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01-02-2011, 09:33 PM   #1
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Flash help

I recently bought a Pentax MX with several lenses and 2 flashes thrown in. I am not new to photography, but I have to admit I never really used any flash that wasn't part of the camera already.

Is there a website where I can get some information on working with older film flashes. I am concerned with metering right now since the flash is totally separated.

Any help would be great, thanks.

01-02-2011, 10:37 PM   #2
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Before we start:

Be aware of trigger voltage if you intend to use these flashes with your dSLR. A high voltage can fry your camera. Search the web for 'trigger voltage' in combination with your make and model of the flash to see if it's safe to use with your camera.

To the question:

They say that the strobist is a good site; just do a search for it on the web.

The basics of flash are quite simple. The distance that you can flash is defined by the guide number divided by the aperture. So a GN of 14 and an aperture of 2.8 will properly expose a subject at 5 meters. Behind it will be underexposed and in front of it will be overexposed; this is, by the way, with any flash (internal or external).

If your flash is an automatic flash, it will measure the amount of light reflected from the subject and cut off the light at the moment that it thinks that the subject is properly exposed. So with your GN of 14 and aperture of 2.8, your subject can be closer and still be correctly exposed. In this case there will also be a minimum distance at which a subject can be properly exposed; as a rule of thumb, it's about 10% of the calculated distance. So a subject closer than that will be overexposed.

And if the flash supports TTL, the camera will determine correct exposure (based on reflection of the light from the film) and cut off the light when it thinks correct exposure is achieved.

You can use any shutterspeed longer than or equal to the sync speed (1/180 on Pentax dSLRs). With that you can influence how much ambient light will be incorporated in the picture.
If you need faster shutterspeeds, your flash needs to support someting called 'high speed sync'.

PS
The guidenumber is usually specified as ISO100. If you set your ISO to 400, the GN will double and at 1600 it will double again.

Last edited by sterretje; 01-02-2011 at 10:42 PM.
01-02-2011, 11:23 PM   #3
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A couple of clarifications, if I may:

If the flashes are from the film era, they may support TTL. If you plan to use them on a modern digital Pentax that is useless (except on one body that went out of production some years ago) as the new protocol is "P-TTL". They are NOT the same thing and they are not compatible with each other. Too many people offhandedly refer to P-TTL as TTL as a sort of verbal shorthand, not considering that not everybody knows the difference. It is important to always make the distinction when talking about the two. Other modes on the flash units, such as auto and manual, are still perfectly usable.

A more practical application of the GN math is to determine aperture based on subject distance, rather than to figure out shooting distance based on aperture. The math is similar: divide the GN by the subject distance to get your aperture. Set that aperture on your camera and set your shutter speed to sync speed (or slower).

A final note is that depending on the system in use in a particular country GN can be expressed in terms of feet or meters. Either way is fine....just know which one you're using so you don't mix units when doing math. The steps between guide numbers are sort of funny and work like the steps between aperture numbers, with each two steps making a numerical doubling and individual steps being roughly the previous number times 1.4 (square root of 2). So you'd have something like GN7 -> GN10 -> GN14 -> GN20 -> GN28 -> GN40 etc. Each step is one EV difference of exposure, or one doubling of the light, but not of the number.
01-02-2011, 11:43 PM   #4
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To be clear I have NO plans of using the flashes on anything but my older manual film cameras. Also neither flash is TTL.

Thanks for the tips though, I am already learning some stuff.

01-03-2011, 12:13 AM   #5
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In that case:

If they have auto modes, just match up the ISO (ASA) on the flash unit with whatever film you're using, note the aperture indicated, and set that aperture on your camera. Set shutter speed to sync speed or slower. For flashes with auto modes you will always find some indication on the back of the flash about the range of distances for each auto setting. It is typical to have two modes, with each having a line under the distance scale in different numbers. Usually red and green or red and blue. Pick one, make sure your subject is within the indicated range of distances, and you're good to go.

If using manual modes, match up the ISO on the flash unit with your film, then either divide subject distance into GN to get your aperture or, more easily, make use of the handy-dandy scale that practically all of those older flashes so conveniently provide and read your aperture off of that. Set that aperture into your camera, put shutter speed to sync speed or slower.

Nothing at all hard about it, and nothing to meter except that every now and then you might have to eyeball guesstimate the distance to your subject....total precision isn't necessary.

Last edited by Mike Cash; 01-03-2011 at 12:20 AM.
01-03-2011, 10:27 AM   #6
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Great Site For Flash Photography.
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