Digital SLR and P-TTL flash guide

MattMatic · 2007-07-10T13:25:52+0100

In response to several people who have had "issues", I'm reposting this note as a sticky. The original was on . Just copied out verbatim. Hopefully, those of you who are experiencing "odd" flash issues can check out this guide to see if there's something you've missed ;) In my experience, P-TTL is very good indeed at coping with dynamic situations (and from initial quick tests it seems that v1.30 is better than before). But my experience comes from the days of Manual only and Auto flash using fully manual cameras (Pentax S1a to be precise) :) So let me share some knowledge... Points (in order of importance): * Flash is limited in power Probably the biggest thing in the P-TTL controversy. P-TTL will sort out the exposure but only within the limits of the flashgun. Let's use a car analogy. Most of us have cars with limited power, so we may see a situation where we'd like to accelerate from 0 to 60 in 2 seconds, but even when we push really really hard even to the floor the car won't go any faster. The flash can be like that - it's limited and has a limited range. Even if P-TTL says "I want loooooads of light for this shot" the flash may very well not have the power and the shot will be dreadfully underexposed. (Ever see people in a stadium trying to use flash photography? How much effect do you think their flashguns will have? ? Absolutely zero.) * Flash distance is dependent on ISO rating and aperture This is where physics comes in and the "inverse square law", ie double the distance and you quarter your light (because it's falling in 2 dimensions not just one). For photography this means you need to balance the ISO and Aperture (note: NOT the shutter speed. More on that later) so you get the correct distance. For those with the AF360 and AF540 there's a little distance scale at the bottom of the LCD. Try the following: - Set the camera to ISO 100 (fixed) - Set the camera to Av mode and rack it up to f/22. Your distance will be 0.7-1.5m. Anything further away than 1.5m will be dark! - Now set the Av to f/5.6 and the distance changes to 0.7m to 6m. - Set the camera to ISO1600 (fixed) and at f/5.6 the distance will change to 3m-22m. Woah! With that setting of ISO1600 and f/5.6 if you have someone standing 2m away they will be drastically overexposed! The flash can't turn off quick enough! (A bit like asking a Bugatti Veyron to nudge forward 2cm - you probably won't be able to stop in time :P ) * Bounce flash messes up the distance This is where the AF360 and AF540 will turn off the distance scale! The flash distance is no longer the camera-to-subject distance, but camera-to-ceiling-to-subject distance. Depending on where you are this distance could be enormous. If you're outside then it's infinite :P * Balancing the light This is where some skill comes in (and a tiny bit of knowledge ;) ). When you're trying to take realistic images using flash you really want to balance the ambient lighting with the flash. For example, let's suppose the ambient reading (ie without flash metering) is ISO200, 1/30s, f/5.6. If you set your flash to use f/11 then you're going to "punch" your way through the image with the flashgun. The flash lighting will be two stops (f/5.6 to f/8 to f/11) brighter than the surroundings and you'll get the "cardboard cutout" look. Bleurgh :( Actually, if you set the K10D to auto ISO in Av mode you'll notice that the ISO changes according to the ambient lighting even when the flash is powered on. The K10D is making some pretty reasonable guesses at balancing the light. * Metering aims for grey Don't forget that the metering aims for mid grey. If you're taking shots of mostly black or mostly white scenes then you'll almost certainly need exposure compensation (on the camera, not the flash) to "inform" the metering system what you're taking. * Shutter speed is (almost) unimportant The duration of the flash is blindingly quick :P Something like 1/10000s. As a consequence, the shutter speed is not that important - especially when you want to "freeze" action. But this is where you need to learn about balancing the light... If you want to freeze action and isolate the subject from the surroundings, then you want to make sure the flash is at least one or two stops more powerful than the ambient light. If you want to make the shot look natural then you need to at least keep the flash and ambient lighting within about -1 to 0 stops. If the shutter speed is very slow (or the subject is moving) such that a non-flash exposure would produce a streak or motion blur then you'll probably want to put the flash in "trailing curtain" mode (slide switch to the position to the right of the green triangle). This mode opens the shutter and just before it closes it fires the flash - the flash will produce its punch at what will appear to be the "head" of the object so the object looks like it's moving in the right direction (exposure is blur-then-flash). Using the green mode makes the subject look as though it's going in the wrong direction (i.e. flash-then-blur). Think about the time ordering of things... ;) * Use fully manual for controlled situations For studio type shots where the conditions don't change - use fully manual. This mode works extremely well when using the Wireless settings of an *ist-D/K10D and AF360/AF540 combination. - Set the camera to Manual, 1/60s, f/8 (a starting point) - Set the flash to Manual, 1/4 power - Take a test shot - If it's too bright do one or more of the following: > Pick a small aperture (ie large f-number) > Move the flash further from the subject > Reduce the flash power - If it's too dark, do one or more of the following: > Pick a larger aperture (ie smaller f-number) > Move the flash nearer to the subject > Increase the flash power - Once you've got a good flash exposure you can balance with the ambient lighting: > Lower the shutter speed to allow more of the ambient light in. (e.g. 1/15s) > Raise the shutter speed to darken the background (e.g. 1/180s) With a bit of practice you can get the right aperture and power combination within a couple of test shots - and having digital preview is extremely helpful indeed! * Pentax DSLR and AF360/AF540 specifics - In the viewfinder the lightning flash icon will be lit when the flash is charged. - Immediately after taking the shot the lightning icon should remain lit. If it goes out the shot will be underexposed - the camera wanted more light than the flash could give. - I would suggest Auto-ISO and Av for general portraiture (e.g. weddings) because then the camera will generally get it right every time. - If you want to override the flash "effect" you can set the exposure compensation on the AF360/540 using the little dial. Something like -0.5 or -1.0 will allow more ambient light in. - If you are shooting outdoors and want fill in flash, set the flash slider to "HS". Note that the distance is halved when in this mode (though if the shutter speed goes below 1/180s the flash will go back to "normal" mode). - If you want attractive catchlights in the eyes (especially for close portraiture say about 2m away) then pull out just the little translucent catch light panel and point the flash straight up (90 degrees). The small amount of reflected light from the catch light panel will produce a subtle fill in with a lovely sparkling catch light. - If you use a Stofen Omnibounce then you'll halve your effective flash distance. But the flashgun won't know - so you'll have to make mental adjustments when you look at the flash distance scale (or consciously check the lightning indicator in the viewfinder). - The same rules apply to using Wireless mode. But remember, the flash distance is the flash-to-subject distance, and not the camera-to-subject distance!! For the K10D you can't use "HS" mode in Wireless (but the *ist-D could, and only the *ist-D). - For controlled situations you can use an "non intuitive" setting :P I have done the following in a number of situations: Turn off the flash; Set the camera to "M"; Meter the scene using spot metering to try and expose near the highlights; Ensure the shutter speed is under 1/180s; Turn on the flash in P-TTL mode; Take the shot. Every time this is a winner - the flash fills in the shadows without changing the exposure. (I have used this for taking shots in dark rooms that have light streaming in and got shots that have everything properly exposed ;)). - For wireless, the "Slave 1" is the "intelligent" mode (for the *ist-D and K10D only). "Slave 2" is the "dumb" mode which will sync to all pop up cameras - however, if the camera has a preflash, make sure the flash only uses up to half power otherwise the main flash will be exhausted (and used up on the preflash). See page 49 of the AF360 manual. Custom options in the K10D and *ist-D can be set to use the pop-up as a trigger only ;) I'm sure there are things I've missed... but hopefully this "brain dump" (to use the jargon) will shed some extra light on flash usage and to show that, actually, the Pentax flash system is excellent :D Matt

