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07-31-2016, 02:18 PM - 2 Likes   #1
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Automatic Flash - Expectations and Realities

The Pentax Digital Camera Flash Lighting System - Planned Developments

Practical Automatic Flash Photography

This is a development thread for my 'Flash Guide', and here I am dealing with my proposed new section on automatic flash workings. We get many requests here and concerns raised about P-TTL flash exposures, and often we find that there are aspects of the equations that are somewhat out of control (I have called it 'Auto-Everything-Chaos Mode' on another thread!).

I have now drafted up the text of my proposed new section, which aims to introduce and explain, in general, non-technical terms, what is going on when we use an automatic exposure mode and switch on a P-TTL flash at the same time. It is my own personal aim to encourage more and more use of Manual exposure mode with P-TTL flash mode, but we have to accept that there are good times to rely on fully automatic operations .... IF we understand them and their limitations.

So that is what this section of the guide is meant to address. A lot of this is somewhat untested .... certainly there's very little documented officially, so like so much about Pentax flash, we are making it up right here as we go along !! But I am asking for everyones advice and experience, and your contributions to making the guide as helpful and accurate as possible. So please read through the text below and feedback your thoughts ..... have I pitched it right, is there any technical aspect that should be added, or better explained. Bear in mind that its meant to be a welcoming introduction to Pentax flash photography for those who may be very new to it, or have been struggling to understand things, as well as a backup resource for more experienced people as well.

Thanks for your interest and comments ... the final version, with amendments based on feedback here, will include the usual pictures of the flashes and camera control panels .....



Automatic Flash Photography, Expectations and Realities
As we have seen there are various exposure mode configurations between both flash and camera that will influence how we balance the flash and ambient exposures.
On Pentax, we can summarise the possible combinations:


1) Auto-Exposure Camera Mode plus P-TTL Flash Mode
(Green, P, Av, Tv, Sv, Tav – P-TTL)

2) Manual Camera Exposure Mode (M) plus P-TTL Flash Mode
(M – P-TTL)

3) Manual Camera Exposure Mode (M) plus Manual (M) Flash Mode
(M – M)

4) Manual Camera Exposure Mode (M) plus ‘Auto’ (A) Flash Mode
(M-A)

5) For High Speed Sync Flash : Manual, Tv, or Tav Camera Mode Plus P-TTL Flash Mode (M, Tv,Tav – P-TTL)

It is certainly easier to understand what is happening and to keep complete control over the exposures with both camera and flash in Manual modes. Each specific part of the equation we adjust (eg aperture, ISO or flash power) will have a proportional impact, and may require a compensating equal adjustment to another setting to maintain the exposure level needed.

When we use a camera auto-exposure mode, or flash P-TTL / Auto modes then we are giving control over to the metering systems and electronics, plus the various software processing algorithms that make the decisions. Unfortunately of course, the designers cannot program in an understanding of what exactly each photographer is hoping for with every scenario. If we expect the system to second guess our own creative intentions, then we are likely to be disappointed. We still need to have a good understanding of how things work, what are the limitations of the equipment, and how we can exert the needed control to make the automatic systems work for us.


Pentax Automatic Flash Exposure / Ambient Balancing
Whilst as users of the equipment, we cannot have an in-depth knowledge of exactly how the various metering and flash / ambient exposures are balanced, a feeling for the general principles is helpful, to understand what the camera is trying to achieve, and why it may not be able to achieve what is needed, without our intervention.
Balance is the key word here, and that is essentially what the camera will be trying to achieve when in both automatic camera and flash modes, a balance between the ambient light (the background) and a flash-lit foreground subject.


There are two key principles that govern the end result :
1) If the possible / allowable ambient exposure settings result in a well exposed foreground subject, then the flash output will be adjusted downwards to provide a supporting, accompanying role (commonly known as ‘fill-in flash’)

2) If the possible / allowable ambient exposure settings result in an underexposed foreground subject, then the system will attempt to light as much of the scene as possible with flash

In general, fully automatic flash photography is most likely to be successful when the situation matches No1 here. It is the No 2 situations where the balance between the two exposures can become too great, resulting in an overpowering, washed out foreground and dark, unattractive background.


