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Shooting a bracketed exposure test to calibrate a hand held meter to your camera.

This page is about how you can use a hand held light meter to gauge correct exposure.
First off, the obvious question that needs to be answered is ‘why can’t I just set the meter to the ISO (ASA) of the film I am using and then go for it?’. This is a natural question. After all, that is precisely what you do with a normal 35mm SLR camera and with most 16mm and 35 mm movie cameras. With a super 8 camera, and with most standard 8mm cameras, however, if you do merely set the light meter to the film’s rated ISO and start filming, the results you get will almost certainly be under exposed. So what is it that is different about super 8 cameras?

Two things actually. The first is to do with how the light coming in the lens gets to both the viewfinder and the film. In a normal SLR camera (or a ‘mirrored shutter’ movie camera) a mirror diverts the incoming light up to the viewfinder. While the mirror is in place, all the incoming light goes to the viewfinder, and none of the incoming light goes to the film. When you release the shutter to take a picture however, the mirror momentarily flips out of the way. When this happens, all of the light now goes to the film, and the viewfinder is completely dark for the duration of the exposure. In mirrored shutter movie cameras this results in a viewfinder that ‘blinks’ 24 times a second – once for each exposure. Super 8 reflex viewfinders (with very few exceptions) don’t ‘blink’ like this. Instead the camera’s reflex viewfinder system employs a semi-silvered beam-splitting mirror to direct part of the incoming light from the lens to the viewfinder while allowing the rest to get through to the film. Thus, some of the incoming light is lost to the viewfinder. Now the question is ‘how much?’. The answer is each camera is different.

The second factor is that while most normal modern hand held photographic light meters have ‘cine’ settings for use when shooting movie film, these meters by and large assume a 180 degree shutter opening on the camera. A 180 degree shutter opening would mean that for every revolution of the shutter, light gets to the film for 50% of the time (50% because 180 degrees is half of the 360 degrees of a circle). Thus, at a shooting speed of 24 frames per second, a 180 degree shutter opening would yield a shutter speed of a 48th of a second per exposure. A 180 degree shutter opening is a reasonable assumption for the meter manufacturers to make as many 16mm and 35mm cameras do indeed have 180 degree shutter openings. With super 8 and standard 8 cameras, however, this is only rarely the case. Many super 8 cameras have a normal shutter opening of 150 degrees. This yields a faster shutter speed at 24 fps than a 48th of a second. Some cameras are designated ‘XL’ or ‘existing light’ cameras. These are ‘low(er) light cameras. Usually an XL camera will have a shutter opening of about 220 degrees.

So there are two reasons why you can’t just use a hand held light meter in the straightforward way: because a certain amount of light is lost to the film by the beam-splitter that sends light to the viewfinder, and because of the difference between the shutter angle assumed by the light meter and the camera’s actual shutter angle.

What to do. All you can do is to shoot a careful test. A carefully shot test will tell you how to compensate for these two factors. It will tell you a ‘compensation factor’ for the particular light meter and camera combination. I find it most useful to apply this compensation factor to the ISO rating of the film. Invariably, given the factors outlined above, the compensation factor will involve setting the ISO rating on the light meter to something lower than the official rated ISO of the film stock. Note that if you use a different meter, or a different camera, once again you should shoot another test.

Here is how to shoot the test. The technique to use is called ‘exposure bracketing’. This means taking a series of exposures of the same scene, each time adjusting the exposure a certain amount. Since the ISO series runs in 1/3rd f-stop increments, it makes sense to similarly conduct your exposure bracketing series in 1/3rd stop steps.

Take some care to choose scenes for your bracketing tests that aren’t too tricky from an exposure point of view. Don’t pick a back lit or a spot lit subject. Someone sitting on grass is ideal. It is best also to use a tripod if you have one so that the framing is identical from exposure to exposure. This will make judging the finished results easier.

To begin, set your light meter to the rated speed of the film. If it is 64t for instance, set the meter to 64 ISO if you are shooting under tungsten lights, or 40 ISO if shooting under daylight (as I recommend you do for this test). (Do I need to explain why 40 not 64 ISO? Simply because if you are filming out doors you will be using the camera’s internal colour correction filter to turn the daylight coloured light into tungsten coloured light. This filter absorbes 2/3rds of a stop of light.). I suggest you start at the rated speed of the film and work downwards from there as we can be pretty sure that the rated speed will be too fast (high) to give correct exposures so you don’t need to try even faster speeds (higher ISOs).

So take a reading at this ISO setting. Lets say you are shooting at 18fps. You have the meter set to the rated ISO rating and you read off against the 18fps cine speed mark on the meter the aperture f-stop setting. Lets say for example it indicated f8. Now, on manual exposure set the camera’s aperture to f-8. Shoot for say 5 or 10 seconds. Next, make another exposure but this time with the aperture 1/3rd of a stop more open. In this example, 1/3rd more open will mean f-5.6 and 2/3rds. Continue the series by shooting at f-5.6 and 1/3rd, then f-5.6, then f-4 and 2/3rds, f-4 and 1/3rd and so on. I suggest shooting the same scene in this way at least 7 times to cover two full f-stops difference. The more the better. It is also a good idea to have a card in the scene (and make it big!) indicating which shot you are up to. The card simply needs to indicate ‘0’ (meaning ‘no compensation’) then ‘+1/3rd’, ‘+2/3rds’, ‘+1’, ‘+1 and 1/3rd’, etc.. There is no need to note the actual f-stop used as it is the degree of variation that is of interest, not the setting itself.

The idea with the bracketing test is that when the film comes back from the lab you will be able to see which compensation factor gave the best exposure. Ideally you would conduct this bracketing test with a few different scenes and average the results.

So lets say you did seven 5 second exposures brackets in 1/3rd stop increments of 3 separate scenes. This would mean 21 exposures in all. Good work. Now lets say on looking back at the test roll you decide that the shots marked ‘+1 and 1/3rd’ are the best. What this means is that in future, when using this same hand held meter and this same camera, you simply need to apply a compensation factor of +1 and 1/3rd stops to the films rated ISO. If using 64t with its daylight rating of 40 ISO, then +1 and 1/3rd stops = 16 ISO. You don’t need to worry if this sounds low. These are just numbers. Besides, the first home movie films had an official rated speed of about 10 ISO. The good thing about applying the compensating factor to the ISO is that if you switch to a different frame rate (say 24 fps) you don’t need to worry about making any further adjustment. With the light meter set to 16 ISO you can simply read off the aperture given by the the 24fps cine speed marking (alright, ‘setting’ not ‘marking’ if you are using a digital light meter!). Similarly if you switch to a different film stock all you have to do is apply the same compensating factor to it – you don’t need to shoot another test.

So shoot a carefully bracketed test. It’s a good investment. If done well, you shouldn’t need to do it again.

Just one last note for those that want to use a normal SLR camera as a light meter. Yes, you can do this too, but again, you have to do the bracketed test. Set the camera to the film’s rated ISO (again, say 40 ISO for Ektachrome 64t in daylight). Set the camera’s shutter speed to a 30th. Now read off the aperture setting indicated by the SLR’s meter. Note that this will only work for the frame rate you shoot the test at (18 or 24). As there is no cine setting on the SLR’s meter, you can’t just readily switch like you can with a hand held meter. All you have to do is remember that 24 fps requires 1/3rd of a stop more light than 18 fps.

Enjoy!


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