ham888
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Reged: 11/06/2008
Posts: 190
Loc: Worcestreshire
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Hi,
Have my D80 set to Iso Auto. However when I use the built in flash (TTL) then Iso does NOT get automatically adjusted. In "P" mode with flash I sometimes get the exposure meter reporting underexposure so why does it not increase the Iso. If I do this manually it helps, or if I allow a slower shutter speed with flash that also helps.
So..why does it not alter the Iso automatically with flash? Any idea's.
Cheers,
Harry
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Harry
My Flickr
Edited by ham888 (29/06/2008 09:46)
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IvorETower
Little Buttercup
Reged: 15/11/2006
Posts: 1629
Loc: Camberley, Surrey
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Sorry, no idea. I set the ISO myself on my D80. It doesn't confuse the flashgun that way
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sillyconguru
enthusiast
Reged: 05/11/2005
Posts: 310
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Quote:
I sometimes get the exposure meter reporting underexposure so why does it not increase the Iso.
The electronic analogue display in the viewfinder only shows how the ambient light relates to the current camera settings, the flash metering is quite separate.
Quote:
So..why does it not alter the Iso automatically with flash?
The ISO will increase if the camera decides (after emitting pre-flashes) that the flash couldn't illuminate the scene with the current ISO.
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IvorETower
Little Buttercup
Reged: 15/11/2006
Posts: 1629
Loc: Camberley, Surrey
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Yes - but what does it take as the "current ISO"? That from the last shot taken, or something else, beacuse if the scene is dark enough to use flash, it ought to be bumping the ISO up to 800 or more, yet clearly it doesn't do this
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ham888
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Reged: 11/06/2008
Posts: 190
Loc: Worcestreshire
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Thanks for replys.
So..it seems that when I make manual adjustments to get the exposure meter to correct exposure I am actually setting the camera manually to take the shot without flash, correct?
Ivor, I assume the "current ISO" is the one selected manually - even with "Auto Iso" switched on then you can still select a default minimum Iso, I guess it trys this first?
Harry
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Harry
My Flickr
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El Sid
Going potty
Reged: 14/04/2003
Posts: 9160
Loc: Sussex-by-the-Sea
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Have you set an upper limit on Auto ISO adjustment? Obviously the camera won't adjust the ISO past the set limit (nor beyond the maximum possible ISO if no limit set) so underexposure will be indicated.
Not sure if the same is possible with Nikon but with my Canons there is an exposure lock button. When a flash is activated pressing this button forces the camera to perform a flash calculation and the results are displayed in the finder for you to check - also as long as the button is held down or the shutter released within 6 or so seconds this is the exposure used.
Are you using the inbuilt flash or a Speedlite? Speedlites are a lot more powerful than the inbuilt one and it's quite possible that the flash is sufficiently powerful to illuminate the scene without recourse to ISO adjustment.
Another thing to point out is that the exposure reading in the finder refers to the ambient light of the scene, all an underexposure reading means is that the background may be underexposed on taking the picture. As long as the subject (which should be the thing you focused on) is within the range of the flash that should be properly exposed. This largely applies to the P, A and M modes only - in S mode the camera will normally set the maximum shutter/flash synchronisation speed and adjust the flash output (and aperture if necessary) purely for the subject and ignoring the ambient light. The result in S mode will usually be a perfectly exposed subject and an underexposed background (and possibly an overexposed foreground if the subject is a bit far back). In P, A and M the camera attempts to balance the flash with the ambient light for a more natural result so you will find the shutter speed is often set well below the max sync speed (beware of camera shake if the speed is too slow). Again I don't know if Nikon are the same but with my Canons there is a menu item that tells the camera that you want to use slow sync speeds otherwise the camera will only go down to the minimum programmed shutter speed allowed for the lens in use.
You are also learning that you can't totally rely on automation but sometimes need to take personal control........... a valuable lesson at anytime... 
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Nigel
Completely BSRIPN 
ElSid Gallery
A camera in the hand is better than one in the cupboard........
