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Canon EF 70-200 f/4L IS and EF 2X

Doug Kerr

Well-known member
Today I received my Canon EF 70-200 mm f/4L IS USM zoom lens (used, excellent condition, from KEH). It is way lovely.

I have a Canon Extender EF 2X II. An interesting issue with focal length extenders is whether the increased resolution caused by enlarging a subject (so it is covered by more pixels) might be counteracted by degradation of the composite lens optical resolution caused by the imperfect performance of the extender.

I did a quick-and-dirty check of this. The results are interesting.

Here is the scene as shot with the lens itself at 191 mm (reduced from original camera resolution for presentation here):

Test_F28848-01A-S600.jpg


Scene as shot at 191 mm​

I had meant to shoot at 200 mm, but evidently fumbled the zoom ring!

The circle shows the general location of the crops we will see shortly.

The scene was also shot with the lens at 200 mm (actually!) but with the 2X extender in place (overall focal length 400 mm).

Here we see, from the 191mm shot, a 150 px x 100 px crop of the license plate area of the vehicle seen above, upsized 2X (to get us ready for the comparison to follow):

Test_F28848-01-C1-2X.jpg


Crop from scene as shot at 191 mm, upsized 2X​

Here we see, from the 400mm shot, a 300 px x 200 px crop of the license plate area of the vehicle seen above, at original camera resolution:

Test_F28847-01-C1.jpg


Crop from scene as shot at 400 mm​

We see that in fact there is a substantial improvement in image sharpness resulting from the use of the 2X extender.

Best regards,

Doug
 

Cem_Usakligil

Well-known member
Hi Doug,

A nice demonstration, thank you. It is a known fact that adding an extender (i.e. more glass) causes a certain degradation to the IQ. However, this is a lesser degradation compared to the case where one would have to employ advanced software tools in order to increase the resolution of the picture which was taken without an extender. In this case, hardware wins from software, because it contains more information to start with.
 
We see that in fact there is a substantial improvement in image sharpness resulting from the use of the 2X extender.

Hi Doug,

Thanks for the demonstration.

While the resolution benefit is obviously there in the center of the image, it's the corner performance that may suffer too much for general use. But that doesn't mean it can't be useful when the center resolution is more important and the reach (magnification) beyond 200mm is needed. One can always stitch multiple frames with liberal overlap between the tiles, and in effect use mostly a cropped image circle to build the field of view. At these long focus distances the parallax is limited so stitching with enough overlap can even produce good results without a pano setup.

Cheers,
Bart
 

Jerome Marot

Well-known member
While the resolution benefit is obviously there in the center of the image, it's the corner performance that may suffer too much for general use.

That is unlikely. Converters work by enlarging the center part of the image given by the base lens. Whatever unsharp corners the base lens may have is rejected outside of the frame. Basically, they are divergent elements, a variation on the Barlow lens.

Because they enlarge the center portion of the base lens, a perfect converter would divide the resolution of the base lens by its power. In practice, however, this is not quite true and some matched converters may be computed to correct some residual aberrations of the base lens. But they do degrade resolution, and stronger converters (x2, x3) degrade it more than lighter ones (x1.4).

BTW: a zoom is little more than a lens with a movable converter. The converter's power varies with its distance to the lens. A telephoto is a lens shorter than the equivalent long focal with a built-in converter.
 

Doug Kerr

Well-known member
Hi, Bart,

Thanks for the demonstration.

While the resolution benefit is obviously there in the center of the image, it's the corner performance that may suffer too much for general use. But that doesn't mean it can't be useful when the center resolution is more important and the reach (magnification) beyond 200mm is needed. One can always stitch multiple frames with liberal overlap between the tiles, and in effect use mostly a cropped image circle to build the field of view. At these long focus distances the parallax is limited so stitching with enough overlap can even produce good results without a pano setup.

Thanks for the observations.

What you say about corner behavior is certainly true, but there is a risk we may misapply it, depending on our situation.

Let me start with a discussion of how a focal length extender works. (Bart, I'm sure you know this; this is for the benefit of the onlookers.)

A rear focal length extender (the type we normally use on SLR lenses) is a non-inverting. magnifying relay lens. With an extender in place, the base lens (this was at one time called the prime lens, before that term was inexplicably hijacked to mean "lens of fixed focal length) still forms its image in the usual place, the back flange distance to the rear of its mount register surface (which now falls inside the extender).

That image, a virtual image (or aerial image) in this case (since it is not formed upon any surface) is the object for the extender.

From it, the extender forms its image, larger than the first image, at the back flange distance to the rear of its mount register surface (that is, on the body focal plane).

A real issue in this whole topic is "imperfections added by the extender"; the result of its aberrations. Although this is an important part of the whole story, it is important to separate it from the matter of the impact of the operation of the extender. Thus, for the moment, I will assume the happy situation in which the extender is an ideal non-inverting, magnifying relay lens.

Now, let's considered the matter of "corner degradation". If we were to test the composite lens (base lens plus extender), we would find certain properties at the corners of whatever frame size we decided to use as the premise for the test - probably the frame size of some particular genre of camera.

But those parts of the image are taken from portions of the image formed by the base lens that would not be at the corners of the frame were the base lens used alone, but father toward the interior of the lens' image circle. The aberrations there would (typically) be less severe than those at the portion of the image that would be at the corners of the frame were the base lens used alone.

