a8 January 2015

Dual Iso Test - 14 EV Dynamic Range on Canon DSLR

Some time ago I wrote a post about my first impressions from using Magic Lantern's Dual ISO feature. At the time of writing I promised you a more thorough test but it took me quite a long time to write it. But here it is finally today!

First a short recap - Dual ISO is a feature that increases dynamic range of Canon DSLR cameras to around 14 EV (similar to what you get in Nikon and Sony's cameras) from just about 11 - 12 EV but unfortunately it comes at a price - resolution in highlights and shadows area is halved. It's because when you capture Dual ISO image, what your camera really does is to use two ISO values and interlace between them. Halving resolution might sound a bit scary, but is it really that bad? Let's find out.

Below are two photos from London taken during sunset. Left one was taken using Dual ISO feature and right one is a normal single exposure photo. Both were shot from same spot, using same exposure parameters (ISO 100, 1 s, f/10) and using the same focal length of 47 mm. They aren't identical as apparently the lights on Big Ben became brighter in a few seconds that separated them, and I missed that when capturing them, but these images will work nicely for the test anyway.

Dynamic Range

First of all let's compare dynamic range of both image.

For this purpose I dragged Shadows slider in Lightroom to +100. I did this because I wanted to open the shadows under the bridge which I found way too dark. On the left you can see 100% crop from Dual ISO image and on the right 100% crop from a single image (note: you might need to click the images to see them in large to see a difference):

Quite a difference! Dual ISO image has very low level of noise (definitely acceptable) while single image is very noise with a lot of ugly colour dots! Single image requires a lot of denoising while Dual ISO none to very low.


So far so good. But just a while ago I mentioned one issue with Dual ISO which is lower resolution in highlights and shadows. Let's see how bad it really becomes by viewing 100% crops once again:

While at normal viewing size halved resolution of highlights and shadows is not a problem at all as it's impossible to notice that, at magnification of 100% the artifacts are clearly visible (there is much less detail in highlights and shadows).

BTW despite I showed it on example of highlights here, the same happens with shadows.


So summing up. Dual ISO works pretty well, by really increasing dynamic range of Canon cameras to values not known to Canon users. I had never dreamt of opening the shadows by dragging the slider to +100! Now it's possible. And that is really impressive.

Of course it comes at a cost of halved resolution of highlights and shadows but it is only visible at maximum magnification. At normal viewing size it isn't a problem at all. However, if you intend to do serious cropping or want to make large prints from your photos this can become a real issue for you.

Just last thought - would I recommend Magic Lantern's Dual ISO? Well, it depends - I'm after best image quality possible so I still prefer to bracket if possible. But there are cases when using such wide dynamic range makes life much easier, for instance: when shooting hand-held, with a lot of moving subjects or when you target photo compositions (which doesn't allow bracketed images most of the time).

So Dual ISO is a really great addition but it's not ideal solution. It doesn't make sensor any better, it just pushes it to its limits. Magic Lantern did great work here but it won't convert your camera into Nikon D810 or Sony A7R. Unfortunately :)

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