Auto flash control, TTL and P-TTL

If any kind soul could summarise what the various flavours of flash control are and how they work, this particular duffer would be most grateful.

I have an AF400FTZ and I'm trying to figure out how best to use it on the K30. I generally use it for a bit of fill, but I can't seem to get a decent effect. A brief reading of the forum suggests that the flash is TTL and the K30 only supports P-TTL, which might be the source of my problems. Does the flash exposure compensation, as dialled in through the body, have any effect, or am I must being dull?

I think for any system the most versatile and controllable approach is full manual. This eliminates any variables caused by the camera and its interaction with the flashgun and the inconsistencies and innaccuracies caused by the 'through the lens' flash metering, which can be negatively influenced in the same way as the ambiant light metering, as well as by reflections and flash hotspots causing underexposure.

Manual control allows better balancing of flash and 'background' light as well. There will certainly be many more expert people here who will understand the specific functions your Pentax unit offers, but for manual work you really need a distance readout scale, or LCD indication of the correct subject distance.

You firstly set the ISO and aperture on camera, then either repeat these settings on the flash (or confirm the flash has read them from the camera), then read off the correct 'flash to subject' distance from the flash display. If you cannot get far enough away for this distance, then you have two options - either stop down the aperture further, or reduce the flash power (eg 1/2, 1/4, 1/8 etc.)

Your choice of which to do would really depend on whether you had a need for a specific aperture for a depth of field effect. If you didn't want to stop down then you must reduce the flash power to achieve the correct flash exposure. Different flash models will have different means of displaying the required information.

For Fill flash use, you would want to reduce the power more than recommended for correct flash exposure (say half or so), also by stopping down or reducing power further, again depending on the subjects need for any particular aperture. I do feel this approach will ultimately give consistent and more precise results than any automated TTL system.

Your flash can be used on auto together with a Pentax DSLR body that supports TTL, these are limited to the early *istD and *istDS bodies. Pentax bodies since then only support P-TTL for auto flash control.

On your camera, with the flash mode switch set to Auto, you will only get 1/1 (100%) flash discharge.

Your flash can however be used on either of it's manual settings but these are very limited at 1/1 and 1/4 output.

It is unlikely that the flash compensation set on the K30 body will actually affect the flash output.

Time to get a new flash I think

Pentax, Metz, and Sigma produce P-TTL compatible flashes.

mattie, for total control Manual is your best option. You did not indicate what you are photographing, but as a general rule of thumb if your flash is not off camera, you will get better results if you bounce your flash. Boucing flash from the side will give you good light to your images. You will need to experiment and it takes practice to get good results.

P-TTL is a bit hit and miss and you wont get constant exposures, but I don't think your flash supports P-TTL?

The link below will provide a wealth of info in relation to Pentax & Flash

http://pttl.mattdm.org

Just came across this
http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/FZ-10/Ext-Flash/Flash-Auto-Mode.html

Well worth a read.

Ta all, useful insights - shame that it seems I'll lose the simplicity of fill-flash through TTL.

As an aside, why did Pentax discontinue TTL support? Cost-cutting, or because P-TTL is seen as the future?

I believe digital sensors reflect the light more than film which alegedly causes issues with TTL (which works by looking at the light reflected by the film during exposure). I strongly suspect that cost saving is also a factor though as it means the camera doesn't need the extra sensor

P-TTL works differently by produceing a preflash & monitoring the illumination produced by that with the camera's main sensor. In use it is generally as simple to use as TTL unless you are using optical slaves to set off other flash guns. (If you try the preflash sets of the slaves, which are then recharging during the main flash). The effect is not dissimilar to the anti-red eye pre-flash.

I have to admit I find P-TTL invaluable for macro, where I hold the flash in my left hand at various angles/distances from the subject. Manual would be totally impactical here as the flash-subject distance is too uncontrolable.

Cheers Mike - would I be right in assuming that you can dial in flash compensation, rather than have to compute what percentage of power would be required for fill?

(I suspect the answer is yes, as it sounds like the difference is more one of how the flash light is measured than anything else, but thought I'd ask to make sure!)

The 'units' (if you like) for flash power and flash exposure compensation are different - power is rated in fractions of full output, so 1/2, 1/4, 1/8, 1/16 etc.

Compensation is rated in 'stops', as in aperture values, so -0.5 (half a stop), -1.0 (1 stop) etc. I use minus because for fill this is the more common direction to head in, as we are not trying to completely illuminate the subject (which will have ambiant light on it), but rather to 'fill in' darker areas like shadows on faces or under the brim of a hat.

Of course, despite the different units of control input, we are effectively doing the same thing. 1 stop less of flash would be the equivalent of 1/2 power I believe.

As far as 'computing' this value, well I'm not sure if there is a formula or not! But, its something that comes with experience both with the subject type and the particular camera also - flash exposure metering is affected in the same way by a subjects 'reflectivity' just as ambiant light metering is. So, for example, a light toned face would require a bit more fill flash than a dark toned one. I've always understood that a good starting point for most fill situations is -1.5 stops. I'm sure others will have other experiences to relate as well.

Just as an extra thought here, it's perhaps worth mentioning that even though we think of Flash Exposure Compensation in terms of 'Aperture Stops', it is not of course actually affecting that actual aperture setting of the lens (only the lens aperture ring or camera control wheel will do that).

Flash exposure Compensation is actually controlling flash power output - so -1.0 stop caused the flash to fire at 1/2 power, just the same as if you dialed in 1/2 power on a flashgun.

All I'm really trying to avoid is the need to work out what the flash power should be at a given given 'length' (or whatever the term for the degree of zoom applied to the flash head is!), range from subject, GN for the flash and given the ambient light (i.e. the exposure setting on the camera). I can do it for still life, but anything candid and I'm toast.

Much easier just to dial in 2 stops' flash compensation and tweak if anything looks washed out or a little flat.

I've serious respect for the guys who could do all the manual calculations 'on the fly' in the days of old, but I'd prefer to leave that sort of thing up to the tools, if at all possible.

I may be barking up wrong tree, but my understanding was that dialling in 1/2 power would be a half-duration of a full flash (i.e. half of the flash capacity), not a half-duration - and therefore half intensity - of the amount of flash commensurate with the scene, which is what flash compensation provides.

Have I got this wrong?

Um... well I hope others more experienced than me will comment too - but 'half flash power' I believe relates to the power output (strength of light) that the unit puts out. How it technically achieves this I believe is to do with the 'Thrystor' circuits that use coils to store charged energy and the amount from each recharge that is released.

Flash duration would have an effect only at longer shutter speeds, ie beyond the 'Flash Sync' speed (180th sec on my K7) - generally the flash duration is significantly less (faster) than the shutter is actually open, so in these conditions just reducing the flash duration would have no effect.

Mattie, not sure if you saw my post just above, but in relation to the power output, I think from looking at your last question we are actually saying the same thing.

Quoted from above ... 'The 'units' (if you like) for flash power and flash exposure compensation are different - power is rated in fractions of full output, so 1/2, 1/4, 1/8, 1/16 etc.

Compensation is rated in 'stops', as in aperture values, so -0.5 (half a stop), -1.0 (1 stop) etc. I use minus because for fill this is the more common direction to head in, as we are not trying to completely illuminate the subject (which will have ambiant light on it), but rather to 'fill in' darker areas like shadows on faces or under the brim of a hat.

Of course, despite the different units of control input, we are effectively doing the same thing. 1 stop less of flash would be the equivalent of 1/2 power I believe'