[thechumpen]

Hello

I'm trying to learn about flash and have been practising with a stuffed toy with my Metz 58-AF1 flash mounted on my K-r. Pentax A-50mm lens set to F5.6 and I'm around 1.5m from subject. I am trying to figure out real world difference between A mode and PTTL mode on flash.

I took a bunch of shots with the flash on A mode and got consistent results every time. I then tried on PTTL and probably 1 in 3 shots is massively over exposed. Therefore TTL seems to be pretty unreliable/inconsistent compared to A mode.

I'm very new to this so does anyone have any info on whether there are certain scenarios where you would use one mode over the other? Or is A always more reliable?

Thanks.

[123jippo]

I find Pttl is ok with popup flash only. With metz AF 58-2 result can be whatever. I use M mostly, at least iget what i want.

[MSL]

Or is A always more reliable?

Yes. Many threads on this topic already. P-TTL can work for some situations but the pre-flash used for metering can be easily fooled.

[maxfield_photo]

A mode is very good, but it has it's limitations. The photcell sits on the front of the flash, and has a field of view, I think roughly 15°. So to use A-mode, you want your subject to occupy most of the center of your frame. Obviously if you are 10 meters away from your subject, they aren't large enough in the frame to give the sensor what it need to make a proper decision, but even in that case, the flash will probably fire near full power, which should at least get you in the right ballpark. Also placing your subject off center, especially with a wide angle lens can cause the A-meter to miss your subject all together and meter off into empty space. A-mode is based on a reflective reading, just like pTTL, so it can be fooled by color and sheen, but unlike pTTL it doesn't use a preflash and complicated algorithms to judge the exposure, it just reads what's happening in real time and when it thinks the subject has received enough light for the given aperture/ISO, it shuts off. Oh, an A-mode works well for 1-flash setups, but when you start adding in more flashes, they can sometimes fool each other into thinking that the subject has received enough light, when really the light is coming from another source.

It's good to learn all the different exposure modes of your flash. My mode of choice from first to last goes Manual>Auto Thyristor>pTTL I only leave manual mode when I don't have time to meter with my Sekonic, like if I have a moving subject. Some folks would choose pTTL>Auto>Manual, and that's fine, but sometimes they don't abandon pTTL when they should, and then when they get to manual mode they have no idea what to do. Whichever is your mode of choice, you should also learn how and when to use at least one fallback mode.