Canon 70-200mm f/4L, 200mm @ f/4, left and right actual-pixels

Contents

Introduction

June 5 + June 20, 2007

Buy a “pro” lens, and get “pro” quality—or so one expects. Unfortunately, the reality is often different. Manufacturers such as Canon calculate MTF (a measure of lens resolution at specified contrast), based on the optical design. If MTF were measured with real lenses bought from real stores, the optical performance picture might be quite different.

Unfortunately, there’s many a slip between cup and lip, and that certainly applies to lens designs and lens manufacturing and endlessly-vibrating oceanic cargo ships. The author doesn’t know if the bad optical samples he’s encountered over the years are due to poor quality control or mishandling after leaving the factory.

Over the years, the author has purchased multiple Canon and Nikon “pro” lenses ($1000+ each) with obvious optical problems—right out of the box. In the worst case, a Nikon 17-35mm f/2.8 AF-S zoom was so out of whack that the camera viewfinder image was visible blurry!

Don’t take thousands of photos, only to belatedly discover that what you thought were the ten outstanding images are all blurry on one side!

What follows is the author’s firsthand experience with the US $1000 Canon EF 70-200mm f/4L IS, a solid example of what can go wrong with a lens whose specifications hint at outstanding results.

The Canon 70-200mm f/4L IS zoom

In June 2007, I purchased the Canon EF 70-200mm f/4L IS, Canon’s latest 70-200 zoom (see my May 4 blog entry discussing the four Canon 70-200 zoom options). As is my habit, I verify that the lens is optically sound (!) by first shooting an assortment of field shots to see if any obvious issues arise. For zoom lenses, using a variety of focal lengths is important, particularly at the short and long ends of the zoom range.

My suspicions were raised when I observed roof tiles on a house that were blurry on one side, but not the other. Not having ruled out user error (though I thought it unlikely), I filed that issue away in my head for later investigation.

A few days later, I carefully tested my new lens. My favorite test scene is a large mural on the front of Memorial Church at Stanford University campus (not shown). Aside from being a beautiful work of art, it makes an outstanding test subject because it is composed of a huge number of small tiles about 3/4" square, providing excellent detail, and also an obvious center, allowing accurate camera placement.

Testing on the Canon EOS 5D

I chose to test the 70-200mm f/4L IS on the Canon EOS 5D because it offers a full-frame sensor; any optical problems tend to be more obvious at the edges and in the corners. Cameras with smaller sensors crop off those areas, making issues harder to detect.

200mm — Obvious problem at f/4

Shown below are actual-pixels crops from a Canon EOS 5D image, taken from the left and right sides of the frame (vertical center). The camera was centered on the subject, was level left-to-right within 0.3°, and was tilted slightly upward. The crops were taken along the plane of focus, so they should be maximally sharp. The left edge should yield the same sharpness as the right edge.

Seen below, the left edge is obviously sharper than the right edge; clearly the lens is optically misaligned, causing blur along one side.

Depending on the focal length and focus plane of focus, the blur could shift between the left and right sides, or appear sharp in the middle and blurry at both edges. The latter is hardest to distinguish from the normal drop-off in optical performance away from the optical center.

Canon 5D with 70-200mm f/4L IS, 200mm @ f/4 — Left edge (sharp)

Canon 5D with 70-200mm f/4L IS, 200mm @ f/4 — Right edge (blurry)

200mm — Still a problem at f/8

Stopping down two stops to f/8 doesn’t rectify the problem (and it’s hardly a solution if a fast shutter speed is required). At f/8 the left side maintains its advantage; observe the loss of delineation in the brown tiles which form the loops. Note also that the left edge crop hardly changes from f/4 to f/8, a sign of an outstanding lens (the brightness change is due to vignetting). If the whole frame were this sharp, the 70-200 f/4L would be outstanding.

Canon 5D with 70-200mm f/4L, 200mm @ f/8 — Left edge

Canon 5D with 70-200mm f/4L, 200mm @ f/8— Right edge

200mm — Still a problem at f/11

At f/11, the left side is still sharper than the right side. In other words, stopping down 3 stops does not equalize the two edges and does not provide sufficient resolution for a 12-megapixel sensor. It’s as if a 12-megapixel camera becomes a 6-megapixel camera! This is unacceptable performance for a professional lens. And it’s a good example of why only the finest optics used with precise focus can extract the full sensor resolution.

