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12-13-2012, 11:21 AM   #31
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QuoteOriginally posted by dosdan Quote
But changing the f-number and/or ISO is not changing the flash power. The GN rating is the same, but the effective flash illumination distance is changed.

Take a GN of 53, rated with a 105mm FL lens, f/1, ISO100, 25% subject reflectivity. Assuming you had a subject of the correct reflectivity and you were spot metering it, if you had a 105mm f/1 lens, at ISO100 and max. flash power, you should be able to correctly illuminate a subject at 53m.

Now close the aperture to f/2. Since 1/4 of the previous light is now reaching the sensor, with the inverse-square law the effective distance is distance/f-number ie. 53m/2 = 26.5m. So the effective distance is reduced, but the GN rating is still 53 (the flash power produced is still the same, but the lens is letting though less reflected subject light.)

Now increase the ISO: ISO100->ISO800. You have not increased the flash power at all. The same amount of reflected light as before is reaching the the sensor. But now you are boosting the sensor output signal, thus increasing the rendered brightness. So a dimmer captured image is now displayed, after post-sensor gain, at the "correct" brightness level. Effectively you are increasing the acceptable flash illumination distance. Since ISO100->ISO800 is 8x, the square-root of this for the change in effective distance is root-8 = 2.8x approx.

So with a 105mm lens at f/2 and ISO800, the effective flash illumination distance is 53m/2 x 2.8 = 74m
approx.

BTW, GN is not just about absolute flash power either, as it also involves the directing effect of the head reflector: the flash is a beam, rather than omnidirectional. The directivity of the flash reflector changes in a "zooming" flash unit and this alters the GN rating at different FLs.

Dan.
I said EFFECTIVE flash power......

12-13-2012, 11:22 AM   #32
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QuoteOriginally posted by Noam Quote
I'm not sure what you mean - there is nothing reflective in the scene (both canvas print and piano are matte finished), and anyway all images in the set should have been affected equally.
reflection off the desk, dont say you didnt notice that
12-13-2012, 01:23 PM   #33
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QuoteOriginally posted by Anvh Quote
I said EFFECTIVE flash power......

Changing ISO and/or f-number is not changing the effective flash power. There is no increase in directed light output hitting the subject.

These are camera-based parameters and are altering the effective flash illumination distance. They are altering either the captured light level (f-number) or the rendered brightness (ISO). Their "effectiveness" relates to the camera, not the flash unit.

I understand the term "effective flash power" to refer specifically to the actual flash light output i.e. to the raw (omnidirectional) flash tube light output level in combination with the effect of the directing reflector and any diffuser or colour filter you have fitted. All flash-unit based. The "effective" part of the term "effective flash power" I consider to be referring to the "actual" light output. It's affected by flash tube aging (see http://laser-caltech.web.cern.ch/laser-caltech/report/Flash%20lamp%20Eg&G.pdf, pp. 17-18), directivity & diffusion, colour filtering and perhaps any change over time in the capacity or leakage of the main flash capacitor.

If you want to include the camera parameters, as well as the flash unit, in the discussion, it's better to use the term "effective flash illumination distance", which includes both ends of the flash-camera system.

Dan.

Last edited by dosdan; 12-13-2012 at 03:27 PM.
12-13-2012, 03:09 PM   #34
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Then you understand it differently then i mean it

With that i mean the amount of flash light in the photo/ captured by the sensor, so the effectiveness of the flash light so to say.

Sorry for the confusing.


Meaning the same thing but using different words, happens when being international.

12-13-2012, 03:40 PM   #35
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Actually, effective flash illumination distance also is affected by the subject reflectivity. So the full system is: flash-subject-camera.

If 18% or 12.5% subject reflectivity is used in specifying the GN then, for the same flash output power, this will result in a smaller GN than what Sigma gets with its 25%. I wonder if there's any standard for this in the GN spec?

Dan.

Last edited by dosdan; 12-13-2012 at 04:01 PM.
12-13-2012, 10:55 PM   #36
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QuoteOriginally posted by Noam Quote
Quicksand and Ray, thank you very much for your comments.
Quicksand, your words are both encouraging (reinforcing my findings) and discouraging (they may never fix this...). Thanks for the thread references. I read through at least some of them in the past.
Ray, can you confirm that you had the same misbehavior with the Pentax 540 flash? This is very important. Thanks for the tip on bright objects. I'll investigate it (although my test scene is not overly bright. The wall is painted green).

