Instead of changing the brush size, you use two fingers to pinch your photo larger or smaller. Your masking brush stays the same size.
There are a few different blur adjustments in Tadaa SLR. Some look more natural than others.

The consumer DSLR may be dying, but that doesn't mean you have to miss out on the gorgeous effects of a high quality lens system. Tadaa SLR, released last week for iOS, is an easy tool for mimicking the shallow depth of field that you can get from a good SLR lens.

Doubtful that an iOS app could give my iPhone photos a natural-looking shallow depth of field and matching bokeh, I decided to give Tadaa SLR a thorough hands-on.

To start, I uploaded a photo of my sister holding a sheer scarf to the sun at a recent music festival. The subject, though isolated in composition, still had some competing aspects in the frame distracting from her face.

The first step is choosing what part of your frame you want to keep in focus. This is done using Tadaa SLR's masking tool. A "Detect Edges" feature helps you keep lines neat as you trace your subject. I chose to keep her face, neck, and chest in focus while letting her arms blur out.

You can choose to either blur out all of your un-masked frame or give a more natural-looking circular or linear blur.
Tadaa SLR also provides basic editing tools like brightness, contrast, and saturation adjustments.

I chose a circular blur because I thought that it gave my composition the most natural-looking effect. A focal box similar to those found in many capture apps helps Tadaa SLR keep the most important part of your subject in focus.

Next, I adjusted the brightness, contrast and saturation of my image, giving my subject a little more light on her face. When you are finished with your photo, you have to upload it to Tadaa SLR's social media tool. From there, you can email a link to your friends or share the photo to Twitter, Facebook or Tumblr.

Like nearly every photo editing app, Tadaa SLR has a package of filters and lets you adjust the intensity of each.
Tadaa SLR's social media aspect lets you share your photo and "like" and comment on your friends' images.

In the right hands, Tadaa SLR has the potential to be a great editing app. Like the original Tadaa, its effects are best used sparingly and carefully. It's too easy in Tadaa SLR to create a jarringly unnatural-looking blurring effect with a strong filter on top.

Despite its powerful editing tools, Tadaa SLR is totally free, (likely because its social media aspect gives the developers hope of future monetization a-la Instagram's ad integration.) It's available on iPhone 4 or newer and you need iOS 6 or 7.

With a little precision and a lot of patience, Tadaa SLR does a decent job of replicating an SLR lens’ depth of field. No matter how hard you try, though, apps like Tadaa SLR will never be as good as a real SLR lens.

My "before" shot.
After applying the effects in Tadaa SLR.