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Optical Tune Ups

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It’s not  falling that hurts, it’s the sudden stop at the end.   

-Author Unkown

 

 

Understandably, people are concerned with whether a new lens they buy is functioning properly and optically within specifications. One thing I harp on is that we also need to watch our lenses over time. Bumps, drops, and normal wear-and-tear can affect a lens optically.

A fair number of pro photographers take advantage of annual ‘clean and check’ services to help keep their lenses in the best possible shape, but other people never find this kind of thing necessary. As usual when there are two different schools of thought, both are correct. Most people don’t ever need to worry about an annual check-up, but it sometimes is worthwhile.

I’m going to use, as an example, some of our data from Canon 70-200 f/2.8 IS II lenses. I choose this lens for a couple of reasons: 1) we have a lot of copies and 2) it’s a high-resolution lens so getting a bit out of sorts may not be apparent at a glance. This lens is NOT chosen because it gets out of adjustment more often than others; it’s about average in that regard.

Let me also make clear that chances are an individual may never see this happen to one of their lenses. I’m always amused when I write about something like this and some fanboy immediately replies ‘my lens never had a problem.’ That’s cool. I’ve got thousands of lenses that never had problems. But about 10% of rental lenses do get out of sorts optically. While rental lenses get abused a lot more than individuals’ lenses, it’s inevitable that this will happen to some lenses in the real word, too.

It’s also fairly easy to fix, although it will usually require a trip to an authorized repair center to get it done.

A Lot of Imatest Data

We test all of our lenses regularly, by photographing test charts, by using Imatest, and more recently by also testing on an optical bench. I’ve summed up the effect of a year’s worth of abuse on slightly over 200 copies of the 70-200 f/2.8 IS II in the graph below. When we put these lenses into service all of them had acceptable results – that is they started the year within the red box.

In the graph, I’ve plotted each copy’s worst results during a year’s heavy rental use. (The graph, for those who aren’t used to my articles, shows MTF 50 in the center of the lens along the horizontal axis, and weighted average MTF 50 on the vertical axis, measured in line pairs / image height from raw images.) I’m showing results at 200mm. The results at 70mm were similar.

 

Worst Imatest Result for 200 copies

 

It’s pretty obvious that 26 copies (a bit over 10% of all copies) lost their optical edge at some point during a year of heavy use with a lot of shipping tossed in.

How Significant a Problem?

How much difference does it make optically? Looking at these graphed numbers you would probably think not too much. You’d probably be right.

First, if you look at the vertical red line I drew, every copy that was out of the box but to the right of the red line still has normal center sharpness. That’s not too surprising; many types of decentering affect the corners and edges but have little effect on the center. Since many people are using a 70-200 f/2.8 lens for action shots, shooting centered subjects, they may not notice soft edges or corners.

Even many of the other copies might pass routine inspections. A resolution of 700 / 500 line pair per image height is as good as a lot of lenses get on a good day. Unless you were comparing copies of the 70-200 f/2.8 IS II you might not notice a problem with copies that were resolving 700 /550 line pairs per image height. If you took careful test chart images you’d see that there was astigmatism (horizontal and vertical lines aren’t resolved equally) or that one area of the lens didn’t resolve as well as others.

The 4 lowest lenses on the graph had obvious problems that anyone would have noticed, although one of those 4 still remained sharp in the center. The 6 lenses above those but well to the left of the red line all had a soft side or corner that you would have noticed if you checked sides and corners. The others you might, or might not, have thought had a problem if you shot with them extensively.

If It’s Broke, You Can Fix It

If, as I claim, the lenses outside the box aren’t as good as they ought to be, then it should be possible to improve them. Aaron and I performed optical adjustments on each of the ‘bad’ copies and the results are shown in the graph below.

 

Imatest results for 26 ‘bad’ copies before adjustment (blue) and after adjustment (red).

 

If you look carefully, you’ll see there is one fewer red dot than there were blue dots. One copy is off at Canon because it needed more help than we could give it.

As far as causes among the 25 lenses we adjusted, three had broken collars (the nylon bearings that hold the elements in their helicoid track). The others simply had an element that required recentering, tilting, or spacing.

Many of these elements slide up and down in helicoid tracks during focusing and zooming; wear and tear can eventually move them a tiny bit and rental lenses are heavily used lenses. Other elements may shift a bit with a bump or jar, affecting the optics of the lens. A tilt or decentering of one element by less than 1 millimeter is enough to devastate the optics of a lens.

Just for another look, I’ve put the ‘after adjustment’ values back into the first graph. Now that’s much better!!

 

The same lenses as in the first chart, after the problem lenses had been adjusted.

 

A Couple of Thoughts

Does this matter to any of you? It will to at least a few of you at some point in your photographic journey. If you have a lens that seems to not be quite as sharp as it once was, a trip to factory service for a makeover is probably worthwhile. Remember, these changes are fairly subtle, often affecting only the corners or edges, or just increasing astigmatism.

