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I'm having an issue with my Canon 6D. Whenever I shoot photos inside my house they turn out so grainy! I've tried turning the ISO down but anything lower than 6400 and my photos are basically black. Mind you this is mid-day, lots of light in our house, and yet the ISO on my camera will not go below 6400 and it is making my photos look terrible because they are grainy instead of sharp! Please help!!

Lens is 24 - 105mm f4.0. I shot at 24mm, f5.0, shutter 1/160th, ISO 6400 (this is on auto, but only because anything lower I set it at makes the photos black!).

I'm still kind of new to this, but I do understand ISO, aperture, and shutter speed need to be in certain places for light to come in!

marked as duplicate by scottbb, mattdm, MikeW Sep 5 '16 at 19:40

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    Why do you need 1/160th shutter speed when shoot at 24mm with a lens with image stabilisation? Is something in your house moving very fast? Otherwise - for every time you halve your ISO, increase your exposure time by two. Use ISO3200 with 1/80, ISO1600 with 1/40 and ISO800 with 1/20 - that should still work at 24mm with IS unless you have very jittery hands. – Erwin Bolwidt Aug 30 '16 at 6:53
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The short answer: It's darker then you think it is. Here's a depiction of various brightnesses and an an exposure value which nominally will give correct exposure at that brightness. Note that these are overlaid — the area of the whole circle is what matters, not the separated rings.

exposure value as circles — this chart by Matthew Miller, CC-BY-SA 3.0

This seems shocking, because our eyes are so good at adjusting, but sunlight is 500× brighter than a typical home interior.

Your settings of "f/5.0, shutter 1/160th, ISO 6400" correspond to a correctly-exposed image at EV 6 — exactly what's labeled as "home interior" on this chart — so theory is meeting reality.

EV 0 corresponds to a one second exposure at a theoretical f/1 and ISO 100. You can calculate in stops from there by hand, or use a site with an EV calculator.

Note that higher ISO isn't, itself, what's causing the noise. That's really caused by lack of light, and, short of getting more light in, it's actually not a bad thing. See this answer for more detail and practical examples.

If you want a lower ISO, you'll need either slower shutter speed, wider aperture, or more light. If you want to go down to ISO 800, you'll need three stops to compensate (because you halve 6400 three times to get to 800). That means you could get equivalent exposure from f/5.0, ISO 800, and 1/20th of a second. You'll probably need a tripod, or good image stabilization, to get acceptable results, though — and if you're shooting people, they'll have to be very still. Opening your aperture all the way to f/4.0 gives you another ⅔ stop — but that only gets you to f/4.0, ISO 800, and 1/30th.

My suggestion is to invest in a low-cost off-camera flash, either optically triggered or as a radio slave.

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