Metering from a Newbie

[AlanAnt]

Having purchased a Super 8mm after 20+ years of photography, I'm wondering about dialing in the exposure. Will my photography light meter yield a decent exposure if the Shutter speed and ASA is a constant? I've read that 24 fps would be close to 1/50 shutter speed. Hypothetically, if I use 50 ASA film, set shutter at 1/50, take an Ambient light meter reading and the display determines f/8, would that be a valid exposure? Hope that makes sense. With the price of Super 8mm film and processing, I'd like to minimize as many errors as possible.

[Paul Thrussell]

The trouble with reflex Super 8 cameras is that the viewfinder eats up some of the light - others can better explain, but basically you can't just dial in a shutter speed and frame rate and use that as-is without some compensation.

[freddenacka]

The best is to try, and change if necesary. Different cemeras have different shutter angel.
What camera are you going to use?

[AlanAnt]

Thanks for your response. I have a Nikon Super 8 Zoom with a shutter angle of 170 and a Canon 814 XL with the Variable Shutter angles of 150 and 220. I own a Sekonic L-358 Light Meter which has CINE Mode, which is based on a 180 shutter angle. Knowing that Canon shoots Daylight at 150 (-1/3 stop) and Low Light at 220 (+1/3 stop) from the Light Meter's 180 default reading, I should adjust the ISO on the light meter either up or down 1/3 stop and take a meter reading to set the Aperture after selecting the fps on the Sekonic. Interesting about the viewfinder needing to be accounted for when taking a meter reading.

[carllooper]

Yes, so most Super8 cameras deflect a certain percentage of the light to the viewfinder. For example, the Leicina deflects 20%.

Light meters with an FPS setting assume a 180 degree shutter as a reference. To the extent that the camera's shutter deviates from 180 degrees you can re-rate the film stock accordingly, and re-rate again for the percentage of light lost to the viewfinder.

So for example, if your camera otherwise had a 180 degree shutter (most don't) but deflected 20% of the light to the viewfinder, then you would set the ASA on your light meter (called 'rating the filmstock') to 80% of the ASA printed on the film.

Here's a formula for light meters with an FPS setting. You set the FPS setting on the light meter and then the ASA according to this:

Lightmeter ASA Setting = (Camera Shutter Angle Degrees / 180) x [ (100 - Viewfinder Loss Percent)/100 ] x ( Film ASA Rating )

Example.

Film ASA Rating = 200
Shutter Angle Degrees = 120
Viewfinder Loss Percent = 20%

(120/180) x [ (100-20)/100 ] x ( 200 )
= 0.66 x 0.8 x 200
= 107 ASA (or nearest light meter setting)

You'll notice in this formula that if the shutter angle actually was 180 degrees, then the first factor would become 1 (ie. 180/180 = 1) and the resulting ASA would be equivalent to just compensating for viewfinder loss only.

The only other factor would be in relation to any loss incurred by use of the orange filter built into Super8 cameras. But in almost all situations you'll want to ensure that this filter is not in play. And can therefore be ignored. To make sure the filter is not in play, set the camera to it's "bulb" setting. The only exception is where you just happen to be using tungsten balanced film (unlikely) and you are using such stock outdoors. For all other scenarios (other than creative ones of course) move the filter out of the way by setting camera to it's "bulb" setting.

If the light meter doesn't have an FPS setting then you'll need to use a formula which tells you what to set the meter's exposure time.

Exposure Time = (Shutter Angle / 360) x (1/Camera FPS) secs

In this case you'd just rate the film to compensate only for the light loss to the viewfinder:

Lightmeter ASA Setting = [ (100 - Viewfinder Loss Percent)/100 ] x (Film ASA Rating)


Now of course the above assumes you know or can find out what the camera's shutter and viewfinder light loss is. Without such information the only solution is to shoot a test roll. You make an assumption about shutter angle and viewfinder light loss, and set the light meter accordingly, and then you shoot the same scene with different aperture settings around that which the light meter is recommending. This is called "bracketing". You alter the aperture in 1/3rd stops or in half stops, and make notes.

The shot that looks exposed correctly (along with the corresponding notes for that shot) will tell you how far out your assumption was. You then readjust your assumption by the exact same amount and use that from then on.

The easiest assumption to use (and it doesn't matter what because you'll be correcting it), is to assume a 180 degree shutter, and a light loss of 0% giving you a factor of exactly 1, ie. no change in ASA at all.

After shooting film lets say you find the best shot is when the aperture is opened by 1/2 stop from what the light meter recommended. This means the film can be rated as "less sensitive" than you otherwise rated it (because to get a good shot you had to open the aperture by half a stop, ie. as if the film were less sensitive than it is otherwise rated. Now it doesn't matter if this is due to more light lost by the viewfinder, and/or to a smaller shutter angle. All we need to know is a composite compensation, in the form of an ASA rating to use in the future (when using a particular camera). And that will do the trick.

The formula for this is:

Light Meter ASA = 2 ^ [ Log2(Film ASA) +/- Compensation in stops ]

where the ^ symbol means "to the power of". And Log2(x) means "the logarithm of x" in base 2.

In the example case the compensation in stops would be -0.5, so the new ASA to be calculated would be:

Light Meter ASA = 2 ^ [ Log2(200) - 0.5 ]

After setting the light meter ASA to this new value, you no longer have to compensate for your original (and incorrect) assumption using the aperture. The light meter will now do that correction for you, and give you the correct aperture to use up front.

Unfortunately I can't find my calculator to calculate what the new ASA rating would be for this example.

...

Ah found it. The answer is 140 ASA (approx).

Log2(200 ASA) = 7.6438
7.6438 - 0.5 = 7.1438
2^ 7.1438 = 140 ASA (approx)

Here's another example. Suppose we need to re-rate the film by exactly -1 stop. Well that's easy of course. We know from general knowledge that the answer will be 100 ASA, but we can use this general knowledge to check the formula:

Log2(200 ASA) = 7.6438
7.6438 - 1 = 6.6438
2 ^ 6.6438 = 100 ASA

Yep. 100 ASA.

C

Notes.

The Log function on calculators is Log Base 10, which you might write as Log10(x)
To get the answer in Base 2 (which is what you need) you divide the calculator's answer by Log10(2):

So Log10(200) on a calculator will give you: 2.3010
Dividing this answer by Log10(2) on the calculator, gives the base 2 result: 7.6438

[AlanAnt]

Thanks Carl for sharing very useful information. I sincerely appreciate it.