Extension tube CA, test images

[rick55]

After I asked about it in this thread, I printed a target and took a couple of test shots with and without the extension tube.

Here's my EF-S 17-55 directly on the camera, no PP, full image:



Here's a full-sized image of the intersection at the upper-right of the same:



Now with the tube mounted, again no PP, full image:



And a full-size image of the upper-right:



The bottom line appears to be that the lens has some CA, which is to be expected. It seems to measure about 4-5 pixels on the blue side. With the tube, that CA at least doubles, to 11-12 pixels.

These were exposed toward the right, but not overexposed anywhere: no blinkies on the camera display, and no blown highlights indicated in ACR.

I wasn't able to focus exactly the same (that's the idea of the tube), so there's a little difference in distance from center, but I wouldn't expect it to make this pronounced a difference.

Cheers,
Rick

[Dave Humphries]

The bottom line appears to be that the lens has some CA, which is to be expected. It seems to measure about 4-5 pixels on the blue side. With the tube, that CA at least doubles, to 11-12 pixels.

These were exposed toward the right, but not overexposed anywhere: no blinkies on the camera display, and no blown highlights indicated in ACR.

I wasn't able to focus exactly the same (that's the idea of the tube), so there's a little difference in distance from center, but I wouldn't expect it to make this pronounced a difference.

Hi Rick,

Hmmm,

Good target and methods employed, but with an unexpected result.

That is classic CA proof, but I really don't understand why it gets worse with the tube, given that's only using the centre of the lens.

Got me stumped

[Dave Humphries]

OK, looking at the EXIF

First (untubed) shot:

Camera Maker: Canon
Camera Model: Canon EOS REBEL T1i
Lens: EF-S17-55mm f/2.8 IS USM
Image Date: 2010-04-29 10:56:28 -0400
Focal Length: 55.0mm
Focus Distance: 0.37m
Aperture: f/9.5
Exposure Time: 0.011 s (1/90)
ISO equiv: 100
Exposure Bias: none
Metering Mode: Spot
Exposure: Manual
Exposure Mode: Manual
White Balance: Auto
Flash Fired: No (Manual)
Color Space: Adobe RGB (1998)

and with tube:

Camera Maker: Canon
Camera Model: Canon EOS REBEL T1i
Lens: EF-S17-55mm f/2.8 IS USM
Image Date: 2010-04-29 10:57:53 -0400
Focal Length: 55.0mm
Focus Distance: 73.00m
Aperture: f/9.5
Exposure Time: 0.011 s (1/90)
ISO equiv: 100
Exposure Bias: none
Metering Mode: Spot
Exposure: Manual
Exposure Mode: Manual
White Balance: Auto
Flash Fired: No (Manual)
Color Space: Adobe RGB (1998)

I wonder if it is possible to provide a sample shot without the focus have changed, which may affect the CA of the lens even without a tube. Obviously the field of view will change which may make interpretation of the results more difficult.

OK, I know I'm clutching at straws

[rick55]

I wonder if it is possible to provide a sample shot without the focus have changed, which may affect the CA of the lens even without a tube. Obviously the field of view will change which may make interpretation of the results more difficult.

OK, I know I'm clutching at straws

Clutching at straws is good: I'd like to be sure I evaluate this properly. I think the answer is that an extension tube visibly degrades the lens, and I need to spring for a real macro lens. Honestly, the advantages of something like the EF 100 f/2.8 are probably enough that it's stupid to try to get by with an extension tube.

But I'm not I understand the other test condition you're looking for: the image plane was at the same place (the camera was on a tripod). The EXIF data is interesting: the lens with extension tube was actually exactly 12mm closer to the target than the lens without the tube. Should I try moving that 12mm?

Cheers,
Rick

[Dave Humphries]

Should I try moving that 12mm?

That's a very short straw to clutch

I can't fault your methods Rick.

Hopefully someone else will come up with something.

However, yes, a 100/2.8 will be a real boon.

I am really fighting to resist treating myself to the Nikon equivalent, a 105/2.8 with AF, metering and IS advantages over the manual 90 I have recently been using.

I must be strong, I must be strong ....

