Top critical review
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GREAT BUILD QUALITY, POOR OPTICS
on July 24, 2013
Opening the box and holding this lens for the first time was one of the most exhilarating feelings I have had since initially touching my first K5. The weight was perfect, the size was perfect, attached to the camera it brought a well balanced feel, and the zoom and focus ring action was more than ideal. The purpose of this purchase was to have a WR lens with GOOD corner to corner sharpness at 20mm-60mm at f5 stops, and GOOD center sharpness from f6-8 stops from 60mm to 100mm not expecting good corners at the longer end. These are NOT unrealistic expectations given the poor history of reviews for this lens elsewhere than here on Amazon. Additionally, only primes give great performance, standard zooms a good performance with some sweet spots, and long zooms like this I only expect half the range to be good. Anything like an 18-200 is not conducive to decent image quality. Its just not going to happen due to the laws of physics.
So, after unboxing the 18-135, I shot the first shot: the DC motor coupled with accurate Auto Focus (AF) left me saying "WOW". Yes, it is that smooth and accurate. It brings back memories of the simplicity of Point and Shoot (PNS) cameras.
Then came reviewing the pictures taken with this lens. Once again, it brought back memories of PNS cameras. NOT GOOD. In fact, PNS cams do not have the issues this lens provides. I returned my first 18-135 and bought a second with similar results as the first lens. Please see specifics below.
I have the following lenses in my arsenal to make reference to:
Pentax A 50mm f1.7 and Pentax M 50mm f1.4
Pentax XS 40mm
Pentax DA 70mm limited
Pentax A 35-105
Pentax DA 16-45
Pentax DA 55-300
Pentax F 70-210
Pentax A 70-210
Lenses used and sold/returned:
Pentax 18-55 ALII
K5 (2 each, both used in testing)
THE SPECIFICS OF PERFORMANCE of the DA 18-135:
18-20mm f3.5 to f5.6: good center sharpness, soft corners, little CA, extreme vignetting
f6-f10: good centers, fair corners, some vignetting
20-22mm f4-6: same as above
f7-10: VERY GOOD
22-35mm f4-f7: EXTREME FIELD CURVATURE rendering images useless. Extreme vignetting. Only shooting a flat brick wall resulted in good corner sharpness.
f8-f10: Sharpest point of the lens - excellent images with no CA and no visible distortions
35-68mm f4.5-f6: Field Curvature cleans up some, Vignetting gets worse at 68mm, all vignetting is visible
f6-f10: Very good sharpness, but one lens had some right side blur which is undoubtedly just sample variation. I will not ding the score much for this.
68-88mm f5.6-f10: Max aperture moves here. Images get worse. Vignetting never goes away, but is best at f10 stops
Center is very good at f6 stops, but corners bad, best at f8 to f10. CA is very noticed, but cleaned up some in RAW (only). JPEGS are rough.
88-100m f6-10: Only Center sharpness is fair. CA is wild, images have a less than mediocre look, poor borders/corners. Vignetting is EXTREME in far corners.
100-135 f6-10: All pretty useless. CA is wild, Vignetting is a bizarre harsh dark triangle in extreme corners & CA and Vignetting is not editable at all.
Superb Color. Contrast is very very good for this type of lens. Overall, contrast fairs better than most lenses I have used.
Problem I found to be the worst was shockingly vignetting. The vignetting at the corners of images at 60-135mm were so isolated in the extreme corner, the darkening so dark (more than 1 EV) that the correction of this flaw was impossible. All editing was just a substitution for the issue and not a fix. Plus, I do not want to have to edit every picture for something this ridiculous. Additionally, the vignetting was extreme at ANY focal length below f6.
This lens is very clean from 20-60, or at least correctable. But it swings the other way on you after 60 and by 80 it is just awful.