Canon 5D with 70-200mm f/4L, 200mm @ f/4 — Left edge

Canon 5D with 70-200mm f/4L, 200mm @ f/8— Right edge

150mm — even worse

At 150mm, the situation has reversed itself; the left side is now blurry and the right side is now sharp; one can see the image go from blurry to sharp across the width of the image. The switcharoo could be due to the lens itself as elements move, or it might just be that focus was slightly to the advantage of one edge over the other. Whatever the cause, the difference is even more dramatic than at 200mm, and f/11 doesn’t cure the problem (not shown).

 
Canon 5D with 70-200mm f/4L, 150mm @ f/4 — left and right edges

Testing on the 1D Mark III

One must rule out a bent lens mount to be sure that the problem is caused by the lens, and not a lens mount/sensor alignment issue. This can be done by using a control lens (preferably more than one) that demonstrates symmetric sharpness, or by observing that the results occur on another camera body. In short, keep the lens or camera constant and change the other.

The images below were taken with a nearly-new Canon EOS 1D Mark III. It has a 1.3X crop-factor, so one could expect improved optical performance with only the central 60% of the lens image circle being used. Alas, the behavior matches that seen with the EOS 5D; the left side is reasonably sharp, but the right side is blurry. Results at shorter focal lengths with the 1D Mark III confirm the problem, though it’s at its worst at 200mm.

Canon 1D Mark III with 70-200mm f/4L, 200mm @ f/4 — left and right edges

Ruling out the lens

The Canon EF 135mm f/2L was used as a control on the Canon EOS 1D Mark III. At f/2, there is just half the depth of field as at f/4, so any problems will be far more obvious. No problems are observed (see below). Given the other tests, we can safely conclude that it is a lens problem—the 70-200mm f/4L is optically misaligned.

 
Canon 1D Mark III with 135mm f/2L @ f/2 — left and right edges

The replacement 70-200 f/4L IS

The replacement lens was considerably better. Any variation from left to right was small enough that variables such as lighting must be accounted for, and precise testing done. Also, it was tested on the Canon EOS 1D Mark III; the Canon EOS 5D with its full-frame sensor would be a better test body. I plan to do so as time allows.

Although it’s close, it does appear to be slightly out of alignment at 200mm, as suggested by slightly lower contrast on the left at 200mm (see below).

 
Canon 1D Mark III 70-200mm f/4L IS @ 70mm, f/4

 
Canon 1D Mark III 70-200mm f/4L IS @ 200mm, f/4
Note less well-defined tiles on the left side

The Canon EF 85mm f/1.8

Prime lenses are not immune from problems!

The photographer who assumes a prime (non-zoom) lens is unlikely to suffer from optical alignment issues is in for some disappointments. It’s difficult to say whether this is Canon’s quality control, or whether lenses are put out of kilter during shipping. Some of both is likely. But the degradation is the same for the photographer who puts a lens into service without verifying its performance first.

I purchased an EF 85mm f/1.8 in February, 2007 (in part to take advantage of a rebate with the Canon 5D), but had not used it other than to verify basic function until June, when I gave it my standard “sanity check” prior to putting it into service.

I particularly like shooting fast lenses wide open and sometimes off-center, so edge performance is of particular interest to me (and at any rate, a customer expects and deserves a lens that performs to specification!). As seen earlier in this article with the 70-200 f/4L IS, stopping down isn’t a solution, and my intended uses for the 85mm f/1.8 rarely include stopping down beyond f/4; I would typically shoot it at f/2 or f/2.8.

Test results — Canon EF 85mm f/1.8

A number of test frames at f/1.8 were taken on the Canon EOS 1D Mark III with the same results: blurred on the left, quite sharp and contrasty on the right. Some frames were acceptable, being only subtly “off”; this is because a zone of sharpness does have some depth, and the plane of focus does achieve a happy medium in some cases. But when focus is spot-on, a loss of detail and contrast on one side of the image is starkly clear. See for yourself below.

The 1.3X sensor of the Canon EOS 1D Mark III chops off any corner and edge “crud”, and Canon’s MTF chart suggests outstanding performance over the area covered by its sensor, with the unusual property of having higher contrast away from the optical center. Color saturation and contrast are indeed very high.

