External light meter - compensation for shutter speed and reflex viewfinder

[David T. Nethery]

Experienced cinematographers, please excuse the gross ignorance exhibited in this post.

 

I will admit it: I'm a lazy shooter . In the past , I've always relied on the internal light meter of my Super-8 cameras (Nikon R10 / Nizo 560 / Canon 814 Auto Zoom) . I'll spot meter with those cameras (zoom-in , take the reading) and then lock the exposure when I need manual control, otherwise I leave them on auto-exposure and the results have always been good.

 

I'm getting back into shooting film after several years of not shooting any film.

 

Now I have a newly purchased eBay camera, a Nikon R10 , which seems to be in perfect condition except for a somewhat sticky meter (sometimes it seems to work ok, other times it "hesitates" or sticks when moving between f-stops when it is on auto exposure ... still waiting to get back the test rolls I shot to see if this is as big a problem as I fear ) and on my Canon 814 AZ the internal meter no longer works , but I can set the manual exposure. The Nizo 560 is working fine on both auto and manual. I'd like to get both the Nikon R10 and the Canon 814AZ serviced , but can't afford it right now and if I could learn to use both of those on manual exposure I'd be happy.

 

To my chagrin, I've never mastered using an external meter to set the aperture manually on my Super-8 cameras because I've always read that the typical external meter will assume a camera with a 180° shutter angle , but the Nikon R10 is 160° shutter and Canon is 155° shutter , so I can never get it straight in my head how to read the external light meter to compensate for that . I have a Sekonic L-158 and a Gossen Scout-2 meter , both of which have a "Cine Scale" with markings for 9, 18, 24, 54 fps (and intermediate markings) , but if both meters are set to assume a 180° shutter angle the recommended f-stop can't be accurate for a camera with 160° and 155° shutter angle , plus I am told I must compensate for between 1/3 - to - 1/2 stop light loss to compensate for light that is redirected to the reflex viewfinder . Ok, maybe it shows how lazy and/or stupid I am, but the mental gymnastics involved in this take all the fun out of it for me.

 

But I want to try to figure it out. Assuming my Sekonic L-158 and my Gossen Scout-2 are still accurate (?) , and assuming both those light meter's Cine settings are measuring based on a shutter angle of 180° (is this known for sure ? it doesn't say anything about that in the user manuals for those meters , but that's what I'm told ... ) is there an "easy" formula for converting a reading for a 180° shutter angle to a 160° or 155° shutter and also add to that the "1/3rd to 1/2 stop loss of light" for the reflex shutter or do I just ignore the 1/3'rd to 1/2 stop loss of light as not that significant (especially with negative film, which has a wider exposure latitude ... but for shooting reversal I would like to be more spot on ...)

 

Also, does anyone know if the "Cine Scale" on meters like the Sekonic L-158 and Gossen Scout-2 took into account the lost light redirected to a reflex viewfinder or did those meters always assume the use of a non-reflex viewfinder , so no loss of light to the viewfinder was included in the Cine Scale reading ?

 

In plain language , if I'm taking a reading with my Gossen Scout 2 and it is giving me an f-stop based on a 180° shutter angle at 1/48th of a second for 24 fps , but my Nikon R10 is 160° shutter angle , 1/54th of a second (24fps) how much do I have to adjust ? Looking at the Sekonic L-158 right now , it is giving a reading of f8 at 1/30 of a second and f5.6 at 1/60 of a second , so around half-way between 5.6 and f8 would be about right ?

 

 

 

 

[David Mullen ASC]

Can't you just set the shutter speed instead of the shutter angle?

[Dom Jaeger]

The difference between 180° (1/48 sec) and 160° (1/54 sec) is pretty small, only about a sixth of a stop.

 

As David mentioned, if you know or can calculate the shutter speed, use that. You can just use the closest shutter speed on your meter, for 1/54 sec the 1/60 setting is fine.

