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01-04-2010, 12:08 AM   #1
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Help: Using Flash with M Lenses (no A position)


Would you please show me how to use flash with M lenses. I have a Sunpak 383 and a Pentax AF-540FGZ.

Thank you.


01-04-2010, 03:04 AM   #2
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For flashes in general:

If the flash has an "auto" setting, then there will be some indicator of the aperture you should use. The flash will fire and a sensor on the front will determine when sufficient light has been reflected from the subject to be right for the aperture you set. This type is convenient because it works automatically across a range of subject distances.

If the flash has no auto setting, then you have to choose some manual flash output level. Some flashes let you choose from a range of outputs. Some have many, some have a couple, and some have only full power output.

If you have to work with manual power settings, flashes typically have some sort of scale on the back that indicate the appropriate aperture for a given distance. Refer to this and set your camera's aperture accordingly.

If you know the "Guide Number" (a number that indicates the flash output power and is expressed in either feet or meters and assuming ISO100) then you can do a little simple math and determine your shooting aperture.

Divide the Guide Number (GN) by the distance from the flash to your subject and the result is your aperture. For example, at ISO100 and with a GN16 (meters) and a subject 2 meters away you get 16 divided by 2 = set your camera on f8. If the subject is 4 meters away, 16 divided by 4 = f4.....see how it works?

Of course, you can use any ISO setting you want to. Just remember that each doubling of the ISO equals a one-stop adjustment to the aperture as well. So for the above examples at ISO200 the first would be f11 and the second would be f5.6. For ISO400 you would get f16 and f8.

The shutter setting must be at or slower than the flash synch speed of 1/180. As far as the light from the flash itself is concerned, it really doesn't matter what it is. The duration of the flash effectively becomes your shutter speed. Imagine for example that you take a shot in a pitch-black room with the flash. So long as the aperture is alright, it really doesn't matter if your shutter is open 1/180 second or 180 seconds....all the light that is affecting the exposure is happening in that ultra-fast burst of light from the flash. The shutter speed does matter, though, for balancing ambient light (all light that doesn't come from the flash, in other words). You can leave the shutter fast to minimize the ambient light, or you can slow it way down to include more of it into the photo.

Here are a couple of extreme examples to illustrate the point about the shutter speed:

The first shot is my hat on a fencepost, taken in the late afternoon....still daylight outside. The flash was placed close by (off camera) and the shutter was at the max for flash, 1/180. The flash output and distance to the subject was close enough that the aperture I used was about f16. Any photo taken without flash would have been just about pitch black, as that would be roughly 5 stops underexposed for that time of day. So the result was that the hat received the right amount of light for that situation and everything else was black.

The second shot was a night time shot and the shutter was open for 15 minutes. The lamp post and the sides of the bridge would have been in shadow even at that long of an exposure, even though the full moon was enough to light up the rest of the scene at a 15 minute exposure. I put a flash inside the lamp post and fired it twice and then crossed the bridge twice with the flash on the end of a monopod and stuck out over the side of the bridge, crossing the bridge twice and firing it at the side a total of 20 times to light it up.

So you can see, by playing with the shutter speed and including or excluding ambient light you can have quite a bit of creative control over your scene. Also, remember that if you don't like the depth of field a given aperture gives you for a flash exposure you can monkey with the ISO to get a larger or smaller aperture.

I hope this is helpful and I am sorry I can't give any information specific to the models of flash you have, as I am not familiar with them.
01-04-2010, 12:28 PM   #3
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Original Poster
Thanks Mike. Your reply is a wealth of information, and your examples are amazing. I will try it out later and post some flash photos. Thank you.


01-05-2010, 01:07 AM   #4
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Now, there is a trick for enabling P-TTL with manual lenses, but it involves:
- drilling another locking hole in the lens, so it locks halfway in the socket and behaves like a m42 stop-down lens (that or just mount it halfway, but beware not to unmount it accidentaly)
- shorting the data & A pins to trick the camera into thinking there is an A lens.
- selecting the widest available aperture on the camera (NOT the lens!)

I still wonder why Pentax has not changed their P-TTL algorithm to solve this, as it would be ridiculously easy to do : just do the P-TTL preflash after stopping down the lens, and not before!

01-05-2010, 01:11 PM   #5
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QuoteOriginally posted by dlacouture Quote
I still wonder why Pentax has not changed their P-TTL algorithm to solve this, as it would be ridiculously easy to do : just do the P-TTL preflash after stopping down the lens, and not before!
I don't know the exact reasons. But this can be one: with the lens stopping down, the pre-flash strobe may not supply enough light for metering. Note that the pre-flash strobe has to be weak so that it does not drain too much energy for the main strobe.

Back to the original question: unless there is a real need to use p-TTL, I prefer flash auto mode (using the flash's light sensor): just match the ISO and the aperture settings on the flash with the settings on the camera and lens and shoot. The exposure is then much more reliable than p-TTL.

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