If anyone could upload some test images it would be awesome.
Two images will be enough: Just put the camera on a tripod, set appropriate manual settings, and shoot one straight-flash and one tilted-flash shot in P-TTL.
Noam,

The bounce flash over-exposure is the same with the Pentax 540. I have two K5s and they both exhibit the same issue.

The k10s and K20s I owned did not have this problem with any flash. I did not shoot much with the K7 I had, but I seem to recall that it had the same problem. I no longer have the K7, so I cannot confirm this.

It seems that after a quick test with that the K01 does not have the same problem with bounce overexposure using the Pentax 540 flash. I shot my kitchen cabinets with both cameras in bounce, and then included my white dishwasher in the scene. K5=serious overexposure, K01=proper exposure.

Ray
12-13-2012, 11:52 PM   #37
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I have a K7 and 2 AF540s, and can confirm that this bug is present. As Noam, I did a bunch of tests myself to confirm this. Simply moving the flash a tiny bit up causes the camera to overexpose by about 1.5 stops. I've taken to dialing back the flash by 1.5-2 stops to give my pictures some hope, but it does result in many way underexposed shots.

I will not buy a new Pentax camera until this bug is fixed.

Anvh, the reflections should not be relevant because with the flash pointed straight you have the exact same reflections.
12-14-2012, 08:15 AM   #38
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QuoteOriginally posted by dosdan Quote
Actually, effective flash illumination distance also is affected by the subject reflectivity. So the full system is: flash-subject-camera.

If 18% or 12.5% subject reflectivity is used in specifying the GN then, for the same flash output power, this will result in a smaller GN than what Sigma gets with its 25%. I wonder if there's any standard for this in the GN spec?

Dan.
No idea.... there is for light meters at least.

12-16-2012, 06:55 AM   #39
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Ummm.. bump?

Reading again some of the posts in past threads about the issue (which are remarkably similar to my findings), and seeing no counter-evidence to these findings posted here or elsewhere, I'm assuming that the bug is real.

So are we Pentaxians really that indifferent about a serious yet elementary bug that was not fixed in yet another product generation (K-7 then K-5 and now K-5II)?

The K-5II is a brand new product, so we still have a chance to convince Pentax to hammer down the bugs. I have no delusions that they'll care to fix the previous models.

I don't really know how to continue from this point. I guess I'll try customer support, but I have a feeling it won't bear fruit. they can't really support me, because it's a bug in the product - not a defect or mishandling. And I have little belief that the information will seep through all the way to the R&D engineers in Japan, based on how the SDM issues were not addressed, as an example.

And I don't want to swamp the forums with posts on this, definitely not on my own. The only thing I'll earn is troll reputation, which will be counterproductive I think.

Can anyone help push this forward? Perhaps some influential members with connection to Pentax reps? Adam, Frank, Falk, Yvon? Ned still around? Any other Pentax bloggers? Can you spread the word on this? Maybe we can create some buzz?


Last edited by Noam; 12-16-2012 at 07:11 AM.
12-16-2012, 12:41 PM   #40
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This issue has been here for... 6 years now you know, you really think they don't know about it?
12-18-2012, 03:28 PM   #41
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Is the K-01 immune from this issue, does anyone know?
12-19-2012, 01:45 PM - 1 Like   #42
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I just tested with the K-01, exposure was perfect with the flash up. I will try to post some pics later.

Also retested with K7 and it did overexpose (with flash dialed back -1.5 exposure was good)
12-19-2012, 02:43 PM   #43
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QuoteOriginally posted by arnie0674 Quote
I just tested with the K-01, exposure was perfect with the flash up. I will try to post some pics later.

Also retested with K7 and it did overexpose (with flash dialed back -1.5 exposure was good)
Thanks Arnie, it would be great if you could post those pics along with an explanation of the conditions they were taken under (not just EXIF-type exposure parameters, but also the angle of the flash head, how far away the ceiling is, what color the ceiling is, and a description of the ambient light).

I know this is technically a thread in the K-5 (family) forum, but I think it's a good idea to try to capture the scope of this problem.

So far, it seems the K-7, K-5, K-5 II, and K-5 IIs are afflicted.

The K20d I used to own, and a K-x I have tested, had no problems with bounce flash exposure. And now Arnie is saying the K-01 is OK too.

I don't know about the K-30 or the K-r.
12-19-2012, 02:50 PM   #44
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What about doing it in Liveview mode? Especially with the k-30, as it shares quite a bit with the k-01.
12-19-2012, 03:51 PM   #45
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Live View on the K7 is no better than regular. I dont have a K5.

I dont really have the time to test all these parameters. All I know is if the flash is bounced, it over exposes. Always.
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