TEST: Sigma 150-500 OS + Extension Tubes (1 Viewer)

The disclaimer – Although I wouldn’t consider myself an idiot by any means, I am in no way an expert regarding optical physics or mechanics. I simply love the technical arena that comprises all things optical. I offer the following info as a shared experience of my own Sigma 150-500mm OS lens, and my new found appreciation for extension tubes. If this helps just one other person out there, great. If not, well…somebody stop me before I do this again.

INTRO
I’m fairly confident that most who own this lens are already aware of its many virtues, namely its versatility. Not only does it reach out and touch those feathered friends we so enjoy with 500mm of focal length, and offer a highly effective stabilization system, but it also gives a reasonable magnification ratio of 1:5 life size. It was the 1:5 image ratio that initially led me to consider doing this test. I have spent some recent weeks with a new found interest in photographing both dragonflies and butterflies. The magnification of this lens seemed useful for the larger varieties, but still lacked the grasp that I lusted for to fill the frame.

So, on a whim, I removed a 12mm extension tube from my pocket, and after stacking the tube behind the lens I not only gained the ability to move closer. I also swore that the image size was noticeably larger, as if it was adding additional focal length to the lens. I suppose this is a concept already understood by some. But for me, at the time, it was a small revelation. I had always been under the impression that a tele-convertor was required to increase focal length, and the thought of adding more glass to the optical train of a 21 element zoom lens doesn’t seem overly appealing to me. So the curiosity got the best of me, and after Google-hunting I came up short with regards to existing “150-500 OS + extension tube” tests.

That’s where this testing comes in. I wanted to know if, and how much various tubes would increase the focal length. Additionally, I was curious as to where both the Minimum Focus Distance and Maximum Focus Distance would be in relation to the sensor. Of course adding any tube would surely eliminate infinity focus beyond some point. So ultimately, my idea was to see if adding a particular length of tube could give me better “macro” (MFD) for large insects, while also giving me more length for birds, all without the loss of image quality.

THE TEST
Equipment used: Canon 20D, Sigma 150-500 DG OS HSM, Kenko Extension Tube set of three tubes (12mm, 20mm, 36mm), Bogen/Manfrotto 055XPROB tripod, Bogen/Manfrotto 393 gimbal head (Monopod Bracket), shutter release cable. All images were triggered in mirror lockup mode to further reduce vibration. OS was turned off for the duration of the test.

A total of five different comparison tests were done:

1. Image quality comparison, mainly as a visual representation of fine resolution and chromatic aberration. I have already run tests in the past to determine that my copy, like many other Sigma 150-500 OS lenses, is sharpest at 400 (403)mm and ƒ8.0. Therefore I used this setting to photograph a page of fine text out of a magazine, comparing the virgin lens against three different extension tubes.

2. Minimum Focus Distance. Four separate focal lengths were repeatedly used throughout the test to reference the changes of adding tubes at those particular lengths: 150mm, 300mm, 400mm and 500mm.

3. Measured image size per corresponding MFD.

4. Maximum Focus Distance. I abridged this test to only the 500mm focal length. As a side note, I did this test while at work, and was unable to fully test the maximum focus with the 12mm tube, but a reasonable estimate is offered.

5. Measured image size, per modification of the lens focal length. I set the reference target at exactly 10 feet (3m) from the sensor plane of the camera, and mathematically determined the ratio of each stacked tube/lens combo photo to the original lens photo.

CONCLUSIONS
The first thing I’d mention is the surprise that this ƒ6.3 lens suffers virtually no loss of auto focus speed or accuracy with any of the tubes attached. Mind you, this is with the center weighted AF on my Canon 20D. I suppose there will be some variation within different camera lines, as well as different manufactures. Although not included here, for fun I ran the tests with all three tubes stacked. Of course AF is ridiculously challenging in this guise, offering maybe a 1 in 10 lock-on success rate. But I digress.

Regarding image quality, to my eyes, there is little real world difference between the straight lens vs tube stacked variations. At the “pixel peeping” level it seems that the images do get slightly sharper with the addition of the tubes, with a peak in sharpness using the 32mm tube. The improvements are mild, but notable. I have some guesses as to why, but in the end I’m not quite sure. So rather than cluttering this test with additional banter, and uneducated speculation, I’ll leave my hypotheses aside for now.

