Underexposure due to flash or body?

audiobomber · 02-07-2009, 10:25 AM

I'm an idiot. I had the camera set for Wireless mode, and the on-camera flash turned off (controller position.)

**Marc Sabatella** · 02-07-2009, 11:12 AM

I looked up the guide number for that flash, and found it's 174 in feet at ISO 100. So at f/5.8, the maximum distance for good exposure would be 174/5.8 = 30 feet. So you were at the extreme limit of distance at f/5.8, and definitely too far away at f/8.

Of course, that's assuming the birds weren't in daylight, which it sounds like they were. So your problem is probably something a bit different. While it should have been *possible* for the flash to illuminate the birds at f/5.8, I can prpbably guess what happened. The flash meter works by doing a pre-flash and measuring light coming off the subject. The snow in the foreground probably reflected a ton of light and told the flash to scale *way* back. I doubt it even noticed the birds. If there is a way to tell the flash to fire full power, you could try that. You'll get nicely exposed birds and a blinding mess of overexposed snow, but it should work.

The trick, of course, would be in figuring out how to light up the birds without blowing out the snow. But I'm no flash expert. I'm just applying a little basic knowledge and some common sense. Most likely, the way to handle this would be to use M mode so you can set an exposure that takes the ambient light into consideration and then force the flash to provide the right amount of fill via flash compensation, or something like that. I actually have no idea how to *fix* the problem. But I have no trouble understanding why you might have problems in this situation.

audiobomber · 02-07-2009, 12:10 PM

Could someone please tell me why this didn't work? The K20D was set the same way for both photos: Aperture priority, Auto ISO, +2 EV, 55-300mm at 300mm, F8.

Flash on camera, Sigma ST 530 Super, P-TTL mode.

Flash off

I need a longer lens

**Marc Sabatella** · 02-07-2009, 12:41 PM

See my reply above. At f/8, the birds were too far away to be lit by flash alone. But the camera didn't know that - it saw that you were using flash and duly chose a shutter speed far too fast for the ambient light to sufficiently illuminate the scene at ISO 100. If you wanted the ambient light to contibute more, you should have used M mode, not Av, so you could have chosen a slower shutter speed. Or fixed the ISO at a higher value rather than let auto pick 100 for you. But you'd probably still have the other problem I mentioned - the flash metering would have noticed the reflections of the flash coming off the snow and therefore cut off the exposure long before it should have. Not sure if even the +2EV would have been enough to counteract that problem.

audiobomber · 02-07-2009, 02:04 PM

I chose Auto ISO, the camera chose ISO 100. The K20D can boost the ISO in flash mode, I've seen it. So why didn't it? I wanted the camera to take the same exposure it metered without the flash, but I wanted to add fill flash. Instead it metered way below.

**Marc Sabatella** · 02-07-2009, 02:58 PM

Originally Posted by audiobomber

I chose Auto ISO, the camera chose ISO 100. The K20D can boost the ISO in flash mode, I've seen it. So why didn't it?

I don't know, but it must have thought it wouldn't need to. And indeed, had your subject not been too far away for the flash to reach, and not you not had highly reflective snow right in front of you that cut off the flash prematurely, that exposure probably would have worked just fine.

How do you respond to my observations about the distance of the subject and the reflectivity of the foreground snow? Do you not see how those two factors complicate the situation? If your flash was a laser beam that could have lit the bird only - and was strong enough to do so at the distance you were - then you'd expect good results, sure. But that flash probably reflected off the snow at your feet, which reached the camera sensor and made it say, "woah - too much light! too much light!" - thus cutting down the flash output far too much to reach the bird. The area at your feet that probably caused the bright reflection reflection doesn't show in your picture, so you aren't aware that this happened, but I'm betting if you had taken the same shot with a wide angle lens, you'd see exactly what I'm talking about - bright snow at your feet, getting progressively dimmer as you go further out. And since the flash would was insufficient to illuminate the bird at f/8 even at *full* power, no surprise that you get an underexposed shot in a case where the camera is scraming at the flash to cut down on its output because of the bright reflections in the foreground.

I wanted the camera to take the same exposure it metered without the flash, but I wanted to add fill flash. Instead it metered way below.

I think my explanation makes perfect sense and probably predicts exactly the results you are seeing, but since I'm not a flash expert, I'll bow out of any further attempts to try to explain *exactly* what was going on, since I don't know for sure. Maybe someone else can explain the exact procedure followed by the camera.

Instead, I'll suggest you look at it a different way. You say you wanted the exposure the same. But you didn't present the same scene, nor did you ask the camera to do the same thing. In the first case, you presented a scene lit by the sun 93 million miles away from every point in the scene and asked the camera to meter off that and set an ISO and shutter speed. That's a straightforward task. In the second you presented a scene that was lit evenly by the sun but also very unevenly by a flash unit about five feet away from the highly reflective foreground but 30 feet from the bird, and asked the camera to not only meter set ISO and shutter speed but also to set the flash output. So you gave it scenes that were lit *very* differently and then asked the camera to do different things, too. I don't find it that surprising that this led to different results.

audiobomber · 02-08-2009, 07:35 AM

Thanks for trying Marc, but your theories are in vain. I decided to do some empirical testing and got out the K100DS. I photographed the same scene with the same settings. It exposed the photo perfectly. Then the K20D took the exposure perfectly. So what I am left with is some sort of malfunction in the shot I posted yesterday. I have had this problem intermittently with the K20, and not that I can recall with the K100DS.

