Pentax Quality Control Strikes Again

Started Apr 23, 2015 | Discussions thread
Timbukto Veteran Member • Posts: 4,988
Re: Pentax Quality Control Strikes Again
5

Joseph Tainter wrote:

Timbukto wrote:

I would suggest you do MFA tuning prior to brick wall tests. This is because if the lens is front focusing, it is possible that due to field curvature, some portions are within sharper focus than others. No lens in existence exhibits absolute perfect flat plane field curvature, but by not calibrating it first you may be seeing the tail end of your field of focus instead of the middle portion, etc.

Likewise my 18-55 by default looks to have soft portions in some areas and not others, but with MFA tuned, it can give me a sharp frame to frame 18mm f3.5 shot.

See here -

http://www.lensrentals.com/blog/2015/03/a-quick-zoom-variation-demonstration

Thanks for the recommendation, but this wouldn't account for unbalanced weakness. That can only come from defective assembly.

I've done this test on many lenses before testing for focus compensation. Nearly all have subsequently needed some degree of focus compensation adjustment.

Joe

It is not a black and white issue. Slight imbalance does not matter and can be exacerbated by not having the lens focus calibrated.

Lets pretend that the field curvature of a lens is like a rainbow...a double rainbow even. Lets pretend the area between a double rainbow is what is in focus. Now lets assume the rainbow is not a *perfectly* concentric cone and there is a slight tilt, but there is a good deal of space in the *middle* portion of the double rainbow. Can you see how back focus and front focus can thus exhibit *uneven* softness in the frame?

Like I said read Roger's article...no lenses are *perfect* at all focal lengths. But you *do* need to make sure your focus is dead on so that you are putting as much as possible in field of focus. My 18-55 clearly has the right side coming into focus slightly after the rest of the frame. I can consider it defective, or I can just make sure the focus is spot on and adjust it for rather good quality at 18mm f3.5.

Case in point...this is an image with the kit lens that looks like the upper right area is a soft blob.

Same lens...corrected focus...sharp all over on a kit lens...hurray for me. An f3.5 shot beating a stopped down f5.6 shot. This is how a lens transforms from junk to pretty damn good.

I've been across many brands and many lenses, preowned and new. I have more thoughts on the matter but my findings are like Rogers...a ton of lenses are imperfect especially zooms. Better is the enemy of good enough is the line he uses...that is exactly why I've gone all over the place from full frame L lenses to a Pentax with a double kit lens.

Overall thoughts on quality vs brands -

It is *largely* no big difference but there are some factors to consider.  IMO Full frame or larger systems have an advantage compared to APS-C and smaller systems in what I consider focal plane tolerance.  Overall it is more difficult to make lenses of wider focal lengths with perfect field flat field curvature with no tilt.  However systems of larger sensors make equivalent focal lengths provide wider AOVs.

In otherwords it *tends* to be the case that it is easier to make a sharp 85mm 1.8 than it is to make a sharp 50mm 1.8.  It is easier to make a sharp 50mm 1.8 than it is to make a 35mm 1.8.  So in otherwords larger sensor systems gain an advantage in terms of optical quality for the same AOV.  Keep in mind that it tends to be that Pentax has been criticized for optics but they have long been only APS-C.  Sony E-mount prior to any full-frame mount has largely been criticized for lens optics as well and deservedly so in some instances.

Very few *full-frame* systems and lens mounts invite as much criticism IMO because in general it is *easier* to build optics for because it tends to be that tele designs are inherently easier to make sharper(yes tele's can still be large and hideously expensive, but overall they provide the sharpest results compared to wide angles).

I've had hit and misses across brands and systems, but I do tend to believe its easier to make a sharper 85 than 50, and 50 than 35, etc.

I also believe that when people spec lenses for smaller sensor mount, they try to maximize sharpness for what is essentially far tighter pixel pitch, but for size reasons also under-engineer edges (to suit APS-C size vs FF size).  So in general many FF lenses aren't necessarily as sharp, but many should provide uniformity.  In addition because the 'peak' sharpness of FF lenses might not even be as high as the peak sharpness of an APS-C designed lens, instances of non-uniformity stand out much greater.  Sony e-mount lenses are designed to be small as possible exactly for APS-C...they have the SAME complaints in satisfying 24MP sensor.  Centers are sharp...borders and edges not close...and uniformity not that great.  IMO Pentax seems to have similar constraints (i.e. the 50-200 tele I have is very very compact...and brighter than the Sony one to boot).  Although IMO my double-kit lenses are better than the Sony's 16-50 and 55-210 double kit.  MFT often gets a pass but IMO the sensor size aspect ratio tends to make for easier to build optics...but even then I've had terrible times finding a good copy of a Oly 45mm 1.8.  Also mirrorless cameras can have terrible times getting everything in focus at infinity at times because many hybrid AF heuristics optimize on speed and the moment the *FOCUS* point in question is sharp that is where it is focused on...but like my image example shows there may be more careful focusing that gives better results.

Overall because Pentax has both focus peaking, catch focus, and focus tuning...it is one of the *best* brands to optimize IQ for at the entry level.

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