Quick theory question

[Chris Adler]

Nope, turned that off. Just the exposure meter in spot mode.

[j1clark@ucsd.edu]

What is your argument regarding the exposure as determined in waveform?

I don't have an 'argument' with waveform monitoring... but for the camera I have it's not available, and needless to say, I don't have a industrial strength external monitor with such available...

I did use "OnLocation" the Adobe app which used the 'live' feed capability of Firewire enabled cameras... but alas... with the race for USB, and no live feed... that tool is pretty useless.

The way I use the waveform monitor now is pretty much a post-processing analysis tool for my 'test' shoots, to understand the interaction between metering the scene, setting the camera (including looking at the in camera histogram with an eye for skewed 'haystacks'...), and the resulting image that I can view in the Premiere waveform monitor tools.

I'm going to read that sony 'white paper' mentioned above about placing the grey card response at 38%. That may only hold for 'sony' and perhaps only for 'broadcast' applications... but if the 'grey card' is 'lower' than close to 50% IRE, that would explain some things that have caused me questions...

In particular if I 'place' the grey card at 50%, I seem to only have about 1 and 2/3's stop before I get nearly 'saturated' white on my GH-1... well on my 'typical' viewing devices...

[j1clark@ucsd.edu]

One question that has always bothered me about T vs. F... is that T seems to include effective 'transmission' along with the F-stop setting. Now, if the 'glass' has a 98% transmission end-to-end... adjusting for that 2% difference, doesn't seem to me to be something that would affect the Depth of Field calculations which has nothing to do with how well the lens 'transmits' light.

So, in your table of 'listed f-stop' vs. calculated t-stop, is most of the 'difference' really because the manufacturer 'missed' on the f-stop, by a 1/3 or 1/2... and 'oh by the way there was also a 2% loss due to the lens glass itself'?

Is that how to look at T-stops?

As for 'how much'... given my financial state... I have to do the 'find out what my lens delivers'... rather than buy a set that are precisely calculated a priori...

But in general I like to be as precise as possible.

And speaking of that sort of thing... there's also some amount of variation of light meters relative to 'calibrated sources'... I think it may be 1/2 to 1/3 stop depending... perhaps the Sekonic cine meter has better factory calibration... but I wonder about the 'usual DSLR camera'...

[cpc]

One question that has always bothered me about T vs. F... is that T seems to include effective 'transmission' along with the F-stop setting. Now, if the 'glass' has a 98% transmission end-to-end... adjusting for that 2% difference, doesn't seem to me to be something that would affect the Depth of Field calculations which has nothing to do with how well the lens 'transmits' light.

So, in your table of 'listed f-stop' vs. calculated t-stop, is most of the 'difference' really because the manufacturer 'missed' on the f-stop, by a 1/3 or 1/2... and 'oh by the way there was also a 2% loss due to the lens glass itself'?

Is that how to look at T-stops?

As for 'how much'... given my financial state... I have to do the 'find out what my lens delivers'... rather than buy a set that are precisely calculated a priori...

But in general I like to be as precise as possible.

And speaking of that sort of thing... there's also some amount of variation of light meters relative to 'calibrated sources'... I think it may be 1/2 to 1/3 stop depending... perhaps the Sekonic cine meter has better factory calibration... but I wonder about the 'usual DSLR camera'...

Differences between f-stops and t-stops are both due to light loss and inaccuracies in aperture mechanisms. F-stops shouldn't be trusted blindly either. An f2.8 lens can be actually f2.9 and T3.3. Manufacturers don't lose sleep over incorrect f-numbers. T-stop labeled lenses are usually much more precise because they are supposedly measured and better quality controlled.

Light meters should always be tested and used in conjunction with a specific camera transfer curve. If you know where a tone falls on your curve, you can do wonders without any tools like WFM, false colors, zebras or whatever.
Meter precision is not very important as any decent meter will provide correction in 1/10 stops. And meter calibration is not very well standardized, or rather, it is defined in a large interval of possible calibration constants. But for practical use it is more important to have a consistent meter than a precisely calibrated meter.