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MattMatic MattMatic

5870 posts
22 years
Sussex,
UK

Posted 03/07/2009 - 21:45 Link

That was quick work Don

http://www.mattmatic.co.uk
(For gallery, tips and links)

Don Don

6213 posts
19 years
Winterpeg Manitoba Canada

Posted 04/07/2009 - 02:22 Link

you wanna know the sick part?
took less time to read it than to print it...
I read it once and it''l take hold for life...

Fired many shots. Didn't kill anything.

Sitav Sitav

47 posts
17 years

Posted 30/07/2009 - 10:16 Link

ewen_r wrote:
I have noticed that my GX20 (K20d equivalent) massively under-exposes PTTL flash shots when I have a split-image focussing screen fitted.

Haven't found any elegant solution to this other than to replace the focussing screen with the default one.

Wondering if any Pentax users have seen this too?

I've had this trouble with my K10, I put the new screen in this morning and wondered why everything was under-exposed, I've put the original screen back in and it's back to exposing correctly.
I've only just bought the flash (Metz 58 ) so haven't really looked into a solution yet, I guess it could be compensated for manually but that somewhat negates buying a dedicated flash doesn't it.

Simon

K10D
www.photonetics.co.uk

MattMatic MattMatic

5870 posts
22 years
Sussex,
UK

Posted 30/07/2009 - 11:57 Link

Simon,
All metering for P-TTL, as well as for non-flash metering, is performed through electronics housed in the top of the pentaprism. The same 16-segment meter sensor is used to evaluate the preflash as is used for normal exposure (though it appears that for flash exposure the whole frame is used, irrespective of the metering mode you select ). Consequently, anything that affects the mirror or focus screen can potentially affect exposure.

If you use the flash on "A", or "M" mode then there's no such problem
Matt

http://www.mattmatic.co.uk
(For gallery, tips and links)

Hiljon Hiljon

42 posts
15 years

Posted 31/07/2009 - 00:58 Link

Thank you so much for this. I have really waiting for this. it is much appreciated. Thank you

rakn rakn

23 posts
15 years
Sweden

Posted 01/08/2009 - 16:24 Link

Hi Matt,

That was an excellent guide !

