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Thread: Depth of field calculator

  1. #1

    Depth of field calculator

    Hi. Has anyone used a Depth of Field Calculator? I wondered if it is a labour saving device or more complicated than it worth

  2. #2
    Don Nelson
    Join Date
    Jul 2008
    Location
    Portland, Oregon
    Posts
    275

    Re: Depth of field calculator

    More complicated than its worth, particularly when you introduce tilts and swings.

    A better method is to do all the view camera adjustments you can, with the lens wide open.

    Now focus on the farthest part of the image (it will be the one that comes into focus with the shortest bellows length, but due to swings and tilts, it may not necessarily be the farthest from your lens....but it will be the focus point that requires the shortest bellows). Note the location on the rail or base.

    Now focus on the nearest part of the image (same rational as above, it will be the place that requires the longest bellows to achieve focus.) note the location on the rail or the base.

    Now reset the bellows length to exactly halfway between the two points.

    Now observe the points on the ground glass that you acheived the two focus points. Stop down the lens until both are within acceptable focus.

    That's the easy way, especially if you have tilts and swings that the dof calculators won't help you with.


    Couple of points here -
    If you camera does not have a measurement scale on the rail or base, you can add one by getting a cheap metal tape measure at the hardware store -- open it up and cut off the section you need. Attach to rail or base at a place out of the way for all movements. You can use rtv -- that will peel off when you wish to remove it.

    Or carry some low tack tape (blue stuff or drafting tape) to put markers, and buy a tape measure or get one of the fiberglass tapes at a fabric store.

    As an alternative to stopping down and watching focus, you can find some precalculated tables for distance vs where you need to stop down to for an acceptable circle of confusion (for a given film format which makes some assumptions about enlargement). Linhof used to give these tables out. I use my own version (hey, when you contact print, the circle of confusion isn't a big factor!)

  3. #3

    Re: Depth of field calculator

    Thanks Don. I was hoping that a depth of field calculator would help find the right tilts and swings to calculate the Scheimpfug angles. I am using a 10x8 Canham but I also have a Sinar F1 which has a method for doing this by using a calibrated screen and a calculator on the rear standard. I wondered if there is a way of doing the same with the Canham 10x8?

  4. #4

    Re: Depth of field calculator

    The Linos pocket Scheimpflug/depth-of=field calculator is very easy to use. It even has the required mm scale on it if your camera does not have a scale on the focusing track.
    Just set the magnification ratio, the approximate angle the camera is inclined to, film size, focus on the far point, focus on the near point and set the standards to the recommended angle and rear standard position.

  5. #5
    Don Nelson
    Join Date
    Jul 2008
    Location
    Portland, Oregon
    Posts
    275

    Re: Depth of field calculator

    Quote Originally Posted by Bob Salomon - HP Marketing View Post
    The Linos pocket Scheimpflug/depth-of=field calculator is very easy to use. ....
    Bob

    is this the same one sold under the Rodenstock name** ?

    Or if something different, do you have a pointer (URL)?
    Don


    ** I have one of these Rodenstock dial jobs. I don't bother using it.

  6. #6
    Don Nelson
    Join Date
    Jul 2008
    Location
    Portland, Oregon
    Posts
    275

    Re: Depth of field calculator

    One other tool I've also got, but also don't use, is from Visual Departures and called the Quickstick. Doesn't do tilts and swings....B&H sells these.

    And here's a link to previous discussions on the Rodenstock dial:
    http://www.largeformatphotography.in...ead.php?t=1414 People were bimodal distribution on whether useful or not. Give it a try and see if you like it...

  7. #7

    Join Date
    Nov 2003
    Posts
    1,218

    Re: Depth of field calculator

    Quote Originally Posted by Paul Greeves View Post
    Thanks Don. I was hoping that a depth of field calculator would help find the right tilts and swings to calculate the Scheimpfug angles. I am using a 10x8 Canham but I also have a Sinar F1 which has a method for doing this by using a calibrated screen and a calculator on the rear standard. I wondered if there is a way of doing the same with the Canham 10x8?
    The calculator in essence uses the focus spread method to determine the proper f-stop for desired depth of field. Here is one version of it which works for 8 x 10. It assumes a coc of 0.2 mm, a common choice. Focus on the nearest point you want in focus and note the position on the rail. Then focus on the furthest point on the rail which you want inf focus. The proper place to place the standard is half way between.

    Measure the distance in mm between those points, which is called the focus spread. Then multiply that by 10 and divide the result by 4. That gives you the f-number to use. For safety's sake stop down a bit more.

    Let's do an example. Suppose the focus spread is 10 mm. You would use 10 x 10/4 = 100/4 = 25. You should probably stop down to f/32 to get everything you want in focus.

    This method does not take diffraction into account, but that is seldom an issue for 8 x 10 photography.

    You should of course check what appears in focus as you stop down. Chances are that if something is in acceptable focus at a given f-stop when using a loupe, it will remain in focus in the final print. Unfortunately, pas a certain point, the image will be too dim to tell whether or not something is in focus.

    You can also apply the above rule when using tilts or shifts. I'll explain it for tilts. You would first settle on where you want to put the exact subject plane and tilt to get it right. (See below.) Then you would focus on the highest point you want in focus and on the lowest point you want in focus, above and below the exact subject plane, and measure the distance on the rail between those points to get a focus spread. You would use that the same way to find the f-stop.

    Focus spread comes in in a different way in trying to set the exact subject plane by tilting or swinging. Let me explain that for tilts. Using high and low points you should usually set the subject plane vertically halfway between them on the rail. You then choose a near point and far point in the desired subject plane. Using those points, you focus on each and then adjust the tilt to reduce the focus spread between them to zero. The rule to remember is the following. First focus on the far point, and refocus on the near point. If doing so moves the standards further apart, increase the tilt. If it moves the standards closer together, decrease the tilt. Usually in a few iteration of this process, you find that both near and far points are simultaneously in focus, so you stop.

    Depth of field calculators or tables are usually of much use in the field. There is no easy way to accurately determine the subject distances you need, and you have everything you need in front of you on the ground glass and camera. Also, you seldom need to know the exact tilt angle, but if your camera has an angle scale, it will tell you want it is. I know of no simple practical way to determine the tilt angle in advance. You would have to make lots of careful distance and angle measurements and use a lot of algebra and trigonometry.

  8. #8

    Re: Depth of field calculator

    Quote Originally Posted by Don7x17 View Post
    Bob

    is this the same one sold under the Rodenstock name** ?

    Or if something different, do you have a pointer (URL)?
    Don


    ** I have one of these Rodenstock dial jobs. I don't bother using it.
    Linos purchased Rodenstock Precision Optical several years ago. The calculator used to carry the Rodenstock name now most are Linos branded. They are also sold by European large format camera makers under their names as well.

  9. #9

    Re: Depth of field calculator

    Thats a very thorough explanation Leonard. Thank you. The method for calculating the tilt angle I will certainly use.


    http://www.digitaldarkroomuk.com

  10. #10

    Join Date
    Jan 2010
    Posts
    5

    Re: Depth of field calculator

    Hi. I was wondering if the delta of 8.1mm (as listed here: http://www.largeformatphotography.info/dofknob/#dofcard) for the scaling of the DOF calculator was between f45 and f45 or f45 and 0 or another distance.

    Any help would be greatly appreciated!

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