Lens Testing Continued; Comparison with Another Copy

www.dennismook.com

I have been going through a process of testing a lens I recently purchased.  This is something I regularly do.  My procedure consists of three phases.  First, I ensure everything that is supposed to be included in the box is there, then I fully check the lens out visually, looking through the glass for any debris, oil on the aperture blades and I check the focusing ring and aperture blades for smooth operation, etc.  I also gently shake the lens to ascertain if anything may be loose, such as a lens element.  Second, I run a series of software based tests on the lens to fine tune the micro-focus, determine focus consistency as well as determine the best aperture as well as the apertures I would consider acceptable to use on the lens (you would be surprised at which apertures quality falls off quickly, both wide open and stopped down).  Last, I use the lens for a period of time to determine if I really like the real world images I make with the lens.  It is only then do I decide whether or not to keep the lens.  It is a lot of work, but to me it is worth while in long term happiness with such a large monetary outlay.  I don't like spending a lot of money and then not being happy with my purchase later on.

Last week, I purchased a Sigma 150mm F/2.8 Macro lens with optical stabilization.  This is the newer of the two designs from Sigma.  The older design does not have the optical stabilization.  The lens looks and feels very well made with a high level of attention to quality.  The lens passed the first test with no issues.  It didn't pass the second series of tests, however.  So, I never got to the third part, using the lens in real world situations.  In my case, I wasn't able to successfully achieve satisfactory micro-focus calibration, could not successfully achieve a high level of consistently accurate focus and got different results when determining the best aperture between two tests.

A good photography buddy of mine also purchased the same model lens last week from a different source.  Yesterday, he came by my home and I ran his lens through the same series of software tests to compare and ascertain if his lens also had the same issues as mine, which may indicate an overall issue with the lens, or if I may have just had a poor copy of the lens.

The FoCal software I use to test lenses, assigns a random, but consistent number across various tests to compare accuracy of focus (AOF).  The number you will see is that accuracy of focus number, which is also indicative of lens resolution, contrast and overall image quality, so in the comparison we are comparing apples to apples.  In looking at the AOF number, higher is better.

My Lens                                                                          His Lens

Sharpest Aperture                       F/3.8                                    Sharpest Aperture              F/5
Sharpest Aperture AOF              625  (2nd test 1050)            Sharpest Aperture AOF     920
Micro Focus Calibration            +2, +7, +8, -2 (4 tests)         Micro Focus Calibration    -2
(two additional tests would not calibrate at all)
Micro Focus Calibration AOF   ~600 (test avg.)                    Micro Focus Calibration AOF  835
Focus Consistency                     91%                                      Focus Consistency             98%

I will also throw in that in the test to determine best aperture, the software focuses the lens to its most accurate regardless of what the camera's AF module says.  I suspect that it locks the mirror up, opens the shutter and uses contrast-detect focus off the front of the sensor, as would be done in "live view."

What conclusions can I draw?  Well, I can't draw any conclusions about this lens model in general with this very small sample of two lenses.  But I can say that his lens is a better copy than mine.  The best I can say about my copy is that I believe the root of all the issues is the inability of the lens to focus consistently, therefore affecting the results of the micro-focus calibration test as well as the focus consistency test as well as the lower AOF (resolution) numbers.  Also, when the lens' optical stabilization is activated, there is a lot of loose mechanism sounding noise, which was not the case in my friend's lens.  I read where optical stabilization noise is normal, but somehow I think mine was not normal.  Additionally, as a comparison, I know that all of my Nikon lenses focus in the 99%plus region for focus consistency.

What next?  I have returned my copy of the lens to the vendor.  It was painless to do so and at no cost.  Will I purchase another copy or buy a lens from a different manufacturer?  I don't yet know.  I may purchase another copy from a different vendor, hoping that the two lenses came from different manufacturing batches.  I may research at the Nikon 105 F/2.8 VR and consider that if I like what I find.  I may decide not to purchase a macro lens at all and ship my slides and negatives out for digitizing.  I'm going to chew on it for a couple of weeks and then make a decision.

Thanks for looking.

Enjoy!
Dennis Mook