(attached images are center crops @ 100%)

Minimum Focus Distance could be shortened in all cases, but was most dramatic at 150mm focal length with the 36mm tube attached. This particular setting enabled a 26 inch (68cm) MFD and an image size of 1:3. Additionally, with the same 36mm tube, but extended to 500mm, the MFD becomes 64” (162cm) and gives a similar image ratio of 1:3.2. This particular setting makes for a useful “butterfly” lens, in my opinion, given the longer distance to subject measurement which allows for that ever important “No Scare Zone”. However if absolute highest detail rendition is preferred ,and you can get closer to your subject, the noted resolution at 150mm will trump 500mm (which is not a surprise when considering the design of this particular lens).

The following attachments show the largest images size for each setup respectively. All images are full frame and resized for web.

Image #1 is 1:5 with no tube @ 500mm
Image #2 is 1:4 w/12mm tube @ 500mm
Image #3 is 1:3.7 w/ 20mm tube @ 500mm
Image #4 is 1:3.2 w/ 32mm tube @ 500mm
Image #5 is 1:3 w/ 32mm tube @ 150mm


(Note the last two images comparing the resolution difference: image #4 is 500mm + 36mm tube, image #5 is 150mm + 36mm tube)

With no IQ loss to my eyes, and continued AF function, adding the tubes for short to medium range use (such as bird photography) offers a perfect solution for increasing the lens length. The math is simply as follows:

@500mm+12mm=550mm ƒ6.9
@500mm+20mm=600mm ƒ7.6
@500mm+36mm=650mm ƒ8.2

The tubes do not correctly report the above apertures, but your camera’s TTL function should compensate automatically based on these values, specifically when used in Shutter or Aperture priority modes.

All images are full frame and resized for web.

There is, obviously, no free lunch. The Maximum Focal Distance becomes effected noticeably as the tubes increase in size. I do believe, though, that all of the tubes are useful as long as you consider your photographic style and/or subject matter before hand. For example, a more restrictive Maximum Focus Distance is fine for butterflies, dragonflies, hummingbirds, etc… Whereas a deeper maximum range might be more beneficial for perched songbirds at medium range. Recorded measurements are as follows:

@500mm+12mm>78 feet (23.8m) (I was unable to setup the reference target farther than this distance due to space restraints)
@500mm+20mm=55 feet (16.8m)
@500mm+36mm=29.5 feet (9m)


For me, it seems that unless I’m going for distant birds in flight, at the very least the 12mm could just about live on the lens. An approximate distance of 80 feet is more than enough, especially considering that I seldom target birds at more than 20 feet. But at least, with 80 feet of reach, one could catch larger birds or land animals at medium distance.

When I get around to it I’ll probably do a test that pits the naked lens @ 500mm against a comparably image sized 403mm setting + 20mm tube, at a real world birding distance of 20-30 feet. I’d put money on the 403mm + 20mm tube setup offering up much better resolution and contrast. But only a field test can bring that hypothesis to justice.

Questions and comments are welcome. Recommendations for additional tests also welcome.

A couple recent images taken with this setup:

Interesting,Jason.So could i add the Kenko ext tubes to my 400F5.6 and be able to increase the magnifaction(mispelt,methinks!!)

christineredgate said:

Interesting,Jason.So could i add the Kenko ext tubes to my 400F5.6 and be able to increase the magnifaction(mispelt,methinks!!)

Click to expand...

You sure can Christine, not sure about increased magnification (it is not much) but I attach a chart of min/max focussing distances with various tube combos on the 400/5.6. I used them on the 400/5.6 to get closer to things like butterfly's.

Christine, Roy answered better than I could have, since he uses the same lens as you (thanks, Roy). A simple and generic answer is "yes" you can put extension tubes on any lens. They work with some better than others, but the principle is the same no matter what the lens. Medium focal length lenses (35mm-100mm) really seem to undergo the greatest magnification increase, by cutting down the minimum focus distance significantly. But then they also seem to suffer most with regards to severity of maximum focus distance losses. Putting the same Kenko tubes on my 50mm lens gives me a truer macro setup (roughly 1:1 or better, depending on the tubes used).

Roy, I like your chart for the min/max focus distances. If you don't mind I may have to borrow that format to make a complete chart for all the data I collected in my testing. It might be easier for others to reference.

I suppose I could add the data from the stacked tube setup as tested (68mm). It is a bit extreme, but still usable if you are good with manual focus, and can stay within 16 feet of your subject.

Fowl Mouth said:

Roy, I like your chart for the min/max focus distances. If you don't mind I may have to borrow that format to make a complete chart for all the data I collected in my testing. It might be easier for others to reference.

.

Click to expand...

No problem Jason.

intresting read Jason i had no ider you could still get 78ft etc with a tube so thanks for this review.
Rob

My pleasure, Rob. That was an eye opening discovery for me as well.