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Disappointed in Kenko DG extension tubes for canon
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Rylee Isitt



Joined: 13 Apr 2012
Posts: 474
Location: Canada

PostPosted: Mon Apr 30, 2012 7:12 pm    Post subject: Reply with quote

Charles,

Nice fix! If mine start to misbehave, I might do something similar. But so far they fit tightly enough for most purposes.

I've found the trick to product photography is massively diffused lighting. Bounce light off of the ceiling if you have to. I have some experience doing product photography with large, set-the-place-on-fire studio lights. Their large size makes getting good images a bit easier, but they aren't very bright (not like a flash). For when I sell stuff on eBay, I typically pop a flash up into a white ceiling.

Here are my own contributions using the ceiling-flash technique, shot in jpeg with +1.67 flash exposure compensation and no post-processing out of camera:





So, extension tube pictures are the new cat pictures!
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Chris S.
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PostPosted: Tue May 01, 2012 12:03 am    Post subject: Reply with quote

Recognizing that this thread is about Canon vs. Kenko, all I have are Nikon and Kenko. In the composite below, can you guess which one is the Nikon (the other three are Kenko)?



Pretty obvious, isn't it? Here's the key:

A--Kenko 20mm extension tube
B--Kenko 36mm extension tube
C--Nikon PN-11 extension tube
D--Kenko 1.5 x teleconverter

By comparison, the Nikon piece seems like a work of art versus the Kenko items. Interestingly, I purchased the Kenko items new, and the Nikon item second hand. Although I use and recommend certain Kenko items, the quality just does not compare with older Nikon equipment (and by presumption, Canon equipment as well).

BTW, no lighting or Photoshop heroics were applied--but the visual quality of the subject makes a real difference, doesn't it? Lighting was a single flash, not diffused, bounced off a partial ring of white paper inserted into the camera side of the various tubes. The flash was blocked from hitting the subject directly with a piece of black paper.

The cantilever separation seems to be roughly consistent between these Kenko samples and the Nikon--something around 0.6 mm. I can't accurately use a caliper in these tubes, but I did insert multiple pieces of paper into each cantilever, and measured the combined paper thickness with a digital caliper. Both Kenko and Nikon items seem similar--if anything, the Kenko cantilevers have more spread. Yet as said, I've found that stacking multiple extension tubes gives me more slop than I'm comfortable with.

Charlie, any idea how much inter-cantilever space you were dealing with for your Contax equipment? Hard to imagine fitting portions of rubber band into the 0.6 mm that I seem to be looking at.

Cheers,

--Chris
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ChrisR
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Joined: 14 Mar 2009
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Location: Near London, UK

PostPosted: Tue May 01, 2012 2:30 am    Post subject: Reply with quote

Clever hack Charles - but I shudder at using rubber bands in lenses . The material is pretty horrible, with a short "design-life". It oxidises, its plasticisers evaporate, polymers break up, cross links break, and after a non-elastic phase you're left with a brittle goo stuck on and likely to crumble off into the nether regions.
I'm no expert, but even those who are can't tell the identity of elastic stuff by visual inspection. ( Melting/burning and sniffing gives them a clue).

If you start with something like "surgical" tubing, or industrial rubber "O" rings you stand a much better chance of getting something which will last.
Some "O" rings of course come with material identified, such as "Viton", "Nitrile" rubber or silicone rubber. The last of those covers a multitude but I believe any of the type would work here.
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Charles Krebs



Joined: 01 Aug 2006
Posts: 5749
Location: Issaquah, WA USA

PostPosted: Tue May 01, 2012 10:33 am    Post subject: Reply with quote

Chris S, ChrisR

What can I say, I was young(er) and foolish Wink , but it worked great!

