How to check camera's meter?
I just recently received my EOS 3, a camera which unfortunately has lots of history of underexposure problems. Is there a way I can check the accuracy of its meter besides sending it to Canon? I've heard of shooting slides, using the F/16 rule or using a grey card to check for meter problems. Which one's the most accurate and the best way of doing so?
Have you got a handheld meter? If so, easiest thing by far is to meter a gray card with this, meter the same card under the same lighting with the camera, and compare. If you don't, then a quick check is to make some exposures outdoors (not necessarily with film in the camera) and see if these are plausible according to the sunny 16 rule. This will give a rough guide, but if a camera has a tendency to underexpose, for example, due to peculiarities of its matrix metering system, then only a film test with varying subjects will really show if the meter is OK.
who says it has a 'history of underexposure'?
under what conditions?
shoot a roll, make your own detrmination
Originally Posted by film_guy
Well, which one involves real pictures? Sunny 16 is a get-you-out-of-trouble measure when the meter doesn't work. Grey cards are a complete waste of time: there's a module about why in the Photo School at www.rogerandfrances.com but the basic reason is that no speed determination system on earth is based on grey cards. This leaves shooting slides, which has the double advantage that you might get some pictures out of it.
Shoot as wide a variety as possible of your normal subjects, bracketing at least +/- 1/2 stop; preferably +/- 2/3 stop (the optimum, if the camera lets you do it); and failing +/- 2/2 stop, 1 stop.
Remember however that the metering algorithms for slides are completely different from those for negatives, because slide exposure is keyed to the highlights (so they don't 'blow' to a featureless white) while with negs it is keyed to the shadows, to get enough shadow detail: it is difficult or impossible with most subjects to 'blow' the highlights with neg, though you may need reduced development, softer paper or dodging and burning to retain the highlight detail.
In other words, if you normally shoot neg, testing with slides won't tell you much except that it over- or under-exposes with slides. If you normally shoot neg, shoot neg film, bracketing +/- 1 stop; look at the shadow detail in your negs (you don't need to print them); and base your future exposures on that.
The few books that I have say that most meters can vary from 1/3 to 2/3 stop from camera to camera even in the same brand. Most of the advice I have found is to meter something that is average in color like green grass or concrete or a gray card if you have one in bright sunlight. Using the sunny 16 rule and adjust the ISO till it reads what ever shutter speed corresponds to the ISO you plan to shoot. I have an F2 with a DP1 and DP2 finder and they are about 1/3 stop different from each other. But they always match my Mint Pentax ZX-L that also matches my Luna Pro handheld meter. And as a note they also meter right about the Sunny 16 rule. I have calibrated both meters to the f/16 rule and my slides come out great and my B&W prints come out splendid as well. That being if I do my part on determining exposure.
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Can't quite agree here. A gray card has the average reflectivity of a normal scene and above all provides a consistent standard for comparison readings which eliminates the effect of variables such as different angles of view of an built-in meter versus and handheld one, different metering patterns (with a gray card, integrating, center-weighted, matrix and spot metering should give the same result each time). I thus regard it as ideal for COMPARISON purposes - it will also give good results for practical photography in most cases, being exactly the same from a measurement point of view as a incident reading. Whether you might get a slightly better result in practical field work with slide film by metering a highlight and placing this on zone VIII (you probably would) or with neg film by metering the deepest shadow and placing this on zone II is another question, but I think to go straight for a film test with varying subjects is to risk confusion (even some matrix metering systems are only semi-intelligent insofar as they only work with horizontal subjects). Above all, gray card metering is quick and gives you an indication in seconds of whether your meter is in the ballpark as regards accuracy - if it is, THEN shoot some film!
Originally Posted by Roger Hicks
Last edited by David H. Bebbington; 02-12-2007 at 03:18 AM. Click to view previous post history.
I have a Nikon F2 with a DP-1, DP-2 and DP-3 finders an EM and FG and a JTL LM-8 Digital Light Meter. I acquired the DP-2, DP-3 and FG recently and to do a quick check of their meter accuracy before purchase I used a simple test in a bathroom in our house that has a light fixture with 10 60W clear bulbs (very bright) and dimmer with eggshell white walls. I tested the F2 finders against the EM the FG and light meter at varying light levels from 1/1000 sec at f22 to 1 sec at f2 and multiple ISO settings, all meters matched every possible ISO shutter aperture combination within 1/3 stop. These are fairly simple metering systems, but I know they accurately measure a simple light source with the same results. Obviously you have to have trusted camera/meter to run a comparison against. The DP-2&3 and the FG have worked with film in the field without any problems.
