How do I use this Portaflash Flash Meter?

Hi.

 

I purchased a used Portaflash Flash Meter but it came with no

instructions.

 

I'm smart enough to figure out that I put this meter by the subject

I am photographing (still life - plants, inanimate objects) and fire

a test flash. I then read it and adjust my aperture accordingly.

This makes sense, but how do I know what shutter speed to select???

 

For example, I received a reading of f5.6 +2/3 @ 100iso. Ok, I

understand this means that I set my aperture at f5.6 or even f8, but

should my shutter speed be set at my camera's maximum X-sync speed?

 

Any help would be greatly appreciated, I just want to make sure I'm

using this correctly, or if my meter is defective.

 

Thanks in advance!

-James

james,

 

A flash meter, used in flash measuring mode, measures only the light created by the flash. You need to use a shutter speed that is slow enough for your camera's shutter synch speed and slow enough for the flash duration of your flash. You can use a slower shutter speed to catch any ambient light that you like, such as light fixtures in an interior space or candle flames etc.

To expand on what Brooks said...

 

You can select any shutter speed you want as long as it's not faster than the top sync speed for your camera.

 

If you use a slow shutter it may also include some of the ambient lighting. The slower the shutter, the more ambient lighting is included. That is often desirable.

 

If you don't want any ambient lighting to be seen in the photo, make sure you are using a shutter that is fast enough to eliminate it. Using a regular light meter, set the shutter speed to 3 stops faster, or more, than the meter recommends for the ISO and aperture chosen.

James,

 

This is how I like to look at the missing shutter speed issue. There are two sources of light when you are using flash: the non-flash ambient portion and the flash portion.

 

Your camera or hand-held ambient meter will allow you to measure the ambient light. You can, for instance, measure that light at your sync speed (say 1/125 for example) and get your ambient reading (say f2.8 @ 1/125).

 

Now back to the flash. Most electronic flash units, whether portable or studio units, have a flash duration of 1/400 of a second or less, and often less than 1/2000 of a second. Unless you are using a leaf shutter lens where you can sync flash at up to 1/1000 of second, the flash has a duration shorter than the period the shutter is open. In a darkened room where the flash is essentially the only light source, you'll get the same film exposure at all syncable speeds because the shutter speed is essentially the flash duration.

 

Now to the readings together. The perfect flash meter measures just the light contributed by the flash burst, discarding the contribution of the ambient light and then calculating an F-stop that would give you correct exposure for a flash pulse of that amplitude and duration. For the sake of argument, let's say your flash meter gives you a reading of F5.6. If you set you camera to 1/125 of a second at F5.6, you will be getting the f5.6 contribution from the flash correct, but you'll also be getting the F2.8 contribution from ambient light. The ambient light is 2 stops or 4 times less bright than the flash, so it will contribute some light but not all that much. If fact. the camera should theoretically be set to F5.6 and just a hair If instead you took your same ambient reading and set your camera shutter to 1/30, you'd have to set you camera aperture to F5.6 for a correct ambient exposure. Now the flash and ambient light will make equal contributions to the exposure. In theory, you should then set set your aperture to F8, because you have 2 X f5.6, or twice as much light. I hope you get the picture and I haven't added confusion,though I probably have. This is sort of hard to understand, and I'm not so sure I'm helping. The bottom line is, when the ambient contribution is small in comparison to the flash, you can just set your camera to the max sync speed and use the flash meter to set the aperture. If you are using slide film however, you may have to be a bit more exact.

 

One last note. No flash meter is perfect. In fact, many flash meters are not that good at separating the flash portion of the light from the ambient light, especially when balancing flash with bright sunlight. The readings you receive under those conditions can result in overestimated flash output. The flash meter readings you get in a normally lighted studio setting should be fine with your Portaflash.

Here is a <A HREF="http://www.shutterbug.net/buyers_guide/lightmeters/charts/flash.html">flash meter chart</A> that list the PortaFlash flash meter was designed for 1/60s shutter speed at ISO100.

Firstly, my thanks to (in order of contribution) Brooks, Jim, Barry and Philip for their replies.

 

Secondly, according to the link at Shutterbug provided by Philip, it states that the PortaFlash has a 1/60 shutter speed. So, my question is that since my camera has a maximum flash sync speed of 1/30, would I have to double the f-stop opening to compensate? eg. if the Portaflash recommends f5.6, would I use that, or f8/f11?

 

I guess you all could tell I'm a real newbie when it comes to flash photography!

 

Thanks again!

James

James,

 

You can use whatever f-stop your meter indicates for flash at 1/60 or any slower shutter speed. As you lower your shutter speed, you will pickup more ambient light if there is any.

 

Why don't you shoot a roll of film at various shutter speeds using your flash and flash meter at the indicated f-stop, in a dark room and then repeat the same series of shutter speed exposures in a room with some ambient light. Then you'll be able to see for yourself what happens as you use slower shutter speeds.

Unless you are in a dark room, there is no doubt the flash meter reading will be affected by ambinet light level. The problem is more pronounced when flash is more or less equal to ambient light level.

<BR>Here is <A HREF="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=0018GG">a similar discussion</A> on the use of <A HREF="http://www.butkus.org/chinon/luna-pro_f/luna-pro_f.htm">Gossen Luna-Pro F</A>.

<BR><BR><B>I wonder how the compensation figures are calculated. Anyone?</B>