Equipment

Measuring Lens Variance

Published June 26, 2015

Warning: This is a Geek Level 3 article. If you aren’t into that kind of thing, go take some pictures.

I’ve been writing and discussing the copy-to-copy variation that inevitably occurs in lenses since 2008. (1,2,3,4) Many people don’t want to hear about it. Manufacturers don’t want to acknowledge some of their lenses aren’t quite as good as others. Reviewers don’t want to acknowledge that the copy they reviewed may be a little better or a little worse than most copies. Retailers don’t want people exchanging one copy after another trying to find the Holy Grail copy of a given lens. And honestly, most photographers and videographers don’t want to be bothered. They realize lens’ sample variation can make a pretty big difference in the numbers a lens tester or reviewer generates without making much difference in a photograph.

It does matter occasionally, though. I answered an email the other day from someone who said, in frustration, that they had tried 3 copies of a given lens and all were slightly tilted. I responded that I’d lab-tested over 60 copies of that lens, and all were slightly tilted. It wasn’t what he wanted to hear, but it probably saved him some and his retailer some frustration. There’s another lens that comes in two flavors: very sharp in the center but weaker in the corners, or not quite as sharp in the center but stronger in the corners. We’ve adjusted dozens of them and can give you one or the other. Not to mention sample variation is one of the causes that make one review of a lens say it’s poor, when other reviewers found it to be great.

At any rate, copy variation is something few people investigate. And by few, I mean basically nobody. It takes a lot of copies of a lens and some really good testing equipment to look into the issue. We have lots of copies of lenses and really good testing equipment, and I’ve wanted to quantify sample variation for several years. But it’s really, really time-consuming.

Our summer intern, Brandon Dube, has tackled that problem and come up with a reasonably elegant solution. He’s written some Matlab scripts that grab the results generated from our Trioptics Imagemaster Optical Bench, summarizes them, and performs sample variation comparisons automatically. We’re going to eventually present that data to you just like we present MTF data: when a new lens is released we’ll also give you an idea of the expected sample variation. Before we do that though, we need to get some idea of what kind of sample variations should be expected.

For today, I’m going to mostly introduce the methods we’re using. Why? Because I’m old fashioned enough to think scientific methods are still valid. If I claim this lens scores 3.75 and that lens scores 5.21, you deserve to know EXACTLY what those findings mean (or don’t mean) and what methods I used to reach those findings. You should, if you want to, be able to go get your own lenses and testing equipment and duplicate those findings. And maybe you can give us some input that helps us refine our methods. That’s how science works.

I could just pat you on the head, blow some smoke up your backside, call my methods proprietary and too complex, and tell you this lens scores 3.75 and that lens scores 5.21, so you should run out and buy that. That provides hours of enjoyment fueling fanboy duels on various forums, but otherwise is patronizing and meaningless. Numbers given that way are as valid as the number of the Holy Hand Grenade of Antioch.

 Methods

All lenses were prescreened using our standard optical testing to make certain the copies tested were not grossly decentered or tilted. Lenses were then measured at 10, 20 ,30, 40, and 50 line pairs per mm using our Trioptics Imagemaster MTF bench. Measurements were taken at 20 points from one edge to the other and repeated at 4 different rotations (0, 45, 90, and 135 degrees), giving us a complete picture of the lens.

 

The 4 rotation values were then averaged for each copy, giving as a graph like this for each copy.

 

The averages for 10 copies of the same lens model were then averaged, giving us an average MTF curve for the 10 copies of that lens. This is the type of MTF reading we show you in a blog post. The graphics are a bit different than we’ve been using, but that’s because we’re generating these with one of Brandon’s scripts now, so they’ll be more reproducible now.

Graph 1: Average MTF of 10 copies of a lens.

 

Graphing the Variation Between Copies

Every copy of the lens is slightly different than this ‘average’ MTF and we want to give you some idea of how much variance exists between copies. A simple way is to calculate the standard deviation at each image height. Below is a graph showing the average value as lines, with the shaded area representing 1.5 standard deviations above and below the average. In theory, (the theory doesn’t completely apply here, but it gives us a reasonable rule of thumb) MTF results for 98% of all copies of this lens would fall within the shaded areas.