A better material choice makes eminent sense. And looking at these pictures the "gap" I filled was larger, but overall the approach might still be valid if done with care. (And it would be not be "in" lenses, but poorly made female bayonet mounts in tubes that might otherwise not be useful). Using a sharp razor to "shave" pieces off of a washer of known composition might solve the "size" and longevity issues.
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ChrisR
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PostPosted: Tue May 01, 2012 5:33 pm    Post subject: Reply with quote

"In" the mirror box wouldn't be great either Wink

Actually be careful with "washers" - many plumbing washers aren't UV stable! Some "O" rings too come to think of it, judging by the ones which come on garden hose connectors which always seem to break up. I have selection kit in which I think are Viton, which last much better.
EPDM is probably OK too.
I suppose you could use a UV filter Cool !
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_sem_



Joined: 04 Aug 2011
Posts: 47

PostPosted: Wed May 09, 2012 6:46 am    Post subject: Reply with quote

Rylee Isitt wrote:
... Mine are also made out of metal, with a metal mount. The plastic is just finishing over a metal barrel.


Are you sure about this? I've got a 2yro set for Nikon, my impression is metal mounts but plastic barrel (or thin metal at best). Haven't had any problems, but I see some flex and wouldn't vouch for them with heavier lenses.
Nikon tubes are reputed to be sturdier mechanically, but lack electrical connection. A $30 "Dandelion" metering chip may be attached to reclaim aperture control and sort-of metering with G lenses / small bodies.

Aside for this, some full-frame Nikon users complain that they vignette with long lenses. On the other hand, a tight opening improves contrast sometimes, so this is a bonus over cheapos that lack any light-baffling with crop-frame cameras.
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Rylee Isitt



Joined: 13 Apr 2012
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Location: Canada

PostPosted: Wed May 09, 2012 11:14 am    Post subject: Reply with quote

> Are you sure about this?

The outside of the barrel is plastic, but when I look inside, the structure is metal. I'm hoping that the metal is load bearing, rather than the plastic!
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_sem_



Joined: 04 Aug 2011
Posts: 47

PostPosted: Wed May 09, 2012 11:39 am    Post subject: Reply with quote

Now I remember. You can see the Nikon-mount Kenko guts in Step 1 of PN-11 electrification guide here
http://damien.douxchamps.net/photo/pn11/
Plastic I'm afraid. Though I wouldn't mind plastic if the thing were stiff enough, I guess the play in mounts is more problematic than the tube.
Btw chipping the PN-11 should be much easier than the surgery displayed. It doesn't allow AF and VR with certain lenses, but that may not be field relevant. [/url]
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Rylee Isitt



Joined: 13 Apr 2012
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Location: Canada

PostPosted: Wed May 09, 2012 2:10 pm    Post subject: Reply with quote

The author refers to the Kenko tubes as "plain plastic", but they contain metal mounts and metal inner tubes as can be seen in his photo. If the inner metal tube screws into the metal mounts at either end, then the metal is the load bearing component.

I don't want to take mine apart to find out, though, and the article's photo of a heap of parts doesn't answer this question very clearly.
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_sem_



Joined: 04 Aug 2011
Posts: 47

PostPosted: Wed May 09, 2012 3:01 pm    Post subject: Reply with quote

Rylee Isitt wrote:
If the inner metal tube screws into the metal mounts at either end,...

You never lose hope do you? Wink
Whatever the inner tube is made of, it doesn't. It ends before the female mount side. No need to take it apart to see the 5mm gap on the longest tube.
I think it is not plastic that flexes. The metallic mounts do not seem to move relatively to the plastic tubes which they're screwed to (at least not yet), and the plastic tubes don't seem to bend. But I can see a thin gap appear between the metal mounts of two tubes upon tension. Not much, but I don't see that between the camera mount and any lens. Btw I've seen much worse-made F-mounts of certain all-metal macro adapters.
But the Canon version seems to be a quite different, less complicated design. F-mount has mechanical coupling for the aperture lever, aperture sensing tab, and the AF screw.
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Rylee Isitt



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PostPosted: Wed May 09, 2012 8:55 pm    Post subject: Reply with quote

Despite the gap between the mounts and inner tube, there's no way to tell if the screws pass through the plastic spacers and bridge the gap unless you take the tube apart. You'll notice that the screw conduits line up throughout the length of the tube. Although we have determined beyond a doubt that they are not solid metal. Even if the screws bridge the gap, I suspect the plastic is still providing plenty of structural support.

