Calculating DOF: from lens or film plane?

[Peter Shier]

Being fairly new to LF, I was looking over one of the excellent DOF tables from the Schneider USA website, and it occurred to me that with smaller formats I nev er concerned myself with the source of the DOF calculation. With LF, it can obvi ously make a significant difference. So, the question is if the table says DOF i s from m to n feet, does that begin at the film plane or at the lens?

Thanks, Peter Shier

[Chris Hawkins]

It is the nodal point of the lens. This usually assumed to be the midpoint between the front and back of the lens. In certain cases this assumptions is flawed. See Stroebel's "View Camera Technique" for addition information on this topic. http://www.amazon.com/exec/obidos/ASIN/0240803450/o/qid=981059217/sr=8 -1/ref=aps_sr_b_1_1/102-4484586-1288959

[Alan Gibson]

I beg to differ. The tables on, for example, http://www.schneideroptics.com/large/depth/300depth.htm, appear to be measured from the film plane, with a 0.12mm diameter circle of confusion. It's a shame that Schneider aren't more explicit.

The tables show the focus distance, and near and far points, measured from the same plane. (E.g. at close focus, all three distances are approximately equal.) This could be the lens or film plane. Because of the difficulty of measuring from the nodal point, this distance is usually the film plane. Calculation confirms that this is what Schneider have done.

I would take these tables with a pinch of salt. The 0.12mm CofC is the conventional figure, but I find it much too generous. In other words, for critical purposes, assume a much smaller depth of field.

[Peter Shier]

Where do you see on their site that they are assuming a 0.12mm circle of confusion?

You say that "Calculation confirms that this is what Schneider have done" to conclude that they measured from the film plane. Can you elaborate?

Thanks.

[Peter Shier]

I called Schneider tech support this morning and asked them about this. They told me that the distance measurements are from the lens.

[Alan Gibson]

I'm sure their tech support is correct. I took the standard figure of 0.12mm (not mentioned, as far as I know, on their site) and plugged this into the standard formulae. This gave exactly the numbers they give for the 300mm lens, 5 feet, f/11.

I repeated the calc from the lens (rather than the film plane), and could not replicate their results, whatever CofC I chose.

But, as I say, I'm sure their tech support must be right.

It doesn't make a huge load of difference. Tables (or formulae) give more precision than may be supported by real photography.

[Peter Shier]

Thanks for the info Alan. I personally wouldn't be so quick to believe anyone's tech support but I am taking their word for it.

I also received a note off-line from someone questioning why I am so concerned about this level of accuracy and that is indeed a good question. I am going to be shooting portraits at an event (Polaroids and either prints or transparencies). I will be setting up in a corner with strobes and people will be coming over in groups of 1-4 to have their pictures done throughout the event. As things will be moving quickly I won't realistically be able to recheck focus on every one so I need to pick a DOF at that close range that will allow a reasonable amount of freedom of movement of the subjects. I will place some tape on the floor to mark the area where they can stand. With a 210mm lens at around 5 feet away at F16 - F22 that allows for less than a foot of DOF so I do need to be quite precise in this calculation.

[adam friedberg]

alan, i agree with your last point. that's why i always affix a dof table to every print. it's proof that my picture is in focus. if someone tries to say otherwise i point to the table and shut them up with pure mathematics. this method works especially well with portraits.

pete, even with a dof printout backing you up i think you'll need a little more than f22 on a 210 to get one person in focus without checking, let alone four. i would consider a shorter lens and/or asking your subjects to wait an extra 30 seconds and actually focussing. it will take less time than reshooting.

[Alan Gibson]

Oops. My spreadsheet had a bug. I was correct that the tech support were right: the figures given in the table are consistent with CofC 0.12mm (which is often quoted for 5x4" film), measured from the object to the lens, not the film plane. Sorry.

But I would definitely NOT rely on tables or formulae for this purpose. Shoot some piccies at the location, or a mock-up in your front room, and get a feel for how far out of focus is OK before it becomes unacceptable. Remember that 'depth of focus' isn't a simple binary good/bad thing, but rather an 'acceptable badness'.

Yes, this testing will cost you time and money. But it will also help you on the day: you are less likely to trip over cables, or forget to bring that vital lens brush, spare cable release, or anything else.

[Peter Shier]

Thanks Alan. I will definitely practice this setup using numerous subjects (roaming family members- before the event.