 

Graph 2: Average MTF (lines) +/- 1.5 S. D. (area)

 

Obviously, these area graphs overlap so much that it’s difficult to tell where the areas start and stop. We could change to 1 or even 0.5 standard deviations and make things look better. That would work fine for the lens we used in this example, but this is actually a lens with fairly low variation. Some other lenses vary so much that they would just make a graph that basically is nothing but completely overlapping colors, even if we showed +/- one standard deviation.

The problem of displaying lens variation is one we’ve struggled with for years; most variation for most lenses just won’t fit in the standard MTF scale.  We have chosen to scale the variance chart by adding 1.0 to the 10lp/mm value, 0.9 to the 20 lp/mm value, 0.75 to 30lp/mm, 0.4 to 40lp/mm, and 0.15 to 50lp/mm.  We chose those numbers simply because they make the graphs readable for a “typical” lens.

Graph 3 presents the same information as Graph 2 above, but with the axis expanded as we described to make the variation more readable.

Graph 3: Average MTF (lines) +/- 1.5 S. D. (area); modified vertical axis

 

You could do some math in your head and still get the MTF numbers off of the new graph, but we will, of course, still present average MTF data in the normal way. This graph will only be used to illustrate variance. It can be quite useful, though. For example, the figure below compares the graph for the lens we’ve been looking at on the left, and a different lens on the right.

 

 

 

Some things are very obvious at a glance. The second lens clearly has lower MTF than the first lens. It also has a larger variation between samples, especially as you go further away from the center (center is the left side of the horizontal axis). In the outer 1/3 of the lens, in particular, the variation is extremely large. This agrees with what we see in real life: the second lens is one of those lenses that every copy seems to have at least one bad corner, and some more than one bad corner. Also if you look at the black and red  areas at the center of each lens (the left side of each graph) even the center of the second lens has a lot of variation between copies. Those are the 10 and 20 line pairs per mm graphs and these differences between copies in the center are the kind of thing that most photographers would notice as a ‘soft’ or ‘sharp’ copy.

The Variation Number

The graphs are very useful to compare two or three different lenses, but we intend to compare variation for a lot of different lenses. With that in mind we thought a numeric ‘variation number’ would be a nice thing to generate. A table of numbers certainly provides a nice, quick summary that would be useful for comparing dozens of different lenses.

As a rule, I hate when someone ‘scores’ a lens or camera and tries to sum up 674 different subjective things by saying ‘this one rates 6.4353 and this one rates 7.1263’. I double-secret hate it when they use ‘special proprietary formulas you wouldn’t understand’ to generate that number. But this number is only describing one thing: copy-to-copy variation. So I think if we show you exactly how we generate the number then 98% of you will understand it and take it for what it is, a quick summary. It’s not going to replace the graphs, but may help you decide which graphs you want to look at more carefully.

(<Geek on>)

It’s a fairly straightforward process to find the number of standard deviations needed to satisfy some absolute limits, for example, +/-12.5%. Just using the absolute standard deviation number though, would penalize lenses with high MTF. If the absolute MTF is 0.1, there’s not much room to go up or down while if it’s 0.6, there’s lots of room to change. This meant bad lenses would seem to have low variation scores while good lenses would have higher scores. So we made the Variation number relative to the lens’ measured MTF, rather than an absolute variation. We simulated the score for lenses of increasingly high resolution and saw the score would rise exponentially, so we take the square root of it to make it close to linear.

Initially we thought we’d just find the worst area of variability for each lens, but we realized some lenses have low variation across most of the image plane and then vary dramatically in the last mm or two. Using the worst location made these lenses seem worse than lenses that varied a fair amount in the center. So we decided to average the lens’ MTF across the entire image plane. To keep the math reasonable, we calculated the number just for the 30 line pair per mm (green area in the graphs) variance, since that is closest to the Nyquist frequency of 24MP-class full-frame sensors. Not to mention, higher frequencies tend to have massive variation in many lenses, while lower frequencies have less variation; 30lp/mm provides a good balance.  Since some lenses have more variation in the tangential plane and others the sagittal, we pick the worse of the two image planes to generate the variance number.

Finally we scale the score to get a reasonable scale.