Two things that may be worth mentioning in the next Flash Guide version is that the lightning icon in the viewfinder can disappear right after exposure for more than one reason. If the camera did not get as much flash light as it wanted (as stated in the guide), or if the flash gun needs a second or two (or three...) to recharge from the batteries. It took some time for me to sort that out... I got perfect pictures but the icon disappeared all the time. And it's hard to tell the reason, a bit confusing I think.
Also the unexpected behaviour of K10D/K20D to use some average flashlight measuring mode in P-TTL instead of the user choosen mode.

If I set the flash compensation to +1 and wireless mode (fn button) and "use built-in flash as master in wireless mode" (custom menu) in my K10D, will I get +1 for both the built-in flash and the wireless one ?

Thanks
Ragnar

K10d, AF-540fgz, some Pentax Tamron & Sigma lenses 20-200 mm, Manfrotto tripod.

Don Don

6213 posts
19 years
Winterpeg Manitoba Canada

Posted 01/08/2009 - 16:45 Link

If you ever add a chapter on advanced multi flash p-ttl set ups let us know... ps it has been a couple weeks now and Gerri is improving! I still go with everything on manual...

Fired many shots. Didn't kill anything.

Edited by Don: 01/08/2009 - 16:46

MattMatic MattMatic

5870 posts
22 years
Sussex,
UK

Posted 05/08/2009 - 05:44 Link

Ragnar,
As far as I am aware, the compensation for slave units is only set on the flash itself. The flash compensation on the camera applies to the pop-up alone (or the flash unit on the hotshoe).

But there are other factors too... Pentax term this "contrast control" flash, IIRC.

TBH, I'll have to try it out. When I use multi-flash it's in a studio-type environment and I have everything on manual Drop me a PM in two or three weeks to remind me if I haven't updated this thread!

@Don: glad it's helped. I'll see what I can do about extra chapters

Matt

http://www.mattmatic.co.uk
(For gallery, tips and links)

ChrisA ChrisA

4839 posts
18 years
South West London

Posted 05/08/2009 - 08:11 Link

MattMatic wrote:
Ragnar,
As far as I am aware, the compensation for slave units is only set on the flash itself. The flash compensation on the camera applies to the pop-up alone (or the flash unit on the hotshoe).

Based on the test I ran and described last year, I believe this is incorrect.

.
Pentax K-3, DA18-135, DA35 F2.4, DA17-70, DA55-300, FA28-200, A50 F1.7, A100 F4 Macro, A400 F5.6, Sigma 10-20 EXDC, 50-500 F4.5-6.3 APO DG OS Samsung flash SEF-54PZF(x2)
.

rakn rakn

23 posts
15 years
Sweden

Posted 05/08/2009 - 08:53 Link

From your test Chris, it looks to me that
- in controller mode the remote flash is affected
- in master mode both flashes are affected
Thanks for sorting this out.

K10d, AF-540fgz, some Pentax Tamron & Sigma lenses 20-200 mm, Manfrotto tripod.

MattMatic MattMatic

5870 posts
22 years
Sussex,
UK

Posted 05/08/2009 - 09:25 Link

Well in that case... the Fn control must be an overall adjustment, while the compensation on the slave itself is an adjustment relative to that slave (perhaps )

As I say, when I get a chance I'll give it a whirl with two remote flashes and see what happens

Matt

http://www.mattmatic.co.uk
(For gallery, tips and links)

stevejcoe stevejcoe

993 posts
18 years
Cornwall,
UK

Posted 05/08/2009 - 09:43 Link

MattMatic wrote:
Well in that case... the Fn control must be an overall adjustment, while the compensation on the slave itself is an adjustment relative to that slave (perhaps )

As I say, when I get a chance I'll give it a whirl with two remote flashes and see what happens

Matt

Matt my experience is in line with your comment.

Steve

Edited by stevejcoe: 05/08/2009 - 09:44

Bluemerlin Bluemerlin

44 posts
15 years

Posted 06/08/2009 - 19:30 Link

Thanks for this I was going through hell trying to guess how to do it.

View My Flickr Photostream
Pentax K20D + BG2- Pentax DA 18-55MM AL

MattMatic MattMatic

5870 posts
22 years
Sussex,
UK

Posted 22/08/2009 - 17:45 Link

Had a quick play on holiday with a single AF540...

It works like this:
* The flash exposure compensation on the camera body sets the overall flash compensation (a base-point if you like)

* The exposure compensation on the slave flash works from the base point that was set on the body.

Consequently, you can set the relative exposure balance of multiple remote flashes using the dial on each slave. You may want a master at +0, with a fill in at -2. Then you can adjust the relative exposure of the all the remotes from the body - adjusting up and down as needed.

Hope that makes sense (and agrees with what you find in practice!)
Matt

http://www.mattmatic.co.uk
(For gallery, tips and links)

Edited by MattMatic: 22/08/2009 - 17:45