Source of Noise With 7d

[Gerry]

I use a Canon 7d and since getting it I have been somewhat disappointed with the amount of noise I get in my shots compared to my Rebel. Tutorials suggest shooting overexposed to reduce noise however when I try that I seem to just get the same amount of noise when the exposure is lowered in post for an acceptable composition. Therefore, my question is, what causes noise, Low light levels on the subject or low exposure level? Will more light on the subject reduce noise (obviously, in studio)? Your thoughts are appreciated. Thank you.

[Colin Southern]

Hi Gerry,

In essence it's caused by 3 things ...

- Significant under-exposure

- Loooong exposures, and

- High ISO

If you're comparing noise with your Rebel at 100% in Photoshop then it's not a fair comparison as the 7D has many more pixels which may well make the noise LOOK worse, even though it isn't.

Send me a RAW image if you like, and I'll take a look at it for you.

[Gerry]

Hi Gerry,

...the 7D has many more pixels which may well make the noise LOOK worse, even though it isn't.

Send me a RAW image if you like, and I'll take a look at it for you.

Hi Colon;

If the noise "looks" worse but isn't, it seems to me it is the same thing a "being" worse! "It all depends on what the meaning of is "is". Seriously, even McQ recommends in his night shooting tutorial to shoot with seriously long exposures to raise light levels to eliminate noise.

I forgot the link to send RAW files. Could you pls. advise and I will send the file that is currently bothering me. Thanks, Colon.

[Colin Southern]

Hi Colon;

I'll be Colin today - I think "colon" is something else!

If the noise "looks" worse but isn't, it seems to me it is the same thing a "being" worse!

Not necessarily. When an image is down-sampled for display on the web, noise is vastly reduced - and when you don't down-sample, but only print a small image (say 6 x 4 or 12 x 8) then the noise is usually too small to be seen with the naked eye. So when people are trying to compare noise they typically get "up close and personal" and look at it in 100% view where a pixel of noise (or image) corresponds with one pixel on your monitor (normally what you see on your screen - if you're looking at the entire image - is only a small selection of the total number of pixels available (unless you have a monitor that happens to have 3 or4 thousand pixels wide, and 2 or 3 thousand pixels high!).

So - if you examine pixels from a relatively low pixel camera with ones from a much higher pixel camera - both at 100% - then you're not comparing apples with apples; you'd need to compare them so that the images (or portions thereof) were the same size which may mean that the lower resolution camera gets inspected at 100%, but the higher resolution camera at only 50% (or there abouts) - and at 50% the noise will appear less.

even McQ recommends in his night shooting tutorial to shoot with seriously long exposures to raise light levels to eliminate noise.

You'll never eliminate noise; the goal is to get it to a level where it's not noticeable or detectable, and correct exposure has a LOT to do with that. Sensors on modern cameras can capture around 12 stops of dynamic range, but we generally only need around 6 or them - which is good because the top 6 are pretty clean (well above the noise floor) but if you under-expose by - say - 3 stops then you're essentailly use stops 9 down to 3 of the sensors dynamic range ... and at 3 stops up from the noise floor there'll be quite a bit of noise creeping in ... and the more you under-expose, the worse it gets. If you under-exposed by 6 stops then the remaining 6 stops that you had remaining would be marginal quality at the "highlight" end, and pure noise at the other. So the first rule is to get as far away from the noise floor as possible.

But wait, there's more ...

When I said that we typically only need 6 or the potentially 12 stops available I lied! (sort of) - that's pretty much the case with a normal reflective scene taken during the day (even with a little backlighting) - but at night things change; the black areas aren't light reflecting off a black object (which normally reflects quite a bit of light) ... it's pure shadow (due to the fact that this thing called the earth is between your subject and the sun) - but - you have some comparitively very bright highlights in the form of lights (eg street lights) (highlights) - and then you have areas inbetween where the light level is somewhere inbetween. All of this is typically too much for the sensor to handle - but - the camera is programed to give a fair amount of weight to protecting highlights ) - not all of them, but it'll usually set the exposure such that the highlights aren't blown too badly ... but at the expense of totally under-exposed midtones. In post-processing you decide "uh - looks too dark overall (you'll be primarily looking at wide-range midtones) so you'll do whatever it takes to raise their levels and thus raise both the under-exposed midtones and a fair amount of noise that's also lurking around those areas.

So the best way to handle night time exposures (in my opinion anyway) is to simply use manual mode and simply increase the exposure so that the MIDTONES look to be an acceptable exposure on your review screen. Yes, you'll have small areas of blown highlights from the like of street lighting, but that's just the way it is ... small areas of blown highlight from lighting looks a whole lot better than large noise areas from under-exposed midtone. Bracketing your exposures is also a good idea until you get the hang of it (and even then it's often still a good idea).

Many cameras are only capable of taking a 30 second shot (maximum) in manual, Tv, or Av modes ... so often you'll need to put the camera in bulb mode (and preferably use a timer, unless you're a glutton for punishment). With a timer you can set something like a 16 minute exposure and then go sit in a warm car while the camera does it's thing - however - if waiting isn't your thing you can shorten the exposure by using higher ISOs - but - they also reduce dynamic range (but long exposures can also increase noise) - so often you have to find a tradeoff ... usually if I'm standing in freezing water then high ISO / less dynamic range wins!

Having just said all that ... another technique for reducing noise is to combine multiple images of the same scene / exposure to reduce noise (it gets averaged out). It works best with high ISO (eg for 1600 ISO take 16 shots) but can also be used on low ISO shots to further improve things (so long as any movement in the capture is desireable (eg clouds) -- this is where expensive tripods win over toy tripods!

I forgot the link to send RAW files. Could you pls. advise and I will send the file that is currently bothering me.

Pop along to www.sendthisfile.com - create a free account - send the file to yourself - and then copy/paste the download link into a PM and send it too me (don't post the link here because on free accounts the file can only be downloaded 3 times, and others will beat me too it!

Thanks, Colon.

You're very welcome, but I'll still be Colin today!

[Gerry]

I'll be Colin today - I think "colon" is something else!



You're very welcome, but I'll still be Colin today!

Jeez,....I have a hard time keeping it straight who you are on daily basis. If you recall, I did that once before.....I'm sooooooo American. My apologies, Colin, we don't have that name in our language....It's funny, I always have remarked how Colin Powell goes by the pronunciation of his first name as COLON....now I'll read the rest of your post!