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

Flash PhotographyFlash Photography

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Diatribe Diatribe

71 posts
16 years
Near Guildford,
but not quite...

Posted 20/02/2009 - 11:31 Link

"(Ever see people in a stadium trying to use flash photography? How much effect do you think their flashguns will have?? Absolutely zero.)"

I'm sorry Matt, but I have to disagree. Anyone using their flash in a stadium knows they will get a brightly lit shot of the back of at least one persons head in the bottom third of the frame.

K20D and a substantial user manual to wade through

MattMatic MattMatic

5870 posts
22 years
Sussex,
UK

Posted 23/02/2009 - 10:00 Link

True enough!

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

Classvino Classvino

248 posts
18 years
Ontario,
Canada

Posted 09/04/2009 - 16:13 Link

Very informative.

Thanks Matt.

Jamie

--------------
MZ-6,K10D,K20D,Grip,DA*Zooms,DA 18-55-200 Kit,FA50,1.7TC,AFZ360
For Saleentax FA28-90,FA100-300,Sigma EX28-80,DC18-200,
Makinon 500mm Mirror

neill neill

50 posts
17 years
Manchester

Posted 31/05/2009 - 00:52 Link

Matt,
now that we know the K7 spec, are you able to speculate how the K7 might handle flash?

Neil

MattMatic MattMatic

5870 posts
22 years
Sussex,
UK

Posted 01/06/2009 - 08:33 Link

Quote:
now that we know the K7 spec, are you able to speculate how the K7 might handle flash?

Neil - no.
My e-book took ages to sort out! You'll notice that much of it isn't even in the manual, let alone the specification!

Still, if anyone wants me to add the K7D to the e-book, I'm happy to do that if they can loan me the K7 Any takers?

Matt

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

Robin128 Robin128

152 posts
16 years
S E Wales

Posted 01/06/2009 - 11:42 Link

So long as it covers my K20D and AF 360FGZ I'll be happy....just ordered your e-book Matt.

Rob

K20D
MZ 50
Pentax lenses....DA* 55mm f1.4 SDM ....F 35-80 f4-5.6....DA 18-250 f3.5-6.3....F 100-300 f4.5-5.6, D FA Macro 100 WR f2.8.

Centon mirror lens 500mm.

Pentax Flash ....AF 360FGZ

JVC 3ccd GZ MC 500EK camcorder

Robin128 Robin128

152 posts
16 years
S E Wales

Posted 01/06/2009 - 20:09 Link

Matt,

Thanks for the quick response, printing it out to beat myself with!

Rob

Robin128 Robin128

152 posts
16 years
S E Wales

Posted 03/06/2009 - 14:59 Link

What an excellent guide...written in simple language so that anyone can understand.

Read it, picked up the camera and flash and cooking already...Thanks Matt!

Rob

MattMatic MattMatic

5870 posts
22 years
Sussex,
UK

Posted 03/06/2009 - 15:17 Link

Rob - really appreciate your feedback
Enjoy your "cooking"

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

Don Don

6213 posts
19 years
Winterpeg Manitoba Canada

Posted 03/07/2009 - 14:49 Link

Matt, check your messages, I'd like to buy your book also.

Fired many shots. Didn't kill anything.

Mannesty Mannesty

7650 posts
20 years
Totana,
Spain.

Posted 03/07/2009 - 14:53 Link

Don wrote:
Matt, check your messages, I'd like to buy your book also.

Buy it here Don. Matt will email it to you.

Peter E Smith - flickr Photostream

MattMatic MattMatic

5870 posts
22 years
Sussex,
UK

Posted 03/07/2009 - 14:55 Link

Don - have PM'd you back BTW - it covers much, much more than just P-TTL Tried to cover pretty well every flash mode on the Pentax with almost any flash! (Phew!)

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

Don Don

6213 posts
19 years
Winterpeg Manitoba Canada

Posted 03/07/2009 - 15:05 Link

My very first Paypal purchase!!!

Fired many shots. Didn't kill anything.

MattMatic MattMatic

5870 posts
22 years
Sussex,
UK

Posted 03/07/2009 - 15:07 Link

Payment received fine Don I'll email the PDF when I get home (about 3hrs to wait!)

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

Don Don