# Advice required on lens calibration with Focal Plus



## Efka76 (Feb 2, 2014)

Dear colleagues, I just recently acquired Reikan Focal Plus software for lens calibration. It is perfectly clear how to calibrate fixed focal length lenses, however I need your advice on zoom lenses calibration. If I understand correctly there are these options:

1) Full manual calibration - to make a lot of manual shots on both end of the zoom and import all such data in Reikan focal. Then software will show what AFMA numbers are for both ends of zoom lenses. I do not like this method as it is very time consuming and there might be some differences due to manual shots.

2) To run full automatic calibration 2 times (for minimum and maximum focal length of zoom lenses). In this case I will get 2 AFMA values. Then there is a question which value should i input into the camera? Its a pity but my camera (canon 7d) does not support input of 2 AFMA values for zoom lenses. 

In my opinion, are the following options:

a) to input AFMA value, which is determined based on long end of zoom lenses (such advice I found in Focal testing guide.

b) To input mid value of both AFMA numbers

c) to run additional test in mid of the zoom and input that value into the camera.


I would really appreciate your advice as tomorrow I will be performing that testing and your advice would be very helpful to me.


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## Canon1 (Feb 2, 2014)

I recommend calibrating zoom lenses at max zoom where DOF is smaller. With the shorter focal length end of the zoom the added DOF will absorb any calibration issues for most shooting situations. 

If you at least know what the optimal AFMA setting is for the short end, you could make adjustments on the fly if the need arises. 

I would not split the difference, as it is more critical to nail AFMA at longer focal lengths.


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## Badger (Feb 2, 2014)

I just went through this exercise with the 6D which allows two values for zoom lenses. I'm not sure what to do with a body that only allows on value. I'll read the Focal manual and see if there is any guidance there.


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## Badger (Feb 2, 2014)

11.6 Analysis at Both Ends of a Zoom Lens

The FoCal Manual Mode test can run two sets of analysis within the data captured for different lens focal length. To use this feature, simply supply shots taken at a spread of AF Microadjustment/Fine Tune values at two different focal lengths and select all the images into the test – the software will analyse the files, group into focal lengths and produce the results as shown:
In FoCal Pro, the vertical lines on the chart indicate the focal length that the result applies to. For all versions, the Predicted Best indicator will show two values for the tested focal length – in the example above, 40mm has a best AF Microadjustment value of -2 and 17mm has a best value of -7. *Standard practice for zooms on cameras with single AF Microadjustment values is to bias the single programmed value towards the telephoto end, so in this instance a value of -4 would be best to enter into the camera for general shooting.*


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## East Wind Photography (Feb 2, 2014)

Badger said:


> 11.6 Analysis at Both Ends of a Zoom Lens
> 
> The FoCal Manual Mode test can run two sets of analysis within the data captured for different lens focal length. To use this feature, simply supply shots taken at a spread of AF Microadjustment/Fine Tune values at two different focal lengths and select all the images into the test – the software will analyse the files, group into focal lengths and produce the results as shown:
> In FoCal Pro, the vertical lines on the chart indicate the focal length that the result applies to. For all versions, the Predicted Best indicator will show two values for the tested focal length – in the example above, 40mm has a best AF Microadjustment value of -2 and 17mm has a best value of -7. *Standard practice for zooms on cameras with single AF Microadjustment values is to bias the single programmed value towards the telephoto end, so in this instance a value of -4 would be best to enter into the camera for general shooting.*



Though you may be better off using -7 as the dof is so wide at 17 that the difference between -2 and -4 wouldn't be noticeable. -7 to -4 at the tele end would almost certainly be noticeable. The best thing to do is to try it and see what your results show.


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## neuroanatomist (Feb 2, 2014)

Personally, I test all lenses at two distances from the target - 25x and 50x the focal length (in addition, I test macro lenses near the MFD). For zoom lenses, I test at both ends and at 1-3 intermediate focal lengths, depending on the 'zoom factor'. For example, a ~2x zoom like the 16-35 gets tested at 16/24/35, a ~3x zoom like the 24-70 gets tested at 25/35/50/70, etc. 4x and higher zooms get three intermediate FLs (even a superzoom like the 28-300L, which I did at 28/50/100/200/300). 

For newer bodies which store two values for zooms (wide and tele), the intermediate FLs should fall on the linear regression between the ends; too far off, the lens should be sent it (or exchanged if new). For older bodies with a single value, I weight the choice based on more used FLs, DoF, and common distances. For example, my 24-70 II at 24/35/50/70 had values of 0, +1, +3, and +5. Fine on my 1D X, I set W = 0, T = +5, and the intermediate FLs fall on the line. For a 7D or a 5DII with those values, I'd choose a compromise weighted to the long end, e.g. +4.


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## Badger (Feb 2, 2014)

> Though you may be better off using -7 as the dof is so wide at 17 that the difference between -2 and -4 wouldn't be noticeable. -7 to -4 at the tele end would almost certainly be noticeable. The best thing to do is to try it and see what your results show.



Maybe, I just copied that out of the Focal manual. I've only had it for a couple of days so I'm not an expert on AFMA yet


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