That said, a WFM and a grey card can solve pretty much any exposure task, including matching exposure across lenses without T-stop markings. And with a through-the-lens spotmeter (like in Magic Lantern) you won't even need a WFM.

[Shooter]

....That said, a WFM and a grey card can solve pretty much any exposure task, including matching exposure across lenses without T-stop markings.

Absolutely.

The wfm and the sensor do not know and do not care..... about F-stops or T-stops.

Dont over complicate this guys.

[brynsank]

They shot cards and scales on neg which we processed and compared to our LAD (lab aim density) via the densitometer and our calculations from our control strips.
(Its more complex than one would imagine)

Haha, given that I have absolutely no idea what you just wrote, I'm imagining something pretty complex already ;)

[thabo]

Ok I've just realized the importance of the 18% gray card so I printed out one on my printer (probably my first mistake and probably needs to be a reflector not paper ???) Now when I look at the waveform monitor on my little HMC40 and adjust it to be half way / 50%, the rest of image is way too dim. I have always tired to get the levels to about 70% but the if I boost the gain to get the important objects at 70% the gray card is at the same level it seems?


What am I missing here / doing wrong; I'm sure multiple things. Also would be fun to find out how to do this with my GH2 as the HMC40 is abysmal indoors in low light.

[j1clark@ucsd.edu]

Ok I've just realized the importance of the 18% gray card so I printed out one on my printer (probably my first mistake and probably needs to be a reflector not paper ???) Now when I look at the waveform monitor on my little HMC40 and adjust it to be half way / 50%, the rest of image is way too dim. I have always tired to get the levels to about 70% but the if I boost the gain to get the important objects at 70% the gray card is at the same level it seems?


What am I missing here / doing wrong; I'm sure multiple things. Also would be fun to find out how to do this with my GH2 as the HMC40 is abysmal indoors in low light.

Well... printing out a test pattern is not exactly the 'best way'... they are expensive... but the 'Official Kodak 18% Grey card found here is what you need to start off, only $15 cheap...

The grey card should be 'placed' around 40-50% IRE. The rule, and I've sort of 'proved' this out for my self, is in the mid-IRE values it's about 15% change for each full stop of lens change. So the 40-50% represents about a 2/3 stop range.

[starcentral]

Ok I've just realized the importance of the 18% gray card so I printed out one on my printer (probably my first mistake and probably needs to be a reflector not paper ???) Now when I look at the waveform monitor on my little HMC40 and adjust it to be half way / 50%, the rest of image is way too dim. I have always tired to get the levels to about 70% but the if I boost the gain to get the important objects at 70% the gray card is at the same level it seems?


What am I missing here / doing wrong; I'm sure multiple things. Also would be fun to find out how to do this with my GH2 as the HMC40 is abysmal indoors in low light.

I suggest you fork out the $10-$20 and buy a proper grey card from your local photo store or order one online and only then use it with your camera & waveform monitor.

Then, once you put that in front of your camera to read 50% on your waveform monitor you must remember that its only that specific spot where your grey card is located that has enough light on it to give you proper exposure, and that it does not mean anything else about the rest of the scene. Your eye can't see it well but different parts of the room will be darker or lighter than the position where you put your grey card in front of your camera. But very likely the scene will be captured quite accurately to how it appears in "real life" ie. dim in spots where there is no light, and bright in spots there is too much light.

Please read this thread below. It got into the same discussion and has links to videos and other stuff that will guide you:
http://www.dvxuser.com/V6/showthread...or-every-scene

[Shooter]

Just for perspective.

There is no reason the grey card has to be exposed to spec always.

It is a starting point (reference) for exposures and depends on what you want to achieve in the over all tone scale.

Its a technical point in levels /exposure ..and not for artistic reasons.