Bolex H16 slow motion speeds?

Hi,

 

If I shoot ASA 100 film at about 16fps, then increase the speed to 32fps, that should halve the light hitting each frame. So that would be the equivalent of filming with ASA 50 at 16 fps?

 

If I double the speed again to 64fps that would be like filming ASA 25 at 16fps?

 

So if I want accurate meter readings for my slow motion footage on ASA 100 film I need to set the film speed from my normal 16 up to 64fps, then set my meter at ASA 25.

 

Is this correct please, nearly correct, or way out?

 

Thanks,

 

Ric

Each film frame will receive the same amount of exposure regardless of the camera speed. Only the speed at which the film is transported through the mechanism changes.

Each film frame will receive the same amount of exposure regardless of the camera speed. Only the speed at which the film is transported through the mechanism changes.

 

Thanks. Odd, when I filmed a lit fairground carousel at night, then upped the speed on the Bolex, that section of the film - now in slow motion - came out darker than it did on the camera using my normal speed setting. Why was that please? Why was the film darker if each frame was receiving the same amount of exposure? The aperture remained constant, wide open, because I was filming fairground lights at night - so the aperture was wide open all the while, whatever film transport speed I chose.

for movies camera the frame rate changes the exposure time. The exposure time is usually half the the frame rate. For example 16fps would be exposed at about 1/30 second exposure time.

Keith, you do realize the OP is asking about a movie camera and not a SLR film camera? I don't do movie cameras but from the online APS Bolex 16mm Reflex Operating Guide:<BR><BR>

 

<I>"When changing filming speeds, do not forget to alter exposure setting. (When changing from 24 to 48 by one stop and so forth.)</I>"

As I said movies cameras are not my thing, but the law of exposure should be the same.

<BR><BR>

 

If I shoot ASA 100 film at about 16fps, then increase the speed to 32fps, that should halve the light hitting each frame."<BR>

-- Correct

<BR><BR>

So that would be the equivalent of filming with ASA 50 at 16 fps?

<BR>

-- Yes, half as much sensitivity, the film twice as dark.

<BR><BR>

 

 

If I double the speed again to 64fps that would be like filming ASA 25 at 16fps?

<BR>

-- Yes, one quarter as much sensitivity, the film is four times as dark.

<BR><BR>

 

So if I want accurate meter readings for my slow motion footage on ASA 100 film I need to set the film speed from my normal 16 up to 64fps, then set my meter at ASA 25. Is this correct please, nearly correct, or way out?

<BR>

-- Correct. The fps speed allows only 1/4 of the light so the 25 ASA will call for four times, or two stops increase of exposure

So if I want accurate meter readings for my slow motion footage on ASA 100 film I need to set the film speed from my normal 16 up to 64fps, then set my meter at ASA 25. Is this correct please, nearly correct, or way out?

-- Correct. The fps speed allows only 1/4 of the light so the 25 ASA will call for four times, or two stops increase of exposure

Please pardon my semi informed reply. I don't know what kind of meter Ric might be using. - I here like Gossens, set them to film speed and can read my shutter speeds for stills above and Ric's FPS in a "CINE" labeled scale below a range of f-stops. Unless we are talking about an ultra crude built in meter we should never even think about ISO with a loaded camera. Set the meter to your stock's ISO, choose your FPS and adjust the aperture according to your metering or rig up more lights when you are hitting a wall because your lens is too slow.

Ric--Have you ever located a manual for this camera? If so, it should tell you the exact shutter speed for a given FPS and that should answer your question definitively. Note to the moderator--Bolex H-16 shutters open less than 180 degrees, so shutter speeds are faster than most movie cameras, and reflex H-16 models use a beam splitter that take roughly 20% of the light form the film and divert it to the finder. If you're exposing color reversal film this is significant.

Thanks to all.

 

 

Light meters are a problem, my cameras don't have them. I have a Gossen Profisix which is excellent but the fps reading is microscopic near the centre of the dial. I am trying to find better light meters for cine film.

 

 

The Bolex guide (p.59-60) says that a 16fps project which wants to shoot at 64fps should open the lens "two full stops, so that the film receives the same exposure", so I was correct in relating the darkening of my film to the increase of camera speed.

 

 

That seems the simplest option for me then, to meter as normal but remember the x2 f.stops for slow motion?

I believe you now have the information that you were looking for. Increasing exposure two stops will do it. Many old time still film camera users would sometimes adjust the ASA to automatically attain a desired exposure. When shooting outdoors in the snow, the meter would indicated a setting that would be two stops underexposed. Changing the ASA from 100 to 25 would get the meter to give the correct exposure setting for that shot and subsequent following shots..

Thanks James,

 

I'm sorted then! ;)

 

Ric

Keith, you do realize the OP is asking about a movie camera and not a SLR film camera?

Ooops! Thanks all posters above for the point made and correction; that is what I get for reading too fast. Sorry Ric,I apologize. Of course many MOVIE cameras are going to change the exposure with camera speed changes. I'll be more careful in the future about reading more carefully! (so embarrassing)

No worries Keith. Just found (and sorted) a Bolex-Gossen light meter, I did not know these existed. Battery on the way. This should make my cine metering a dream compared to using the Gossen stills meter I have, which is excellent but was not designed for motion cameras.

 

https://farm8.static.flickr.com/7510/15713337084_b6584215d1_b.jpg

 

Ric

Hey again Ric. I see from the photos that the meter takes the old PX13 1.35 volt Mercury battery, long since obsolete. Do you know that there are two battery types available today to use? One is the WEIN 1.35 volt air-cell. It works OK, but does not last very long, and will get a surface corrosion of sorts rather easily. The other method is to use a Duracell 301/386 cell that you put into a ~$22 "MR-9" battery voltage-drop converter that you can get from China through ebay. It has a Schottky diode in it that drops the battery voltage (under circuit loading) to 1.35 volts. Using a 1.35 volt power, the meter does not require recalibration. You could also use (without any adapter) a 625 type Alkaline cell, but you would likely want to have the meter recalibrated for accuracy, plus the metering may change somewhat as this battery type's voltage depletes. The Duracell 301/386 will pretty much maintain its voltage close to the point where it dies. Hope this information helps make up a bit for my earlier fiasco. (I have about 8 of these $22 voltage converters for use with some older Canon cameras that originally had the 1.35 volt PX13 battery. Even though they are expensive, they are far better than the short life of the WEIN cells. The WEIN cells are very cheap, however.) Also - you may want to check the meter internally around the battery contacts and into the meter body for wetness / corrosion from the old PX-13's. This is almost a certainty from those cells.

Where did you get a battery for the meter, and what type is it? Just curious.

Where did you get a battery for the meter, and what type is it? Just curious.

 

Hi Keith,

 

Thanks, I ordered several MRB625 WeinCell 1.35V batteries, will update.

 

Ric