 

Most Super 8 cameras lose light both to the viewfinder and the zoom. Opening up the iris by about half a stop more than the meter reads should pretty much cover it. Or you can adjust the meter ISO setting to compensate (either a third or two thirds of a stop slower - so for 200 ISO film, set the meter to either 160 or 125). You could shoot a test to check what works best for your particular camera.

 

Meters usually don't compensate for viewfinder light loss, since professional movie cameras have spinning mirror shutters that don't lose light. The only one I know of is a special Bolex meter made by Gossen that allowed for the prism light loss.

[David Mullen ASC]

Simplest thing might be to just rate the stock 1/3-stop slower on your meter, or use 1/60th instead of 1/50th. Then test to see if that's correct.

[Ruben Arce]

There are a lot of factors in every camera, some manufacturers publish important information like how much light is lost on the prism & T vs F stops for example, but Super 8 cameras were not intended to be used by professionals back in the day.

 

The info that you are looking for can be reverse engineered (Depending on your knowledge and experience) by using a gray card and a light meter. When it comes to light meters nobody can give you straight facts, some manufacturers use 18% some others use 13, 12 and 12.5% so at the end of the day I have learned to just call it middle gray, whatever it is and it works for me.

 

The Nikon R10 is rated on F-stops not T stops so you can typically say you are loosing 1/3rd of a stop there (-1). The shutter angle is 160° instead of 180° on most professional cameras. On my Sekonic 558 I would just adjust the shutter angle and it would not be a factor anymore, but if you have a fixed shutter angle of 180° on your meter, I would compensate another 1/3rd for that (-2) That's because I tend to put my exposure on the hot side but you could consider it 1/48th of a sec or 180° as mentioned before. The third factor would be the prism and since I couldn't find the "filter factor" of the prism (like in the Bolex) I would say add another 1/3rd. (1 full stop)

 

Every time I shoot negative I "over expose" it by two thirds, no matter what just because there is were I like my exposure and because rating it that way I get less grain. So I would personally compensate by one and a half stop. If I were shooting 250D for example, I would just tell my meter I'm shooting ASA 80 and I would forget about all those factors.

In the case of the Canon Scoopic for example, the manufacturer rated the lens on T-Stops meaning they already considered all the light loss factors on the prism and the lens. In the case of mirror shutter cameras like the K3 that is not a factor, but you may still have to compensate if your lenses are not marked on T-stops.

 

Finally, I would test those meters against a modern reliable meter. I haves some of those on my collection and I can tell they are not accurate anymore. I have a Sekonic like that one and I know it doesn't use batteries, they use selenium cells or something like that which is not accurate this days. You can always find the difference against a modern/reliable meter, compensate for that difference using ASA or ISO and use it normally.

 

Just my two cents. This video is very informative and it has a formula you may find helpful,

 

[David T. Nethery]

 

if you know or can calculate the shutter speed, use that. You can just use the closest shutter speed on your meter, for 1/54 sec the 1/60 setting is fine.

 

Thank you, that's good to know that the variable between exposing for 1/54 and 1/60 won't make that much of a difference.

 

 

Meters usually don't compensate for viewfinder light loss, since professional movie cameras have spinning mirror shutters that don't lose light.

 

Right, that's what I've read before , which is what made me nervous about trusting the "cine scale" on either the Gossen Scout 2 or the Sekonic L-158 (as it would be applied to manual metering for a Super-8 camera) , but I was wondering if perhaps the cine scales on some of those meters were ever designed to take into account viewfinder light loss for movie cameras with a beam splitter (like the typical Super-8 camera) , but thanks for confirming that they do not.

 

 

The only one I know of is a special Bolex meter made by Gossen that allowed for the prism light loss.

 

Oh, yes, I remember reading about the Gossen light meter for Bolex cameras, the one that clips on to a bracket on top of the camera . But I suppose one of those would only be accurate to use with the Bolex prism system and a Bolex 130° shutter angle.