In-camera vs. external meters

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Actor
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In-camera vs. external meters

Post by Actor » Sat Jun 06, 2009 9:59 pm

I have a variety of movie and still cameras which have two things in common: they all have internal light meters and they all produce good results when the internal meter is used. From this I conclude that the internal meters are correctly calibrated.

Over the years I have owned three separate external light meters that have one thing in common: they do not agree with the internal meters of the cameras. The latest is a Sekonic Twinmate L-208 which disagrees with the internal meters by about 2 or 3 stops, indicating less light than the internal meters.

Since the internal meters produce good results, even with unforgiving reversal film, I must assume that these are "known good" meters and the external meters are giving false readings. The question is why? If it were only one meter that was giving false readings I'd simply assume I got a lemon, but three seems to be improbable.

Someone's going to ask if the external meters agree with each other. I don't know. The first ceased to work and was replaced rather than repaired. I lost the second. I was never in possession of all three at the same time.

One possible explanation is that, at about $100, these are cheap meters and you cannot expect great performance from them. I don't buy this explanation because (1)even a cheap meter should not be off by more than 2 stops and (2)one of the "known good" meters is in my Promaster PK2500 which costs $175 for body, lens, meter and all, so it's also a "cheap" meter, yet it's spot on.

Is there something I'm missing here?

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Re: In-camera vs. external meters

Post by MIKI-814 » Sat Jun 06, 2009 10:29 pm

Remember that the internal meters are compensating for the loose of light that goes to the viewfinder of the camera (around 1/3 of the light I think, depending on each model)and not to the film itself.

If you are obtaining good results with those cameras in the automatic mode, everything should be ok.

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Re: In-camera vs. external meters

Post by Actor » Sun Jun 07, 2009 1:44 am

MIKI-814 wrote:Remember that the internal meters are compensating for the loose of light that goes to the viewfinder of the camera (around 1/3 of the light I think, depending on each model)and not to the film itself.
1/3 of the light is still less than one stop, while the discrepancy between the internal and external meters is over two stops. And this still does not explain the discrepancy with the still cameras which use a flip-up mirror with no loss of light to the viewfinder.
MIKI-814 wrote:If you are obtaining good results with those cameras in the automatic mode, everything should be ok.
But that still leaves a problem in cases where I want to, or have to, use an external meter. E.g., using E64T or a camera that does not have an internal meter. (I did not mean to say that all my cameras have internal meters.)

The entire problem seems to refute those who disparage the use of internal mertering in favor of external meters.

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Re: In-camera vs. external meters

Post by richard p. t. » Sun Jun 07, 2009 4:32 am

g'day actor,
Everything you are saying makes sense. Also, given what you are saying it is most likely that all of the meters (internal and external) you are using/have used are indeed working correctly.
The explanation is basically what MIKI-814 said: the camera's internal meters are taking in to account the amount of light actually getting through to the film. With super 8 cameras, light 'loss' due to the reflex viewfinder splitting off light as well as the particular shutter angle in the camera typically amounts to between 1 and a half and two stops deviation from what an external light meter indicates. And yes, its always in the direction you experienced: a hand held meter will indicate a higher (smaller sized) aperture than the camera's internal meter. Last week I shot an exposure calibration test for a customer with a nice Sekonic meter and a Beaulieu 4008. By bracketing in 1/3rd stop incraments downward from the rated asa of the film we found that the correct exposure was given by a compensation of 1 and 2/3rds stops more open than the light meter suggested. As I say, this is typical with super 8 cameras. One simply can't use an external hand held light meter with super 8 cameras without first shooting a careful callibration test on reversal film (you shouldn't do such a test with neg because neg is too forgiving). This issue isn't unique to super 8 cameras - for instance with non-reflex Bolex cameras there is a 1/3rd of a stop deviation between what an external meter on a cine setting will indicate and the correct exposure - and 2/3rds of a stop if its a reflex bolex camera. The situation would be different if the internal meters on super 8 cameras indicated an f-stop that took in to consideration the light losses of the reflex system and the deviation between the actual shutter angle and an 'idealised' shutter angle of 180 degrees (which is what most cine meters will assume). But then, the lenses wouldn't look so good as they would have maximum corrected apertures with numbers like f4 and f5.6. No so sexy. Also, these light loss issues aren't to do with the aperture and only show up when using an external meter. So what are hand held meters for? Well you can use them with super 8 cameras if you shoot the appropriate test. And you can use them with serious cameras (which super 8 cameras aren't!). Hand held meters can make meter reading much more 'precise' - especially since you can use your meter as an incident meter (measuring the light falling on the scene) rather than as a reflected light meter (which is what all through the lens meters are). You can also use spot meters for greater precision. However it is also the case that an internal ttl meter will probably be on average, and depending on user skill, more accurate (rather than precise) than a hand held meter.
So trust your internal meters. And shoot a test so you can use an external meter with super 8 cameras by all means.
good luck with it,
richard
I run Nano Lab - Australia's super8 ektachrome processing service
- visit nanolab.com.au
richard@nanolab.com.au