For those who speak computer better than we can explain the formula in words, here’s the exact Matlab code we use:

T3Mean = mean(MultiCopyTan30);
S3Mean = mean(MultiCopySag30);
Tan30SD_Average = mean(MultiCopySDTan30);
Sag30SD_Average = mean(MultiCopySDSag30);
ScoreScale = 9;
if T3Mean > S3Mean
 TarNum = 0.125*T3Mean;
else
 TarNum = 0.125*S3Mean;
end
if Tan30SD_Average > Sag30SD_Average
 ScoreTarget = TarNum*T3Mean;
 VarianceScore = ScoreTarget/Tan30SD_Average;
 MTFAdjustment = 1 - (T3Mean/(0.25*ScoreScale));
 VarianceScore = sqrt(VarianceScore*MTFAdjustment);
else
 ScoreTarget = TarNum*S3Mean;
 VarianceScore = ScoreTarget/Sag30SD_Average;
 MTFAdjustment = 1 - (S3Mean/(0.25*ScoreScale));
 VarianceScore = sqrt(VarianceScore*MTFAdjustment);
end
VarianceNumber = VarianceScore*ScoreScale;

(</Geek off)

Here are some basics about the variance number —

  1. A high score means there is little variation between copies. If a lens has a variance number of over 7, all copies are pretty similar. If it has a number less than 4, there’s a lot of difference between copies.  Most lenses are somewhere in between.
  2. A difference of “0.5” between two lenses seems to agree with our experience testing thousands of lenses. A lens with a variability score of 4 is noticeably more variable than a lens scoring 5, and if we check carefully is a bit more variable than one scoring 4.5
  3. A difference of about 0.3 is mathematically significant between lenses of similar resolution across the frame.
  4. Ten copies of each lens is the most we have the resources to do right now. That’s not enough to do rigid statistical analysis, but it does give us a reasonable idea. In testing 10 copies of nearly 50 different lenses so far, the variation number changes very little between 5 and 10 copies and really doesn’t change much at all after 10 copies. Below is an example of how the variance number changes as we did a run of 15 copies of a lens.
How the variance number changed as we tested more copies of a given lens. For most lenses, the number was pretty accurate by 5 copies and changed by only 0.1 or so as more copies were added to the average. 

Some Example Results

The main purpose of this post is to explain what we’re doing, but I wanted to include an example just to show you what to expect. Here are the results for all of the 24mm f/1.4 lenses you can currently buy for an EF or F mount camera.

First, let’s look at the MTF graphs for these lenses. I won’t make any major comments about the MTF of the various lenses, other than to say the Sigma is slightly the best and the Rokinon much worse than the others.

 

 

Now lets look at the copy-to-copy variation for the same for lenses. The graphs below also include the Variation Number for each lens, in bold type at the bottom.

 

Just looking at the variation number, the Canon 24mm f/1.4L lens has less copy-to-copy variation than the other 24mm f/1.4 lenses. The Rokinon has the most variation.

The Nikon and Sigma lenses show an interesting point. Looking at the graphs the Sigma clearly has more variation, but the Sigma variation number is only slightly different than the Nikon number.  That’s because the average resolution of the Sigma is also quite a bit higher at 30lp/mm  and the formula we use considers that.  If you look at the green variation areas you can see that the weaker Sigma copies will still be as good as the better Nikon copies. But this is a good example of how the number, while simpler to look at, doesn’t give the whole picture.

The graphs show something else that is more important than the simple difference in variation number. The Sigma lens tends to vary much more in the center of the image (left side of the graph) and the variation includes the low frequency 10 and 20 line pairs per mm areas (black and red). The Rokinon tends to vary extremely in the edges and corners (right side of the graph). In the practical world, a photographer carefully comparing several copies of the Sigma would be more likely to notice a slight difference in overall sharpness between the lenses. The same person doing careful testing on several copies of the Rokinon would probably find each lens has a soft corner or two soft corners.

Attention Fanboys: Don’t use this one lens example and start making claims about this brand or that brand. We’ll be showing you in future posts that at other focal lengths things are very different. Canon L lenses don’t always have the least amount of copy-to-copy variation. Sigma Art lenses in other focal lengths do quite a bit better than this.  We specifically chose 24mm f/1.4 lenses for this example because they are complicated and are very difficult to assemble consistently.

And just for a teaser of things to come, I’ll give you one graph that I think you’ll find interesting, not because it’s surprising, but because it verifies something most of us already know. The graph below is simply a chart of variation number of many lenses, sorted by focal length. The lens names are removed because I’m not going to start fanboy wars without giving more complete information. And that will have to wait a week or two because I’ll be out of town next week. But the graph does show that wider-angle lenses tend to have more copy-to-copy variation (lower variation number), while longer focal lengths, up to 100mm, tend to have less variation. At most focal lengths, though, there are some lenses that have little, and some lenses that have a lot of copy-to-copy variation.

