a nifty little field guide, but I noticed it didn't inform the photographer about determining the effective ("real") f/stop when using the scale. (I'd guess this is true for all Depth of Field and Scheimpflug tables and calculators.) In the example illustrations on the guide, I'd say the calculations will be off by about an f/stop given the camera-to-subject distance. Something to correct if Rodenstock ever reprints these?

As I mentioned in my first post, I prefer making decisions visually with a loupe. Kinda like cooking to taste rather than following a recipe. Besides, as much of my work is either soft-focus or wet plate, it's seldom simply "how do I get everything in focus", but rather "what degree of focus do I choose?"

[Dan Fromm]

Mark, one of the really cute things about Rodenstock's calculator is that magnification is an input. I take that to mean that it automatically uses effective aperture internally, shows aperture to set. Bob, is that right?

Mark, most of my closeup work is with flash illumination at magnifications where DoF is too small to be worth thinking about. I use my wonder-working macro flash rig that gives correct exposure over a wide range of magnification with constant flash output and a fixed aperture set or I use GN arithmetic with an adjustment for magnification. All mental arithmetic, its not that hard.

[Bob Salomon]

Mark, one of the really cute things about Rodenstock's calculator is that magnification is an input. I take that to mean that it automatically uses effective aperture internally, shows aperture to set. Bob, is that right?

Mark, most of my closeup work is with flash illumination at magnifications where DoF is too small to be worth thinking about. I use my wonder-working macro flash rig that gives correct exposure over a wide range of magnification with constant flash output and a fixed aperture set or I use GN arithmetic with an adjustment for magnification. All mental arithmetic, its not that hard.

correct

[Steven Tribe]

I can't turn the dial on Bob's figures!

Hint for an animated version on a website?

[Dan Fromm]

Only 44.95 at B&H, Steven, and requires no batteries.