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11-10-2013, 10:36 PM   #1
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K50 flash overexposure in TAv problems & questions

Similar to the K30 and off-camera flash overexposure, I have found the K50 does the same in TAv mode. I quite like the camera and have to agree with the PF in depth review in all other respects.

Has anyone run into the same issues with the K50? There are 2 main things I've run into.

1) Firstly, flash won't even work with the AF360FGZ. I have to use the AF540FGZ.

2a) Changing the flash mode out of P-TTL or dialing in lower flash comp (on the 540) still results in overexposure. This happens in indirect & direct flash uses.

2b) And here's another twist... at close and medium distances using flash causes massive overexposure. At telephoto distances there doesn't seem to be problem. As you can see from the test pic below, the ISO drops down nicely to 4500 when at short telephoto. Can anyone explain?



2c) Switching to Av fixes the problem, but the shutter speed seems to default to the lowest ISO setting. It's a pain to dial the ISO from 100 where I usually have it to something higher as I find the shutter speed is still a bit low even with SR.

2d) Switching to Tv also fixes the problem. I have to keep it under 1/350 to prevent underexposure even with the flash set on HSS.

2e) Switching to M/M fixes the problem as can be seen in the pic below. The test subject is my oldest son.




Last edited by tranq78; 11-10-2013 at 11:15 PM.
11-16-2013, 11:45 AM   #2
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First the advice: don't use auto iso when using flash. It's the mother of all evil

Next the explanation:
  1. In TAv the camera will try to achieve normal exposure as if there is no flash. So with e.g. 1/60s and f/4 in TAv it might think that it needs ISO 1600 (an easy number as an example). It will use the flash as fill-in flash.
  2. There is a direct relationship between the guide number of the flash and the ISO; at ISO 1600 and assuming the zoomhead is set to maximum, the guide number will be around 200 for the AF540 (check the manual; assuming the flash head is at maximum zoom).
  3. There is a direct relation between the maximum subject distance that can be properly exposed, the guide number and the aperture; max subject distance equals guide number divided by aperture; so with the given numbers, roughly 50 meters.
  4. There is also a (lesser known) minimum flash distance; as a rule of thumb I personally use 10% of the maximum if I can't find it in the flash manual.; a flash simply can't flash shorter than a certain duration so there will always be a minimum amount of light coming out of the flash.
With the given numbers, your subject must be between 5 and 50 meters for correct flash exposure when using the AF540 either as automatic flash or as pTTL flash. If it's closer, overexposure is the result; if it's further, underexposure is the result. ISO 4500 makes issue obviously worse.

Note:
HSS reduces the guide number (I don't know how much).

Last edited by sterretje; 11-16-2013 at 12:52 PM.
11-16-2013, 12:17 PM   #3
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To add a little

Your camera has some built-in rules as to determine the shutterspeed in Av. If the camera decides on 1/60s, nothing stops you however from changing it to 1/180s by using Tv. In Tv however, the camera will change the aperture depending on the ambient light. As a result the maximum flash distance will change and you might end up with overexposed or underexposed pictures. The solution is to use M mode on the camera so you can set the shutterspeed and the aperture to your needs.

When you enter that hall where your kid sports, just set the ISO to something suitable for the event; it's basically a once-off so can't really be a pain if you don't try to change it for every shot. I'm not familar with the K50, but you might be able to assign a button to the ISO functionality.
11-21-2013, 07:51 AM   #4
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I don't have the Pentax dedicated flashes and my unit is the K30, but I suspect my experience using a generic pTTL flash will be roughly similar. As suggested, M mode at 1/180 with a set ISO is essential; otherwise the camera tries to go for the highest ISO and cannot properly account for the flash (resulting in overexposure). First, you will want to use the larger flash in any event because you want to limit flash duration to freeze the action. Second, you will want to set the aperture and ISO to at least three stops underexposure (before flash) so as to isolate the action (limit or eliminate trails of ambient light in the action). If you want some faint trailing (I don't mind it but that's a matter of personal preference), you'll prefer to set the camera and flash to trailing curtain flash to give you trails that follow the action rather than precede it.

Here is the key rule. Do some test exposures with the selected settings. If the exposure is off a bit, but you want to keep the particular ISO setting, change BOTH the flash output setting and camera exposure settings accordingly. Obviously if you don't like the amount of ambient light / trailing adjust your aperture accordingly (and you very likely will have to readjust your camera/flash compensation settings). In my experience with the generic flash in pTTL, you can nail it if you are within about 2.5 stops of full output of the flash, otherwise it is going to be off (typically overexposure).

If it was me, and I had 540 flash, I'd work in Auto flash at around 400-800 ISO with whatever gives you the 5.6-8 aperture setting using about 2/3rds of the full flash range to keep flash duration short. If anything, this will be more consistent than letting pTTL over-think the situation. I shot an old Vivitar unit in dim gyms in Auto for many years - most of them in film days - and had no troubles. Sometimes simpler is better.

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