It appears to me that the flash didn't fire during the bad exposure. I did watch the flash and it fired during the spoiled photo session, but I can't say for sure that it fired during the 1/180s exposure time. The camera obviously calculated an exposure that included the flash, but somehow either the fill flash didn't fire during the exposure, or it was at a lower level than the camera expected. I am certain I'm experiencing a hardware problem, but I don't know if it's due to the K20D or the Sigma 530.

**Marc Sabatella** · 02-08-2009, 12:05 PM

Originally Posted by audiobomber

Thanks for trying Marc, but your theories are in vain. I decided to do some empirical testing and got out the K100DS. I photographed the same scene with the same settings. It exposed the photo perfectly. Then the K20D took the exposure perfectly. So what I am left with is some sort of malfunction in the shot I posted yesterday.

Or - trying vainly some more :-) - whatever reflection triggered the premature cutoff of the flash the previous time didn't happen this time. You were standing in just a slightly different spot, or the snow melted just enough that there was no longer a perfectly reflective surface pointed right back at you. I can totally see that being kind of random.

It appears to me that the flash didn't fire during the bad exposure.

The EXIF says it did. Clearly, it just didn't fire *enough*. Were your successful flash pictures this time at the same ISO and shutter speed as the failed one last time? If so, clearly, the difference was the flash not firing as powerfully.

somehow either the fill flash didn't fire during the exposure, or it was at a lower level than the camera expected. I am certain I'm experiencing a hardware problem, but I don't know if it's due to the K20D or the Sigma 530.

I still say it sounds like a freak reflection causing the flash to cutoff way prematurely, but the silence from the people who actually use flash a lot and hence might have some *real* insight is deafening...

Rob · 02-10-2009, 04:45 PM

Originally Posted by audiobomber

I need a longer lens

What you need is a larger bird.

An ostrich, for example could also carry your gear.

Rob
just trying to help.

jeffkrol · 02-10-2009, 05:16 PM

Originally Posted by audiobomber

Thanks for trying Marc, but your theories are in vain. I decided to do some empirical testing and got out the K100DS. I photographed the same scene with the same settings. It exposed the photo perfectly. Then the K20D took the exposure perfectly. So what I am left with is some sort of malfunction in the shot I posted yesterday. I have had this problem intermittently with the K20, and not that I can recall with the K100DS.

It appears to me that the flash didn't fire during the bad exposure. I did watch the flash and it fired during the spoiled photo session, but I can't say for sure that it fired during the 1/180s exposure time. The camera obviously calculated an exposure that included the flash, but somehow either the fill flash didn't fire during the exposure, or it was at a lower level than the camera expected. I am certain I'm experiencing a hardware problem, but I don't know if it's due to the K20D or the Sigma 530.

Weather the flash fired on the first one or not it exposed more as I would expect the camera to expose without exposure compensation. W the 20 don't you have to set "flash ev" compensation in the menu or something?
As to Sigma flashes they are reverse engineered and can suffer malfunctions on newer bodies that they are incorrectly chipped for.
BUT one thing is it seems the flash is ignoring the +2 EV.......
First one has a normal non-EV adjusted histogram.

After fixing it, to me it looks better then your +2EV at iso 650 (whatever). still needs a bit of a fix though.

FunkyMonk · 02-10-2009, 06:34 PM

Originally Posted by audiobomber

Could someone please tell me why this didn't work? The K20D was set the same way for both photos: Aperture priority, Auto ISO, +2 EV, 55-300mm at 300mm, F8.

Flash on camera, Sigma ST 530 Super, P-TTL mode.

Flash off

I need a longer lens

What metering mode were you using?

audiobomber · 02-10-2009, 09:48 PM

Originally Posted by FunkyMonk

What metering mode were you using?

Multi-meter.

Marc came up with the answer, the subject was too far away for the flash at ISO 100. The second shots I took, I was trying to match the settings on the K100DS, so the shots were successful. I still think it should increase the ISO to give proper exposure though.

jeffkrol · 02-11-2009, 07:00 AM

Originally Posted by audiobomber

Multi-meter.

Marc came up with the answer, the subject was too far away for the flash at ISO 100. The second shots I took, I was trying to match the settings on the K100DS, so the shots were successful. I still think it should increase the ISO to give proper exposure though.

Hmmm... in that case a pre-flash would register nothing. If the camera honored the +2EC it should still have come out looking OK.... So you didn't use exposure comp w/ the flash? If the flash hit the target w/ 0EC it would still look more like 1 then 2. What am I missing here?

audiobomber · 02-11-2009, 08:24 AM

Originally Posted by jeffkrol

Hmmm... in that case a pre-flash would register nothing. If the camera honored the +2EC it should still have come out looking OK.... So you didn't use exposure comp w/ the flash? If the flash hit the target w/ 0EC it would still look more like 1 then 2. What am I missing here?

I did use +2 EV with the flash. I agree with you, the photo should have turned out, at least it would have had I used a Canon. It seems that Pentax only accomodates fill flash in Hyper-Program mode. I'll have to try this same setup in P mode and see how it works.

audiobomber · 02-11-2009, 08:27 AM

Originally Posted by Rob

What you need is a larger bird.

An ostrich, for example could also carry your gear.

Rob
just trying to help.

LOL! I think I'll use a swan, because then I wouldn't need the *%(@#^! flash.