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12-29-2011, 09:16 AM   #1
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Off-camera cord question

Hi Pentax community,

this Christmas, I received a flash (Vivitar DF-283-PEN) for my Pentax K20D. I am new to flashes and trying to understand the TTL and the P-TTL differences. I also received a JYC Universal TTL Off Camera Shoe Cord.

1- When I have my flash on my camera (directly), in Av mode, the shutter speed is automatically setup depending on my zoom (ranges between 1/60 to 1/125), which I presumes is the P-TTL function.

2- But when connected to the JYC off-camera cord, the Av mode does not receive feedback from the flash, the shutter speed will be as if the flash was not connected/detected. But the flash will trigger when taking a picture, which means the signal is being sent to the flash (TTL-function).

Questions:

a) Is understanding of P-TTL and TTL correct as above ?
b) How can I know if in condition #2, my flash is outputting always maximum or the camera is controlling the output level ?

Thank you for your help

12-29-2011, 09:38 AM   #2
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you have a pentax dedicated flash, but a universal cord.

so, the cord cannot give all the data needed for the flash to work in p-ttl.

the k20d does not have ttl, just p-ttl.

so, the cord is only sending a trigger signal and the flash will only work in whatever mode you have it set.

you need to spend a bit more to buy a dedicated pentax flash cord, then the flash will work with all functionality.

i bought one of the cheap ones from ebay and it works fine.
12-29-2011, 08:18 PM   #3
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  • a) With a DSLR, the original TTL requires additional photo diodes, measuring the light reflected from the sensor surface. This proved to be unreliable and too expensive, so all manufacturers have skipped this feature some time ago. They have all replaced it by systems called P-TTL (with Pentax), E-TTL, etc. Now the flash will fire twice; once (with reduced energy) for the measurement, then the real one for the exposure. So they can use existing light sensors. So, if the flash does not fire twice, P-TTL does not work.
  • b) The company Metz (a German company producing flashes for all brands, since flashes exist) generally does not recommend using extension cables, as there are HF signals transferred, which need sophistated shielding and drilling of the multiple lines in the cable, otherwise signals may get malformed.
The fact that the camera is temporarily correcting exposure time, has nothing to do with P-TTL. The camera is doing this when it receives an analog flash ready signal at the first additional contact (which was introduced with the M-series - ME or ME Super). Pentax hasn`t changed the function of this contact since more than 30 years. Every Pentax-dedicated flash ever built offers this signal. And every Pentax camera with an electronic controlled shutter since the Super Program/Super A reacts on it.

Last edited by RKKS08; 12-29-2011 at 08:26 PM. Reason: Some wrong information removed
01-03-2012, 02:00 PM   #4
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QuoteOriginally posted by kaarde Quote
1- When I have my flash on my camera (directly), in Av mode, the shutter speed is automatically setup depending on my zoom (ranges between 1/60 to 1/125), which I presumes is the P-TTL function.
Actually, in Av mode the camera is setting the shutter speed so that the flash will balance with the ambient light. It's probably just that as you're zooming the overall reflectance value of the scene is changing so the camera is raising or lowering the shutter speed so that half the light comes from the scene, and half will come from the flash when it is fired. Try focusing on an evenly lit, solid colored wall. Stand reasonably close so that when you zoom in and out you still have nothing but wall in the frame. Does the shutter speed change?

I don't really advocate using Av mode with flash though because it relies on reflectance values, so if you have a particularly light or dark or shiny subject, the camera, and therefore the flash can be easily fooled. Try this:
1) With the flash on your camera and turned on, set your camera to Manual mode, set it to matrix metering, and compose your shot.
2) Now, without moving the camera, select the AF point in your viewfinder that lines up with your subject, the camera will give more "weight" to the exposure value of that point. Hit the green button.
3) Now check, is the aperture/shutter speed something you can live with? If not, hit the AE Lock button and adjust either the shutter speed, or the aperture and the other variable will move in the reciprocal direction to maintain correct exposure. Now take the shot.

That should give pretty good results for an average scene/subject, but if the background looks to bright or too dark, just adjust the shutter speed. It will not have moved because you are in Manual mode. If the subject looks too bright or too dark but the background looks ok, adjust the flash power either on the flash itself, or in the Fn menu on the camera. If both subject AND background look too bright or dark, adjust the aperture or ISO.

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