do we really need to worry about this? The answer of course is yes, but it is so simple and uncomplicated to deal with, that by taking a few simple precautions you can forget about the potential problems associated with film holders and film flatness.
When using a technical camera "Consistent accuracy" is the name of the game if you want to achieve consistently good quality negatives.
How do you achieve that consistency?
Well one of the simplest of the options is to ensure that the film plane is in the correct position relative to the ground glass. By using one of the Fuji or Kodak readyload film holders you immediately eradicate any inconsistency across different film holders. So simple, and this does not require any technical knowledge or understandanding of dogmatic approach that others would have you beleive is the reason for using these holders. Its just plain common sense that if you are using just one film holder, ( a second as back up is obviously useful) then your film depth will be consistent, especially where a pressure plate is used such as in the fuji quickload system.
Of course if the fuji or kodak systems don't allow the film of your choice then you have to look elsewhere. The old Grafmatic film holders have consistently been reported as providing the best consistency of film plane but they are no longer in production so secondhand is the only option here(if you can find one in good condition).
Otherwise you are left with the ususal fidelity, lisco, toyo and other brands of darkslides.
Here again, consistency is simple to achieve. Using holders of the same make and vintage is likely to give better consistency. My tests indicate significant variations across the brands I tested which were fidelity and Toyo. From then on I have stayed with Fidelity only. Toyo would have been just as good but different.
Which ever make or type of holder you use it is very simple to check that your film holder will put the film in the same plane as the ground glass.
To do this you will need a micrometer type depth gauge. These are cheap and readily available from hardware stores.
remove your camera back and measure the depth from where the film holder will rest to the ground glass. Make 9 measurements. 3 across one end, 3 across the middle and 3 across the other end. A sheet of glass or stiff metal to rest across the back and give a base from which to measure will aid greatly in doing this. These measurements should tell you if your Ground Glass is the same depth across its entire area. You will likely get minor variations but if there are significant variations then you will need adjust the glass depth accordingly but before making any adjustments you must first check your film holders.
Check the depth of the film holder in the same way but with a sheet of film in the holder. Only measure just to the film. Do not tighten the micrometer until it clicks but only to the point where it first touches the film. If you have a fuji or kodak holder this is very quick to do.
For normal darkslide holders average the 9 readings from each holder to make sure they all have the same average. Replace any holder wich has a slope end to end or side to side.
Next you compare the average depth of your ground glass against the average depth for all your film holders. If its the same then there's no problem. If its different then you need to consider adjusting it.
How this is done depends on your back. Some use adjustment screws which are simple. Others use shims which requires calculating new shim thickness. This is where you need to be very careful because you will need to remove the shims keeping note of their original position. Then measure their thickness and calculate each new thickness required and where they should be positioned. If you are not confident about doing this then get a suitable camera store/repairer to do it for you.
So what does all of this achieve? Well it allows you to identify faulty film holders. It identifies a badly aligned Ground Glass. It will give you CONSISTENT film plane position realtive to Ground Glass. It will give you the CORRECT film plane position relative to the Ground Glass. It eradicates one possible cause for non sharp negatives. It ensures that depth of field near to far positions will not be shifted from where you expect them to be. It removes the possibility of compounding focus errors with a film plane error.
The tests take little time to do. They require no technical knowledge. And best of all, you only need to do them once(unless you change anything such as Ground Glass) and then you can forget about them safe in the knowledge that your film plane is where its meant to be.
A note on ANSI standards for film holder depth:
Forget them. The fact is that there are very large numbers of cameras and film holders on the new and second hand market that do not conform to the standard and/or have been adjusted from their original settings. There is no policing/enforcement of standards and besides, you now have the knowledge to set your Ground Glass depth correctly. If you need ANSI standards then you are going to have to test for conformity to those standards which still does not put your Ground Glass in the correct position without you also checking that.
In other words doing your own tests and settings superceeds any ANSI standards on Ground Glass/Film Holder depth.