Why does my 540 flash under expose???!!!

I have been using my flash on the K10 a lot recently and have connected it off camera via the cord. All my shots are under exposed. I have to over expose the flash by a stop or the camera. I am using a stofen plastic diffuser - any ideas people.

A flash at this price should just work...

ps it did bounce off the floor a while ago!!

A diffuser will reduce the flash power to some extent. Although I would expect PTTL to compensate, I think it still has some effect.
Actually I find photos taken with or without a diffuser to be rather consistently underexposed -- but not always, of course.
I suppose better to be underexposed, than to blow the highlights. I will just generally kick the flash or the camera exposure compensation up a stop.

Something to keep in mind is that PTTL uses multi-segment metering for evaluating the exposure. Not spot meter nor center weighted. So if you're expecting a smaller portion of the viewfinder to be used for obtaining exposure, this may be part of the issue as well.

[quote="amoringello"]A diffuser will reduce the flash power to some extent. Although I would expect PTTL to compensate, I think it still has some effect.

thanks for the post... Yes I will push my exposure up a stop but I shouldn't have to... it's PTTL after all.

somewhat disappointing. In some ways when I used my Me super I new I had to tweak but now we expect out bucks and the new technology to really do the work for us.

my experience of pentax flash guns has not been positive so far.

[quote="Bluewhistle"]Did you read Matt's post in a different thread? He went through all the possible causes of flash exposure problems. It's quite a complex subject, and I'm afraid that you need to know quite a lot about it in order to ensure that your flash exposures work as expected.

One possibility is that you simply aren't outputting sufficient power for the conditions. Don't forget that diffusers cut the light output quite considerably, and if you are using a smallish aperture there may not be enough power.

It's one thing to use f11 in the studio with studio flash, but it just won't work with on-camera guns.

G

To give you a rough idea of this BlueWhistle, the 540 has a guide number of 54. This means that at f1 and ISO 100 the flash will output to a range of 54m.

so, considering the Stofen diffuser will decrease range by 2-21/2 stops, you're already down to 13.5m @ f1. Shoot at f8 (and ISO 100), and you're down to 3m or thereabouts.

That's the problem with measuring light. Each f stop higher results in 1/2 the light reaching the sensor. Go to f16 in the above demonstration, and you'll be lucky to light your shoes up..

Hope that helps.. (and I'm sure someone will correct the maths if I slipped up somewhere..)

Cheers

Liam

The maths sound about right Liam

FYI, the topic is here:
https://www.pentaxuser.com/forum/viewtopic.php?t=4950

Wish I could post some AF360/AF540 shots I did recently... have to get approval first though. But, yes, as George says, you've got to understand what's actually happening and in my experience it's rare that the AF540 and AF360 get it wrong....

... assuming you are using them within their limits.

All too often people slap a flash gun on the camera and expect it to magically solve every lighting condition.... a bit like expecting a pocket torch to light up the whole house and blaming the torch when it doesn't

Have a read of the thread and see if it sheds any more light (pun intended!)

Matt

This has been bugging me as well-

I too use a K10D (John Riley's fault!)
I am getting better results with the camera flash than with the AF540FGZ in auto pttl digital mode (with the camera settings unchanged).
I am not using any diffusers or fancy techniques, except for reading and following the manual supplied both with the flashgun and with the camera. I even bought the Magic Lantern guide for the K10D.
Although I printed off and read the comprehensive flash guide from this forum ( thanks to the author whose name escapes me), I cannot get my head round this "problem" of underexposure.
Are there any known problems with compatability or any faulty batches I should be aware of?

Neil



Nil Desperandum

Just make sure you've got firmware 1.30 for a start.
('Twas me that did the guide too!)
Matt

Yep, got 1.3.

Glad you acknowledge your work, except that it didna help me.

Could there be a problem with the hardware Matt, or is it a software problem between my ears?

Neil

Statistically speaking (and almost always, in my own case), the latter is more likely, although the former is sometimes tricky to rule out

Care to post an example or two just in case there's anything obvious in the frame that would account for the underexposure?

Smallish, darkish objects with a largish, brightish background frequently end up underexposed on auto in my limited experience. Which is annoying, since of course they're often the reason for using flash in the first place.