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Re: In-camera vs. external meters

Post by Patrick » Sun Jun 07, 2009 6:23 am

Ive found that if I use the internal light meter in one of my 35mm SLR cameras to take a light reading for my Canon 1014 Autozoom Electronic, I can get correctly exposed footage when making certain compensations. From my tests with a grey card, Ive concluded that if I set 60th of a second with the still camera, I close down half a stop from whatever reading I get and this will provide correctly exposed footage for the 1014 when running at 18fps. Theoretically, the same strategy should also work for single frame shooting because both single frame and 18fps on the old 1014 use the same shutter speed, 48th of a second...if I recall correctly.

For others reading this. don't take this as absolute truth for all super 8 cameras because different models may use slightly different shutter angles and beam splitter light losses. I would recommend that even owners of 1014E cameras to do their own testing to see what works for them.

Actor, out of curiosity, do any of your external light meters disagree with your still camera meters as well?

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Re: In-camera vs. external meters

Post by MovieStuff » Sun Jun 07, 2009 7:20 am

Okay, here's the deal:

The internal light meter of any camera is an integral part of the entire exposure system. In other words, the light meter and its "brains" have been tested and calibrated by the manufacturer to work with that particular camera. If you were to remove the light meter assembly from, say, a bell and howell and transplant it into a Nizo, you would not expect it to give you correct exposures.

Likewise, a Sekonic hand held meter is not part of the camera you are using and, to be sure, I have never seen two Sekonic meters of the same make give the same readings. That is why in my past life as a cinematographer, any time I picked up a new meter, I always did tests to see how it responded in the real world. The meter is just a gauge; a measuring stick to tell you how much light is present. If the meter is off by even two stops it is only relevant if you have never tested it and are depending on it right out of the box. But once you test and calibrate out the error, then you can depend on the information that it gives you to calculate your exposures for each shot. The biggest mistake that people make is letting the light meter tell them what to do when, in reality, it is just one of many tools that provide information that you combine to make your exposure judgment.

My two cents.....

Roger

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Re: In-camera vs. external meters

Post by Actor » Sun Jun 07, 2009 7:52 am

Patrick wrote:Actor, out of curiosity, do any of your external light meters disagree with your still camera meters as well?
Yes, they do. In fact, it was the disagreement of the external meters with the internal meters of my still cameras that first alerted me to the problem.

For the moment let's forget the movie cameras and discuss the problem in terms of just the still cameras. I have four: three SLRs and one TLR. Each of these have view finding systems that do not rob light from the film in order to supply light to the viewfinder. In the case of the SLRs there is a mirror that swings up out of the way when the shutter is released. During view finding 100% of the light goes to the viewfinder. During exposure 100% of the light goes to the film. This is also true of the TLR since the viewfinder is completely separate from the camera proper.

Given this the external and internal light meters should agree, but they don't. What's distressing is the magnitude of the disagreement (in excess of two stops) and the fact that the disagreement exists, not in just one meter, but in three different ones from three different manufacturers (Sekonic, Capital and Archer).

Back to movie cameras which use a beam splitter to divide the light between the viewfinder and the film. The point is well taken, but the amount of light diverted to the viewfinder is not enough to explain a discrepancy as high as two or three stops.

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Re: In-camera vs. external meters

Post by Jim Carlile » Sun Jun 07, 2009 9:40 am

Actor wrote:But that still leaves a problem in cases where I want to, or have to, use an external meter. E.g., using E64T or a camera that does not have an internal meter. (I did not mean to say that all my cameras have internal meters.)

The entire problem seems to refute those who disparage the use of internal mertering in favor of external meters.
No, not at all. It just means you have to experiment in order to calibrate your camera's f/stops with that of the meter (s).

If you know that f/11 on your meter actually means f/5.6 on your camera, then you know it's two stops off and will stay that way consistently. If so, an easier way to use the meter instead of always self-calculating for each shot is to adjust the meter's film speed setting the two stops differential. That way the meter and camera will read the same number.

