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Testing for a Decentered Lens: an Old Technique Gets a Makeover

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What is Decentering and What Does it Do?

Strictly speaking, decentering would involve one or more of the lens elements being off of the central axis of the lens. This would prevent the curved surfaces of the lens from bending the light properly. In severe cases it could result in halos or ghosting. In most cases it causes softness, especially away from the center of the lens. A decentered lens may be normally sharp in the center, but very soft in the corners. Or it may just be soft and blurry everywhere. Most lenses have one or more elements that are adjusted to correct centering. Which element that is varies depending on the lens type and design. The front element is often a centering element, with the rear element being the second most common centering element.

Diagram of a Perfect Lens

Lens with the Front Element Decentered

An element can also be tilted to one side or another. Strictly speaking this is not decentering, but it can have similar effects, so people often say a lens is decentered when in fact it’s tilted. In this case one axis may remain sharp, but the other will be out of sorts. If the tilt is side-to-side, the top and bottom of the image might be fine, but both sides soft. If it is corner-to-corner the top right and lower left corners might be fine, while the top left and lower right are soft. High quality lenses usually have one or more elements on which the tilt can be adjusted by two or three elliptical collars.

Lens with the Front Element Tilted

 

The third problem that can occur with lens elements is spacing. If elements aren’t the proper distance apart the lens may not focus the image sharply, or might not focus all the way to infinity. But the lens is not decentered and the tests we’re describing would be normal. There are usually a couple of elements that have ‘critical spacing’ within the lens. Theses are adjusted when the lens is assembled either by removable shims or by installing an element on a ‘ramp’ so that rotating the element moves it forward or backward.

Lens with a Poor Spacing of a Central Element

 

Some Generalizations

It would be nice if we could say “a decentered lens looks like this” and “a tilted lens causes that”. Unfortunately lenses are too complex for that. But one common issue people ask me about is a lens that seems OK in the center but is very soft in the corners. Sometimes that’s just how the lens is designed. But if the lens doesn’t have a reputation for soft corners, it may well be that the copy in question is decentered.

The Way It Used to Be

Back in the days of film and manual focus lenses, most repair shops had a centering collimator. It shined a star chart or a chart of concentric circles through the lens. If an element was decentered the chart would flare or be distorted in one direction. The technician would then adjust those elements that could be adjusted until the lens was properly centered. Obviously in film days you didn’t take a test shot, send it off to be developed, make an adjustment, take another shot . . . . it was all done off camera.

When lenses became more automated, so did testing: Lenses are mounted to the manufacturer’s electronic test system and most of the adjustments made electronically – or the computer report suggests which lens elements need be adjusted. The equipment is breathtakingly expensive and only the factory and some (not all) factory authorized centers have access to it. Standard centering collimators became a thing of the past, except for some specialty shops. (You can find them on eBay every so often if you want one to keep around the house.)

A Simple Test for Decentering

If you want to correct a decentered lens you need an optical bench, a computerized MTF program, or at the very least a lens projector and a lot of knowledge about which elements can be adjusted to correct an abnormality. But if you just want to check and see if your lens is centered properly (at least for most lenses) you don’t need much equipment at all. Now that we have live-view focusing and the ability to look at images in real-time, we nearly have the same thing as a centering collimator built into our camera and lens. You need just a couple of accessories: a tripod to give your camera a stable platform and a simple chart.

The screening test I’m going to describe is not perfect: a few lenses (particularly ultra-wide and 10x zooms) will give false-positive results; and this test won’t detect other causes of softness like problems with spacing of elements. But it’s at least 95% accurate for detecting decentering in our experience (which is for several thousand lenses tested over-and-over).

We use a the Zeiss modified Siemens Star Chart. Star Charts are often used as focusing aids, which is one of the reasons we put them on the resolution charts we use for Imatest and our other testing setups. You determine the lens is properly focused as the rays of the stars get closer and closer to the center. The Zeiss version adds a small white circle around a small black dot in the middle of the star chart. You can buy them for about $30.

Zeiss Siemens Star Chart

 

If you manually defocus the lens just a bit, the star rays and the white and black circles in the center blur, of course. If the lens is in proper alignment and pointed directly (lineup isn’t critical, you can eyeball it) at the star, the white and black circles remain circular as they blur. But if the lens is decentered or significantly tilted the center blur will ‘flare’ out in one direction or another as you defocus.

For example here are star charts shot just out of focus using four Canon 24-70mm f/2.8 L lenses at 70mm.

