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10-26-2007, 02:29 PM   #1
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TTL - P-TTL flash?

What exactly is the difference between TTL and P-TTL flash??
Would TTL external flash work with K100D? I'd imagine there would be some limitations, what would they be?
I don't own any external flash, and ain't big fan of a flash photography, but I'd still like to know...
would anybody be so kind and explain...
many thanx
Pete

10-27-2007, 11:24 AM - 1 Like   #2
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Classic TTL used light reflected off the surface of the film to determine flash power. All the DSLR manufacturers seem to have decided this was not workable on DSLRs (though the first 3 (?) Pentax models retained this system) so they now use pre-flash systems like P-TTL, iTTL, E-TTL etc. Essentially, the flash fires twice for every exposure. Once for metering (the very short preflash) and once for the exposure itself.

THe K100D does not support classic TTL so you would only be able to use a TTL flash in manual mode or (depending on model) in Auto mode. Auto mode uses the flash's own sensor to decide when to stop nuking your subject.

As a cheap alternative to P-TTL, a flash with an Auto mode might be your best bet. They are generally fairly accurate for indoor work. The drawback is that they won't do much thinking for you so outdoor fill flash and other special applications will be much less straightforward than with a more sophisticated system.

The most important thing to be aware of if you're looking around for cheap flash options is the trigger voltage. You should research any flash that looks interesting before buying because if you get it wrong you could easily fry your camera.

Matthew
10-27-2007, 12:08 PM   #3
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QuoteOriginally posted by Matthew Allen Quote
THe K100D does not support classic TTL so you would only be able to use a TTL flash in manual mode .....
Matthew
What exactly you mean manual mode?!?
I'm completely new to falsh, and everything that surrounds it...
thanx for explaining...
10-27-2007, 12:11 PM   #4
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just to expand a little on P-TTL and TTL

The reason that manufacturers went away from TTL for a DSLR was that the reflectivity of the sensor surface was different than film, and it was possible (so I have heard) to get exposure errors as a result, although I have always been extremely happy with TTL metering on my *istD.

P-TTL uses a different exposure sensor, not off the sensor, but off, I believe, the focusing screen. P-TTL takes one flash with the lens wide open, and therefore must know the wide open apature as well as control the lens apature electronically. It measures the exposure wide open with the preflash, then calculates the flash intensity required for the lens being stopped down. This is why K mount (i.e. manual apature lenses) do not work in flash mode on P-TTL cameras.

10-27-2007, 05:36 PM   #5
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Pardon my stupidity then, but I have a K100D and a Sigma 500DG Super, which is supposed to be PTTL compatible. When I use this I get proper exposures, but I don't notice any pre-flash. Is this flash actually being metered by the camera or merely by a sensor on the flash itself? The instructions with this flash are pretty vague. Perhaps the two flashes are very close together and I'm only seeing them as one, but I think that's not the case. The more they automate these things, the more difficult they are to understand.
10-27-2007, 05:50 PM   #6
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QuoteOriginally posted by G_Money Quote
but I don't notice any pre-flash. Is this flash actually being metered by the camera or merely by a sensor on the flash itself?
In P-TTL mode, you'll see the first flash in the viewfinder. This is because it's before the exposure, so the mirror is still down.

The camera does its metering based on what it reads from the return of the pre-flash. It tells the flash unit how much to output appropriately, based on the selected aperture, and then opens the shutter.

While the shutter is open the main flash fires, which you don't see because the mirror is up during the exposure.

TTL flash metering, by comparison, takes place during the exposure, and the flash output is quenched when the camera decides it's had enough light back.

If you are only getting one flash, it is not P-TTL.
10-27-2007, 06:15 PM   #7
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So it's really just behaving in an emulation mode, the flash doing its own metering then. Thanks.
10-27-2007, 06:37 PM   #8
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QuoteOriginally posted by axl Quote
What exactly you mean manual mode?!?
I'm completely new to falsh, and everything that surrounds it...
thanx for explaining...
In this era of modern photography we are accustomed to having either the camera or the flash unit figure out for us just what our shutter speed and aperture should be to properly expose a shot. It is so common that people typically take it for granted.

