Digital SLR and P-TTL flash guide


MattMatic

Link Posted 10/07/2007 - 13:25
In response to several people who have had "issues", I'm reposting this note as a sticky. The original was on this thread. 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 )

* 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

* 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 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 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

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

MattMatic

Link Posted 25/07/2007 - 15:38

golfdiesel

Link Posted 04/09/2007 - 15:54
thanks Matt, this guide is really helpfull.
Without it I would never have gotten to the results I am getting now. Especially the short chapter on using full manual control.
The example below would have gone bad if I had left the camera and flash automatic mode.




Camera: *ist DS
ISO: 800
Exposure: 1/50 sec
Aperture: f/5.6
Focal Length: 45mm
AF360 set to 1/1 power.

This shot was taken during the night parade at the 35th aniversary of Keep Them Rolling (http://www.ktr.nl)
Camera:K20D|Ist*DS|Spotmatic II|MZ-10
Pentax Lenses: DA16-45|DA50-200|50A 1.7
Tamron Lenses: 28-200
Takumar Lenses: SMC 55 1.8
Sigma Lenses: EX DG 50-500 'Bigma'|EX 50mm Macro
Flashes: Metz 58 AF-1|Samsung SEF-36PZF|Pentax AF-220T

MattMatic

Link Posted 11/12/2007 - 10:19
Just an addendum to the "non intuitive" mode...

You can also:
* Set the camera flash mode to "Slow sync"
* Switch to Manual on the camera
* Meter as before and use P-TTL flash.

By enabling one of the slow-sync settings (available on the K10D and probably on at least some of the other DSLRs) the camera will continue to meter the ambient light even when the flash is powered.

Works a treat

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

MattMatic

Link Posted 25/02/2008 - 10:10
Update: It is possible to use the normal "A" mode of the AF360 & AF540 when using any of the DSLR bodies. See the thread HERE

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

DaveR

Link Posted 26/02/2008 - 10:15
Gudday Mattmatic (& anyone else),

Many thanks for posting the 'sticky' - very concise and informative, particularly:

"- 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 was just about to post this:

I recently bought a second hand *ist D in very good condition but am not entirely familiar with it.

When using my AF-360FGZ flash I canít get the flash confirmation indicator on the flash or in the viewfinder to function. I have tried many variations of settings and different lenses. When using the same flash, lenses and settings on my MZ-S both indicators work fine. My ĎDí owners manual says that the flash confirmation indicator function is supported when using this flash.

Any clues?


I now realise that I was looking for the flash confirmation indicator in the form of a 'blinking' icon as in my MZ-S. I've checked and yes in the *ist D it is either off (not enough flash) or it stays on = enough flash.

Thanks again,

Dave.

Mannesty

Link Posted 26/02/2008 - 18:26
*istD manual available here. Enjoy the camera.
Peter E Smith

My flickr Photostream

MattMatic

Link Posted 03/03/2008 - 12:08
Just an update on some tests that I've been running... here...

The Pentax flash exposure can be completely thrown by any amount of reflected light. Typically you'll get a very underexposed image as the camera attempts to preserve the highlight detail (incidentally this is often a good thing, especially when dealing with wedding dresses!). Note that the reflection can be almost any size, in any part of the viewfinder (it doesn't appear to be matrix-metering related).

My advice is this:
* If at all possible use bounced light. It'll improve the quality of the image no end, and cuts down on the possibility of reflections. Point the flash straight-up and pull out the little white catch light panel (you'll have to slide the diffuser back in). If you have a white ceiling then the result will be excellent indeed. If you need a little more "throw", then tilt the flash head forward a touch (60 or 75 degree setting).

* If you have to use straight-on flash (yuck!) then shoot RAW so you can adjust for any exposure issues after the event... but try and use the bounced light with catch-panel (even OUTDOORS! Ok, the bounce doesn't work off the open sky, but the catch-panel does a good job!).

* If you need further reach, then push up the ISO in the camera. ISO400 won't hurt that much, and even ISO1600 with a well lit scene is better than an ISO100 with a horrible flash result


I was reading the info on a website: http://www.dembflashproducts.com/diffuser/
Do note that the Pentax catch-light panel can offer at least some of the effectiveness of those products But I'm still tempted to either buy or make my own adjustable panel & diffuser

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

MattMatic

Link Posted 19/06/2008 - 09:47
You might find this resource helpful (although C&N based) - full of useful tips about balancing ambient light & flash:
http://planetneil.com/tangents/flash-photography-techniques/

If anyone has specific questions how you do this on a Pentax system, then drop me a PM or post a question
Matt
http://www.mattmatic.co.uk
(For gallery, tips and links)

ewen_r

Link Posted 14/10/2008 - 09:25
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?

the_drewster

Link Posted 25/11/2008 - 11:56
Hi All,

This seems like a natural place to ask the question. I've decided to buy a SEF-54 (AF540) flash. Are there any other accessories I should get whilst my wallet is unlocked? Like diffusers, brackets?? I don't want to look like a portable studio when going to the park with the kids, but I would like to buy with the flash anything that may make life easier taking photos.

Many thanks

Drew

Mannesty

Link Posted 25/11/2008 - 12:52
the_drewster wrote:
Hi All,

This seems like a natural place to ask the question. I've decided to buy a SEF-54 (AF540) flash. Are there any other accessories I should get whilst my wallet is unlocked? Like diffusers, brackets?? I don't want to look like a portable studio when going to the park with the kids, but I would like to buy with the flash anything that may make life easier taking photos.

Many thanks

Drew

Sanyo Eneloop batteries and a Sto-Fen Omnibounce diffuser.
Peter E Smith

My flickr Photostream

the_drewster

Link Posted 25/11/2008 - 14:17
Wow at the Eneloop. I'd never heard of those before. Thanks

MattMatic

Link Posted 25/11/2008 - 16:02
Quote:
Wow at the Eneloop

Or the "Hybrio" batteries... same kind of thing.
Matt
http://www.mattmatic.co.uk
(For gallery, tips and links)

the_drewster

Link Posted 25/11/2008 - 18:46
MattMatic wrote:
Quote:
Wow at the Eneloop

Or the "Hybrio" batteries... same kind of thing.
Matt

Even cheaper!

Well, there you have it. Just ordered an SEF-54 (AF540) flashgun. Doesn't get delivered till xmas, but hey at £188, what you gonna do?
Last Edited by the_drewster on 25/11/2008 - 18:48
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