Even at the low resolution of blog-post graphics you should be able to tell that one of these things is not like the other: the lens in the lower left has a blur which is flared out toward 9 o’clock while the other three lenses have nice round blurs. The nice thing about this test is it’s not very set-up critical. The chart doesn’t have to be exactly in the center of the lens, you don’t have to line the lens up at exactly right angles to the chart, it doesn’t even matter which way you go out of focus (near or far) or exactly how far you go.

Let’s look at the resolution tests we did on the same 4 lenses using our Imatest lab – keeping the graphs in the same positions as the star patterns were above. Yellow areas are the highest resolution, blue are worst.

Imatest results for the 4 lenses

You probably notice that our lower left lens (the one with the flared star chart) has a pattern much softer on the right side. Also note the vertical axis (which shows the peak resolution) is different for this lens (the program automates the axis). The other three lenses peak near 800 line pairs, while the lower left lens peaks at about 600. It’s significantly worse than the other lenses.

Our star-chart flare did a nice job of identifying this decentered lens. The other thing that’s nice is the expensive Imatest lab shows me exactly how much the lens is affected, but it doesn’t show WHY it’s affected. The star chart made it pretty obvious the lens had a centering problem. We recentered the front element (the most common place for decentering on this particular lens) and the lens returned to perfect resolution.

If you don’t have $30 to spend on a Zeiss Star Chart, you can make a reasonable substitute yourself: just stick some white rings (like notebook paper reinforcing rings) on some black posterboard.

 

The flare isn’t as easy to spot as with the star chart, but it’s still noticeable. Here are the same four lenses that were used for the example above. Look particularly at the black center and see how it bleeds out onto the white circle at 5 o,clock — the opposite direction from the white flare noticed above. There is still some white flare noticeable: compare the outside of the white circle at the lower right and upper left areas. It’s not as easy to spot as the Star Chart flare, but it’s there (and this chart is free).

Uses and Limitations

Using the Star Chart as a poor-man’s centering collimator is a nice screening tool. It’s not perfect by any means. Some consumer grade zooms (particularly superzooms), some retrofocus lenses, and a few others show a pattern like this even when they are perfectly aligned, but those are the exception. For the majority of lenses, seeing a decentering pattern when the lens seems soft provides you some confirmation that the lens has a problem and may need a trip back to the factory. It can often answer the ‘is it me, or is it the lens?’ question. It may provide some further data when you’re trying to decide if the corners on your new lens are supposed to be sharper than they seem.

Because I know some people are going to ask, I don’t recommend trying to adjust lens elements at home using this method. Centering the lens element to remove the flare can be a good starting place and we do it here. But it’s just a starting place and you need a LOT of other equipment to fine tune the resolution (especially in a zoom). There are some lenses that don’t have any elements that allow tilt or centering — a factory rebuild is the only option when it gets out of sorts. With others, nearly complete disassembly is required to make such adjustments. And, of course, opening up your lens voids any warranty.

 

Roger Cicala
Lensrentals.com
May 2012

21 Responses to “Testing for a Decentered Lens: an Old Technique Gets a Makeover”

Samuel Hurtado said:

Very cool tip.

I’ve solved a few badly assembled vintage lenses, but only of the “poor spacing” type. This issue seems to be very common on vintage lenses: I’ve had to correct it on 2 out of the 5 vintage Leitz primes that I own.

The most obvious issue that this causes is “no infinity focus”, but it also affects corner sharpness, and even bokeh.

Vintage primes can be cheap and awesome, but they’re also kind of a lottery…

http://www.similaar.com/foto/lenstests/lenstestsc.html

CarVac said:

My Contax Zeiss 50/1.4 is actually somewhat decentered. I can see it in the bokeh: the ring around the very edge is considerably fainter on one side.

Mashuri said:

Wow! This was tremendously helpful. Even using the star pattern image on my computer monitor worked well enough for testing. I had just picked up a Tokina 16-28/2.8 and was worried about centering issues, since it seems to be a problem reported by a few testing sites. Can I assume, since everything blurred nice and even in the center (different story in the corners, but I think that’s a trait of UW lenses, since my Sigma 12-24 behaved the same way) that the elements are properly centered?

LensRentals Employee

Roger Cicala said:

Mashuri,

You can assume there’s no decentering – we’ve tested that lens here enough to know it does exhibit that pattern when decentered.

zajcev said:

I know it could be hard to judge but maybe you could help me. My 35L behaves strange. When focusing with center af point it’s ok. When using side left AF sensor it is ok, but using side right AF sensor the lens frontfocuses. Checked on other bodies and the lens beheve similar, so I assume it is not the camera problem. What defect could it be?