 

 

What Are We Going to Do with This?

Fairly soon, we will have this testing done for all wide-angle and standard range prime lenses we carry and can test. (It will be a while before we can test Sony e-mount lenses – we have to make some modifications to our optical bench because of Sony’s electromagnetic focusing.) By the end of August, we expect to have somewhere north of 75 different models tested and scored. It will be useful when you’re considering purchasing a given lens and want to know how different your copy is likely to be than the one you read the review of. But I think there will be some interesting general questions, too.

  • Do some brands have more variation than other brands?
  • Do more expensive lenses really have less variance than less expensive ones?
  • Do lenses designed 20 years ago have more variance than newer lenses? Or do newer, more complex designs have more variance?
  • Do lenses with image stabilization have more variance than lenses that don’t?

Before you start guessing in the comments, I should tell you we’ve completed enough testing that I’ve got pretty good ideas of what these answers will be. And no, I’m not going to share until we have all the data collected and tabulated. But we’ll certainly have that done in a couple of weeks.

 

Roger Cicala, Aaron Closz, and Brandon Dube

Lensrentals.com

June, 2015

A Request:

Please, please don’t send me a thousand emails asking about this lens or that. This project is moving as fast as I can move it. But I have to ‘borrow’ a $200,000 machine for hours to days to test each lens, I have a busy repair department to run, and I’m trying to not write every single weekend. This blog is my hobby, not my livelihood, and I can’t drop everything to test your favorite lens tomorrow.

Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

Posted in Equipment
  • Scott Kennelly

    I’m really surprised nobody has posted a comment here. I wanted to say thank you for posting this. It’s very interesting stuff. I know others find it interesting too, because I cam across a link to this page in a post on a forum somewhere.

  • Leonid Xaxax

    What is a distance from a matrix to a target?

    How many lines per mm should allow a lens for Sony a7rii?

  • Adam Fo

    The German magazine ‘Color Foto’ have been testing lens centering for years. Leica lenses made in Germany have the most constantly high scores for this going back to 1996 (the earliest issue I have.) So, although they don’t test multiple copies,you see pattens emerge.
    Canon (non-L) and Nikon AF wide angles from 1996 to around 2007 or so, can be dreadful.

  • Brandon

    Samuel,

    Here is a very very beta chart I wrote the software for at 1am this morning: http://i.imgur.com/Dlrr5oT.png

    I won’t tell you what lens it is, but the Y axis it the # of samples and the x axis is number of standard deviations from the mean. There’s a histogram and moving average fit curve. Each of the plots is one of the five spatial frequencies measured.

    You’ll notice that it’s bimodal and there’s not really much of a normal distribution. In retrospect, assigning something like “90% of samples fall within this range” was a little silly since it seems the lenses are not at all statistically “normal.”

    Best regards,
    Brandon

  • I revisited this, to read the methodology behind the range charts, and I think you got your probability wrong: with a normal distribution, the range of +/- 1.5 standard deviations contains 86.6% of the realizations, not 98% (that would be +/- 3 standard deviations).

  • carypt

    i cannot determine the manufacturers will to work good .
    i think the variation-number is confused by the relative mtf-result connection . the number should give information only on the build-quality of the lens ( accuracy in spacing , lenses-cut , glass formula ) and not on resolution .

    the variability is because of coincidence of tolerances in build or sloppiness . but yes , design ( quantity of inbuilt lenses ) can increase the tolerance-induced variability , too .
    the fact that high-end-lenses are more vulnerable for mtf-variability-impact should not be hidden by a possible quality bonus , whereas the variability still has heavy impact on the lenscopy .
    the green 30lines mtf indicator is influenced by the mtf-values-height , it should better have a reciprocal mtf-correlation , or something like that , it should not give credit to good resolution .
    well , i dont know how to consider mtf-values fair on variational spread .

    i write this because the sigma 24mm 1.4f looks more variant than the nikkor 24mm 1.4f to me , and so i cant follow the variability-number .

    sry for bad english , sry too , if i got it wrong .

  • Chris Newman

    Thanks Brandon for the clarification. I thought cameras’ image processors combined the data from each photosite to interpolate the missing colour channels and produce a reasonable approximation of a full-colour pixel for each photosite. But your explanation explains the difference between your value and others for the Nyquist frequency.