People did this all the time in the old days. They learned how their lenses worked with whatever meter thay had in their hand and then followed the lesson the same way all the time.

In some camera-lens combinations, f/8 may not really be the same f/8 as another external lens. Hollywood studios had this problem all the time too, which is why it was tricky for them to always have to coordinate each individual camera and lens setup.
Given this the external and internal light meters should agree, but they don't. What's distressing is the magnitude of the disagreement (in excess of two stops) and the fact that the disagreement exists, not in just one meter, but in three different ones from three different manufacturers (Sekonic, Capital and Archer).
Don't be distressed. There's a reason for the discrepancy, and it is that each f/stop number is a just mathematical designation of the proportion of the amount of light that the mechanical iris in the lens will allow through. It is not a measurement in any way of the actual amount of light that is being caught by the film. So, that particular amount of light can differ in each lens even if the number is the same. This is apart from any other light loss in the path.

The same is true of the meters. In that case, f/stops are just proportions of the amount of light that is detected by the cells. They are not measurements, only relationships. And there is not necessarily a one-to-one relationship between any of these tools for that reason, either -- in fact, you're lucky if there is. It always involves experimentation.

But there is a solution to this discrepancy. The Hollywood studios (20th Century Fox) came up with a way to measure the actual amount of light that was passed through each f/stop in various lenses, and created a coordinated system where each lens' specific f/stop setting passed the same amount of measured light.

In other words, they calibrated each lens aperture setting so that f/8, say, was the exact same in each one, when it came to the amount of light that hit the film. They calibrated their meters the same way, too (of course, they had to so that it would all work), and later, the other camera and lens companies joined in.

This created the idea of T-stops, which is simply where the f/stop settings are based upon actual measured light, and are thus uniform across all ranges.

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Re: In-camera vs. external meters

Post by T-Scan » Sun Jun 07, 2009 5:47 pm

I tested the internal meter of my Nizo 481M vs my Seconic the other day. The Nizo internal meter is pretty accurate. I metered a Kodak gray card with the camera meter, then metered the light falling onto it with the Sekonic... The difference was exactly one stop.
100D and Vision 3 please

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Re: In-camera vs. external meters

Post by Actor » Mon Jun 08, 2009 7:47 am

Jim Carlile wrote:In some camera-lens combinations, f/8 may not really be the same f/8 as another external lens.
But it's supposed to be. The definition of f/8 is a lens whose effective diameter is 1/8 it's focal length. The general definition is that the aperture is the ratio of focal length to effective diameter. Any decent text on basic photography will tell you that.
Jim Carlile wrote:... each f/stop number is a just mathematical designation of the proportion of the amount of light that the mechanical iris in the lens will allow through. It is not a measurement in any way of the actual amount of light that is being caught by the film. So, that particular amount of light can differ in each lens even if the number is the same.
Again, wrong, according to the definition of f/stop. The ratio is between focal length and effective diameter, not light intensities.

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Re: In-camera vs. external meters

Post by Jim Carlile » Mon Jun 08, 2009 10:19 am

Nope.

And photo books never tell the whole story. They make it sound like f/stops are regular measurements, that were somehow magically discovered. That's where the confusion always arises.

What's going on here is that the ratio formula is the same thing as a proportional representation-- f/stop calculations are just arbitrary designations of the proportion of light that passes through the lens. It's merely an easy way of mathematically dividing up light in some meaningful way.

True, it has nothing to do with light intensity or measurement of the amount of light that hits the film, and that's the problem and the confusion. F/stops really don't mean a thing unless they can be related somehow by experience to film and lens and lighting conditions.

Here's how it all got started-- it was largely hit and miss. There were no light meters in the 19th century, and for a long time there were not even lens irises. Light was controlled either through time exposure or by the use of Waterhouse stops-- these were little circular cards placed in the light path that controlled the amount of light hitting the film. The Canon 310 still uses one for its separate light meter.

The f/stop formula was just a standardized way of dividing up the amount of light. What photographers would do is they would experiment over time to see what iris stops corresponded best with whatever film and light conditions they encountered, with whatever particular lens and camera combination they had in front of them.

In time, certain standardized combinations became the popular norm. Other manufacturers would duplicate these conditions so that users could realize some kind of consistent result. When exposure meters were invented later on, the companies would calibrate their cells and their measurements to correspond to the prevailing standards-- but these standards were totally ad hoc-- they were not based upon any science of light measurement or anything like that.

In fact, sometimes in the old days different brand meters would give totally different results with the same film stock. Certain brands began to dominate the market, and through experimentation and testing both these meter companies and film manufacturers would come to a consensus about what settings gave what results in what conditions.