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El Sid
Going potty
Reged: 14/04/2003
Posts: 9160
Loc: Sussex-by-the-Sea
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Quote:
Yes - but what does it take as the "current ISO"? That from the last shot taken, or something else, because if the scene is dark enough to use flash, it ought to be bumping the ISO up to 800 or more, yet clearly it doesn't do this
As I understand it with Auto ISO the ISO is set to the lowest setting which allow the indicated aperture/shutter ratio to be used. In each mode there is normally a default shutter/aperture combo which I presume take precedence over the ISO setting. The 'current ISO' should therefore be the one which is required to achieve the indicated S/A combination - be that the program default or a personal selection - according to the ambient light being metered.
Of course if the ISO is already at maximum (manually pre-set or absolute) then it can't be adjusted upwards...
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ham888
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Reged: 11/06/2008
Posts: 190
Loc: Worcestreshire
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Thanks again for replys.
My Iso is set at 100 - with auto Iso allowing it to be increased to a max of 1600.
Now, without flash if the Iso is incresed then this is indicated in the viewfinder. However, with flash the Iso does not seem to increase - maybe with flash it doesn't need to?
I didn't know the meter was reading ambient light, have to say thats a little confusing.
The flash is in-built and set to TTL, so I guess the camera will adjust the flash exposure to correctly expose?
Jeez, this flash thing is confusing especially for a newbie to DSLR's like me!
Harry
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Harry
My Flickr
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IvorETower
Little Buttercup
Reged: 15/11/2006
Posts: 1629
Loc: Camberley, Surrey
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Quote:
Jeez, this flash thing is confusing especially for a newbie to DSLR's like me!
Don't worry, it's not just newbies who find it confusing 
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El Sid
Going potty
Reged: 14/04/2003
Posts: 9160
Loc: Sussex-by-the-Sea
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Quote:
Now, without flash if the Iso is increased then this is indicated in the viewfinder. However, with flash the Iso does not seem to increase - maybe with flash it doesn't need to?
It's possible then that the flash overrides the AutoISO setting. The flash is very bright and will satisfactorily illuminate the subject - as long as it's within range - without recourse to ISO increase though your background exposure may suffer. It's also possible that with flash enabled the AutoISO only comes into operation if the metering pre-flash determines that the subject is outside the maximum range of the flash at the indicated ISO setting - in this case you wouldn't see the ISO value change as the pre-flash happens milliseconds before the actual exposure, the only way you'd know is to check the image info either on the review screen or via the EXIF info on your PC. IF the ISO was higher than the finder said then the AutoISO function has operated.
Quote:
I didn't know the meter was reading ambient light, have to say thats a little confusing.
TTL metering has always only been able to measure the ambient light. In the days before TTL flash metering you had to meter the ambient light then set your flash output manually. When TTL off-the-film flash metering and dedicated TTL flashes came along this made life so much easier as the camera meter actually measured the small amount of light reflected from the film surface during the exposure and shut the flash off as soon as the correct amount of light (flash and ambient) had been received. When digital came along it was discovered that the sheer reflectivity and and nature of the reflective surface made 'off-the-sensor' TTL unreliable so the digital system uses a preflash fired milliseconds before the main exposure and measured by the normal metering system. The net result is the same as the film method by and large.
Quote:
The flash is in-built and set to TTL, so I guess the camera will adjust the flash exposure to correctly expose?
I believe so or at least as far as it can within the limited range of the on-board flash. The flash probably only has a Guide number of 12metres at ISO100 - this means that at f2.8 and ISO100 the maximum flash range is 12/2.8 = 4 metres (approx) and IIRC about twice that at ISO400
Although it deals with Canon EOS systems Nick Guy's Flash Photography explains a lot about the basics of how flash photography works these days - most of which will be broadly applicable to the Nikon systems too.
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Jeez, this flash thing is confusing especially for a newbie to DSLR's like me!
Far less than it used to be. Nowadays to some extent all you need to do for basic flash is to turn the flash on and shoot, the camera takes care of all the tricky bits while you get a good exposure(within the limits of the flash of course). Previously you had, as I said, to manually set the flash power and if you go back far enough you actually had to use the flash guns guide number to calculate your require aperture from the focus distance indicated on the lens!
The downside is that if the automation is fooled by something it's often much harder to figure out what went wrong and how to compensate for it... 
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Nigel
Completely BSRIPN
ElSid Gallery
A camera in the hand is better than one in the cupboard........
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ham888
member
Reged: 11/06/2008
Posts: 190
Loc: Worcestreshire
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Wow, great and informative reply, very much appreciated.
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Harry
My Flickr
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