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07-23-2012, 11:21 AM   #1
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What really triggers a new exposure set?

Forgive me if I cannot be very precise in exposing my issue. I know I should have done a lot of tests to troubleshoot this, but it always happens randomly and in moments when I could not do further tests. So is more an impression that I have and I want to check if it is just me or is common or maybe my camera is broken.
I have a K5.
I set auto iso exposure from 100 to 3200.
I always shot in TAv mode.
Now, sometime happens that for a mistake the ISO jump to 3200 immediately, for example I switch on the camera with the lens hood mounted, in a sunny day, and I expect to see 100 ISO and instead it says 3200, so I check, find the mistake, remove the hood, pres the shutter again half way and... it remains at 3200 iso. No noise (AF seems to remember the object I just pointed to and does not change adjustment) and no change in ISO.
Then I think, strange, there is a lot of light, anyway I pull out the flash, set 1/180 and try again. And again remains at 3200. So I start taking exposure to other objects around me, trying to find much more light or less light and see if it changes. Eventually after few try maybe it goes down to 1600 but never drastically change. I am sure that with the flash, outside, in a cloudy day, I should be able to use 1/180, f8 and maybe 200 ISO but unless I use 1/50 or 1/60 the ISO doesn't go down too much.

As said I cannot reproduce this, it is just a feeling that I have sometime that there is something wrong with exposimeters or the ISO auto setting or it does not understand that I am going to use the flash and does not decreases the ISO. Maybe after one shot with the flash the next shot has a lower ISO. I cannot swear on this. I can only say that if this were a consistent problem I would be 100% sure it is broken, but every time it happens maybe it is 2-3 weeks that I don't notice it so I forgot about when and how happened the previous time.

But now I am sure there is something strange, so if this problem description rings any bell please let me know. I cannot bring it back to the shop saying "there is not enough light".... I need to be sure it is an HW issue (or FW maybe, it is the latest).

Thx

07-23-2012, 11:35 AM   #2
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Setting aside your question, and more to the point, what do your photos look like? Please attach some where you think they demonstrate the issue.
07-23-2012, 11:43 AM   #3
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QuoteOriginally posted by quintinir Quote
But now I am sure there is something strange, so if this problem description rings any bell please let me know. I cannot bring it back to the shop saying "there is not enough light".... I need to be sure it is an HW issue (or FW maybe, it is the latest).
I just think you underestimate how little light there can be in a cloudy day.
When using built in flash with auto-ISO it will operate in fill in mode, so ISO will not change.
07-23-2012, 12:31 PM   #4
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Thanks for answering so quickly.
JohnX: Ehm....Good question :-) I am afraid I don't have any. As said I have not done really tests... when I notice that I get 3200 (consider I always start from 1/250) I just start playing with all the settings and different objects and even switch off and on the camera again, until I find a better compromise time/iso/aperture and at the end I shot only with something that "sounds better". For example If I switch on the camera and have (from the last time) 1/250, f8 and gives me 3200, then I try 125 then 60 and maybe f5.6 until I see ISO 400. So I don't think I have at hand example shot with the first exposure proposed by my camera. The point is that sometime I have the feeling that I change speed and aperture and 3200 remains there. But you are right, next time I will shot anyway and see what happens. I suspect that if the pictures looks good then I have to trust my camera right?

Fogel70: will do the obvious test mentioned above, so maybe you are right and is my underestimate. The reason why I don't think so, is that I had a K10D before the K5, and with that I could really never use more than ISO400, never the less I was shoting as much as now and I leave in the same place and do the same things... so I don't want to say that the K10D can make the same picture with ISO400 while K5 needs 3200, but just that my personal "feeling" is based on the fact that I used to take shot in similar condition with lower ISO. But as you say is subjective.... so I might be wrong.
As far as the hint on the flash.. this is a great info! I didn't know this thing and it explains a lot of other situations.... So if I want to set time and aperture without using auto ISO what should I use? Full manual? Or better question... how can I have auto ISO understand that there will be a lot more light when I shot? there must be a way to use the flash to make light (not just autofill) and therefore predict how much ISO is needed (not based on the ambient light I have before shoting but on the light I will have durig the shot).
Hope this question is clear... not sure I phrased it clearly (obviousy I am not native english speaking).

anyway thanks again

07-23-2012, 01:29 PM   #5
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QuoteOriginally posted by quintinir Quote
As far as the hint on the flash.. this is a great info! I didn't know this thing and it explains a lot of other situations.... So if I want to set time and aperture without using auto ISO what should I use? Full manual? Or better question... how can I have auto ISO understand that there will be a lot more light when I shot? there must be a way to use the flash to make light (not just autofill) and therefore predict how much ISO is needed (not based on the ambient light I have before shoting but on the light I will have durig the shot).
Fully manual is one way of getting full control of the exposure, but if you are in TAv mode you can use exposure compensation to get the camera to select lower ISO, You set the exposure for how much ambient light the camera should capture, then the flash fills in with the rest of the light for correct exposure. But as the built in flash is not very powerful there are limits on what it can do.

Any advanced flash photography really require more powerful external flash.
07-23-2012, 03:30 PM   #6
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Your problem, I believe, is using TAv mode with flash. Camera selects something like 1/2f for shutter speed, you select F8, where does the ISO go?. If it takes a correct exposure at those settings without flash, then your body made the right choice. It doesn't change when you engage flash, because how is the camera to know that you want something different?

I've gone into detail about how P-TTL flash works elsewhere so here's the condensed form. Your camera starts off with an exposure closest to correct without flash that it can manage within the constraints of the selected mode. If the mode doesn't allow you to control shutter speed, it gets fixed at the slower of 1/2f and 1/180. If it doesn't allow you to control aperture, it gets set to a value that will get you closest to the correct exposure for your chosen shutter speed and ISO. TAv doesn't work because ISO goes all over the place. You can work around this by limiting the auto-ISO range (and maybe devote a user mode to it) but there's an easier way.

In case it isn't obvious, you should use manual mode with flash because it's the only one that gives you control over all parameters. If your flash supports high speed sync, manual is the the only shooting mode that works with it. Dial in the shutter speed, aperture and ISO that you'd like to be able to use and let your flash take care of making it possible.

One other thing: if you've selected the menu option that allows you to shoot while your flash recharges, then you may well see your settings change radically in between flash shots.
07-24-2012, 12:20 AM   #7
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Again thanks both. Basically I understand that the camera is not broken and TAv is not the best with flash. So I am happy with that. Will learn the rest.
I always thought that flash (beside increasing or decresing the power manually from the menu) send out a fixed ammount of light (which the camera FW should know in advance I hope) and so set the rest knowing what is going to happen. You are saying that flash adapts its power to the setting (" the flash fills in with the rest of the light for correct exposure" and "let your flash take care of making it possible") that I select (and of course if I chose 1/180 with ISO100 my flash probably is not enough).
For me this is really a big new thing, so I will try to do real tests with this in mind.
Bye
Riccardo
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