Pentax-DA 18-135mm F3.5-5.6 Review

Distortion

Like most lenses, particularly those venturing towards the wider end of the focal length spectrum, distortion is usually present in some way, shape, or form. Coming in several variations -- "barrel" or "pincushion" are the most common ones and the types we'll be focusing on -- distortion in the context of photography is an optical aberration (deformity) that results in bending and deforming straight lines in a scene so they appear to be curved or slanted in the resulting image.

Distortion Explained and Generic Examples

To illustrate what this actually looks like in practice, let's take a look at the following series of grids.

No distortion Barrel distortion Pincusion distortion

Barrel Distortion

Barrel distortion is aptly named because its straight lines (either horizontal or vertical) bend in such a manner that they give off the impression they are "bulging" towards you, similar to the way a barrel would look from the side. Notice how the lines through the very center of the image are straight yet increasingly distort the further they are from the center. This is because they are aligned with the optical axis of the lens whereas the edges have reduced magnification. Barrel distortion is most prevalent in wide angle lenses because the field of view is wider than the size of the image sensor, causing the edges to be "squeezed  in," and hence why the effect is much more pronounced on the super wide focal lengths (16mm and below) on APS-C sized sensors as opposed to their equivalent field of views on 35mm Full Frame (24mm and below). Also, fisheye's are the kings of barrel disortion, and it's their purposeful implementation of it through an optical design that purposefully distorts in this manner that allows for such a mind-bending wide field of view.

Pincushion Distortion

The exact opposite of barrel distortion, straight lines curve upwards in the corners. Common in zoom lenses, especially those with longer focal lengths, it isn't uncommon for superzooms to begin with barrel distortion in their wider focal lengths and then transition to pincushion towards its telephoto end.

While it may seem like all hope is lost, the above examples were exaggerated to clearly see the effect of either distortion. And thanks to modern post processing software, such as Adobe Photoshop, the less of an issue these maladies have become. But of course it's always better to start with the best file possible before your image file ever leaves your memory card. In reality, the actual degree of distortion in any given lens is usually much less, and even the best lenses will exhibit some sort of this deformity, minimized as much as possible thanks to careful attention to optical engineering. The better controlled these lenses are, the more of a premium you'll have to pay for them - both in price as well as bulk.

Back to the crux of this review - how well does the DA 18-135 WR lens hold up in terms of distortion?

Brick Wall Distortion Test

For our controlled "brick wall" test (because what would a lens review be without a brick wall), we shot through the focal length range. Each of the series of shots below incorporate an "Uncorrected" and "Corrected" version displayed in the same frame.

For all the below images, you'll notice a green icon in the middle of them, illuminating once you move your mouse over the image. Grabbing it and sliding the bar to the left or right will allow you to alternate between the two versions for a much easier and dynamic juxtaposition.

Additionally, please ignore the white balance issue - the bricks are a deep red as you'd expect, but the color differences (and the green cast, a reflection off the light green wall to the left) weren't noticed until the images were loaded on the computer and the wife pointed it out to her colorblind husband. Because they were accidentally shot in JPG rather than RAW, they were left as is - the purpose of this section is distortion anyway.

Without further ado, 18mm:

before
after

For this image (and all the rest), it was manually corrected using Adobe Photoshop Lightroom. The correction needed was +7 to straighten the lines. Don't be alarmed if you noticed the image "shrink." That is normal for correcting barrel distortion - think of it as you're compressing the image from the top and bottom to straighten out the lines and remove the aforementioned bulging.

The rest of the barrel corrected images don't display the shrinking of the frame, and this is by design - there's an option that allows you to correct distortion by stretching the image so it maintains the original crop (at a resolution and sharpness cost). It wasn't activated on the above image so you can see the effect correcting barrel distortion has on the overall image as compared to its original state.

For our next focal length, the 18-135 WR was set to 24mm:

before
after

As you can see, zooming in the lens just that little bit has resulted in a drastic reduction in distortion, with only a correction of +2 being required. And even then one could argue personal preference between the two.

Third, we visited 35mm:

before
after

Between 24mm and 35mm, clearly the DA 18-135's "distortion middle ground" had been reached, trading barrels for pincushions. For the correction, imagine a thread passing through the very center of the image and you pulling up on it, "raising" the middle to straight the lines. A correction of -5 was needed for this test image.

Our penultimate focal length in this test, 55mm:

before
after

With a negative two (-2) adjustment to correct for the pincushion effect, it's negligible in our eyes.

Finally, another -2 adjustment for the 18-135 WR's max focal length:

before
after

Real World Distortion Examples

Shooting bricks walls is fun and all, but how do the above examples extend to the real-world (unless bricks are your thing...)? The following examples taken with the DA 18-135 WR show its entire focal length range in actual shooting situations so you manage your own expectations on how distortion from the 18-135 will affect your photography.

Example One

For our first "real-world" example, we revisit the scene used in the General Image Quality page, specifically the 18mm frame. It required a +7 adjustment in Lightroom to get its verticals aligned.

before
after

Example Two

Next we take a look at a common indoor shooting environment - one's living room. This was again shot at 18mm and required a +7 adjustment as well.

before
after

Example Three

For our final 18mm example, we once again take a look at an indoor scene, but a much more challenging one. Stairs as well as a long hallway are always a very difficult situation to shoot with a wide angle because the scene will exude barrel distortion, such as below:

before
after

Example Four

Next, we leave the safety of our humble abode and visit the cold subarctic tundra of Finland for this photo taken at 53mm. Once again, because the type of distortion changed, the value of the correction needed changed from being position to negative. In this case, -5.

before
after

Example Five

Third, this image was captured at 68mm, and a -7 adjustment seemed to affect the horizon line most optimally.

before
after

Example Six

Our final real-world distortion test, we revert back to the scene we started with - only this time at the 18-135's maximum zoom. The distortion correction reads negative five.
before
after

Verdict

In real-world shooting, the 18-135 WR displays some slight barrel distortion at its widest setting in an outdoor, environment with wide vistas. Take the lens indoors and twist it all the way to the left and it becomes more exagerrated. This is especially true of situations are that inherently challenging, such as the aforementioned staircase+hallway example. Zooming in but a hair renders barrel distortion all but indiscernible, especially in wide open spaces.

As expected, the lens transitions from barrel distortion to pincushion distortion between 24mm and 35mm. We weren't able to discern the exact focal length, as the differences became so minute, but we estimate it to be at approximately 26-28mm.

From these focal lengths all the way to its very end, there's some corrections that can be made, but one could argue they aren't necessary. In fact, while making the corrections to straighten the lines and horizons in the telephoto shots, an onlooker mentioned that some of them looked better in their original state.

So what does this all boil down to? Pincushion distortion will have a negligible effect on your output with the DA 18-135 WR, especially so if you are using post processing software to finetune the image to your liking. But that's just it - the vast majority of the time, distortion correction was not necessary at the lens' longer focal lengths.

Conversely, barrel distortion can be a nuissance, and even a problem, if one chooses to use the lens with abandon. It should be avoided in those very tight situations, and if necessary to do so, then used deliberately to minimize the bulging aberration.

Additionally, should you not want to correct for distortion in your favorite program, then you're in luck! The DA 18-135 WR is a Pentax lens. This means it can take advantage of how all of your Pentax DSLR's are capable of in-camera distortion correction with this lens, automatically adjusting the pre-programmed lens profile for the situation and focal length you are shooting at. For all recent Pentax cameras, this function can easily be accessed to turn it on or off by pressing the Info button.

On the following page, we take a look at how the DA 18-135 WR handles vignetting across the frame.


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