I'll agree on two points. First, a quick-and-dirty comparison before you shoot film is indeed a good idea. Second, a sheet of grey card is indeed a useful neutral target -- but no more useful than any other neutral target such as a sheet of white paper, or indeed a field of grass, though coloured targets should be treated with caution because meter cells respond differently to different colours. An old sensitometrists' trick (which I leaned from my frends at Ilford) is to put the test target well out of focus in order to ensure an absolutely even field of illumination.
Second, 18 per cent is NOT the reflectivity of an 'average' scene. This is a widespread myth which I believed myself until a few years ago, and I must confess to my shame that it is propagated in some of my early books -- the ones from before the days when drafts were read through as a matter of course by Mike Gristwood, long-time member of the ISO standards committee.
Kodak's original work in the 1930s indicated that the overall reflectivity of an average outdoor scene is 12 to 14 per cent (indoors, all bets are off). An 18 per cent grey card is a Munsell mid-tone, i.e. the card that most people will pick out as a mid-tone when shown a range of cards from as back as possible to as white as possible.
It has acquired some sort of totemic status because of its adoption by Ansel Adams, but its relevance to metering is slight, as no speed criterion ever has been based on a mid-tone. Speeds for negative materials are based on shadow detail, while speeds for transparencies are keyed to the highlights.
Kodak first sold grey cards for use in determining exposure in studio colour photography only, in the days when incident light meters were rare but reflected-light meters were relatively common, at least among professionals. If I recall aright (I lost the instructions for my grey card some decades ago) the card was supposed to be angled between the subject-camera axis and the subject-key light axis, at which point the question of how much light it reflects becomes quite interesting. I suppose it it pretty much governed by Snell's law. Before they sold grey cards, they recommended the use of a Kodak yellow film packet (in my 1941 Kodak Dataguide, for example).
There's one free module on ISO speeds (including history) in the Photo School at www.rogerandfrances.com, and another on why grey cards are of limited usefulness.
I agree with Roger on the grey card being a problematic thing, sometimes.
I also agree with David that the grey card is extremely useful, sometimes.
But have either of you used, a Wallace Expo/Disc?
These are quite brilliant, they turn any in-camera meter into an extremely accurate incident light meter.
I have had two of these for about 15 years and their accuracy for obtaining, among other things, a perfectly exposed, colour neutral, 18% grey C41 negative, for calibrating purposes under an enlarger, are unbelievable.
The Wallace Expo-Disc is in my humble opinion, the modern grey card!
I have two of them, 52mm and 72mm for my lenses. Using my system I am able to print a colour perfect and density perfect print, first go almost every time. Artistic interpretation of the negative means that I will change things a bit, but for a standard neutral 18% frame it cannot be beaten.
I also use it for B&W.
History? That's my cue to trot out this picture again. Here's Prof Karapetoff with the Neutrowe Gray Card in '39 or '40. It was 14% grey, and that was decided on by looking at pictures made by metering different cards and deciding which ones looked right. As Roger says, it was intended for use with colour film: Kodachrome.
If you want a standard reference, the palm of your hand makes a convenient alternative to a grey card, if you don't have an incident meter. The incident meter, with a dome (cardioid responce) instead of a flat (cosine response) receptor, has the advantage of being better able to measure light from a variety of directions at once.
"...its relevance to metering is slight, as no speed criterion ever has been based on a mid-tone. Speeds for negative materials are based on shadow detail,..."
Well, that's a bit of a grey area. There is a technique known as Aim Density that uses the density of a grey card exposure as the determining factor for speed. It's used in cinematography with colour negative materials, and there are a few variations. It uses midtones, because they tend to be the most important tones in most movies. Well anyway, that's what I think. There is no ISO for the speed of colour negative motion picture film, by the way, so a manufacturer could use midtone criterion for the determination of a recommended EI if they so wished.
This is how one variation it works, in brief. Years ago EK were kind enough to give me a week's (free) workshop in the practical use of it for conventionally printed film and for telecine'd film.
The film manufacturer gives you red, green and blue Status M aim densities for a grey card. You expose a grey card at different effective meter settings, using your meters, in third-stop intervals. Then you measure the Status M densities and find the setting that came closest to the recommended values for each layer. The speed is calculated by voting: eg 250, 320, 320 means 320. Then you use that as your basis for the rest of your film testing, which is a mixture of numbers and appearance, and which is the larger part of the exercise.
Last edited by Helen B; 02-12-2007 at 07:04 AM. Click to view previous post history.