LensRentals Employee

Roger Cicala said:

It could definitely be a decenter, or tilted element causing the phase to not be equal on both sides. Have you checked it carefully to see if one side is softer than the other, or focusing at a different distance?

zajcev said:

I don’t find any of the side softer when fousing with center AF sensor. Image of a text taken a 1.4 at close distance gives a pretty straight dof. Only when focusing with right af points there is a frontfocus. And the text is out of dof.

LensRentals Employee

Roger Cicala said:

zajcev, that leaves me totally puzzled. I’ve no idea what it might be.

sergejv said:

zajcev, the same problem is with all my five nikkor 24-70/2.8 lenses on D300s body.
Left and center AF sensors is OK, but outer right has back-focus. Tested on 50 and 70mm.
Roger, can check your af-s 24-70/2.8 lenses for this issue?

LensRentals Employee

Roger Cicala said:

sergejv, I don’t think that can be a lens issue = on 1 or even 2 lenses, perhaps, but not 5. But to answer the question we haven’t seen any problems with 24-70s and lateral autofocus. Excepting, of course, a couple of copies that were decentered.

Vlad said:

Should there be decentered flaring on the circles other then the one in the center of the lens?

I’m asking because I have a Sigma 50mm 1.4 which suffers from erratic AF an I tested using the black background and white circles. When slightly OOF I can see that the center is ok but the rest whould be decentered going worse to the edges.

Thanks

LensRentals Employee

Roger Cicala said:

Vlad, worse towards the edges, if it’s symetrical, is pretty normal. All the various abberations can make that happen away from center.

Raul said:

I’m trying to determine if a Sony SELP 16-50 is defective (kit lens on NEX-6). In the middle of the zoom range, the left side of the image is significantly softer than the right. It affects about 15 % of the width, an does not improve much when stopping down. This is my second copy of the lens. The first one had this problem at the wider end of the zoom range. Am I unlucky, or this lens has poor manufacturing tolerances? Roger, have you tested any of these lenses?

Pro said:

Hallo, a couple of years ago I bought a D700 with a Nikon 24-70 2.8. The left side of the frame was out of focus but not always so I suspected that an element must been loose. I sent it to Nikon service and after 15 day the said that they calibrate the af of the lens and everything was ok. I find that answer idiot. Since the lens was new I demanded a replacement and after a month a new lens arrived. Better copy but same problem.One shot perfect , 9 shots average .The body was wonderful providing nice pictures at 2500 ISO but the lens…So I sold all the Nikon gear and continued with my reliable 17-55 on my 30D!( yes I bought the D700 instead of the 5DmkII because of the better ISO performance)

Bradley said:

I just picked up a Zeiss Siemens Star Test Chart to check for lens focus and decentering. Well, I just ordered it. I’m assuming that it is just a piece of paper or thin cardboard, right? If so, can it be laminated in plastic without any effect on it’s quality or use?

LensRentals Employee

Roger Cicala said:

Bradley, I wouldn’t laminate it unless you can get a low-glare covering and even then I’d be hesitant. It’s made to be the first surface and adding a diffraction layer might cause an issue. It might not, too, but for critical testing I think it best not to add another variable.

Kate said:

I am testing a Sigma 24-70mm because lately when I am shooting it seems that the center of my image is sharp, and the focus gets softer on the left side. When doing your test with the circles, the corners seem to only show slight change in the center dot. Is this normal? Should the center target be the only one showing misalignment??

LensRentals Employee

Roger Cicala said:

Kate, only the center target is pertinent with this test. Coma and other aberrations will affect the off center dots in a normal lens.

Norbe said:

I just sent my Sigma 70-200 f2.8 HSM II back to Sigma in NY for the decentered problem. Center is shape, but left and right are blurry. They emailed back saying the lens is being sent to Japan for repair. So, it seems that they do not have the equipment in the US to make the adjustment. Just reserved one on rental to fill the gap.

Fabrizio Giudici said:

Many thanks for all the info. I’ve just ordered the Zeiss Siemes star chart, but in the meantime I’ve run a quick test with the image above on the monitor. I’ve just received a brand new SEL-1018 and it looks as it’s decentered (confirmed also by the quick test described at photozone.de). Do you have any experience with this lens?

Jon said:

For the purpose of testing would it suffice to print the start chart on A3 size paper using a laser printer?

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