We have to bear in mind that the camera cannot know exactly the limits as to where you want your flash lighting to cover. If the ambient brightness is low and it is not possible for the camera to compensate and record more of that ambient light, then the flash is going to try and do it instead. Because the flash is trying to illuminate a greater area of the scene like this, then objects closer to the camera are more likely to be over-lit, creating a harsher ‘flash look’, and not a good balance at all. Bringing factors such as bounce into the equation also can really open up the variations to what we can expect the system to achieve.


Our PentaxForums member Beholder3 has carried out various tests and determined that on average the system tries to provide 2EV worth of flash light in these situations (No2 above) … for example, in Av mode, with no flash a certain scene required 1/50th sec / F8 / ISO 6400 for good exposure. With the flash activated the settings were 1/50th sec / F8 / ISO 1600.
None of this means of course that the 2EV worth of flash light is correctly distributed or provides the creative balance that the photographer wanted.


Controlling and Limiting Factors
The automatic system is constrained by three key conditions :

1) Lens Focal Length
2) The Auto-ISO Curve
3) The Program Line

All of these are user configured, but only the first would be actively adjusted when taking pictures. Nos 2 and 3 are pre-set, and are not normally actively managed when out taking pictures, although it would be good practise to review them before starting an important shoot. However they all play a role in the resulting settings from automatic metering and flash / ambient exposure balancing.


The lens focal length will actively restrict the longest exposure time that the camera can set when using the following automatic modes - Green, P, Av, Sv. This is designed to allow for handholding and prevent camera shake when shooting automatically, a noble aim. However, with flash in the equation as well then this restriction can impact negatively by making the ambient exposure too dark, again ruining any nice gentle balance between the flash and background exposures.


We can override this automatic exposure time limitation if we want by setting ‘Slow Speed Sync’ in the camera Flash Mode screen. This setting will allow the exposure time to lengthen significantly and gain a correct exposure for a dark background, while the flash will take care of the foreground subjects.
Without Slow Speed Sync we can expect the following limits to shutter speeds based on focal length …

Thanks again to Beholder3 for his testing and reporting of this information ….
8mm --> 1/25
16mm --> 1/50
24mm --> 1/60
43mm --> 1/100
85mm --> 1/160

Both the Program Line and Auto-ISO parameters will impact on how the camera will be able to successfully balance flash and ambient exposures. Certainly Auto-ISO ca often be seen to lead to problems if the range is not set appropriately to the circumstances. What is right is generally dictated by the levels of lighting, distances to the subject, bounce surfaces and angles, all of which can drastically influence how much flash power is needed. Where distances are greater, bounce surfaces are dull / non-reflective, and bounce angles are more extreme, then higher ISOs will be called for. Each photographer will need to develop a good sense of their camera’s ISO capabilities in terms of the quality to be expected at different ISOs, and where adjustments to other aspects of the shot might be preferable instead.


Where the ISO range is allowed to go too high, then exposure problems can occur. For example, in a dimly lit room, an auto-ISO ‘curve’ might calculate and set ISO 6400 in order to balance a smaller aperture and keep the shutter speed good for handholding …. However, that high ISO may be totally wrong for the flash in use, especially a high powered one, which in certain configurations may be unable to cut its output enough, resulting in flash overexposure.


Unfortunately it is when trying to reconcile these often conflicting aims that our automatic systems and calculations can fail. It is only a tiny camera brain after all!
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.
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.


Nigel McGregor, Copywrite July 2016


Last edited by mcgregni; 07-31-2016 at 02:50 PM.
07-31-2016, 04:09 PM   #2
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Love the thread title!

Good luck and best wishes as you move forward on this next phase of the guide.


