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Tech Podcast Network
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Wednesday
Feb032010

Noise And ISO Test On Canon EOS 350d / Digital Rebel XT dSLR

Let's face it, of the three pillars that make up exposure in photography, ISO is the boring one, and the one that spoils the party.

Shutter speed is cool. You can freeze time, stopping a droplet of water mid-air as it splashes upward from a glass. You can stretch time with a long shutter speed, blurring clouds, the sea, and even people. If you pan while using a longer shutter speed you can bring a real sense of movement to your images, making the background blur as a race-car speeds past.

Aperture is King. Tighten up your aperture and keep everything in your photograph sharp, for an Ansel Adams like depth of field, letting the viewer marvel at the intricate details they see before them. Or perhaps open up your aperture, forcing everything in your photo blurred, while keeping your main subject in sharp focus. Magical.

ISO however, apparently has little artistic value when you change it. ISO is how sensitive your camera is to light, and in the digital SLR world we can simply increase our ISO so we can shoot in lower light, keeping our shutter speeds up to avoid camera shake or movement blur in our subjects. There is an unfortunate side-effect to this - noise.

In the old days of film if you wanted to shoot with a higher ISO you had to change the film in your camera, and higher ISO film usually had larger grain, and in digital cameras we have a similar problem, noise. As we increase our ISO our photos have more "speckles" in them. Details start to become lost, colours become muted, and the quality of the photograph diminishes rapidly. Or does it?

Let me back-track a little and explain the relationship between Shutter Speed, Aperture and ISO when it comes to working out Exposure, or how bright or dark a photo will turn out. To obtain an "acceptable" level of exposure (or brightness) in a photo, we have to let a certain amount of light into our camera. The amount of light that is let in, and hits our digital sensor, is governed by the Shutter Speed, which is how long the camera stays open to let the light in; Aperture, which is how big the hole is in the front of the camera that lets the light in; and ISO, which is how sensitive the camera is to that incoming light.

In all the modes on our cameras except Manual, our camera's help us obtain that "acceptable" exposure by juggling those three settings, usually with a bit of input from us. Lots of photographers, including myself, enjoy using Aperture Priority Mode, where I choose the size of hole (or f-stop) on the front of the camera. By using this mode I can control the depth-of-field in my photographs, having everything sharp, just my subject, or something in-between. As I change the aperture, my camera adjusts the shutter speed to keep the exposure (brightness of my photo) OK.

The problem comes when I'm shooting hand-held. If it starts getting dark, or I use a small aperture, my camera will choose a shutter speed that is too slow. I can't tell you the number of times I've got home to look at photos on my PC only to discover that loads are blurry because I've let the shutter speed drop too much - especially when I've been using longer focal length lenses and zooms.

(A quick hint here - never, ever, trust the LCD screen on the back of your camera to judge sharpness. That screen makes everything look sharp!)

The answer to this is to increase your ISO as soon as your shutter speeds drops below a certain amount. There's a simple way to work this out - if your're shooting hand-held with a 50mm lens, you don't want your shutter speed to drop below 1/50th of a second. If you're shooting with your zoom at 200mm (look on the lens barrel) you don't want to drop below 1/200th of a second. If you're at 500mm, don't go below 1/500th. Easy.

(Or of course use a tripod, but that won't eliminate the blur if your subject is moving).

I'm rubbish at doing this. Maybe its years of reading books and magazines, and listening to Podcasts where the advice is to always shoot at the lowest ISO setting your camera allows. It's meant to give the best photo image quality, and heck, I've even said so myself many times. It's like I'm locked into believing that I can get sharp shots from a 300mm lens handheld at 1/100th, if I just try harder, but that isn't ever going to happen, its just an impossible task. I need to bump up my ISO!

In order to free myself of this hang-up I resolved to take some test images with my Canon EOS 350d / Digital Rebel XT, at all the different ISO's it offers. I wanted a dull, overcast day, because that's what its like in England most of the time, and its also the occasion when I need those higher ISO's. I wanted to really get to know what images shot at ISO 800 and 1600 actually looked like, to see if my phobia of those levels was justified or not.

Here's the result:
Canon EOS 350d / Digital Rebel XT ISO Test


Wait a minute! Don't get disappointed. You can't see any detail in the above photo, you need to see the original to get an idea about the different qualities of the various ISO's, and understand what the photo-montage is. What I did was to take a number of shots of the same scene, at various ISOs. When I got home I zoomed into 100% and copied the middle bit out of each photo and created what you see above.

The bottom right part is ISO 1600, but given the Noise Ninja treatment, a piece of software that does its best to reduce noise from inside Photoshop, Elements and I believe Lightroom. (Or you can get it as a stand-alone program too).

So now you know what the above photo is all about, take a look at the original.

Have a scroll around. Look at the ships hull and super-structure, the sky and the background, at the different ISOs. What do you think?

If you want to see the photo's of the different ISOs in more detail, I've uploaded all the originals to this Flickr set.

Other Things That Can Increase Noise In Your Photos

Sadly, ISO isn't the only thing that can make your photographs noisy. Any sort of post-processing - changing levels, curves or contrast will often amplify noise. HDR is notorious for being a "noisy" technique. Make sure you save, or export to, .jpg files at the highest possible quality setting, or you'll end up with the even worse "artefacts" which can ruin the cleanest of photographs. Make sure you've turned on "Long Exposure Noise Reduction" (in the custom settings on the 350d), or that'll degrade quality too. Here's a comprehensive list of the possible pitfalls.

My Conclusion

The higher ISO settings on my 350d / Rebel XT are not as bad as I thought they'd be. OK, I won't get any images accepted into Istockphoto if I shoot above ISO 200, but they're good enough for most of my needs. (Plus when printing out photos lots of noise just disappears). I'll no longer shy away from bumping up my ISO when I need to. I now know more about my camera and feel more confident in getting the most out of it in challenging lighting condtions. My ISO phobia is cured!

Cheers, Rob.

Reader Comments (2)

Hiya Rob,

Good little foray into the world of ISO. Like you I think that some of us worry about it too much although others don't even tend to think about it. Noise reduction software both in camera and post processing can help out matters a lot more than we could back in the film days. (Although with film the grain was truly random this appears less so with digital and hence visually 'noisy' rather than grainy.)

If I'm just snap shooting I don't even worry about ISO, 400 to 800 is more than acceptable on my Oly for such shots even without further noise reduction. It's only when I have a particular setup shot in mind that the ISO is bumped all the way down where darker areas suffer most, (the exact opposite of what you naturally want to do of bumping the ISO up because of the dark areas!) Even then I'll sometimes bump up to 200 knowing I can get away with it.

Good analysis Rob and I enjoyed reading about it.

All the best,

Victor

February 6, 2010 | Unregistered CommenterVictor

Hi Victor,

I think I'm getting over my ISO-phobia, shrugging off the "ISO 100 only" brainwashing..... hopefully!

Cheers, Rob.

February 7, 2010 | Unregistered CommenterRob_Nunn

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