BOLEX REX5 SHUTTER SPEED CARVED IN STONE

[Giovanni Savino]

What I really would like to know- carved in stone- once and for all, is:

 

The bolex REX 5 manual says that the shutter speed is 1/80th of a second at 24fps but because of the prism deflecting 20-25% of the light passing the actual shutter speed to be used(when using RX lenses that compensate for the light loss) is 1/65th of a second.

 

So: I am using a REX5 with a RX lens at 24fps and I set my hand held light meter at :

Shutter angle 130 degrees

Shutter speed 1/65th of a second

 

is this CORRECT ?? :blink:

[Clive Tobin]

...So: I am using a REX5 with a RX lens at 24fps and I set my hand held light meter at :

Shutter angle 130 degrees

Shutter speed 1/65th of a second

 

is this CORRECT ?? :blink:

No you have it backwards. The physical speed is 1/65 but the adapted speed correcting for the prism is 1/80. 1/80 is less than 1/65.

[Giovanni Savino]

No you have it backwards. The physical speed is 1/65 but the adapted speed correcting for the prism is 1/80. 1/80 is less than 1/65.

 

Thank you Clive ! I thought there was something wrong!

However, let me send you the page of the BOLEX manual I was reading and tell me what you think.

 

Thanks

 

Giovanni

 

Snapshot_Page_3.tiff.pdf

[Clive Tobin]

However, let me send you the page of the BOLEX manual I was reading and tell me what you think.

...

 

The manual is completely wrong. If this is the one I think it is we have notified the author of the error but it appears he has not corrected it yet.

[Tim Carroll]

This is covered on page 9 of the free PDF download of the Bolex EBM manual which you can get HERE!

 

-Tim

[Giovanni Savino]

The manual is completely wrong. If this is the one I think it is we have notified the author of the error but it appears he has not corrected it yet.

 

 

THANK YOU Clive,

 

I was waiting to hear from you before throwing that manual in the trash.

As a matter of fact I shot a test today following your advice and I am picking it up tomorrow from the lab.

Unfortunately the POE lens galvanometer must be out of order as it keeps consistently showing 1 to 2 stops more.

I am wondering, and I'd like your opinion on this, if I re-calibrate the galvanometer on the lens (by setting the ASA dial to a lower ASA setting) so that it matches my hand held meter, can it then be trusted to expose automatically adjusting its iris in a correct way?

 

many thanks

 

Giovanni :) :)

[Tim Carroll]

The meters in alot of those old lenses were designed to use mercury oxide (I think that is the proper term) batteries which had a very unique power loss curve. They gave a very consistant voltage and then pretty much went dead. They are no longer made because of the mercury and environmental issues.

 

The alkaline batteries, even though the same size, have a very different power loss curve. Say it is a three volt battery. An alkaline will start off at say 3.5 volts and steadily lose power until it is dead. Where as a mercury battery started off at three volts and kept putting out three volts until it died. The old meters were designed to work with this consistant voltage, so when you put alkaline batteries in them, they are very inaccurate.

 

-Tim

[Giovanni Savino]

The meters in alot of those old lenses were designed to use mercury oxide (I think that is the proper term) batteries which had a very unique power loss curve. They gave a very consistant voltage and then pretty much went dead. They are no longer made because of the mercury and environmental issues.

 

The alkaline batteries, even though the same size, have a very different power loss curve. Say it is a three volt battery. An alkaline will start off at say 3.5 volts and steadily lose power until it is dead. Where as a mercury battery started off at three volts and kept putting out three volts until it died. The old meters were designed to work with this consistant voltage, so when you put alkaline batteries in them, they are very inaccurate.

 

-Tim

Hi Tim,

 

I am sure you are right about the battery. The original one (no longer available) was 1.35V and the replacement I could find is 1.5V and obviously alkaline....

What i remain curious about is if I correct the 1,5 to 2 f stop discrepancy between the lens meter and my reliable hand held meter, by lowering the ASA setting on the lens dial, can I then trust the lens to expose properly while in automatic mode ?

 

thanks

 

Giovanni

[Tim Carroll]

Giovanni,

 

What I was trying to say is that your meter is probably off now, because the battery is not 1.35 volts, it is 1.5 volts. But if that alkaline battery is brand new, and rated at 1.5 volts, it is probably putting out 1.55 volts or higher. Now after an hour of use, it might be putting out 1.5 volts, after two hours 1.45 volts, three hours 1.4 volts, four hours 1.35 volts and so on until it dies. It just keeps steadily losing power. So the voltage getting to the meter is going to be constantly changing.

 

With the old mercury batteries, when you first put the battery in, it was giving off 1.35 volts. After one hour, it was giving off 1.35 volts, after two hours it was giving off 1.35 volts, and so on and so on until it "suddenly" died. So the meter was getting a constant level of voltage, the level of voltage the meter was calibrated to.

 

The meter is now getting a fluctuating voltage, so if you try to compensate for the meter when the batteries are fresh, in a hour's time, your compensation will be wrong, and if you re-compensate (is that a word?), then an hour later it will be off again, and so on and so on.

 

Make sense?

 

-Tim

[Charles MacDonald]

Giovanni,

 

With the old mercury batteries, when you first put the battery in, it was giving off 1.35 volts. After one hour, it was giving off 1.35 volts, after two hours it was giving off 1.35 volts, and so on and so on until it "suddenly" died. So the meter was getting a constant level of voltage, the level of voltage the meter was calibrated to.

 

I have no idea of the size of the batery we are talking about here, but their are "zinc_air" bateries which have simalar output to the old Mercury bateries, although they have a shorter life.

 

They come with a tab over a hole, and depend on a supply of air to work, which means that is is sometimes nesissary to drill a small hole in the batery compartment door.

 

The major use is hearing aids, bateries for that application have fairly large air holes and a corespondingly shorter life than the ones made for camera use which are sold under the name WEIN cells.

 

Some folks will also use a silver cell and a voltage regulator made out of a forward biased Diode, but that is contriversial as the forward voltage drop of a diode although fairly constant is subject to current load and temperature.

[Clive Tobin]

...The meter is now getting a fluctuating voltage, so if you try to compensate for the meter when the batteries are fresh, in a hour's time, your compensation will be wrong...

 

Not only that, but the exposure will not track properly if the voltage is high. You will tend to get good exposure in dim light where the lens is nearly wide open, but you could be a couple of stops underexposed in bright light. So a simple ASA adjustment will not work.