Chris Adler
Veteran
Nope, turned that off. Just the exposure meter in spot mode.
Quick theory question
- Thread starter Finnegan
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j1clark@ucsd.edu
Veteran
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
Veteran
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'...
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
Well-known member
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.
Haha, given that I have absolutely no idea what you just wrote, I'm imagining something pretty complex already
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.
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
Veteran
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
Veteran
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.php?288957-White-balance-every-shot-or-every-scene
There are newer version of the older Kodak Grey Card that are also color neutral (The Kodak card had a decided color cast. Not sure if they still do.)
Not only can you set the "midpoint" exposure, but you also have a "neutral" grey that you can use to set the correct color balance in Premiere/SpeedGrade/Photoshop etc.
There are many versions of these "cards" out there (there is also a white balance lens cap/cover.) I have been using a WhiBal card for alomost 10 years I think to set my Custom White Balance in camera:
http://www.amazon.com/GENUINE-WhiBa...F8&qid=1344725237&sr=8-6&keywords=whibal+card
I use the largest size. On a Canon camera, you take an image with the card exposed somewhere near mid grey. Then set Color Blance to Custom, and also select the image of the card you just took (filling the full frame) as the target for the custom balance.
Usually best to get the color balance nailed in camera where possible, rather than post. (Unless you want to preserve the relative color of the light in a scene - sunset, etc.)
You might look up information on the "Zone System" to understand the relation between exposure, gamma, and the placement of the values on an "output" scale. As long as you don't get too deep into the jargon/religion. :huh:
Michael
Not only can you set the "midpoint" exposure, but you also have a "neutral" grey that you can use to set the correct color balance in Premiere/SpeedGrade/Photoshop etc.
There are many versions of these "cards" out there (there is also a white balance lens cap/cover.) I have been using a WhiBal card for alomost 10 years I think to set my Custom White Balance in camera:
http://www.amazon.com/GENUINE-WhiBa...F8&qid=1344725237&sr=8-6&keywords=whibal+card
I use the largest size. On a Canon camera, you take an image with the card exposed somewhere near mid grey. Then set Color Blance to Custom, and also select the image of the card you just took (filling the full frame) as the target for the custom balance.
Usually best to get the color balance nailed in camera where possible, rather than post. (Unless you want to preserve the relative color of the light in a scene - sunset, etc.)
You might look up information on the "Zone System" to understand the relation between exposure, gamma, and the placement of the values on an "output" scale. As long as you don't get too deep into the jargon/religion. :huh:
Michael
j1clark@ucsd.edu
Veteran
Of course. I view some of these discussions as a way people who may not have access to 'film school' to get technical info/methods. While the truly 'intro' to photography classes spent some amount of time on all things ranging from the camera to the final print... the second level classes... back in the olden days... covered such things as calibrating, processing, using a densitometer, etc.
The Zone system had come into 'vogue', in many places, and so all lot of the classes had taken up running Zone System type calibrations.
The basic 'take away' message of the Zone System is know what your camera/film/process produces as the 'final image'... and then be able to predict outcomes more accurately.
For 'stills' one need not hang around a set of trays with stinky chemicals... or for moving pictures, wait for the film to be returned... to perform these tests... They can be done pretty 'quickly' these days...
What was confusing me was that my grey card was at 50% IRE, so was the rest of the scene. Upping the light intensity made the card's IRE go up too so I didn't see a way to light the scene at 75 IRE and have the card at 50 IRE. I have ordered one of those fold up cards and will try again, I'm sure it was my piss poor attempt to make a grey card and the lighting that were to blame.
j1clark@ucsd.edu
Veteran
The 'thing' to remember about the grey card... while it may be at 50% IRE... nothing in your scene may be 18% reflective... You may have very dark background, very dark foreground... and a person with 'Caucasian' skin complexion, which is only one stop more 'open' than the grey card... would be about 65% IRE... dark complexion may be one stop 'less' than 18%, or about 35% IRE.
A 'white' sheet of paper typically is about 96-99% reflective, depending on the type.
starcentral
Veteran
First, definitely repeat your steps with a proper grey card. For example let's say if your printed grey piece of paper is even slightly brighter a few shades, or your paper is slightly reflective, or..... or... or.... then you will be closing down your aperture to get 50 IRE in your monitor and in reality darkening your whole scene to what information can be captured within the dynamic range of your camera.
And now the next aspect of using a grey card (or light meter) to figure out your exposure:
Knowing where your grey point is often only a starting point. It doesn't necessarily mean that you light a scene, get grey @ 50IRE, then walk away and press record. Sometimes you have to consider a little more about the subject(s) and what's going on in the scene. Some sources suggest 65% to light a caucasian person but as you develop your own style and learn to work with your specific camera you will learn what works for you. Where you decide to put your "skin" level in relation to middle grey will also depend on your preference and your camera's dynamic range and what you have to work with and what scene information you want to preserve vs. don't mind losing.
WHAT YOU'RE KIDDING? IT'S NOT JUST A SIMPLE MATTER TO USE A GREY CARD?
Yes and no. If you're shooting with a camera that has a very high dynamic range and a file format that will endure extensive grading without falling apart - then YES you really can just go by grey @ 50, or grey @ 40 and shoot your whole film that way knowing you have 6 stops below 50 and 6 stops above and let the colourist adjust the mid, low and high tones. This would be like shooting on RED, SLOG, or to some extent FLAT picture profiles and hacked DSLR's to give you higher bitrates. You can also shoot to an external recorder in 10-bit 422 or 444 that will give you additional headroom over camera's internal codecs.
Anyway, you definitely came to the right place to learn all this stuff.
And now the next aspect of using a grey card (or light meter) to figure out your exposure:
Knowing where your grey point is often only a starting point. It doesn't necessarily mean that you light a scene, get grey @ 50IRE, then walk away and press record. Sometimes you have to consider a little more about the subject(s) and what's going on in the scene. Some sources suggest 65% to light a caucasian person but as you develop your own style and learn to work with your specific camera you will learn what works for you. Where you decide to put your "skin" level in relation to middle grey will also depend on your preference and your camera's dynamic range and what you have to work with and what scene information you want to preserve vs. don't mind losing.
WHAT YOU'RE KIDDING? IT'S NOT JUST A SIMPLE MATTER TO USE A GREY CARD?
Yes and no. If you're shooting with a camera that has a very high dynamic range and a file format that will endure extensive grading without falling apart - then YES you really can just go by grey @ 50, or grey @ 40 and shoot your whole film that way knowing you have 6 stops below 50 and 6 stops above and let the colourist adjust the mid, low and high tones. This would be like shooting on RED, SLOG, or to some extent FLAT picture profiles and hacked DSLR's to give you higher bitrates. You can also shoot to an external recorder in 10-bit 422 or 444 that will give you additional headroom over camera's internal codecs.
Anyway, you definitely came to the right place to learn all this stuff.