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04-06-2019, 10:31 PM   #1
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How is P-TTL supposed to work? How can I tell if it's working?

I just bought a Godox TT350 flash for my Pentax K-1. The instruction manual for the flash says that the TTL function should work with this camera. I have never used a flash before, but from what I've read is that when using TTL, the camera is supposed to look at the brightness of the scene with a test flash and then adjust the shutter speed when the actual photo is taken. However, when have my flash set to TTL mode, with the camera in aperture priority, the shutter speed is always 1/80, no matter how bright the scene is. Naturally, sometimes the scene is too dark and sometimes it is too bright.

04-07-2019, 12:58 AM   #2
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No, what you've said's not right.

TTL will only change the flash power, never the shutter speed or ISO or aperture.

04-07-2019, 01:10 AM   #3
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QuoteOriginally posted by multivac Quote
I just bought a Godox TT350 flash for my Pentax K-1. The instruction manual for the flash says that the TTL function should work with this camera. I have never used a flash before, but from what I've read is that when using TTL, the camera is supposed to look at the brightness of the scene with a test flash and then adjust the shutter speed when the actual photo is taken. However, when have my flash set to TTL mode, with the camera in aperture priority, the shutter speed is always 1/80, no matter how bright the scene is. Naturally, sometimes the scene is too dark and sometimes it is too bright.
When using a flash in P-TTL mode and Av mode (aperture priority), the camera will measure the pre-flash and then adjust the flash output to get a correctly exposed subject. The shutter speed will have no affect on the light falling on the subject from the flash because the flash fires in excess of 1/1000 second.

When your scene is also lit by ambient light the camera will attempt to give a balanced lighting in any of the auto modes, ie it will operate as a fill flash.

I find manual exposure mode works best for full control. use the shutter speed at sync speed (1/180 ) or below, to control the ambient light and the flash power in P-TTL to light the subject
04-07-2019, 02:52 AM - 1 Like   #4
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QuoteOriginally posted by multivac Quote
I just bought a Godox TT350 flash for my Pentax K-1. The instruction manual for the flash says that the TTL function should work with this camera. I have never used a flash before, but from what I've read is that when using TTL, the camera is supposed to look at the brightness of the scene with a test flash and then adjust the shutter speed when the actual photo is taken. However, when have my flash set to TTL mode, with the camera in aperture priority, the shutter speed is always 1/80, no matter how bright the scene is. Naturally, sometimes the scene is too dark and sometimes it is too bright.
QuoteOriginally posted by pschlute Quote
When using a flash in P-TTL mode and Av mode (aperture priority), the camera will measure the pre-flash and then adjust the flash output to get a correctly exposed subject. The shutter speed will have no affect on the light falling on the subject from the flash because the flash fires in excess of 1/1000 second.

When your scene is also lit by ambient light the camera will attempt to give a balanced lighting in any of the auto modes, ie it will operate as a fill flash.

I find manual exposure mode works best for full control. use the shutter speed at sync speed (1/180 ) or below, to control the ambient light and the flash power in P-TTL to light the subject
I too am new to using flash/speedlights I would suggest to follow this link to mcgregni post on "Pentax Flash" and download his comprehensive guide.
Comprehensive Pentax Flash Guide - PentaxForums.com

Here is the link to the article posted on this forum: https://www.pentaxforums.com/articles/photo-articles/comprehensive-pentax-flash-guide.html


Last edited by SharkyCA; 04-07-2019 at 02:55 AM. Reason: add a link and update message.
04-07-2019, 03:49 AM   #5
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First thing I would suggest is to make sure your K-1 flash mode setting is set to 'Auto Flash Discharge', or 'Flash On' if you want flash all the time (p-65 of the manual).

Then set the camera mode dial to 'X' mode, which fixes the shutter speed to the flash sync speed. Then mount the flash, turn it on, set your Godox flash to Auto TTL (or whatever Godox calls the auto setting), and then half-press the K-1 shutter button a bit to get the flash and the camera to briefly introduce themselves to each other (sometimes you will hear a beep, or the flash head zooming a bit, which shows they recognise each other). From that point P-TTL should work, so fire away..

Last edited by rawr; 04-07-2019 at 03:54 AM.
04-07-2019, 08:15 AM   #6
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Have you checked the flash is set to TTL and not M ?
04-07-2019, 08:57 AM   #7
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Welcome to the Pentax Forums!

