Are You Getting the Maximum Sharpness Out of Your Lenses?

There really is a corner in Winslow Arizona that tourists come and have their pictures taken.  Also,
there is a red flatbed Ford parked on the street in front of this building.  All of the detail you see in the
building is painted in, fake windows and all, as it is a solid brick wall.  If you ever want to make this shot,
it is a morning shot. I arrived late in the afternoon and you see it is backlit.
Arrive in the late morning, at the latest.

It was a few years ago, I bought a particular lens that I had really wanted for a long time.  After I received it, and as I usually do, I went out and tested it.  I made some images at the various apertures and focal lengths, from relatively close to infinity focus.  When I looked at the results, something just didn't look quite right about some of the images.  From everything I had read, this particular lens was supposed to be stellar, but I wasn't seeing it.  Hmmm.  What to do?

To make a long story short, after additional testing, I determined that this lens on my particular camera was not focusing accurately.  The result was the images were not quite as sharp as they could have been.

I'm not talking technique when I ask the question, "Are you getting the maximum sharpness out of your lenses?"  In this case, I'm talking about electro-mechanical issues with the gear itself.  Do your lenses focus optimally on your particular camera body(ies)?

My eyes were opened to just how most lenses don't focus optimally on phase detection based focusing systems. (contrast detect focusing system focus off the front of the sensor itself and, by definition, are more accurate) Cameras can be within spec, say from -10 to +10 on a hypothetical scale when it comes to focus accuracy.  Lenses can be in spec, again, say from -10 to +10 on that same hypothetical scale.  If either is not too far from the neutral or null position (0), you probably won't notice and degradation in image quality.  Also, for example, if the camera is far to the plus side and the lens far to the minus side, they would tend to even each other's "off null spec" out.  How about when the lens and camera are both far to one end of the scale, say both are -8 and -8?  Then you may see that focus is out of spec, in this particular case.

In most of the medium to higher end digital SLRs, there is a way, in the camera's software, to adjust the focus point so it can be fine tuned to a particular lens.  In fact, in my Nikons, the ability exists for the camera to remember a particular focus micro adjustment for a number of lenses, all of which may have to be fine tuned differently.  When you put an individual lens on the camera, the camera remembers which lens it is and automatically adjusts the focus fine-tune to the setting you made and saved.  I thought that I would try and use this feature to improve my new lens' focus point so I could fully realize the potential of the lens for sharpness in focus.

Doing some research, I bought a tool to manually correct my lens/camera body combination that I determined was not focusing as well as it could.  The device looks like a ruler that you set up at a 45 degree angle and, by focusing on the "0" point and taking a number of images at different micro fine-tune settings, you can adjust the focus of the lens/camera combination to focus exactly at the "0" point.  I really had a hard time doing this.  It just didn't work well for me.  I found the procedure tedious.  Back to the drawing board.

Looking some more, I found FoCal software by Reikan, a UK company.  Their software, which is geared for Canon and Nikon cameras, allows you through software analysis, to determine optimum focus micro-fine tune for each lens, determine best aperture for any particular lens, determine camera/lens focus consistency as well as some other features.  I took a chance and purchased the software.

When you buy the software, you give the company the serial numbers of your camera bodies and the software only works with those serial numbers.  Thus, you cannot share the software and test your friends' cameras.  FoCal was easy to set up.  You tether your camera body to your computer with the cord that is supplied by the camera manufacturer.  Reikan supplies a chart in PDF form, which you print out at the highest quality. You affix the chart on a wall or surface parallel to your camera's sensor.  You then follow their directions as you go through the tests.

For Canon cameras, I understand the procedures are totally automated (most of the models, I now understand).  For Nikon cameras, it is semi-automated.  By that I mean that the software tells me where to set the micro-fine tune, I set it, then the software trips the shutter an analyzes the exposure.  After many exposures at different micro focus settings, the software produces a graph showing you the setting where best focus is found.  Best focus gives you the highest resolution.  The best aperture and focus consistency tests are totally automated.  I found the software easy to use.

