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06-01-2019, 07:10 PM   #1
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Flash exposure using PTTL

Hi, I recently purchased a Yongnuo 585ex to use with my Pentax K5ii. It's my first foray into decent flash photography, and one of the reasons I went the 585 is the appeal of PTTL and auto exposure of the flash. As I understand it, in PTTL mode (using AV mode on the camera) the system should adjust itself to give you correct exposure for each shot. However, my flash seems to be behaving more like manual unit - in portrait shooting, direct flashes tend to wash out the subject, and bounce flashes often under expose. I have my flash set to PTTL mode, Flash comp. at 0 and exposure comp. also at 0.

Is there something I'm doing wrong and/or don't understand about flashes and PTTL?

06-01-2019, 08:09 PM   #2
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Make sure you're not using old manual lenses (without the A setting - is the 42 in your moniker a hint as to what you're using?) and remember you decide with the flash exposure dial whether to go up or down a stop or two in compensation.
06-01-2019, 08:35 PM   #3
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The 42 is just a coincidence haha. So far I've tried with the 18-135 and the 35mm 2.4. These tests were usually from a distance of about 1 metre - is that too close for the flash to work properly??
06-01-2019, 10:43 PM   #4
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Try setting the camera to X or Green mode. See if that works better.

Also if you mount your 18-135, then mount the flash, then power up camera, then turn on flash, then gently press the shutter button to get the camera to start talking to the flash, then start zooming the lens in and out. You should hear the flash head adjusting to the current lens focal length.

The simplest cause of many flash problems is often a poor connection between flash and camera. Check that there is no crud on your flash or hotshoe contacts, and the flash is sitting snugly on the hotshoe.

06-01-2019, 11:43 PM   #5
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QuoteOriginally posted by kman42 Quote
The 42 is just a coincidence haha. So far I've tried with the 18-135 and the 35mm 2.4. These tests were usually from a distance of about 1 metre - is that too close for the flash to work properly??
Usually, yes. Until you learn and get a feel for these things, I recommend you set the flash power manually, starting at maybe 1/8 and then adjusting from there.

Only *you* know if you are lighting the whole scene or just the subject, the camera doesn't.

When you get the hang of it (especially the ISO settings) return to P-TTL mode and ride the FEC dial.

06-02-2019, 02:59 AM - 1 Like   #6
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QuoteOriginally posted by kman42 Quote
Hi, I recently purchased a Yongnuo 585ex to use with my Pentax K5ii. It's my first foray into decent flash photography, and one of the reasons I went the 585 is the appeal of PTTL and auto exposure of the flash. As I understand it, in PTTL mode (using AV mode on the camera) the system should adjust itself to give you correct exposure for each shot. However, my flash seems to be behaving more like manual unit - in portrait shooting, direct flashes tend to wash out the subject, and bounce flashes often under expose. I have my flash set to PTTL mode, Flash comp. at 0 and exposure comp. also at 0.

Is there something I'm doing wrong and/or don't understand about flashes and PTTL?
I had a problem with a new Yongnuo with PTTL and downloaded/installed their latest firmware. It works fine now.
06-02-2019, 04:01 AM - 1 Like   #7
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Thanks all for the advice. I will check the firmware on my flash. Also, I did some more shots with the flash and realised that I often put ISO into manual mode without thinking about it. Since aperture is also set manually, and the shutter speed was maxed at flash sync speed, the camera had nowhere to go and was forced to over expose. I'll have to keep practising and also try manual flash.
06-02-2019, 04:42 AM   #8
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QuoteOriginally posted by kman42 Quote
Thanks all for the advice. I will check the firmware on my flash. Also, I did some more shots with the flash and realised that I often put ISO into manual mode without thinking about it. Since aperture is also set manually, and the shutter speed was maxed at flash sync speed, the camera had nowhere to go and was forced to over expose. I'll have to keep practising and also try manual flash.
Yeah, Youtube tutorials and advice from us on the Internet is well and good but don't help you put the hours of practice in!



06-02-2019, 05:18 AM - 1 Like   #9
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QuoteOriginally posted by kman42 Quote
Thanks all for the advice. I will check the firmware on my flash. Also, I did some more shots with the flash and realised that I often put ISO into manual mode without thinking about it. Since aperture is also set manually, and the shutter speed was maxed at flash sync speed, the camera had nowhere to go and was forced to over expose. I'll have to keep practising and also try manual flash.
Have a look on the flash display. As you change aperture there should be a change in the distance display which will show a range in metres or feet where your subject will be correctly exposed. Beyond the max range the flash is not powerful enough to light the subject, and below the min range the flash cannot lower its output any more and you will get overexposure. As you open the lens up you will get a longer range, and vice versa as you stop down.

