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06-05-2010, 12:11 PM - 4 Likes   #1
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Interesting thread; adjusting output on the Pentax flashes

Samsung SEF-36PZF seems to has one, too: Pentax SLR Talk Forum: Digital Photography Review
AF540FGZ hidden adjustment trimpot: Pentax SLR Talk Forum: Digital Photography Review
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Just found a tiny adjusting potentiometer on my Pentax AF540FGZ flash unit, hidden (but easily accesible) near the hotshoe, under the "FIX" sticker -tricky, isn't?
QuoteQuote:
I'm glad you did and shared your find. I fiddled with it on mine, it seems to adjust how bright the flash fires, like an exposure compensation. It might help me with mine, I'm not sure if its me or the Pentax way of avoiding overexposure, but previously I've had to always use exposure compensation in the camera to avoid having to fix in PP when it normally underexposes my indoor photos when I need the flash. Perhaps this will make the flash exposure more reliable for me.
Thank you
Russell

06-07-2010, 09:26 AM   #2
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Bookmarked! I'm going to be trying this later this week, my 540 has always needed a minimum of +1 FEC.
06-07-2010, 10:16 AM   #3
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Since I have two of the AF540's, I'll be very interested to see how/if this works without damaging the circuitry in any way?
06-20-2010, 05:16 PM - 1 Like   #4
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It definately affects wired P-TTL exposure, it seems to be a secondary exposure compensation function. I'm kind of surprised that in a microcontroller-controlled digital flash unit, one of the primary adjustments is an analog trimpot that can lose its setting as opposed to an EEPROM calibration value. However, that's good for us, especially since it's right under the "FIX->" sticker. (Yes, it's just a sticker, easy to pry off with a precision flathead screwdriver without any damage.)

You do not need to disassemble the unit at all other than removing the sticker. The glue is pretty weak so in my case I can put the sticker back and it will look exactly like it was to begin with.

It appears that a #0 precision Philips screwdriver is the best size for adjusting the trimpot. Turning the potentiometer clockwise will reduce light output, turning it counterclockwise will increase exposure output. I dialed mine CCW while repeatedly taking pictures of a carpet until exposure looked correct, and then on my next outing with the flash I brought the correct screwdriver to adjust as I shot to get the adjustment dialed to where I wanted it to be.

Right now I have it running a little on the "hot" side, since Pentax unwisely limited FEC to only +1 stop in the positive direction, but -2 stops in the negative direction. Previously I would consistently get underexposure even with +1 FEC dialed in at all times.

Do the 360s have this trimpot too?

06-20-2010, 05:40 PM   #5
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No overheating problems?
06-21-2010, 02:34 AM   #6
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QuoteOriginally posted by Entropy Quote
Do the 360s have this trimpot too?
Yes, but buried. Disassembling the flash foot is required. See the first link in the OP.

Thank you
Russell
06-21-2010, 06:58 AM   #7
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QuoteOriginally posted by Arpe Quote
No overheating problems?
I'm 95% certain this doesn't alter the capacitor charge voltage (and hence maximum flash output.) It just alters the P-TTL exposure control calculations. Not sure if it affects the classic "A" flash exposure mode. (Using an on-flash sensor with no preflash instead of TTL.) I'll do some testing to verify this soon.

So it shouldn't cause any overheating problems as it doesn't "overclock" the flash, it just changes it to expose correctly when conditions are within the unit's ability to expose properly. It won't help situations where the flash is underexposing due to lack of available power, only situations where the flash is dialing farther down than it should. (The latter is a common problem with 540s IMO with LOTS of "severe underexposure" reports and people like myself constantly dialing in +1 FEC and still needing to compensate further in PP.)
06-21-2010, 03:23 PM   #8
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Ahhh. I thought it was upping/downing the power of the flash, but it's just altering its metering - cool.

