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Does Focus Calibration Make a Difference?

March 10, 2016

Most consumer cameras don’t offer focus calibration (auto-focus fine-tune).  Many users of enthusiast/professional cameras ignore focus calibration as “a waste of time”.  After all, the factory makes nearly every lens and camera fully calibrated, right?  Fully not.

 

Top ten things that can use tuning:

 

Cars

Musical instruments

Husbands/boyfriends

Skis

Lasers

Voices

Radio stations

Software algorithms

Computers

Cameras/Lenses

 

Notice that cameras and lenses just made the list.

 

I have never shot with a camera/lens combination that needed a focus calibration of “zero”.  Maybe they exist somewhere.  Even the 85mm f/1.4 AF-S Nikkors (I’ve tested two of them) need focus calibration at some of their wider f-stops.

 

What kind of price do you pay by not calibrating?  Can you even tell the difference?  This article shows you what kind of price you pay.

 

My lenses use focus calibration values that range from as little as “+1” to as much as “-20”.  The full range of calibration values among all of my lenses varies from “-20” to “+15”.  Since the Nikon range is +/- 20, I’m right on the hairy edge.

 

So what happens if you don’t bother to calibrate?  You pay a big price in resolution, that’s what.   If you only own really slow lenses or only shoot with the lens stopped down quite a bit, then it might be a truly “don’t care” situation.  The higher-resolution camera sensors are even more picky about focus calibration.

 

I did a set of tests with my Nikkor 35mm f/1.8 AF-S (DX) on a D7100.  It falls into the category of “average” focus calibration, needing a value of “+7”.  Not too much fine-tuning compared to my 60mm Micro-Nikkor that needs “-20”, but still nothing to sneeze at.  My D7000 needs a fine-tune adjustment of “-5” with the same 35mm lens on it.

 

The following are my test results, conducted with the MTF Mapper program and using an A0 size resolution chart.  I turned on phase-detect auto-focus and then switched auto-focus fine-tuning OFF for half of the tests.  I tested shooting wide-open, and then I tried stopping the lens down a bit more than 2 stops and re-tested.  I forced the lens to minimum focus distance, then used the “AF-ON” button assignment to re-focus between every shot.

 

Take a look at my MTF Mapper Cliff's Notes article on how to perform focus calibration using a proper focus chart and software that can provide definitive answers about if you're in focus or not.  

 

First Test: Focus Fine-Tune is OFF

 

f/1.8 Lens Center MTF50 lp/mm Measurements

32

34

32

36

34

 

f/4.0 Lens Center MTF50 lp/mm Measurements

65

70

70

70

70

 

Second Test: Focus Fine-Tune is ON (+7)

 

f/1.8 Lens Center MTF50 lp/mm Measurements

50

50

50

50

50

 

f/4.0 Lens Center MTF50 lp/mm Measurements

70

70

70

70

70

 

The average f/1.8 MTF50 lens center resolution with no focus fine-tune was 33.6 lp/mm.  The average f/1.8 MTF50 lens center resolution with focus fine-tune ON was 50 lp/mm.

 

The average f/4.0 MTF50 lens center resolution with no focus fine-tune was 69 lp/mm.

The average f/4.0 MTF50 lens center resolution with focus fine-tune ON was 70 lp/mm.

 

At maximum aperture, the resolution change using fine-tune calibration was HUGE (it went from 33.6 to 50).

 

With the lens stopped down, the resolution change was minimal.

 

So there you have it.  If you camera doesn’t have focus fine-tune, then get one if you can afford it.  Or else buy Sigma lenses that support their USB calibration dock for in-lens calibration.

 

If you only shoot lenses stopped down, then you can probably safely ignore focus calibration.

 

 

 

 

 

 

 

 

 

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