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Yesterday I took several photos using my Nikon D3100 with my brand new SB700 flash. I was shooting with my lowest iso of 100. The flash was set to TTL. I noticed that when I started editing the photos these had too much noise in them and I could hardly edit them. I noticed, once I posted them on Flickr, that the iso had been set to 1600. This is probably the reason for the amount of noise, a high iso. But how did this happen if my camera was set to iso 100?

  • Could you eventually post a sample image and its EXIF info ? – Dragos Jan 6 '16 at 22:16
  • what do you mean? – Brian Jan 6 '16 at 22:18
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    So... we're supposed to search for every photo taken by someone named Brian on Flickr and Twitter and somehow magically know that's the shot you're talking about? :) Can you at least edit your post to add a web link, if not the actual image and EXIF information? – inkista Jan 6 '16 at 22:41
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    What we're primarily looking for in the EXIF is what shooting mode the camera was in as well as the exposure settings and whether the flash actually fired. Automated modes (like P or TTL) can shift settings without your realizing it. It's easier for us to have access to the EXIF than to ask you each of these issues/settings one at a time. – inkista Jan 6 '16 at 22:51
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    Are you sure the ISO was set to 100 and not "auto"? – Dan Wolfgang Jan 6 '16 at 23:01
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My assumption is that your situation is explained if you are using camera Auto mode. Your desire for low ISO will need to turn Auto ISO off with flash. However, if you are in camera Auto mode, you cannot turn Auto ISO off, so you have to get out of Auto mode, because, well, because Auto is automatic... auto ISO, auto White Balance, auto everything in Auto mode. That is its purpose.

The camera menu called Auto ISO is for camera A, S, P, or M modes (see the manual). But in Auto mode, everything is Auto, including ISO and WB, etc.

So you might have set ISO 100, and that does work in camera A, S, P, or M modes, but if you watch it, ISO will vary in Auto mode, and then ISO will be high indoors where you need flash, regardless if flash is used or not.

Nikon iTTL DSLRs have had three ways Auto ISO worked with flash. Cameras of the D3100 era are the middle way, and they increase Auto ISO for the ambient, regardless if flash is used or not (more detail on models at http://www.scantips.com/lights/flashbasics4b.html#iso2 .) Of course, Auto mode uses Auto ISO. So that means flash indoors will always be seeing high ISO if in Auto mode. You need to turn Auto ISO off for flash, which means you have have to get out of Auto mode if you want low ISO.

The earlier iTTL DSLR models did not do Auto ISO for flash, and the latest models have corrected it again, but the several models in the D3100 era sets Auto ISO determined only by ambient, regardless if flash is used or not. The Exif will show the ISO value it actually used with flash. You can turn Auto ISO off in camera modes A, S, P, or M. Or, if you put the flash into its Manual flash mode, ISO should always remain at minimum (manual flash cannot react to Auto ISO).

So you can use camera A or P mode, and set any ISO, and a reasonable aperture, and TTL will take care of the flash. The shutter speed will likely always be 1/60 second with flash. And you may want to set White Balance and Picture Control, because these are not necessarily auto in modes A, S, P, or M.

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