Now, to speak a little casually, those aberrations are indeed "magnified" (in terms of the camera image space) by the magnification of the extender. (They are not magnified in object space; that is, as related to the scene features.)

So we have "more benign" aberrations at the corners of the image of the composite lens), exacerbated by the effect of the magnification of the extender. The net result might be "better" or "worse".

In any case, note that this would be as Judged by a test of the composite lens across the camera frame size.

Now let's consider one actual situation in which we might contemplate using an extender. Lets say that the base lens focal length is 200 mm, and the extender magnification is 2X.

We are interested in a scene that would be embraced by the field of view (on this camera) of a 400 mm lens (let's say, without much margin). (If not, then this whole exercise would be moot.) We might shoot it with the base lens (200 mm), and crop as required (that would be to perhaps a little less than 50% of the frame dimensions).

Or, we might install the 2X extender, and shoot at 400 mm. Then the crop for delivery would be a little less than the full taken frame.

Now, let's first consider the image of the wanted portion of the scene as formed by the 200 mm lens when it is used alone. Next we consider the image of that scene material as formed by the 200 mm lens when used with the 2X extender. They are identical (the optics of the base lens have no idea what is to its rear). There will be certain imperfections of that image at any place on it (owing to the finite MTF performance of the lens, the geometric distortion of the lens, and so forth) - on any particular scene feature.

Now, again, I continue to assume here prefect behavior of the extender (for the reason mentioned at the outset). Then, in the case where the extender is used, the image formed by the base lens is doubled in linear size, but otherwise unchanged, by the extender, and falls at that doubled size on the focal plane.

If we could perfectly capture the image of our intended scene region on the focal plane in the 200 mm case, and the image on the focal plane, perfectly (say, with ideal film of fabulous resolution) and enlarge them to the same size for viewing, they would be identical. The imperfections at a certain place in one would be precisely the imperfections in that same place in the other.

Thus we see that, setting aside imperfections in the extender, the use of the extender does not, in any way that matters to our result in this scenario, in exacerbating the imperfections of the base lens.

Now, of course the real imperfections in the extender add into the practical result.

What this means is that, in my test, if I present the entire image taken at 400 mm, a crop covering the same scene material from the 200 mm frame, and arrange to compare them by way of sophisticated upsizing of the latter, the only difference we should see, at any place over the image, center or corner, are:

• From the imperfections of the 2X extender (appears in the 400 mm case).

• From the different scaling of the sensory resolution with respect to the image (better result in the 400 mm case, which of course is why we would even bother with the extender!).

We should see no difference that results from the basic process of magnifying the base lens image with the extender.

Said in a simpler way, in this scenario we never see the behavior of the base lens, with or without the extender, in the portions of its image that would fall in the corners of the frame when the lens is used by itself. Why? Those portions of its image are of scene materiel we have declared to be of no interest to us. Were they of interest to us, we could not have shot the scene at 400 mm.

Best regards,

Doug
 

Doug Kerr

Well-known member
Here we see the "corner" results, again with the EOS 40D, the EF 70-200mm f/4L IS USM, and (in one case) the Extender EF 2X II.

Actually, in the "scene of interest", there was no object at an actual corner that was good for this purpose. But I chose a suitable region near the corner, as shown by the white rectangle to the upper right in this, the "400 mm" shot (presented here at reduced resolution), which defines the "scene of interest".

Test_F28847-01A3-S600.jpg

"400 mm" shot, showing location of "corner detail" region (upper right)​

Here we see a 96 px X 72 px crop from the 191 mm shot, upsized to 300 px X 225 px for presentation here:

Test_F28848-01-C2-S300.jpg

Crop of "corner detail" (191 mm shot)​

Here we see a 200 px X 150 px crop from the 400 mm shot (same scope), upsized to 300 px X 225 px for presentation here:

Test_F28847-01-C2-S300.jpg

Crop of "corner detail" (400 mm shot)​

We see that again there is ample benefit of the improved exploitation of the sensor resolution with the extender in place for this well off-axis scene region.

We of course cannot from this tell exactly what the composite lens performance is at this subject location compared to the performance of the base lens alone at the same subject location (i.e., the impact of imperfection in the extender).

Let's hear if for "half pint" (our name for the Extender EF 2X II).

Note that I have no AF nor focus confirmation on the EOS 40D with this rig (owing to the f/8 aperture of the composite lens).

Best regards,

Doug
 

Bob Latham

New member
Thanks for the interesting discussion guys. I'm probably alone in wondering how much more simple it would be to design a high quality 2x converter/extender if the image was allowed to be inverted?
Clearly it would necessitate a view finder inverter or maybe reporting its use to the camera body could allow it to correctly display the resultant image when using Liveview.
Prime lens aberrations would still be magnified but the simplified converter/extender would be adding less additional ones (or of a lesser effect) to them.

Bob
 

Doug Kerr

Well-known member
Hi, Bob,

Thanks for the interesting discussion guys. I'm probably alone in wondering how much more simple it would be to design a high quality 2x converter/extender if the image was allowed to be inverted?

Well, if we didn't have to worry about that pesky spherical aberration, it would be simple indeed - one positive element!

Best regards,

Doug
 
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