EXPLANATION OF FIELD CURVATURE (FC):
I suspect many do not understand this issue and mistake it for border softness. Border softness and FC (field curvature) are related, but there is a difference. Border softness is somewhat relative to field curvature since both are being caused by the shape of a semi-rounded piece of glass stuck between a flat sensor and a relatively broad span of subject matter; all being crammed through this little piece of glass. The flat sensor and rounded glass do not and cannot perfect proportion, thus distortions arise. Manufacturers and engineers provided elements and methods for correcting these distortions, but when a lens does not correct softness caused by FC correctly or within reasonable standards, the term field curvature comes into play.
In the case of field curvature (in practical terms of what happens and not explaining physics), what happens is the edges of the field of view ACROSS A DISTANCE cannot be rendered without blur. So, photo a brick wall from a small distance, you will note the wall is sharp but the ground before it at the corners of the image are not. Move towards the wall a few steps, and more of the ground is sharp. Picture just the wall, and all corners of the image will be sharp.
This happens more due to the structure of the lens - its filter size, how many elements in place, etc. Prime lenses (fixed focal length) have fewer elements to deal with, so distortions are more easily corrected. Move to the standard zoom, it suffers more, but can be managed. Get a bigger longer lens with a slew of elements inside, things get tough. The usual border softness we see in lenses is caused by this, but when it is extremely severe, we now make reference to "field curvature"; we state that the "normal" and acceptable levels of border softness is so bad that field curvature is kicking in. The 3 Dimensional field is just to much for the design of the lens.
THE PENTAX 18-135 AND FIELD CURVATURE
This lens is the worst I have ever seen for this. I had to return one Tamron 17-50 for field curvature, but other Tamron 17-50's were okay on this subject. My DA 16-45 gets a little field curvature shooting at low to the ground or in near focus situations (less than 7 feet from subject) and f4-6 stops at 20-30mm. Some FC is normal (particularly at the wide angle of lenses, more so in super zooms). This 18-135 was so bad I sent the pics to Pentax and they said "well, it shouldn't be that bad" concerning my first lens. But the second lens was almost as bad. But this second one was behaving consistently in sharpness across the frame with exception to certain angles and shooting from 30-50mm or so. Bottom line, FC is severe with this lens and unless you can accept shooting at f8 stops all the time or memorizing where "the right spot" is given distance, focal length, and aperture, do not buy this lens.
TOO MUCH FIDDLING
I felt like I was constantly trying to guess when I could get a decent shot from this lens. Shot after shot after shot, I would review distorted, vignetted, soft images from this lens that just made me sick. I got to the point that I felt hesitant to shoot with this lens. Thus, if I hesitate my shots, what is the point of shooting with it?
Optical performance dictates 70% of the score of a lens, EVERY TIME. Give me a beat up manual focus lens, if the images look good, the lens is good. Alternatively, the lens needs to mechanically function first. But that is evident. So, though this 18-135 is a 5 star constructed lens with 5 star AF, the optics are about the worst I have ever seen, competing only with the new Sigma 17-70 C which suffers some of the same issues with field curvature from 18-30mm. (see my review on the Sigma 17-70 - its another "ho-hummer" of a lens very similar to this Pentax model).
I suspect the good reviews for this lens are a result of most of these shooters taking "center subject focused" pictures in which case many photos will be good. IF you just shoot a flower, a face, a something in middle of the frame, you will like this lens most of the time.
If you shoot across a distance of more than 15 feet from a subject at large apertures expecting to capture a shot with some depth, or shoot a small landscape, or shoot a building with flowers in front, you will receive hell. This lens is just that bad.
This lens is actually quite good for portrait work. However, this is a contradiction to the purpose of using this lens for this lens is a WR lens and why keep it indoors??? Most will buy this for outdoor landscape and expect good borders and use this lens to its intended function: a WR lens to resist the outdoor elements while being able to shoot with larger apertures for the lower light that comes with in climate conditions or in forests and trees when the sun is not so prominent and rain is (thus the need for WR).
If you stop down, shoot between f8-f10, keep it between 20mm and 50mm, then you can do this provided you get a good copy. Folks, that's A LOT of stipulation for 450-500 bucks. For me, it is just too frustrating.
I will not be trying another 18-135 nor do I recommend this lens for its intended purpose. Great build, Poor optics.