    Chris

  • Brandon

    Chris,

    The Nyquist we refer to here is closest to the “true” or “adjusted” Nyquist of this sensors. The bayer array *immediately* throws away 50% of your spatial resolution, as you need a 2×2 chunk to make a single pixel. From there you have sensor losses, typically in excess of 15%.

    -Brandon

  • Chris Newman

    This is excellent information for those of us wanting to choose the most suitable lenses, far more dependable than the single-copy test results from other sources, although it would be great if equivalent data for the lenses at their sharpest apertures was available. I’m delighted that similar data on zoom lenses seems to be following. I’m particularly grateful to LensRental for generating this information, as these studies seem peripheral to the core business of distributing and maintaining lenses. Most test data comes from organizations that earn their bread and butter from publicising their results.

    I feel uneasy quibbling with work I appreciate so much, but I was surprised to read “we calculated the (‘consistency score’) just for the 30 line pair per mm (green area in the graphs) variance, since that is closest to the Nyquist frequency of 24MP-class full-frame sensors.” This is only 720 line pairs on a picture height of 4000 pixel rows. In contrast http://www.imatest.com/docs/glossary/ states “Nyquist frequency fN = 1/(2 * pixel spacing) = 0.5 cycles/pixel.” and http://bobatkins.com/photography/digital/eos20d.html showed the system response for the Canon 20D (8 MPx, equivalent to about 20 MPx full frame), with EF 50mm f/1.8 lens at f/11, as falling to 0.5 at 50 cycles per mm, and quotes a Nyquist limit for the 20D of around 75 cycles/mm. The review includes similar data for the Canon 10D.

    Chris

  • Roger Cicala

    Sombody, there’s just a point, with what’s approaching and will soon exceed 100,000 data points, that we just have to start compressing and averaging things to keep it anywhere like reasonably manageable. Not saying the method we chose is the best, just saying we had to do something and this seemed practical. We knew going in we weren’t going to be able to do a scientifically valid 95% confidence interval on what is significantly skewed nonparametric data. That would require resources that we’re a few million dollars short of having.

    But compared to the “0” amount of data that existed before we started, we still feel this is worthwhile. Hopefully somebody someday will come behind us and build on this.

  • Sombody

    Brandon (or LR team): One thing that I remember from statistics is that averaging averages is a bad idea. Specifically, averaging the four runs of one lens into a single MTF chart and then averaging those to calculate the baseline and sigmas. Would it be more accurate to treat each measurement run (as your get four sets from each lens) independently instead of averaging them together? If you’re talking about one lens and provide the chart based off the average of the four, that might keep things simple, but when comparing against all other lens, I’m not so sure. Ideas?

  • Blame

    Rodger. Glad to hear it. It was looking at this blur plot for a top quality Zeiss lens that made me suspect that the worst blur could be deliberately shoved to top & bottom where it wouldn’t matter.

    http://slrgear.com/reviews/zproducts/sony135f18z/ff/tloader.htm

  • Roger Cicala

    Biame, we have. It’s obvious when a rotation cust a corner (it hasn’t yet for internal baffles, but does for some permanent hoods) and we drop those numbers – which are always only 20mm at one or in one case two rotations.

  • Blame

    Not sure about your using 4 rotations. (0, 45, 90, and 135 degrees). Two of them will extend beyond the limits of a standard 24x36mm full frame sensor.

    This will penalize manufacturers who ether put a rectangular mask to the back of the lens (presumably for a little extra flair control) or carefully rotate glass elements to push the lowest resolution to the unused area at top and bottom.

    Have you tried a comparison of diagonal against vertical rotation results?

  • Lester

    Great to see serious work on copy variation. Two comments. One is to note the common rule of statistical thumb — if more than two standard errors separates data points, they may be said to be “significantly” different. So it is common in graphs to illustrate error bars or error bands as plus and minus one standard error, so if there is no overlap it is visually obvious that the difference is “significant”. Two is to note that, in a Gaussian distribution, plus and minus 1.5 standard deviations covers around 87% and not 98% of the data (wry smile).

  • Brandon

    F.M,

    Manufactures update the firmware for lenses periodically, especially third parties. Sigma and Tamron are currently updating their lenses to function on the 5Ds/5Dsr for example. Each body has a lookup table stored in it to “fix” autofocus. It factors in the lens model, focusing distance and the aperture and gives a small correction value for AF. Third parties have to report their lenses as a certain first party lens for this to work right, and the codes for the lenses change with every camera, so things get tricky.