 
Canon 1D Mark III with 85mm f/2L f/1.8 @ f/1.8— left and right edges
(blurred left side and crisp right side)

Too much time has elapsed to return the lens, so off to Canon Service it goes (June 20, 2007). We shall see how Canon fares at correcting the problem.

Testing guidelines

What to look for

Look for any of the following:

• asymmetric sharpness left to right or top to bottom;
• asymmetric color fringing left to right or top to bottom;
• a double image, halo or blur in one corner but not the others
• a noticeable shift in image position when swapping two lenses of identical focal length.

Some of these will be hard to detect unless subjects are chosen that have uniform detail. A brick wall or building with a mosaic or anything of that sort will do. So will a newspaper taped to a wall, but that brings in the risk of misalignment of subject to camera, so it’s best to stick with more distant targets that allow near-infinity focus.

Field shots

When you first get a new lens, take some “field shots” (normal subjects)—just about anything will do, but seek out subjects with uniform detail. Shoot at various focal lengths. And shoot wide open so that depth of field doesn’t mask problems. A “slow” lens like the Canon 70-200 f/4L masks problems with its significant depth of field at f/4; an f/2 lens has twice the depth of field as an f/4 lens.

Even a blind squirrel finds an acorn once in a while. A shot like the one below is a poor choice; there is no detail to inspect at the edges and corners. There is subject motion. And even a badly misaligned lens can produce a sharp image in one area of the frame (eg the roof tiles).

Poor choice of test subject

Check your images; is the sharpness symmetric left to right, top to bottom? You must account for subject-to-camera alignment; don’t be misled by shooting a building at an angle for example. Remember, the focal plane is a plane (flat surface through 3 dimensional space). The focal plane and subject plane should be as close to perfectly parallel as feasible.

Further testing

If your field shots reveal a possible optical problem, find a subject that will clearly reveal it. Brick buildings, mosaics, rooftops, a lens test chart, etc can all make excellent choices. Be aware that depth of field (diminishing sharpness in front of and behind the plane of focus) is very shallow with today’s high-resolution digital cameras. A slight upward or downward camera tilt shifts the plane of the image sensor relative ; be aware of how the subject aligns with it.

Focus errors can be detected by aiming the camera up slightly at a building, then verifying that the plane of focus is in the center of the building (focus there!); if it’s sharper at the top of the building then the camera is back-focusing. If it’s sharper at the bottom of the building then the camera is front-focusing. (Visualize the plane of focus as the camera is tilted).

Turn off optical image stabilization if the lens supports it. You are shooting on a sturdy tripod aren’t you? Don’t confuse the image stabilization system.

Zoom lenses should be shot at the wide, middle, and long end. It is not uncommon to see variation as the lens is zoomed; testing at one focal length is generally not adequate. Most problems are most obvious at the long end, so start there.

Finally, examine test images (TIF, not JPEG) at the edges and corners (you did shoot RAW, didn’t you?!). All aspects of optical performance should be symmetric about the center of the frame. If this is not the case, something is “off”.

Conclusions

Don’t assume that your brand-new lens (or one you’ve banged around) is optically good. Test it and see for yourself. Today’s modern lens designs are outstanding, but real lenses must be manufactured and transported, offering many possibilities for theoretical performance to drop considerably.

If you find that your new lens is optically out of whack, you might be able to exchange it for another copy—one good reason to work with a reputable vendor. Or you can send it in for service, which nearly always resolves the problem (in the author’s experience). Both approaches require retesting the lens; there is no guarantee with either approach. The risk in sending it in for service is exceeding the return period of the vendor. Be sure to be as specific as possible about the problem, including photographs that show it clearly. This will help ensure that the problem is fixed.

See also:

Focus Accuracy
Lens Mount Misalignment
Lens Reference
Consumer Digicams and Diffraction

...and other Free Articles. And Paid Reviews.

Contact: email a comment on this article.

---

Enjoyed this article Please use an affiliate link to help support the free content on this site. Affiliates include amazon.com and the Apple Store. Also, the Recommended Products and Vendors page offers discounts to diglloyd.com readers on certain products.

© Copyright 2007 digLloyd, Inc.  All Rights Reserved. Reproduction in any form requires explicit, written permission. No part of this document may be copied, distributed, disseminated in any form, electronic or tangible,  whether in whole or in part, without the prior written authorization of digLloyd, Inc or its authorized representative. [Last updated: 26 December 2006]