T-Stops VS F-Stops in calculating Depth of Field

[Brad Hoover]

Although it is obvious that you would always use T-stops for setting exposure, is it not the actual iris apeture size (F-stop), not transmitted light (T-stop) that is used to calculate depth of field (along with focal length and focus distance)? I ask this because I have several lenses that are marked with both F-stops and T-stops. I also have some lenses that are marked ONLY in T-stops. When using a depth of field chart and only having a T-stop reference, it seems that the DOF calculations would be inaccurate, admittedly on the safe side, but still inaccurate. On most lenses the difference between F-stop and T-stop might be small and not have a substantial effect on the DOF calculation. Other lenses such as long complex zooms might have a considerable difference between F-stops and T-stops ( a full stop or more) which would throw the calculation off considerably.
Is this true or am I completely wrong? If it is true how would one calculate the difference?
Thanks in advance for your insight and experience.
-Brad Hoover

 

[David Mullen ASC]

Yes, use T-stops for exposure and f-stops for depth of field calculations. However, depth of field is inherently imprecise -- you can never factor in all the variations of display formats, degrees of enlargements, lens sharpness & contrast, etc. all of which affect the circles of confusion, which in turn affect depth of field calculations. I mean, technically something going onto a 100' screen should have a different circles of confusion than something going on TV, but now "TV" includes HDTV, DVD, soon HD-DVD, DLP projection, blah, blah, blah... Technically we should have different charts for zoom lenses, Cooke S4's, Zeiss Super-Speeds, anamorphic lenses, etc.

So for the most part, you can use the T-stop for calculating if the f-stop is unknown, or factor in that a lens generally loses light in transmission, not gains light, so a T/2.8, for example, must be slighter wider-open than an f/2.8 if it is compensating for light lost.

As you can tell, I'm somewhat jaded on this subject -- depth of field charts and calculations, unless you are doing miniature efx work (and even there you are most likely to just use much light possible for the smallest aperture that is feasible or practical), are only meant to give you a ballpark idea of what will be "acceptably" sharp-looking outside the point at which the lens is focused, which is an inherently imprecise and subjective notion. Can you imagine a hard demarkation between the point where an out-of-focus object still looks sharp and then looks unsharp?