Steve
07-31-2016, 11:24 PM   #3
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Just some quick notes before I find more time to test and elaborate:

  1. The 1/180 is 1/200 for a K-1
  2. My shutter times were APSC related, so now we should add FF ones (I'll test that with the dfa 24-70) as well.
    They also only applied to "normal" AutoISO line.
  3. It is important to teach beginners that a flash adds a fourth variable to an equation which previously had "only" three. Now it is T+A+I(SO)+F(lash)=18% grey. So unless you set 3 in M mode so there is only one correct answer for the fourth, you always have many correct answers made out of doubles or triplets of settings. And: Av with AutoISO and Flash leaves exactly the same amount of lottery freedom to the camera as Green mode.
  4. I believe this part of PTTL logic was changed between K-5II and K-3/later, so the results might differ.
  5. As this scenario often lets the camera decide I assume it does use the Program line. I always use MTF-line, but most people probably never change it from Auto-line. I fear (not tested) that Auto-line might use some scene detection and thus might be the culprit behind some weird flash behaviour reported, which I personally never suffered from. This might be an interesting area for a lot of testing.
This really is the most complex subtopic.

Side note: Canons always nails the "AutoISO" to a fixed ISO 400 in Av mode with flash, regardless of other things. And there is a good reason for this: get rid of one unwanted variable. So much for a more sophisticated flash system. ;-)
08-01-2016, 12:49 AM   #4
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QuoteOriginally posted by stevebrot Quote
Love the thread title!

Steve
yes, lets get back to reality! Thanks for the support Steve.

Thanks also Beholder 3 for those in depth points. I am sure you are right about the various permutations of program line effects, and your tests so far are very revealing.

I want to try and keep the information in the guide to a general summary style, to give an overview of the forces at play, to help with the intelligent intervention needed by the photographer. Do people feel that the level is pitched correctly for this ? I realise that I should perhaps condense some parts of the text ... When I draft up something like this I tend to ramble and get very thought down!

08-01-2016, 08:07 AM   #5
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QuoteOriginally posted by beholder3 Quote
Av with AutoISO and Flash leaves exactly the same amount of lottery freedom to the camera as Green mode.
"Lottery" is the key term here


Steve

(...auto-ISO is evil...mutters to himself...)
08-01-2016, 10:44 AM   #6
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I will certainly have to find a place for that phrase in the guide Steve ... Can I quote you... ?


I have been aware there are many threads started by people new to flash photography ever the root cause of problems is a lack of firm user control over the whole business. This is usually underpinned by the expectation that the camera would be able to take all the necessary decisions on its own when in automatic modes. Then we get to the reality ....

Last edited by mcgregni; 08-01-2016 at 10:56 AM.
08-01-2016, 10:58 AM   #7
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Did my checking now and I learned that they changed the behaviour in the K-1 again in Av mode.

1) Shutter times = normal AutoISO line.
They kept the behaviour of the K-3 that the shutter times when using a flash are set by the "normal AutoISO line" regardless of what you set the camera to. It's neither "fast" nor "slow".
And that is actually about 1*5 times the focal length with a very short focal length still sticking to a faster speed.
I have to admit that since this is the same logic as on the APSC cameras, I beleive they simply forgot to change the AutoISO lines back to the olden times rule of thumb, where "normal" means shutter is 1/focal length and no need to use the crop factor.

2) AutoISO = off, set to ISO 100
In contrast to K-5 and K-3 the K-1 now hard switches off AutoISO and sets it to ISO 100. At least I always got ISO 100, even if without flash I got ISO 6400. While that is more foolproof and predictable (reducing the lottery) this means flash will very likely cancel out any ambient light in Av mode unless you take control yourself.

Here are my tests:
Attached Images
 
08-01-2016, 11:19 AM   #8
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I'm sure that assumption is right ! Thanks for the table and additional points, I will find a way to incorporate it into the section.

08-31-2016, 12:18 PM   #9
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@mcgregni

Do you have a planned closing date for suggestions to the new revision? When are you planning to release the next revision?
08-31-2016, 10:53 PM   #10
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Hi, there's no specific date yet, and plenty of time to include any further suggestions, so keep them coming!

I've been very busy lately and the school holidays are still in full swing .... I've still got to try out the ring flash and write about that, but most of the other sections I've planned are more ready. Being realistic I'd hope to have a supplement PDF ready this winter, if things go well then it would be good to have it out around Christmas ...
08-31-2016, 11:40 PM   #11
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Merci for the info.
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