    Physical variation in terms of focus shift is definitely possible, but I do doubt it since the 50mm Art is the lowest variance 50mm-ish lens I have tested yet, and there aren’t many left I haven’t completed.

    Regards,
    Brandon

  • Brandon,

    (about the focus shift question)

    Thank you very much for your answer.

    I remember getting a bit puzzled with some reviews of the 50 1.4 Art stating different results about these characteristics and wondering if it would be due to some strange sample variation (as all reviews stated stunning sharpness), but I understand that this would be very unlikely?

    Thanks again

  • Roger Cicala

    BigEater, that might cause me to accidentally read a page from the Comical Appeal.

    Roger

  • BigEater

    And by the way, why not just tape a sheet from the Commercial Appeal to the wall and shoot that?

  • BigEater

    I’m sure that camera manufacturers think you’re the Devil himself come to torment them, but you are doing the world a great service. Keep it up. Thanks….

  • DXO doesn’t actually test lenses and sensors. They calculate their scores.

    “The DxOMark Score corresponds to an average of the optimal quantity of information that the camera can capture for each focal.The quantity of information is calculated for each focal length/aperture combination, and the highest values for each focal are weighted to compute the DxOMark Score”

    This is why DXO has such wildly off numbers when comparing the FE55 across the Sony A7r and NEX7

  • jankap

    Wouldn´t it be a good idea to offer this testing to us users? Imagine one pays a fee, send his lens to you and become it back after some (not so long) time with a report.
    The report would tell us: a good buy, use it and (do not) wonder or put your lens into the bin.
    A nice study, by the way.
    Jan

  • Thanks for all your work!

    And I totally think you should also rent legacy lenses from the 70’s and 80’s. What could go wrong? 😀 You wouldn’t even have to worry about electromagnificient focusing when generating all the information to still my curiousity.

  • Chris

    All I’m concerned about is the Canon 16-35mm F4 IS cause that’s the lens I own. =) I imagine Canons newer lenses are more consistent than older ones. Though it is strange DXOMark rates the 16-35 F4 IS equal to the 17-40 in sharpness. Some of its sharpness maps even suggest the 17-40 edges the 16-35 out in some areas. I don’t get it.

  • Brandon

    Nightphotographer,

    We are looking to include *every* lens we can in the database. Already discontinued lenses are out of the question, many low price lenses are difficult (e.g 50mm f/1.8 STM) because of low stock, and rare lenses like the Schneider tilt-shifts and leicas are difficult, but all mainstream lenses we can test will be done.

    We have already completed almost the entire ZE line. The 135 requires the larger collimator so it is backburnered for now.

    As of this very moment, I have tested 385 copies across 68 models for the database. I will pass 400 copies today, and there is a new model or two in the latest group of lenses I have pulled. We’re getting there, it just takes time.

    Regards,
    Brandon

  • NightPhotographer

    Thanks for the time and effort you put into this. For a long while, I was looking for such data base but, apparently, what you are doing is unprecedented. I am not asking for including a specific lens in your test, as you have warned readers not to do so but I think it might not be a bad idea to include some semi affordable Zeiss lenses such as 135 APO in your test to show if the high price that they ask for their lenses is justified.

    Cheers

  • Madwyn

    I’ve had a bad copy of SONY Zeiss Distagon 24/2, unfortunately SONY’s engineers with trained eyes and equipments can’t tell the tilted glasses.

    Since then I realised the importance of copy quality.

    I’m really glad to see you are doing this. Fanboys will love you and your results will be quoted a lot.

  • Brandon

    Charles,

    We looked at several options – the best two were taking the log or taking the square root. With the log we would have to multiply by a larger constant, but the effect is similar enough with sqrt that it works out okay.

    Here’s an example of the simulation – https://dl.dropboxusercontent.com/u/8482172/variancescorecorection.png

    The log correction is perhaps better, but the lens must be truly exceptionally good to score high enough that things aren’t linear (higher than 20 with the final scaling choice). Note the vertical log axis.

    -Brandon

  • Matt

    Do you tend to source your lenses all in on go, or buy from a range of sources and over time? I’m interested if you see much variation between manufacturing batches or factories.

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