Laszlo Pusztai

My Mark II Artist’s Viewfinder app has a unique feature from day one. Something that no competitor managed to copy: real-time distortion correction for wide converter lenses. In the past distortion correction was available through the profiles we made for all the wide converter and device combinations the app supports. Creating these profiles involves tedious measurements, and of course a purchase of each and every converter lens and iPhone/iPad we support.

The profile approach was good a few years ago with a limited set of converters on the market, but these days you can find at least a dozen different converter lenses at online retailers. This, together with the product lineup splits Apple already implemented with the Plus phones and Pro iPads, renders the previous modus operandi financially unsustainable.

So we decided to address this situation in a way that is beneficial for both our users as well ourselves: by allowing users to create a custom wide converter. And of course we do support simple distortion correction with this! Being a simple correction, it does not always produce the almost perfect results the profile-based approach is capable of, but as you’ll se in the examples later on, very capable nevertheless.

We’ll continue to support the profile-based approach for existing combinations, but will add new profiles only if:

• we already have the converter and the device in the lab, or
• we receive several customer requests to support a converter – as it was the case with the Moment Wide lens, or
• we cooperate with the converter’s manufacturer.

OK, with all that said, let’s see how it works. Once you select Custom for the wide converter, a new menu item – Custom Converter Setup – will appear on the menu.

Tapping it will bring up the converter setup screen that you can see on the left.

You can set the converter lens’ actual conversion factor with the first slider. Since this is almost always different from what the manufacturer says, you’ll definitely need to fine tune it for your actual phone (since the conversion factor also depends on the phone’s lens as well as the distortion correction amount – be prepared for a little trial and error). We support converters with factors between 0.45 and 0.7.

The second slider is used to specify how much distortion the converter lens has. It is an arbitrary scale from 0 to 30, zero meaning that you will not want to correct the distortion (suitable for well-corrected lenses such as the Moment Wide).

The setup procedure

Here’s how I recommend to do the setup. Set distortion correction first (if needed at all). I’d recommend 15 as the starting point for 0.65x-class lenses and 20 for 0.5x-class lenses. Check how the main screen looks, and watch for overcorrection in the corners. In case you still have barrel distortion, go back to the menu, and increase correction. If the corners are overcorrected, back off a little. A tiled wall, a window or a tall building are good test targets.

Once you’re happy with the correction, mount the phone steadily, turn off Wide Mode, and set the main screen so that you have some objects or markings at the edges of frame lines. Now turn on Wide Mode and check whether the objects/markings are still at the frame lines. Go into the menu and adjust the conversion factor if not. Repeat until you are satisfied with the result.

You can create only one custom wide converter, so it is advised to take note of the parameters in case you need to restore them later.

Correction examples

Walked up to Hallgrímskirja in Reykjavík to make some examples with my iPhone 6s Plus and two olloclip converters. Here is how the 4-IN-1 Wide lens looks with no correction.

olloclip 4-IN-1 Wide on iPhone 6s Plus – No distortion correction

As you can see below, this lens has a simple spherical distortion, which can be corrected almost perfectly with the custom converter setup. My experience is that most 0.65x lenses behave the same way and would only marginally benefit from a profile. Note that I set the conversion factor ad-hoc in these examples, so you may see some discrepancy in frame line positions.

olloclip 4-IN-1 Wide on iPhone 6s Plus – Custom wide converter with distortion setting 13

Now to the olloclip Ultra Wide. Distortion is huge, and without correction renders the lens pretty much unusable for viewfinder purposes.

olloclip Active Ultra Wide on iPhone 6s Plus – No distortion correction

And this is how it looks with the custom wide converter’s correction. You see some mustache-like corners as the simple correction is not able to eliminate complex distortions (which profiles can do), but the lens becomes usable.

olloclip Active Ultra Wide on iPhone 6s Plus – Custom wide converter with distortion setting 22

Just like with profiles, the simple distortion correction works only with wide and ultra wide lenses – not with fisheyes.

When will this be available? The feature is currently in testing in version 4.7, and is planned to be released later this summer. In the meantime, comments are welcome.

The updated app is now available for download.