But it doesn't matter really, since the body of the tubes are not flexing. It's apparently a lack of tension in the metal mounts that causes the flex, and bending out the cantilevers at least temporarily solves that problem.

Quote:
But I can see a thin gap appear between the metal mounts of two tubes upon tension.


Yup, same here.

I think people have different degrees of flex that they will accept. For what it's worth, I used Kenko tubes with a 400mm lens, and the flex was noticeable but not too bad. Mind you, I didn't hold on to just the camera body with a heavy lens dangling off the end. I always hold both the body and the lens in those sorts of circumstances.

I'd prefer to have better quality tubes, sure. But they do the job.
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rjlittlefield
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PostPosted: Wed May 09, 2012 9:57 pm    Post subject: Reply with quote

_sem_ wrote:
But I can see a thin gap appear between the metal mounts of two tubes upon tension. Not much, but I don't see that between the camera mount and any lens.


Rylee Isitt wrote:
...the body of the tubes are not flexing. It's apparently a lack of tension in the metal mounts that causes the flex, and bending out the cantilevers at least temporarily solves that problem.


Let me try to quickly summarize what's going on.

In these bayonet mounts, the flat mounting faces of the lens and camera are normally held tight together by the tension of three springs on the back side of the female part. The springs are compressed when the lens is mounted. As long as the pitch/yaw torque between the mating parts does not exceed the tension of the springs, the two parts stay mounted in perfect contact -- no gap at all. Whenever the pitch/yaw torque exceeds the tension of the springs, the faces of the mount separate by a distance equal to the difference in thickness between the retaining tabs in the female (camera) part and the matching groove in the male (lens) part. The stronger the springs, the more torque can be tolerated before the gap forms; the more precise the machining, the smaller the gap will be when it does.

With this design, there is always going to be some torque at which a gap forms. So the question is a matter of degree: how much torque is needed, and how big is the ultimate gap?

With my Canon T1i body and 18-55 mm kit lens, the gap is easily visible but it forms only with more torque than the weight of the lens or body. With my Canon T1i and Canon 100 mm f/2.8L IS USM macro lens, the gap is much smaller and it's well camouflaged by the black weathersealing ring on the lens. The play between lens and body is clearly visible when observed through a 10X loupe under good light, but I would never see it without specifically looking for it. And it still forms only with more torque than the weight of that lens.

With my Kenko tubes, less torque is required before the gap forms, and when it does, the gap is bigger. As a result, they feel "loose and floppy". For my applications, this is no concern because it's easy and natural to support the lens and camera in such a way that there are no gaps while the picture is being taken. I believe this is the same situation that Rylee describes, just spelled out in more detail.

--Rik
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DQE



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PostPosted: Thu May 10, 2012 5:43 pm    Post subject: Reply with quote

Excerpt from Rik's last post:

"Let me try to quickly summarize what's going on."

"In these bayonet mounts, the flat mounting faces of the lens and camera are normally held tight together by the tension of three springs on the back side of the female part. The springs are compressed when the lens is mounted. As long as the pitch/yaw torque between the mating parts does not exceed the tension of the springs, the two parts stay mounted in perfect contact -- no gap at all. Whenever the pitch/yaw torque exceeds the tension of the springs, the faces of the mount separate by a distance equal to the difference in thickness between the retaining tabs in the female (camera) part and the matching groove in the male (lens) part. The stronger the springs, the more torque can be tolerated before the gap forms; the more precise the machining, the smaller the gap will be when it does. "
-------------------------

After reading the many informative posts on this thread as well as Rik's summary, and thinking about my own seemingly different impressions about my Canon tubes, my guess is that my springs (and perhaps other subcomponents) are slightly different from other people's tube components.

