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Jordan at Admiring Light reviews Sony FE 70-200f/2.8 GM
  
 
Brandon Dube
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p.4 #1 · p.4 #1 · Jordan at Admiring Light reviews Sony FE 70-200f/2.8 GM


Ayoh wrote:
Ok thanks, so the reported MTF measurements for a single lens are averages at multiple orientations (and poor uniformity brings the MTF down for that lens). But are the deviation bands shown in your variation plots (linked example below) the deviation in the lens-averaged MTF, or the deviation of all your individual MTF data points? i.e. is it analogous to the standard deviation of all your individual MTF data points across many lenses, or the standard error in the averaged MTFs for each lens?







I guess it is the standard error since the maximum values in the 50LP/mm variation bands for
...Show more

Drawing conclusions about a population based on an observation of a single member is kind of silly. The fact that the particular lens Joey did his test with is good on the right hand side doesn't really tell you anything about the lens. If it is bad on the left and good on the right, it is good on the right because it is bad on the left, and a perfectly aligned one might be "meh" or "okay" on the right, but won't be great.

If we computed the average charts in a way that was indicative of maximum performance, that would be very dishonest.

The aggregation is done based on the assumption that the lenses should all be rotationally symmetric. Per lens, we get 8 data points for 2mm .. 20mm. We take these vectors of length 8, and concatenate them all together for each lens and are left with an N*8x1 vector for each field point, where N is the number of lens copies tested. We then average and compute stdev over the N*8 dimension.



Sep 07, 2017 at 02:08 PM
Ayoh
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p.4 #2 · p.4 #2 · Jordan at Admiring Light reviews Sony FE 70-200f/2.8 GM


Brandon Dube wrote:
Drawing conclusions about a population based on an observation of a single member is kind of silly. The fact that the particular lens Joey did his test with is good on the right hand side doesn't really tell you anything about the lens. If it is bad on the left and good on the right, it is good on the right because it is bad on the left, and a perfectly aligned one might be "meh" or "okay" on the right, but won't be great.

If we computed the average charts in a way that was indicative of maximum performance,
...Show more

You missed my point. To reiterate:
1) at the moment your variation bands correspond to the standard error in the means of the MTF values averaged for each particular lens
2) the variation bands could be set so they correspond to the standard deviation of the total set of all unaveraged MTF values recorded for all data points across all lenses

For scenario 2 the maximum and minimum values of the variation bands would have higher magnitudes and would be a closer indication of the max/min observed MTF values. As you plot case 1) the max/min values are reduced due to averaging. Therefore high MTF outliers like ones for the right side of the particular Sony lens above are not captured in the current approach. This is not necessarily a criticism of your approach, just an observation that due to the averaging the variation bands are narrower.


Edited on Sep 07, 2017 at 03:44 PM · View previous versions



Sep 07, 2017 at 03:20 PM
Ayoh
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p.4 #3 · p.4 #3 · Jordan at Admiring Light reviews Sony FE 70-200f/2.8 GM


Brandon Dube wrote:
Drawing conclusions about a population based on an observation of a single member is kind of silly. The fact that the particular lens Joey did his test with is good on the right hand side doesn't really tell you anything about the lens. If it is bad on the left and good on the right, it is good on the right because it is bad on the left, and a perfectly aligned one might be "meh" or "okay" on the right, but won't be great.


Wel since the Sony lens on the right side appeared marginally sharper than a sharp well-centered Nikon benchmark lens, it tells you that the Sony lens is capable of high maximum MTF values but maybe not high uniformity. But through the standard error averaging of left and right sides in the lens rentals plots, the maximum exhibited MTF is not captured.

And as to "it is good on the right because it is bad on the left" then if you are only comparing the left and right sides to one another, then by semantic logic that is inevitable. But if you are comparing a particular lens' left and right side performance to some external benchmark (like an ideal MTF) than why would the right side performance have to decrease to increase left side performance? if the left side of the image is soft, because say the test chart photographed in the image is titled, then if the tilt is corrected the left side softness would be removed without impacting the right side sharpness.



Sep 07, 2017 at 03:43 PM
 

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Brandon Dube
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p.4 #4 · p.4 #4 · Jordan at Admiring Light reviews Sony FE 70-200f/2.8 GM


Ayoh wrote:
Wel since the Sony lens on the right side appeared marginally sharper than a sharp well-centered Nikon benchmark lens, it tells you that the Sony lens is capable of high maximum MTF values but maybe not high uniformity. But through the standard error averaging of left and right sides in the lens rentals plots, the maximum exhibited MTF is not captured.

And as to "it is good on the right because it is bad on the left" then if you are only comparing the left and right sides to one another, then by semantic logic that is inevitable. But if
...Show more

Optical misalignment (decenter, tilt) generates coma and astigmatism that are constant over the field of view. The rotationally symmetric nominal aberrations will have opposite sign on the left and right. On one side, the misalignment contribution will cancel the nominal one and you get great MTF. On the other, they add and the image quality is bad. If you want to know more, search for Nodal Aberration Theory in optics journals.



Sep 07, 2017 at 04:00 PM
Brandon Dube
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p.4 #5 · p.4 #5 · Jordan at Admiring Light reviews Sony FE 70-200f/2.8 GM


Ayoh wrote:
You missed my point. To reiterate:
1) at the moment your variation bands correspond to the standard error in the means of the MTF values averaged for each particular lens
2) the variation bands could be set so they correspond to the standard deviation of the total set of all unaveraged MTF values recorded for all data points across all lenses

For scenario 2 the maximum and minimum values of the variation bands would have higher magnitudes and would be a closer indication of the max/min observed MTF values. As you plot case 1) the max/min values are reduced due to averaging.
...Show more

#2 is the scenario in play, as I mentioned in my previous comment. Please read before replying.



Sep 07, 2017 at 04:01 PM
SoundHound
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p.4 #6 · p.4 #6 · Jordan at Admiring Light reviews Sony FE 70-200f/2.8 GM


Whaaa, wimper! If only my Nikkor 70-200 FL would AF with my Vello adapter on my A9. Or if only Sony had decided to copy the Nikon instead of the Canon zoom?


Sep 08, 2017 at 01:03 PM
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