Some meter companies-- like Weston-- would even recalculate Kodak and Ansco's own ASA ratings because their meters gave different f/stop results. One of the reasons for this is that f/stops are completely arbitrary when it comes to actual light measurements (the other is that Weston Co. was also cranky about film speeds.)

The whole point here is that f/stops really don't mean anything. They are just the result of a formula, a mathematical representation, usually calculated in halves. That's why f/8 in one lens may pass a different amount of light than f/8 in another. And this doesn't even address the light loss that can occur within the camera itself.

To put it simply, when you pull out a meter and it says "f/8," that doesn't mean there is an f/8 amount of light. What the meter is telling you is that over a period of many years, this reading with this particular set of cells has been agreed upon to give a consistent result within a range of film speeds, and that the reason this is the case is that all of the parties have worked together to come to this standard f/number for this combination of conditions. The film companies adjusted their emulsions, the lens companies adjusted their coatings and glass elements, and the meter companies adjusted their whatever they do to make their meter work. They all consistently calibrated and recalibrated their products over time.

It wasn't until T-stops were developed that a consistent, science-based way of coordinating the prevailing f/stop methodology with light and film sensitivity came into play. It was simple: they started measuring the light.

So the moral is-- don't worry at all about f/stops. Just like the 19th Century, if you want to manually gauge light and film, you have to experiment with the settings and keep track of what works. But the settings are totally arbitrary-- the only good thing they do is just divide up the light in a consistent way.

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Re: In-camera vs. external meters

Post by BetterSense » Mon Jun 08, 2009 4:00 pm

You are quite wrong on several points.
f/stop calculations are just arbitrary designations of the proportion of light that passes through the lens. It's merely an easy way of mathematically dividing up light in some meaningful way.
No, they are not. Fstops ARE a geometrical measurement that is based on the physics of the optical system. It's very simple...it's the focal length divided by the iris diameter. You make it sound like F/ratio was invented in the context of photography. The world of optics is more diverse than photography; I work in a laboratory and F/numbers are used for everything from spectrometer optical systems, lasers, rifle scopes, and even IR imaging. That it is useful for photographic purposes is only because of convenience.

F/numbers are related to the amount of light coming through the lens only theoretically, not fundamentally. It's a close approximation to say that twice the iris area admits twice the light. The reason that this scale is geometrical and not calibrated to the amount of light coming through a lens, is that for one thing, it was a difficult thing to measure light for centuries, and F/ratios relate to geometrical parameters of the optical system...things like DOF. Like capacitance, F/numbers are a purely geometrical measure. Any two optical systems with the same f/ratio will have the same DOF characteristics; the amount of light coming through the lens may be different between them. If you really want to be precise in comparing the light transmission of different optical systems, you can use T/stops, which Hollywood invented because in their application, exposure was more critical than DOF.
f/22 and be there

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Re: In-camera vs. external meters

Post by aj » Mon Jun 08, 2009 5:24 pm

I many years of photography I never noticed a difference in outcome of lightmeters and certainly not like full stops. What would otherwise be the purpose of ISO. To make us experiment? :)

My Gossen Lunasix just line-ups with every other meter like a Sekonic, a cheap Gossen or any internal camera meter-system.
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Re: In-camera vs. external meters

Post by MovieStuff » Mon Jun 08, 2009 6:21 pm

aj wrote:I many years of photography I never noticed a difference in outcome of lightmeters and certainly not like full stops.
I've seen it many times. It's pretty common, actually. I know of no professional cinematographers that trust a meter right out of the box.
aj wrote:What would otherwise be the purpose of ISO. To make us experiment?
Well, yeah. An ISO rating is only another point of reference but it doesn't mean anything, aesthetically. When Kodak puts an ASA of 500 on a film stock, that is a reference for the least amount of light it can shoot under and still get an image they consider is usable. However, everyone knows that rating it at ASA 250 will tighten the grain and give more pleasing results. The only way to know is to experiment and see what you like.

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Re: In-camera vs. external meters

Post by christoph » Mon Jun 08, 2009 8:04 pm

well, if your internal SLR/super8 and external light meters are off by two stops there are basically three possibilities:

- miscalibration of one or all the meters
- user error in setting compensated shutter speed, T-stop and the proper ISO (last part is black-magic with super8 cameras, even more so if you include built-in filters)
- user error in metering technique

judging from my problems with wrong meter readings and the huge difference, it's most likely a combination of all three.
++ c.

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