P-TTL settings for flash


RobL

Link Posted 10/05/2016 - 19:56
If I use any of the automatic settings like aperture priority or X-sync with a Pentax flash I notice that by default the exposures all minimise the amount of flash output at the expense of speed, aperture or ISO depending on which setting is used. As a result the camera will apply typically 1/50 sec with f2.8 and the maximum ISO. X-sync only appears to set the speed at 1/180 or 1/200 depending on the camera and again applies minimal aperture and maximum ISO. The only way over this is using manual. Surely an auto setting should give the optimum exposure?

McGregNi

Link Posted 10/05/2016 - 21:20
Rob, in automatic modes with flash the camera is aiming, where possible, to balance the flash brightness with the ambient (background) exposure. It tries to do this with the secondary aim of providing an exposure time that allows steady handholding.

To do this it works within some quite severe constraints ..... The focal length of the lens determines the shortest exposure time allowed ..... The lens itself provides the possible aperture choices. ... And the ISO figure, either fixed, or the possible ISOs available if auto-ISO is active ....

These are the constraints the system must work with. If there is sufficient available light to record at a time value that can be handheld then the system will try and record that .... If it's quite low light then it must record that by increasing the ISO and opening the aperture. This is what is giving you F2.8 and a high ISO, an attempt to record enough ambient light but steady for hand holding.

If enough ambient light can be recorded in this way then the system will aim to provide a secondary, supporting light source from the flash ... This is commonly known as 'fill in flash'.

Unfortunately this is where problems can start with automatic modes generally, and auto-ISO in particular. If you already have F2.8 and a high ISO for recording the ambient light, then it is very easy for flash to subject distances to be too small and cause overexposure and erratic flash results. The system has no way to reconcile these quite conflicting demands and simply fails. .... This leads to the very common complaints about P-TTL reliability.

The reality is that it is unrealistic to expect the technology to be able to make so many decisions .... Flash and ambient light balancing is no simple matter and needs many factors taken into account. The human brain is far more suited to manage all of this than a tiny camera brain I'm afraid, so there's no easy auto solution that will prove reliable in all situations.

In lower light, to try and prevent ISOs going too high, there is an override of the exposure time restriction available ..... This is known as ' Slow Speed Sync' and is a flash mode set in the flash mode screen. This will allow longer exposure times at any focal length, to record more ambient light. In low light you need to brace the camera though.

For great reliability and full control, I recommend no auto -ISO, ever, for flash work, and manual camera mode combined with P-TTL or manual flash exposures. Av can also be intuitive so long as you keep a close eye and grip on ISO and shutter speeds for ambient/flash light balancing.

If there's no ambient light to record , or you don't want any to record, then you can shoot at short exposure times up to 1/180th for steady shots and the P-TTL system will revert to trying to illuminate the whole scene with flash light. But again, a fixed ISO complementary to the distances involved or bounce surfaces is going to give better results.

One final point .... Everything here ONLY applies to autofocus lenses. P-TTL with automatic camera exposure modes is only compatible with autofocus lenses. If you are using a manual focus lens then no consistency can be expected.
My Guides to the Pentax Digital Camera Flash Lighting System : Download here from the PentaxForums Homepage Article .... link
Pentax K7 with BG-4 Grip / Samyang 14mm f2.8 ED AS IF UMC / DA18-55mm f3.5-5.6 AL WR / SMC A28mm f2.8 / D FA 28-105mm / SMC F35-70 f3.5-4.5 / SMC A50mm f1.7 / Tamron AF70-300mm f4-5.6 Di LD macro / SMC M75-150mm f4.0 / Tamron Adaptall (CT-135) 135mm f2.8 / Asahi Takumar-A 2X tele-converter / Pentax AF-540FGZ (I & II) Flashes / Cactus RF60/X Flashes & V6/V6II Transceiver
Last Edited by McGregNi on 10/05/2016 - 21:49

McBrian

Link Posted 11/05/2016 - 06:50
^^^^^

That's a superb explanation Nigel.

Having not explored it, does the selected program line (Normal, MTF , Speed, etc. ) have an influence on shutter speed/aperture selection in P-TTL flash?
Cheers
Brian.
LBA is good for you, a Lens a day helps you work, rest and play.

RobL

Link Posted 11/05/2016 - 08:34
Thanks very much Nigel, I was beginning to think there was something wrong with my equipment.

McGregNi

Link Posted 11/05/2016 - 11:11
No problem Brian and Rob ... Glad it was helpful My flash guide is an ongoing project so this issue will form the basis of a new section dealing with automatic modes and behaviours in the next edition, so I always appreciate having the points raised.

To be honest there's very little authoritive documentation on theses questions, certainly not in the manuals of the equipment concerned. I don't know at this stage what direct influence a program line setting would have in the flash scenarios we're talking about. There may have been some testing done previously and reported on, and I'm interested in getting more information out there via my guide, so I'll certainly look into it.

I may be able to get some testing done at PentaxForums with different cameras to be sure the behaviours are the same. I understand that the program line only applies to P and green modes, is that right? My instinct is that it might exert a small influence with flash, but the shutter speed restraint based on focal length is likely to be the overriding limitation, and that in order to get the ambient light balanced with the flash then we would still see increases in ISO and wide apertures to do this....

But if anyone knows more for sure, please let us know!
My Guides to the Pentax Digital Camera Flash Lighting System : Download here from the PentaxForums Homepage Article .... link
Pentax K7 with BG-4 Grip / Samyang 14mm f2.8 ED AS IF UMC / DA18-55mm f3.5-5.6 AL WR / SMC A28mm f2.8 / D FA 28-105mm / SMC F35-70 f3.5-4.5 / SMC A50mm f1.7 / Tamron AF70-300mm f4-5.6 Di LD macro / SMC M75-150mm f4.0 / Tamron Adaptall (CT-135) 135mm f2.8 / Asahi Takumar-A 2X tele-converter / Pentax AF-540FGZ (I & II) Flashes / Cactus RF60/X Flashes & V6/V6II Transceiver
Last Edited by McGregNi on 11/05/2016 - 11:11
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