A couple of things to keep in mind, however.  You can only save one setting per lens in your camera.  If you have a zoom lens, you have to decide at which focal length you will test and make that setting.  I found that different focal lengths in all my zoom lenses have different micro-focus fine tuning adjustments.  What to do?  I chose the longest focal length.  Why?  My reasoning was that at any particular distance, the depth of field will decrease with an increase in focal length.  So, the most critical and least forgiving focal length, which would have the shallowest depth of field and be more prone to focusing errors, would be the longest for each zoom lens.  So, I used the longest focal length to set my camera for the zoom lens.  You choose for yourself.

I have owned different camera bodies and different lenses at different times, so some lenses were not tested with a particular camera body, depending upon which I owned at a particular time.  My first reaction was astonishment as to how different each camera body calibrated so differently with each lens. Here are some of the details:

First, Nikon allows a focus fine-tuning from -20 all the way to +20.  The factory default is 0.

Nikon D800E                                    Micro-focus Adjustment                       

Nikon 16-35mm f/4 G VR                                  -4    
Nikon 20mm f/2.8 AF-D                                   -20    
Nikon 35mm f/2 AF-D                                      -20    
Nikon 24-70mm f/2.8 G                                     -7
Nikon 24-120mm f/4 G VR                                -3    
Nikon 28-300 f/3.5-5.6 G VR II                        -3      
Nikon 50mm f/1.4 G                                            0    
Nikon 70-200mm f/2.8 G VR II                         -6                                                      
Nikon 70-200 f/4 G VR                                     +3    
Tamron 90mm f/2.8 macro OS                            -2  
Sigma 150mm f/2.8 macro OS                            +2  

Nikon D700                                      Micro-focus Adjustment     

Nikon 16-35mm f/4 G VR                                  +2            
Nikon 20mm f/2.8 AF-D                                      0              
Nikon 35mm f/2 AF-D                                        -8              
Nikon 24-70mm f/2.8 G                                     +2
Nikon 24-120mm f/4 G VR                            Not Tested
Nikon 28-300 f/3.5-5.6 G VR II                          0                
Nikon 50mm f/1.4 G                                         +12              
Nikon 70-200mm f/2.8 G VR II                           0              
Nikon 70-200 f/4 G VR                                 Not Tested
Tamron 90mm f/2.8 macro OS                       Not Tested
Sigma 150mm f/2.8 macro OS                        Not Tested

As you can see, the calibration, to optimize best focus of any individual lens with either camera body, varied widely.  That was surprising.  Did I see a difference in image quality after calibration.  With some lenses, yes I did.  With others, no I did not.  But those in which I didn't see a difference were already optimal close to or at the default "0" setting.  I'm sure this system is not nearly as good as an Imatest system or other testing procedures where sophisticated optical benches are required.  But it seemed to work fine for the purpose.  Others may have had different experiences.

For all the lenses, the focus consistency was in the 95% and above.  Most were in the 98-99% range.  In other words, very consistent focus.  Best aperture, in many cases, did not follow the "two down from wide open" rule we often hear.  On several of my lenses, it was less than one stop down or three stops down.  For example, the 70-200 f/2.8 VRII's best aperture was f/3.2.  For others, it was considerably stopped down to get best aperture.  The Nikon 28-300 f/3.5-5.6's best aperture was f/10!  It depended upon each individual lens.

The upshot of all of this is that if you think your camera and lenses are calibrated at the factory to produce optimal focus out of the box, you are probably mistaken in most cases.  If you are stickler for creating the best possible images, you may want to look at a program such as this to test and calibrate you own camera/lens combinations.  If nothing else, I now know that my camera/lens combinations are matched the best I can match them and I know the sharpest aperture as well as what I would consider "acceptably" sharp apertures for each lens.  I feel better anyway.

Thanks for looking. Enjoy!

Dennis Mook

Many of my images can be found at www.dennismook.com.  Please pay it a visit.  I add new images regularly.  Thank you.


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