This would explain why my tubes almost seem too "tight" and are somewhat difficult to engage and to disengage. Somewhat difficult, and quite noticeably firmer than any of my Canon camera lenses but not disturbingly so. The tubes are enough tighter than my camera lenses that at first I was a little concerned that I might overstress the camera's bayonet mount or a lens' bayonet mount. Yet they do remove without any visible damage and without *that* much rotational force to "unbayonet" them from the camera body or a lens.

As a result, when I mount my two Canon tubes to my 5D or 5DII camera bodies, there is no perceptible flex with any practical/sane amount of hand-applied stresses from any direction that I could notice. Same thing occurs when I attach any of my lenses to these tubes (previously mounted on a camera body).

Previously, I had just guessed/assumed that this is the way Canon decided to build and spec their tubes, in order to avoid risking flex or whatever. I've had them for 6 years now, and they have never noticeably loosened by my (uncontrolled experiment-based) subjective impressions.

I wish I had a sample of say 10-20 Canon and Kenko tubes to do some simple statistics on how variable different manufacturing runs are. It could be that I have just accidentally acquired an unusually tight set of Canon tubes, for example.

It would also be great to have a lab or machinist do some measurements on things like flex in various directions, testing several samples of several brands.

Can anyone think of a simple test for tube flex that each of us could easily do, and that would also be reasonably relevant to normal use and/or our various subjective impressions? Perhaps a simple torque wrench could be used to see how much force is needed to rotate the tubes after unlocking? Or, perhaps a measure of how much vertical flexing under load while attached to a camera body would be useful? I worry about non-lab measurements without a good way to calibrate and verify the integrity of the test setup, though...

In the meantime, I've tried to use carefully chosen qualitatively descriptive words about how tight I believe my Canon tubes are, with the hope that this conveys the same impression each of you would get if you worked with my tubes yourself.

I just can't manually detect any flex of any type, in any direction after I hook up my tubes to my camera bodies. It doesn't matter if I evaluate this with or without my Canon 100mm (non-IS, non-L) macro lens. This impression is so strong, FWIW, I have the impression that my camera body bayonet mount is stronger after I hook up my tubes, based on how firmly the lens attaches to the tubes vs to the camera body directly.

Hope these added subjective comments are helpful. My impressions sure seem disjoint from many or most other tube flex impressions reported in this thread.

------------

Hmmm....I wonder if bhphotovideo.com would effectively guarantee that an additional 25mm tube (which I've wanted for some time) will feel as flex-free as my current tubes?!?! It would be just my luck that current manufacture is less rigid. The $139 price is as a matter of principle annoying, though...
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Craig Gerard



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PostPosted: Thu May 17, 2012 11:40 pm    Post subject: Reply with quote

A recent observation indicated another area that may be contributing to the 'jiggle' in adapters. The groove designed to accommodate the locking pin is rather wide on a number of adapters and as a consequence there is potential for undesirable 'play'.

Reducing the width of the 'groove' with JB Weld has crossed my mind.

Has anyone tried a similar approach?


Craig
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Charles Krebs



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PostPosted: Fri May 18, 2012 8:38 am    Post subject: Reply with quote

Craig,

There are two parts to this particular "problem". As you mention sometimes the hole into which the locking pin drops is too large for the pin (made that way Crying or Very sad , or has worn with long-time usage). The other is that the tolerances and/or design of the locking pin could be better, and the pin itself can wobble back and forth a little. Annoying enough for a single lens, but multiply this effect several times over for a set of tubes and it can become really annoying.

On one occasion (very long, heavy Nikon telephoto with tripod collar, heavy camera bodies, and lens rotated between vertical and horizontal on the by moving the body) the hole in the lens mount became elongated rather than circular and I decided to have the mount replaced.

But if the locking pin "jiggles", there is really not much you can do besides trying to make the three internal tensioning springs as tight as possible to make rotation "harder".

One of the reasons why "screw mounts" and breech-lock mounts are superior to "bayonet" mounts if multiple connections are made.
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