06-21-2010, 03:36 PM   #9
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QuoteOriginally posted by Entropy Quote
Bookmarked! I'm going to be trying this later this week, my 540 has always needed a minimum of +1 FEC.
I tend to agree with you - don't know if it is a camera feature or flash feature to protect the flash from clipping highlights.
06-21-2010, 03:58 PM - 1 Like   #10
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QuoteOriginally posted by Arpe Quote
Ahhh. I thought it was upping/downing the power of the flash, but it's just altering its metering - cool.
Yup, it only affects P-TTL metering. I just confirmed it.

If the flash is in M mode at full power, the trimpot has zero effect.

If the flash is in A mode (metering using a sensor on the flash), the trimpot has zero effect.

If the flash is in P-TTL mode, the trimpot seems to get you at least 1.5-2 stops of adjustment. It's VERY sensitive, a tiny nudge leads to a noticeable shift in the histogram when shooting my living room carpet. This probably explains why so many Pentax flashes are maladjusted. Also, the trimpot doesn't have stops at the end of the adjustment - it'll just go to the other end of the range if you turn it too far.
06-21-2010, 05:17 PM   #11
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Any idea on how much of a turn = 1 stop (roughly of course as a starting point).

Boy, all this technical talk and NO PICTURES. Get out there and show us your carpet pictures, c'mon people it's photography! (JK)
07-12-2010, 09:01 PM   #12
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AF-540 adjustment!

I cannot emphasize enough, the value of this thread!!!

I'd be willing to bet that most of the complaints about this flash unit under-exposing in PTTL, can be solved by following the directions in this thread.

I own two 540's, but I rarely use them both. Well I decided to test them and adjust as necessary, based on this thread. I was surprised to find that the one unit was about a full stop under-exposed compared to the other one, and even at that, the better of the two still under-exposed.

On the one flash, I had to adjust the little screw about 1.5 turns CCW, on the other one only about half a turn. Like was mentioned earlier, if you turn it too far you'll drop back to underexposure. At one point all it took was about 1/16 turn and the peak on my histogram dropped all the way to the far left quarter of the graph, and the resulting image was almost too dark to make out. (I shot my living room curtains instead of the carpet. )

Now I have each flash's P-TTL adjusted equally and optimally. The histogram peak is just past halfway on the graph, into the third quarter. That appears to be as good as it gets. I have yet to try either flash out in any other situations, but I'm hoping that the "AF-540 under-exposure problem" is a thing of the past. BUT, you'd think that for such an expensive flash, that they might do this adjustment at the factory before they ship them out, rather than leave it for an ingenious Pentax owner to discover! Again, thanks to the OP for this EXCELLENT information!

Last edited by NeverSatisfied; 02-08-2011 at 08:11 PM.
11-02-2010, 11:59 AM   #13
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A tutorial on the utility of this trimpot adjustment should be made into an article for the forum. I'd wager this to satisfy a number of disgruntled P-TTL users and have them using it effectively after optimising the flash output.
11-02-2010, 02:46 PM   #14
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QuoteOriginally posted by Ash Quote
A tutorial on the utility of this trimpot adjustment should be made into an article for the forum. I'd wager this to satisfy a number of disgruntled P-TTL users and have them using it effectively after optimising the flash output.
My second post in this thread pretty much sums up the procedure. About the only thing not covered (but covered later) is that a tiny adjustment goes a LONG way. Just "nudge" it between adjustments. Counterclockwise increases exposure, clockwise drops it. There is NO indication of when you exceed maximum adjustment and drop to minimum other than a drastic change in results.

Also to the one guy that had 1.5 turns CCW adjustment - Not sure if this is possible. I think one turn covers the full adjustment range before going "back to minimum power".
11-03-2010, 05:26 PM   #15
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Yea I don't remember for sure (and I'm not going to re-do it now that it's set! ), but you're right 1.5 turns does seem a lot because I do remember how touchy it was. Probably I went all the way past and back and didn't realize it. Anyway, the point is that people need to check and adjust the trimpots to optimize the 540, thanks Entropy for sharing your experience!
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