For most predictable results try Manual exposure mode on the camera. Set the shutter speed to your camera's sync speed 1/180 (or slower if you want to record ambient light in your shot too). Choose an ISO and aperture that is also suitable for any ambient light you want in the scene. Then switch on the flash and look at the range display. Shooting a subject from 1 metre will be a challenge.
06-02-2019, 06:25 AM   #10
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One more thing.... if your are using auto iso change to setting your own. Even with more modern cameras sometimes I have problems with flash and auto iso. I almost always use PTTL even with macro and with the flash 12" or less from the subject (with a flash bender or some sort of modifier). and get consistent results though i do change the flash power as needed and sometimes change the iso.
06-02-2019, 06:42 AM   #11
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QuoteOriginally posted by kman42 Quote
Hi, I recently purchased a Yongnuo 585ex to use with my Pentax K5ii. It's my first foray into decent flash photography, and one of the reasons I went the 585 is the appeal of PTTL and auto exposure of the flash. As I understand it, in PTTL mode (using AV mode on the camera) the system should adjust itself to give you correct exposure for each shot. However, my flash seems to be behaving more like manual unit - in portrait shooting, direct flashes tend to wash out the subject, and bounce flashes often under expose. I have my flash set to PTTL mode, Flash comp. at 0 and exposure comp. also at 0.

Is there something I'm doing wrong and/or don't understand about flashes and PTTL?
Having a good picture with flash is more than simply putting the unit on the camera and expecting it to give exactly the intended results, no matter if it's P-TTL or not. You should give a good read or two at Strobist (mostly manual flash) and NeilVN Tangent's (lot of good tutorial on TTL falsh) websites to learn the basics.

But, as a crash course, what you should do is:

1) Decide the wanted ambient exposure. This is done by using exposure compensation or M/X mode. The more the picture is underexposed, the more the contribution from the flash will be important.

2) Adjust the output of the flash. This is done with flash exposure compensation or putting the flash in manual mode. You can (and often have to!) use negative FEC to make the flash more subtle, particularly if ambient light contributes significantly. There's no good or wrong way, or one-size-fit-all settings here. It all depends on the scene and what you're trying to achieve.

Said otherwise, you should think of flash photography as blending two pictures in one: one taken without flash and the other with the flash. If you have a washed out result, as you descrbed, it means the blend is not good. Usually it means too much and you should try underexposing the ambient or flash contribution.

For the specific issues you're mentioning, it would help a lot have a lot to post pictures to identify what can have be wrong.

Also remember that P-TTL flash exposure will be affected by the target reflectivity. Dark subject will tend to lead to overexposure while white subject will lead to underexposure. Higly reflective surfaces (like mirror, glass, polished metal...) can also completely fool the P-TTL system, sending back a lot of the light back and thus making the flash lowering its power level to compensate.
06-02-2019, 09:02 AM - 2 Likes   #12
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QuoteOriginally posted by kman42 Quote
I did some more shots with the flash and realised that I often put ISO into manual mode without thinking about it.
This should not be a problem in most cases as long as the ISO is not set very high with the subject very close. The K-5II manual suggests ISO 1600 as an upper limit. Whether auto-iso will respect that upper limit, I don't know.

QuoteOriginally posted by kman42 Quote
Since aperture is also set manually, and the shutter speed was maxed at flash sync speed,
This also should not be a problem for general shooting as far as the flash portion of the exposure is concerned. If the ambient exposure is set to overexpose, say full sun on the subject at high ISO and maximum aperture in Av mode with the display blinking, the flash will only make things worse.

There is a lot of good advice in the comments above. I can only add that for general shooting, P-TTL requires very little user input and works in all camera exposure modes. Yes, it even works in "B". That being said, there are limits. Those limits are:
  • Maximum flash output too low for subject distance at set aperture and ISO
  • Minimum flash output too high for subject distance at set aperture and ISO
Here is where a decent understanding of manual technique can help one anticipate when and why things don't work. This is also where the camera's P mode comes into its own. My suggestion is to spend some time with the camera's built-in flash shooting a variety of subjects and lighting conditions to build confidence. Once that is accomplished, learning the ins and outs of using the Yongnuo will come easier.