Light is light, whether it be from the sun, lamps, or a flash unit, and exposure has to be adjusted (as you already know) according to how much of it there is.

The problem with flash is knowing how much of it there is. And the problem with not knowing how much of it there is is that we can't adjust accordingly.

Modern flash units typically work in conjunction with the camera to meter the amount of light from the flash and adjust the output power of the flash to adjust everything so that all we have to do is push the shutter button. This automation function comes in many flavors and goes by many names, each taking a slightly different (and sometimes proprietary) route to get to the exact same place....proper exposure.

But what do we do when we don't have a flash unit that speaks the same "language" as our camera? For example, we have a TTL flash and a P-TTL camera. The two are not going to play nicely together and figure out our exposure and make the settings for us, so we can forget about that.

But......

The flash does have a "manual" option, which will fire the flash at its maximum power output. For folks only familiar with automated flash and automated aperture setting, a common complaint is that the flash overpowered everything and made a perfectly solid white photo. Look around, there are plenty such posts right here on Pentax Forums.

What we have to do in manual mode is figure out the appropriate aperture and set it ourselves. This is where we have to either use the handy scale on the flash unit (if it is old enough to have one) or do math using the flash's Guide Number.

If using a scale, your job is easy. Just set the ISO of your film, then read off the correct aperture for the distance to your subject.

If using G.N., you have to keep a couple of things in mind.

1. G.N. is expressed in terms of feet or meters. Know which you're dealing with.
2. G.N. is expressed in terms of ISO 100. If using something else, remember to adjust accordingly.

What you do with the Guide Number is to divide the distance into the Guide Number. The result is your correct aperture for a photo at ISO 100.

Example:

I have a flash with a (meters) G.N. of 28 and I want to take a photo of something 10 meters away: 28 / 10 = 2.8 so my aperture would be 2.8. Except I'm using a K100D and my minimum ISO is 200. The difference between ISO 100 and 200 is one stop, so I need to adjust my aperture one stop from f2.8 to f4 instead.

Even modern electronic whizbang flash units with a dozen buttons and an LCD display on the back can still be set for "manual" and used in just that way if necessary or if desired.

Many flashes give one the option of various levels of manual output, typically expressed in fractions: 1/2, 1/4, 1/8, 1/16, etc.

For those situations, you don't have to do any on the spot, in-your-head dividing by fractions. Just remember that each of those is one stop apart and adjust that way.

There is an alternative to all this, though. Enter the "Auto" flash.....more typically called an "Auto Thyristor" flash. This sort of flash will automatically adjust flash output without the need to communicate with your camera. You still have to set an aperture....but it covers a broad range of distances and uses its own sensor to determine when it has received back enough reflected flash to make the exposure right, then cuts off the flash output in midstream.

For example: A glance at the scale on my 25+ year old Vivitar 2500 shows me that at ISO 400, on the "Blue" setting I get f8 exposure with my subject between 1 and 5 meters. With the "Red" setting I get f4 exposure on my subject between 2 and 10 meters.

Or I can put it on "Manual" and do it all myself.

The main and large points that come out of all this long-winded filibustering is that just because a particular flash and a particular camera seem not to be compatible doesn't mean that they can't be made to work together in some fashion and that just because one has an ultra-modern state-of-the-art camera doesn't mean one has to pay top dollar for an ultra-modern state-of-the-art flash to use with it or go without flash altogether. There are alternatives.

10-27-2007, 08:50 PM   #9
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P-TTL Vs TTL

QuoteOriginally posted by axl Quote
What exactly is the difference between TTL and P-TTL flash??
Would TTL external flash work with K100D? I'd imagine there would be some limitations, what would they be?
I don't own any external flash, and ain't big fan of a flash photography, but I'd still like to know...
would anybody be so kind and explain...
many thanx
Pete
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