Canon EF 1:2.8/300 mm L IS USM - Forums

Topic starter

Aug 28, 2022

Hi all,

as a pro photojournalist I have used this lens (1st version, ~14 years old) for work. Since I started with mobile AP about 18 month ago, I also use it here – see my profile / gallery.

One thing I really miss is a useful dovetail to mount it on my iOptron CEM26EC. For the time being, I always used the built-in lens clamp together with a Manfrotto plate intended for use with a normal Tripod. Though it seems, that small plate can hold the heavy lens tight enough for a night while also "surviving" a meridian flip, I face two/three challenges:

first and foremost: a dovetail with 2 fixing points (instead of 1 screw) would hold the lens' optical axis perfectly parallel to the dovetail itself and thus to the mount's implied pointing direction.
second: it still may happen sooner or later that the whole thing crashes to the ground or – less dramatic – annuls itself from slightly (or heavily) drifting away from proper pointing over time
third: thinking about buying a dedicated astro camera in the future, I certainly still want to use this lens with that new camera. So the dovetail thing remains an issue for my needs.

My question is:

Does anyone know or has experience of how to use this fine (but heavy) lens with 2 fixing points along the optical axis with a dovetail to get a reliable sturdy mount?

Of course, I searched the web and some dealer's sites but couldn't find something promising to date.

Thanks for listening and all helpful hints!

Frank aka firstLight

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kuechlew

Aug 28, 2022

Before I moved on to astrocam + scope I used an own construction where I mounted my camera to a vixen style dovetail with a 1/4" screw and the lens was fixed with a tube ring mounted to the dovetail. I actually used another tube ring clamped to the focus ring for fine focusing. It helped that I had a battery pack mounted to my camera.

It may be difficult to find a large enough tube ring for your 300mm f2.8 though.

Clear skies
Wolfgang

firstLight

Topic starter

Sep 2, 2022 · Edited: May 25, 2024

Thanks for your reply!

I never bought or used additional battery packs for my EOS 5D4 or EOS 7D and don't intend to, because I won't need them. The EOS 1Ds3 has a big battery built into it's huge body, weighing some ~1.4kg (body alone). I don't know whether adding a battery pack to either of 5D4 / 7D would level nicely on a dovetail together with an additional tube ring.

Unfortunately the lens tube of the 300mm f/2.8 L IS USM is stuffed with electronics and several rings / buttons, so fixing an additional tube ring seems next to impossible. Also, the tube itself has rounded transitions between the functional parts:

The (more or less) best position to place a second tube ring would be around the 'focus memory' action ring to the right of the red line. The original tube ring allows 360° rotation for the camera body which is very useful. This option would be lost when a second tube ring wouldn't allow easy rotation as well.

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kuechlew

Sep 2, 2022

There is no free lunch. You mentioned the advantages of a second mounting point yourself. You won't use the ability to rotate the lens completely because you can always loosen the screws in the tube ring and rotate your lens. Loosening and fixing will of course change the parallel adjustment of the lens to the Vixen plate, so it's a bit of a mess. I only see two possibilities for a two point connection: Lens + Body (some smallrig components may help you to deal with the height difference between lens collar and body) or lens collar + tube ring. The first option will lose the option of lens rotation but it may be the more reliable way to go. Losing the option of lens rotation may not be as bad as it sounds if you decide to crop the fov.

Both options have drawbacks but sound better to me than having the setup drop to the ground at some point.

Clear skies
Wolfgang

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firstLight

Topic starter

Sep 2, 2022 · Edited: Dec 21, 2023

Both options have drawbacks but sound better to me than having the setup drop to the ground at some point.

Haha ... you name it and you are right!

Option 1: Thinking about the 'second tube ring' option again:

plus: a second tube ring would be completely independent of the camera (DSLR in my case) mounted.
plus: a long spacer ring and screw is probably available without pitfalls.
plus: everything is only attached to and dependend on that single lens,
plus: the camera body's weight serves as a counter load behind the balance point. This implies a rather short dovetail an most probably no collision risk with the mount at slewing or meridian flip.
minus: a thin enough and nice fitting tube ring might be nowhere 'in stock', thus probably time consuming and hard to find and/or rather expensive when it has to be custom crafted by a supplier (offers exist). Of course there will be no return policy when it doesn't fit as expected, eg. in case when I overlooked something.

Option 2: Using the camera body as a second fixing point:

plus: maybe more sturdy due to the longer distance between the two fixing points (but maybe not)
minus: no field rotation possible
minus: the balance point needs to be moved farther backwards, thus longer dovetail needed
minus: having to move the camera body farther out (behind the balance point) could yield mount collision at slewing or meridian flip.

So, to my own surprise, I currently vote for Option 1: This should be safer and more versatile with the single drawback of getting a nicely fitting tube ring somewhere.

Did I miss or overlooked something?

CS Frank aka firstLight

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kuechlew

Sep 2, 2022

Based on your first name I assume you're located somewhere in the German speaking regions. Astroshop, Teleskop Service and Lacerta offer a broad range of tube rings and dovetails, so it should not be too much of an issue to find a clamp which fits. A "cheap" clamp with 3 clamping screws may be sufficient since your lens collar already offers a very good primary support to the lens. These usually come in pairs, the second one you may use for improving manual focusing like in this video: (9) A simple trick to improve manual focus on stars - YouTube

Not 100% sure whether moving the balance point back is a drawback with this long lens. Which option is the more balanced one depends on whether the current balance point is in front or behind the lens collar. If it's in front, supporting the lens at the front is better, if it's behind supporting the camera is preferable. Both options should work well though.

I would expect Option 1 to be cheaper.

If you have access to someone with 3D printing skills this may help for each of the two options and may offer additional possibilities.

Clear skies
Wolfgang

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Tareq Abdulla

Nov 12, 2022

I have this lens and not much i can talk about it, the tripod collar of the lens just broken from inside so now i can't lock it secure around the lens rear, and i couldn't find a replacement anywhere, so either i use a generic another model or just use a ring mount instead.