Light meters and t-stops question

QuickHitRecord

QuickHitRecord

Veteran
After years of using waveforms and false color, I am trying to make a light meter part of my regular workflow. But I keep coming back to one question: Since different lenses set to the same f-stops will often have different t-stops, isn’t using anything but a t-stop marked lens almost always going to give an inaccurate reading?
 
I use a meter, an old analog light meter. At the moment of exposure at the camera, the waveform and false color are the right tools but they are relative to ISO, stop, nds, etc. What they don't tell me is the quantity of light there. Now I scout with a meter and it tells me well, ok, at ISO 800, I'm at f1.4. Then I think, do I want to shoot at that level of light, say hand held, that far open? If not, what can I do to augment this light to get me where I want to be? Punch of the ISO? hmmm... 2 stops get me up in the 3200 range, not great for noise, ok, how can I light this to keep this feel, get what production wants from this location (I shoot on location at this point ) set the right mood, etc and get me to 5.6? The light meter gets me to think about light before I point a camera at it.

Sorry, I'm not really addressing your question. For me, the light meter does something else other than the on camera tools. I would say that it's close enough, though.
 
QuickHitRecord said:
After years of using waveforms and false color, I am trying to make a light meter part of my regular workflow. But I keep coming back to one question: Since different lenses set to the same f-stops will often have different t-stops, isn’t using anything but a t-stop marked lens almost always going to give an inaccurate reading?
Click to expand...

In short, yes.

As you know, (but some readers may not), the whole reason we have T-stops today, is because F-stops were not very reliable for cinematographers who needed or wanted the absolute ability to maintain consistency and accuracy in their exposures from shot-to-shot, lens-to-lens. This is very important today, but it was even more so back when things were shot photo-chemically and without the modern technologies we have today, such as all the in camera exposure tools and monitor exposure tools. An F-stop on a lens is just a mathematical calculation of how much light should pass through a lens. It however does not take into account many small factors, and ends up being just a rather accurate estimate. It should also be known that it also depends on the lens design and coating technology as to how much different a T-stop will be from an F-stop. For instance, a modern day prime lens with sophisticated lens coating will likely have an F-stop rating much closer to it's true T-stop value versus a vintage zoom from the 1970's. The former could be just an eighth or quarter of a stop off, while the latter could be half a stop or more!



Back to the OP, I understand your dilemma, but perhaps you can continue to implement your light meter into your work by using it for finding your lighting ratios on set. If you are going to continue to use the same set of F-stop lenses, you could perhaps actually find the T-stop value of your lenses and document them. Would take some careful work but it might be worth it. Maybe that can help you set exposure on the lens with more confidence.
 
Most professional lenses are actually going to have t-stop markings. Most of the lenses that don't have t-stop markings aren't there because it's not likely you'll be using them on a project that requires that much accuracy, or because the light loss is very small. Most modern lenses are a lot better about light transmission than the old ones* (which used t-stop markings), so you're not likely to have a problem with it. Just set it to where the marking is, whether it's t-stop or f-stop it'll be pretty close.

Or like Ryan said, shoot some tests and make a note.

*this is pretty generalized, there are a lot of factors but on the whole it seems to be true.
 
Thanks for the replies and confirming my suspicions. I'm going to try to find someone with a t-stop tester (I believe that AbelCine has one), find the t-stops that correspond to my f-stops, and re-mark my prime set (maybe with a simple label maker).
 
Unless one has taken one's DSLR lenses to a service, had them tested, and remarked for T-stops, one will be dealing with F-stops.

But this is no different than back in the olden days, and one did not have a 'fancy' SLR that had metering through the lens...

Basically one would 'calibrate' lens, meter, exposure, resulting negative density, and ultimate print density, and come up with an effective Exposure Index.

One would check 'f-stop' relationships, so, one would 'stop down' in 1/3 f-stop steps, or up, etc. and measure the differences in density.

The other thing that could screw up the works was that shutter speeds were sort of not all created equal as well, so one would have the camera service test the shutter speeds, and one would have a table to make adjustments.

One of the things that hasn't been clarified for me, is how t-stops relate to Depth of Field calculations, does each lens have a table of corrections, or is the error small relative to the actual f-stop.
 
dxomark gives you a T-stop reading for the lenses they tested, but I'm not sure they do it at all apertures, maybe it's wide-open only (and that matters a lot because I'm not sure how it's usually treated but vignette must be a factor).
 
I got a quote from Abelcine in Burbank, who have the only Zeiss T-stop tester in the country (that the rep at Zeiss knew about):

- $130 gets me the accurate wide open T-stop on my set of five Nikkor AIs primes.

- For $260, they will use a paint pen to make T-stop dots on the focus ring.

- For $390, I get the paint pen markings, and a chart that shows the f-stops as they correspond to T-stops.

Not too bad!
 
My camera is a T-stop tester, as long as I have one lens marked in T-stops to begin with. Aim at a white wall with the lens marked in T/stops, look at the waveform at different T values, take notes. Now start playing with the lens marked in f/stops, and mark where the waveform matches your notes.
 
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