I suspect sometimes people don’t really notice their lens is slightly decentered. They just don’t like the pictures it makes quite as much as they once did and use it less frequently. People may just decide they’ve gotten tired of that focal length or that particular lens without really checking to see if it’s slightly out-of-spec. Lenses with astigmatism will do that, particularly. The images just don’t quite look right but it’s hard to put your finger on why.

I’ll also answer a couple of questions I suspect some people will be asking. The first will be: Can you do this at home? I don’t think so, at least not on a lens like this. Optical adjustment on this lens, for example, requires disassembly, partial reassembly leaving off part of the barrel to expose the adjustable elements, then some careful time with a lens test projector or other optical testing equipment (shooting test charts won’t cut it for this), and finally reassembly.

That may also answer the question I see online every so often, “Why did they charge so much for an optical adjustment”? It takes us 1.5 to 4 hours to adjust one of these lenses. I’m sure the factory service center can do it faster, but it’s still a fair bit of work.

The other question someone will ask is: Can adjust the lowest ‘passing’ lenses to be better and bring them all up to the top scores? To some degree it’s possible. But remember adjusting to be better at 70mm may cause problems at 135mm or 200mm. Adjusting them to be great at 20 feet focusing distance may adversely affect infinity. It’s a balancing act, trying to get the best resolution overall.

A saying from one of my previous careers is really appropriate to lens adjustments — Better is the enemy of good enough. I can’t tell you the number of times we’ve decided that a lens was pretty good, but maybe we could just tweak that upper right corner at 70mm a bit. After we ‘improve’ that corner at 70mm, we spend the next 3 hours desperately trying to fix the center at 200mm, which was ruined by that 70mm corner adjustment.

 

Roger Cicala and Aaron Closz

Lensrentals.com

February, 2013

22 Responses to “Optical Tune Ups”

Mike G. said:

I’m curious, did you folks ever think of branching out into the “lens testing/tuneup” business?

I’m not a pro, and any problems with my photos are much more likely to be due to the guy clicking the shutter than to the lens, but I’d think this kind of tuneup work would be valuable for pros that rely on one or two lenses a lot (like let’s say a wedding photographer), if you could turn it around quickly.

Will said:

@Mike: A great idea IMHO and one that fits naturally with the rental business. Offer a discounted rental of an equivalent (or upgraded) lens while it’s in for repair and even the turn-around time wouldn’t matter much.

Tom A. said:

I have asked this question a couple of times in the past. Why not offer calibration and optimization services? Will, your suggestion of a loaner to ease the pain of separation is a great point. Though… Roger does like to tear things down completely. I’m not sure I have ever seen photos from those lenses after they were put back together. :-)
I suppose, with most people not being able having much more variability in their technique, than their equipment. It might be impossible to make the average consumer of this service happy.

Josh S. said:

I was thinking the same thing reading through the article! I know I have a lens I would love to have tuned up.

Anthony said:

“Can you do this at home?”

Would you say the same for most lenses, or are some easy enough to work with that one could do this at home? Obviously if you’ve got a $5k+ L-series or the like and you can get it fixed under warranty, that’s the way to go, but I wouldn’t mind taking apart cheaper lenses (older primes, kit lenses, etc.) to try stuff like this.

And if I may axe two questions, any thoughts on putting together a simple home optical station for such work? My last setup was for testing focal lengths and involved a darkroom, a pair of calipers, and projecting an iPhone display through lenses onto a wall. Managed to get fairly useful numbers, but I don’t see that setup being super useful for adjusting lens elements.

LensRentals Employee

Roger Cicala said:

Anthony,

with older lenses, especially primes, it might be doable to some degree at home with a tripod squared to a reasonable test chart (ISO12233) at a minimum. That would let you visualize corners and compare horizontal and vertical lines to get an idea about astigmatism.

But adjustments are very lens specific. Every one has slightly different adjustable elements and methods to adjust those elements (shims, concentric collars, ramps, centering slides, etc.) If you’re comfortable taking lenses apart you could probably figure it out for each one eventually, but it would be time consuming. There are many lenses we don’t adjust here just because we haven’t the time to figure it out and we don’t have that many copies.

John said:

It seems to me that one of the very competitive online sellers of lenses could gain an advantage by testing and certifying both new and used lenses it sells as “good” examples.

Anthony said:

Thanks, Roger!

I wouldn’t try anything like this myself on any lenses I depend upon, but I can’t leave all the fun to you guys. ;)
I think I still have a friend’s busted canon 50 1.8 somewhere, (the front element is tilted a very noticeable ~15° to the left) so that should be safe to start with.

And if I don’t get anywhere with it, thanks for taking things apart for us and sharing pictures!