[JonathanC]

I'll have a guess here at a possible cause. Do you have a filter fitted? The issue that came to me is that as you get closer, the incident angle from subject to filter gets steeper, and as the filter is unlikely to be more than a single element you'd expect to get some CA.

I hope its as simple as taking the filter off.

BTW I looked at Ken Rockwell's review of this lens, and though he says it works well, there's a load of CA in some of the shots.

Jonathan

[Steaphany]

Could the added light path brought on by the extension tube simply exacerbate the CA already present with this lens ?

When the lens is mounted directly to the camera, the light travels a shorter distance and the disparity between the red and blue is not as significant.

Remember, the EF-S mount has a flange focal distance of 44 mm, adding the 12 mm extension tube is a 27% increase.

[peterb533]

This link may be useful in understanding what is going on
http://toothwalker.org/optics/chromatic.html

Peter

[rick55]

I'll have a guess here at a possible cause. Do you have a filter fitted? The issue that came to me is that as you get closer, the incident angle from subject to filter gets steeper, and as the filter is unlikely to be more than a single element you'd expect to get some CA.

I hope its as simple as taking the filter off.

BTW I looked at Ken Rockwell's review of this lens, and though he says it works well, there's a load of CA in some of the shots.

Jonathan

Hi, Jonathan;

I didn't have a filter on (unfortunately), so no easy fix.

It was at 55mm, so the CA according to tests should be reasonable but not great, .03 - .04% of frame height, according to slrgear. I think I'm seeing double that with no tube, but I'm measuring to the edges, whereas they may measure to a 3dB down point or something, so it's probably pretty close.

Cheers,
Rick

[rick55]

Could the added light path brought on by the extension tube simply exacerbate the CA already present with this lens ?

When the lens is mounted directly to the camera, the light travels a shorter distance and the disparity between the red and blue is not as significant.

Remember, the EF-S mount has a flange focal distance of 44 mm, adding the 12 mm extension tube is a 27% increase.

Hi, Steaphany;

That's what I'm guessing, but that it magnifies it pretty heavily. It certainly increases it more than 27%. It appears to more than double, but that could well be. Since the image plane is moving, I can easily believe that any mis-focus of different light frequencies moves as some trig function, since the light rays will be coming in at an angle.

Cheers,
Rick

[rick55]

This link may be useful in understanding what is going on
http://toothwalker.org/optics/chromatic.html

Peter

Thanks, Peter;

I'll read this when I get back today (have to run out for a bit). I also repeated the tests with a 12mm offset, but need to process them.

Cheers,
Rick

[arith]

I think if an extension tube increases CA the so will a multiplier, the reason I mention it is because I was thinking of getting a multiplier. Keep up the good work and I read through half of Peters link and will look at the rest later.

[rick55]

Peter, that's a wonderful discussion of CA: I've read articles about it before, but I think this is the best one I've seen so far.

It seems clear that the CA shown in the test images is transverse (lateral color). If we look at the center of the target, where the lines are perfectly radiating from the center of the image, there's no visible fringe at all.

Here's detail from the center without the tube:



And with the tube:



If there was longitudinal CA (axial color), I'd expect to see fringe in this part of the target. This seems to strongly support the idea that the tube is increasing the existing CA in the lens, rather than introducing a new degradation of quality. Of course, it could be that the lens, and adding the tube, contribute longitudinal CA. But I think it makes sense that the tube is multiplying a lens effect, since the tube is just moving the lens elements away from the imaging plane.

Cheers,
Rick

[Steaphany]

Rick,

In the CA examples, you chose the upper right test pattern intersection. When you examine the lower left intersection, does the CA red and blue switch positions ?

I would expect that CA would be symmetrical relative to the central optic axis.

[Dave Humphries]

I would expect that CA would be symmetrical relative to the central optic axis.

Hi Steaphany,

I agree, it certainly looks that way in the full size shot with tube, where it is so bad it is visible without magnification.
The blue is always 'inside' and the red 'outside' the black lines.

Peter,

Thanks for the link, that was quite informative.

Cheers,

[rick55]

You're right, Steaphany. And Dave is right in what he sees in the uncropped image.

Here's the lower left detail without tube:



And with tube:



The fringes do trade sides compared to the images above.

Cheers,
Rick