You are correct that P-TTL works by taking a light measurement from a short preflash and applies that measurement to determine the flash exposure. Where there is a disconnect is in regards to how flash exposure works. Here are a few points regarding flash, P-TTL, and your camera...
  • For most cases,* shutter speed does not affect flash exposure. It is only a consideration for the portion of the exposure contributed by ambient light (the amount of light on the subject other than the flash).
  • Flash exposure is determined by distance, flash power (duration, actually), lens aperture, and ISO.
  • The total exposure of the photo is a combination of ambient and flash exposure
  • Within limits, P-TTL flash is capable of balancing flash vs. ambient light exposure as well as supporting several special creative features. What features are supported depends on the flash being used and the camera mode. Your flash may not support all of the possible range of features.
  • "Fresh out of the box", your Pentax camera should provide a fairly seamless P-TTL flash experience in all modes using default settings. The same is true of your flash.
  • As noted above, your K-1 provides several flash settings. Which settings are available depends on camera mode. The K-1 manual is helpful.
  • Your flash supports several modes as well. Consult its manual as to how those are used.
  • The general rule is that your camera will not trigger the flash at shutter speeds higher than 1/200s unless the flash supports and is configured for so-called high speed sync (HSS). For auto exposure modes, your camera will automatically set a shutter speed at or below 1/200s if a P-TTL flash is detected and is ready to discharge.
  • On the K-1, you can set whether to always have a mounted flash discharge or to let the camera decide whether flash is needed. The setting may be toggled either way by either the record menu or the control panel. If the flash does not fire in bright conditions when desired, check that the setting is to always fire.
  • Remember that distance and maximum flash power will limit the flash range as might bounce technique, filters, flash modifiers, and use with wide-angle lenses. On the other side of concern, there are limits to minimum power as well as maximum. As a result automatic flash control with P-TTL may not work as expected when doing close-up and macro photography.
Of course there is a lot more (flash photography is a deep subject), but this should be a good start. Have fun and don't sweat the details.


Steve

* When the flash is configured for high speed sync (HSS), shutter speed does effect the flash exposure at speeds higher than 1/200s. This is something to keep in mind if using HSS.

Last edited by stevebrot; 04-07-2019 at 09:52 PM. Reason: grammar and proper terminology
04-07-2019, 06:06 PM   #8
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QuoteOriginally posted by multivac Quote
I just bought a Godox TT350 flash for my Pentax K-1. The instruction manual for the flash says that the TTL function should work with this camera. I have never used a flash before, but from what I've read is that when using TTL, the camera is supposed to look at the brightness of the scene with a test flash and then adjust the shutter speed when the actual photo is taken.
As everyone else is mentioning, the flash's TTL setting adjusts the flash's power setting (within the flash's limits). Your camera's exposure mode still does the adjusting of iso, aperture, and shutter speed.

With flash, however, exposure isn't as simple as the triangle with ambient-only photography.

In flash, there are two sources of light: the flash, and the ambient (all the light in the scene that isn't from the flash). And the two sources have different controls.

Ambient exposure is controlled by iso, aperture, and shutter speed.

Flash exposure is controlled by iso, aperture, flash power, and flash to subject distance.

[this is assuming you're at a shutter speed of sync speed or slower. At higher shutter speeds, with HSS, shutter speed can affect the power output of the flash; at sync speed and slower, the flash burst is so brief that it's shorter by a good ways than your shutter speed will ever be, so leaving the shutter open for longer won't grab you any more light from the flash, just the ambient].

And the differences between these two sets of controls mean that you can balance the flash against the ambient however you like. So long as you're within the power limits of the flash, you could go all the way from a black background with a lit subject (aka "killing the ambient") all the way to a silhouetted subject with a well-exposed background and everything in between. So it's not about getting the needle on your meter to "0" (or thereabouts), as with ambient-only photography. If you wanted to kill the ambient, you'd probably be around -5EV on the ambient and that's where you want to be.

QuoteQuote:
However, when have my flash set to TTL mode, with the camera in aperture priority, the shutter speed is always 1/80, no matter how bright the scene is. Naturally, sometimes the scene is too dark and sometimes it is too bright.
Basically, how you know TTL is working is if you shoot something with the flash, and you get good flash exposure, and then you move closer / farther to your subject, or adjust the iso or aperture setting on the camera, shoot another shot and get the same flash exposure. TTL lets you drag your iso and aperture as well as your shutter speed.

But. If you're overexposed or underexposed in the ambient, the image will probably also look over/under exposed.

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