Is P-TTL totally trouble-free? The answer is obviously no. Nothing in photography is totally trouble-free. Here is a list of common causes of P-TTL grief:
  • Poor support from third-party flash, particularly for features such as multi-flash master/controller/slave relationships
  • Flash not in P-TTL mode. It is never automatic except for the built-in flash in green mode.
  • Attempts to use P-TTL when using the aperture ring to control the iris diaphragm. While this is theoretically possible, your camera will politely disagree and will fail over to full flash output.
  • Having flash exposure compensation set (body or flash) and not remembering such
  • Combination of subject distance, ISO, and technique beyond the maximum flash output. This is most commonly encountered when attempting bounce flash with a very high ceiling. Note that this is a general flash concern and not limited to P-TTL.
  • Combination of subject distance, ISO, and technique beyond the minimum flash output. This is commonly encountered when attempting narrow DOF (wide aperture) close-up work. Again, this is a general flash concern and not a P-TTL thing.
  • Pre-flash output is too high for set ISO, aperture, and/or subject distance. This case creates an error condition with the system failing over to maximum flash output. This is a corner case that primarily affects attempts to do high ISO close-up work at wide apertures.
  • Use of KA lenses. Yes, all lenses allowing body control of the aperture are P-TTL compatible and are listed as such in many Pentax body and flash manuals, that for the K-5II included. However, recent guides have a footnote that exposure errors may occur with non-AF lenses. The short explanation is that the P-TTL pre-flash output is tailored for subject distance when that is known. Auto-focus lenses provide an estimate of such through the mount data pin. Without this information, pre-flash will always be at maximum output, resulting in the error in the bullet point immediately above under some conditions with resulting maximum flash discharge. When using non-AF lenses, it is good to be aware of this weakness and scale back ISO or wide aperture on even moderately close subjects when doing P-TTL flash.
  • Problems with master/controller/slave when doing multi-flash P-TTL work. Everyone should try this at least once if they have both a master/controller and a potential P-TTL slave. Setup is complex. I have used the feature on occasion, but prefer manual technique for off-camera flash.
  • Mixing P-TTL master/slave and non-P-TTL optical slaves. Yes, this can be done and can be done with brilliant results, but requires the slaves to be properly configured.
I think that about covers it.

Again, P-TTL flash is pretty easy and transparent for uncomplicated stuff. It only gets strange when the edges of functionality are probed.


Steve

Last edited by stevebrot; 06-02-2019 at 03:46 PM.
06-02-2019, 03:58 PM   #13
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QuoteQuote:
Use of KA lenses. Yes, all lenses allowing body control of the aperture are P-TTL compatible and are listed as such in many Pentax body and flash manuals, that for the K-5II included. However, recent guides have a footnote that exposure errors may occur with non-AF lenses. The short answer is that the P-TTL pre-flash output is tailored for subject distance when that is known. Auto-focus lenses provide an estimate of such through the mount data pin. Without this information, pre-flash will always be at maximum output, resulting in the error in the bullet point immediately above under some conditions with resulting maximum flash discharge.
I did not know the pre-flash power is controlled by the estimated subject distance.

Does this mean bouncing to a high ceiling may fool the pTTL system, as then the pre-flash output may be too weak to deliver usefull results?
That is, if the flash does not inform the camera that bouncing is used. I don't know whether this is the case.

Of course I could find out the answer by doing some tests, but at home the ceilings are not high enough, and you (or someone else) may already know the answer.
06-02-2019, 06:06 PM   #14
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QuoteOriginally posted by RKKS08 Quote
I did not know the pre-flash power is controlled by the estimated subject distance.
There was a thread a few years back in regards to "A" series lenses not working properly with the AF160FC ring flash. The flash manual has an end-note to indicating proper P-TTL exposure with AF lenses only. The camera manuals, on the other hand have no such note and neither did promotional materials, IIRC. Long story short, the flash would only fire at full power if used at macro distances when using P-TTL with other than AF lenses. It was fine at longer distance, but not up close.

I did not have that flash, but did a little research with my Sigma EF-610 DG Super and by using a bit of trickery* was able to view the pre-flash directly with A-series and FA-series lenses. Keeping aperture and ISO equal, the FA-series had shorter duration pre-flash at short focus distance while the A-series had the same long duration at all distances.

I would encourage anyone who regularly uses A-series lenses with P-TTL flash to explore the system behavior at the settings/set-ups used for various "creative" intentions.

QuoteOriginally posted by RKKS08 Quote
Does this mean bouncing to a high ceiling may fool the pTTL system, as then the pre-flash output may be too weak to deliver usefull results?
Good question. One might imagine that something like a Sto-Fen Omni-Bounce might be a little hobbled at common head/shoulders distances. I don't have time to test it out this evening, but it is something worth considering.


Steve

* When using the 2s delay timer, the pre-flash lights at the beginning of the two seconds with the main flash following for the actual exposure. The difference in pre-flash between the A and FA lenses is pretty obvious.

Last edited by stevebrot; 06-02-2019 at 10:27 PM.
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