Alex said:

I have a Nikon 20mm whose images went very bad very suddenly. Turns out the entire front cell had come loose and was unscrewing itself. The interesting part was the area of the image that stayed in focus wasn’t the center of the image, but rather offset towards a corner. I guess the element was tilting as well as spaced incorrectly…

Opened up the lens and snugged things back down and all is well. That lens is the one thing that keeps me daydreaming about an FX body–I love that viewing angle for landscapes. If anyone starts making a 10mm prime for m4/3 it’s all over.

Matt said:

Thanks for all the wonderful articles you write. Everyone of them is terrific. When you sell a used lens you print next to “sharpness” something like 20/20 lines pair/mm (horizontal/vertical) and then say in the comments it is a high resolving lens. Does this description relate in any way to the MTF results you publish from your testing of new lenses, and the subject of many of your articles about how well a lens is going to reproduce the scene we shot? Thanks.
Matt

Nqina Dlamini said:

Pity you don’t have a branch in my country. Thanks for the articles.

derek said:

thanks Roger , hope start somekind of lens testing business soon.
but any way, a great write-up!

Mario D. said:

Roger,

How does the failure rate of prime lenses compare to zooms?

And how do you align a lens/camera to a flat test target? I am actually thinking of macro shots of flat subjects, but I suppose the principles are the same. I tried iterative changing of the angles (so many degrees of freedom!) as well as measuring distances using a stiff ruler.

Cheers,
M

LensRentals Employee

Roger Cicala said:

Mario, primes don’t get out of sorts as often as zooms, no question about that. It does happen, though, and wide angle primes seem a bit more likely to have issues.

Squaring to targets is harder than people realize. The simplest way is to use an exactly rectangular target and then analyze the image in photoshop to make sure the edges are all exactly the same length and therefore square.

Olavi.P said:

Hi
But when we have non AF lens? How change these kind of lens?

Thank You for answear.

LensRentals Employee

Roger Cicala said:

Olavi,

These adjustments have nothing to do with focus mechanism or autofocus. They are simply optical adjustments to the elements. The same principles apply for manual focus lenses.

Marshall Black said:

My Canon 70-200 f4 went out of sorts a while back. Wish I could have noticed the problem before my trip to the Lake District and Scotland though. Canon UK put in a new focusing motor or something along those lines, plus moved an out of alignment lens element. Funny enough, it didn’t seem as bad on my 60D shots as the 5DII.
Would you have any tips on how to spot a problem before making your trip, particularly as I have a trip to NZ in planning for October? Of course I have plenty of time to spot and sort out before jumping on the plane….
Cheers
Always interesting stuff on here. Thanks for that.
Marshall

Chris said:

I bought a used 70-200 f/2.8L IS a couple of years ago. After using it a bit, I felt it wasn’t nearly as sharp as ones I had rented from you guys. When my pop-up flash began getting flaky, I sent both my 7D & the 70-200 to Canon, asked them to repair the flash and also to check the lens with the body. When they sent the lens back, they stated it was “within specification”. I’ve never been happy with this lens.

Last fall, I used this same lens on a T3i and was surprised by how sharp it seemed to be. However, at the same time, I seem to find myself unhappy with the sharpness of the photos I take with a 3 lenses I use most often on the same body, so now I’m starting to suspect the body and not the lenses. One of the 3 lenses I’ve been less than pleased with ever since I bought it and I bought it brand new (but others have told me the same thing when they owned the same lens).

Assuming the chances of 3 of my main leses all being out of whack are slim, what sort of cleaning/tweaking could be done on the camera besides a wet-cleaning of the sensor?

Thanks,
Chris

LensRentals Employee

Roger Cicala said:

Chris,

First and foremost, I assume you’ve done a good backfocus / frontfocus check of the lenses on each body? Chances are highest it’s an AF thing.

If there is no backfocus / frontfocus, the next most likely issue is a camera mount – camera sensor misalignment. Only the factory can do that kind of adjustment, but if you test carefullly you might see some side-to-side variation that affects all of the lenses, although it will be most pronounced for the wide angles.

Roger

Chris said:

Hi Roger,

Thanks for taking the time to reply.

I have tried to micro-adjust the lenses before, but never feel I get the results I expect (maybe I’m being unrealistic).

When it comes to the 24-70 f/2.8L, I’ve discussed with others that have had the same lens and their feedback indicated that they experienced the same frustrations with that lens that I have.

I need to sit down and do a thorough, controlled test

Regards,
Chris

Lee Saxon said:

The most surprising bit about this to me is that the “normal” box is such a wide spread; 200 lp/ih seems like a lot to me.

BozillaNZ said:

Hi Roger,

Could you shed some light on how did you manage to adjust the 70-200 IS II? I had fixed many lenses but not this one, fearing that I might break something because there is no schematic diagram and no where to start. Where in the lens are the adjustment screws? Front/back/middle? Any pointer would be appreciated!

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