Where can I find the maximum camera weight for my lens?

Question

I have a somewhat unique question here, and I haven't been able to get a clear answer elsewhere. There is a question with a similar title, only with the camera and lens terms swapped. My question is different, in that I really do need to know how much weight a Canon white telephoto prime L series lens can handle on its mount.

I am looking to use a particular CCD camera with my 600mm f/4 L II lens, and probably with a 300mm f/2.8 L II lens in the future. The camera with all the various accessories attached may weigh in at about 6lb, maybe more. The heaviest DSLR camera with accessories that I've been able to identify weigh in at around 4lb.

I just need to know what the acceptable limits are. Once the camera is attached, it won't really be touched or moved or anything like that...it will only track, with the lens, on an equatorial tracking mount for astrophotography. If the mount cannot handle 6-7lb of weight, then I think I'll need to figure out some kind of custom support rig, so the entire weight of the camera is not hanging off the mount alone.

UPDATE:

I use a custom rig to attach my lens and a guidescope to my mount. This is what I use:

So the weight of the entire system will not be hanging off the tripod foot on the lens. Actually, the tripod foot is not used at all...the lens is supported in both the front and the back by scope rings, attached to a large Losmandy D-type dovetail at the base, and a Vixen V-type dovetail at the top, for both support and accessory attachments.

The entire weight of the camera will really be hanging off the lens mount. It will usually not remain in the orientation you see above...that is the park position in polar alignment pointing at Polaris (well, the NCP). The mount will reorient the setup to point wherever I want it to in the celestial hemisphere, and then track my object across the sky throughout the night. So the force of gravity on the camera will change, frequently sometimes, and as the mount tracks.

Maximum tracking speed is about 3.5 degrees per second...not terribly fast. Normal tracking rate is sidereal, extremely slow (it matches the rotational speed of the Earth...so, extremely slow. :P) It moves pretty smoothly, comes to a stop slowly. There shouldn't be any jerking actions...unless I experience a pier crash, where the scope or camera run into the pier or tripod the mount sits on.

UPDATE 2:

Here is an example of the size of the camera:

It is largish, maybe about the size of a 1D X. However, that is not its total size. It will also be used with a filter wheel, which is actually larger in area than the camera, and offset down from the center of the camera a bit:

The trick here is getting the camera supported...despite the filter wheel. More so than that...I need to be able to rotate the lens with the camera in order to properly frame my targets. So the filter wheel won't necessarily always be hanging over the camera down towards the ground...it could be pointed to the side, up, at a 45 degree angle or anywhere really.

...hence the reason I am wondering whether the lens mount can handle it on its own, safely, without any risk. I can't risk this lens, so if there is any risk, then I'll probably have to build some kind of support rig for the camera.

Answer 1

Using Catia (Mechanical engineering software), I tried to answer your question by retro-engineering the canon parts.

I found a 3D model of a canon lens from grabcad.com and isolated the lens mount geometry. I compared its dimensions to the ones of my lenses and found them accurate enough. On the following view, you can see the lens mount.

I used brass as a material (Young modulus = 1,31 10^11 N/m2, elastic limit = 3,5 10^8 N/m2), as cited here : http://photography-on-the.net/forum/showthread.php?t=1118809 (you can find information about dimension too).

Then I applied a surface load representative of a force of 100 Newtons, so about the weight of a 10 kg body hanging vertically from the lens mount (considered as fixed, as if the lens is looking toward the sky). I set the surface connecting the screws to the body as anchor points.

As expected, higher constraints (effort) are found on the screw fixations.

As you can see, with a 100 N load, the Von Mises criterion stay under 9 10^7 N/m2, which is under the elastic limit of the brass (about 3,5 10^8 N/m2).

This is good news, it means that your lens mount will not break and that you won't stress the material above its elastic limit. Moreover, you have a safety factor of about 4.

With a load of 5 kg (50 N), the maximum constraint is about 4,5 10^7 N/m2, about 8 times less that the elastic limit.

Given the geometry, this constraint will probably go up if your lens is horizontal because only 1 or 3 fixations will work on traction. I will make a second simulation to look at this case (maybe this weekend if I have time).

Answer 2

It's not just the weight of the camera that's a concern, but the torque that it will apply to the mounting ring on the lens (and in the camera, of course).

Torque is turning force, and it's calculated by multiplying force (weight, in this case) by distance from the center of rotation. So, if we guess that your camera is 4" deep from the mount to the back of the camera so that the center of mass is about 2" from the flange, and that the camera weighs 7 lbs, the torque at the mount would be around 14 in-lb. That's probably comparable to an EF 70-200 f/2.8 being mounted with no support: that lens weighs around 3 lbs and is about 8 in long, giving a torque of around 12 in-lb.

Even so, I'd want to support the camera for two reasons:

• Safety. You don't want to take chances with a lens that costs $11K.

• Stability. Putting a relatively heavy, unsupported object on the end of the lens will shift the center of mass of the whole system. Even if the mount itself is fine, expecting a rock solid connection to the equatorial mount through just one point might be asking too much of the lens' tripod foot. Connecting both camera and lens to a support system will help eliminate unwanted movement.

In the movie industry it's common to use a rail system as a chassis to assemble lens, body, and other camera components into a single unit. There's lots of good info online for DIY rail systems.

I just need to know what the acceptable limits are.

Reach out to Canon. If you're not already a Canon Professional Services member, you should be with a lens like that, and the CPS folks will be able to find out for certain whether you're pushing your luck with a 7 lb camera or if you're completely within bounds.

Reach out to the camera manufacturer. The thing you really care about is whether it's safe to mount the camera and filter wheel on your expensive lens -- you don't need to know about the actual limit so long as your use comes in under the limit. Considering that the manufacturer designed the camera to use an EF mount in the first place, they've probably done some research on the subject and may even have licensed the use of the EF connector from Canon, in which case they may have access to Canon's specifications for the hardware. At any rate, they should be able to tell you if it's safe to let the mount support the camera or if you need to provide additional support of some kind.

Answer 3

The 1D Mark II was 55.5 oz. The 5D Mark III + BG-E11 grip + 2 LP-E6N batteries is about 59 oz. The heaviest flash Canon has sold in the recent past is the 600EX-RT which is 18.5 oz with batteries installed. The WFT-E8A is another 1.35 oz. Add that up and you're at 79 oz. which is about 17 oz. less than a 6 pound CCD camera.

The EF mount should be able to handle that, but you might be approaching the design limits. It's interesting to note that the EOS C100 + battery + grip comes in right around 51 oz. - just less than the same weight of most of the 1-series bodies and the grip is designed and wired to accommodate a boom microphone which would add another few ounces.

It would probably be safer to use a rig that supports both the lens and the camera. However, if the manufacturer sells the camera with no fittings for support of any kind beyond the mount interface, which in this case is a Canon EOS EF mount, then it would seem at the least they are confident the mount can handle the stresses put upon it by the use for which the camera is intended.

Answer 4

I don't have the answer for you but the answer is maybe obvious : if it were a classic mechanical problem, I would say that the lens capacity shouldn't exceed the capacity of the body (using no lens collar):

• If you body can support a 2 kg lens on the body mount, then the same weight is supported by the lens mount ;
• If your lens comes with a collar for your tripod, it's probably safe to say that your body isn't designed to support the lens weight for long (even if using a collar has probably more to do with torque and [camera+body] stability).

So a safe limit should between the 70-200 f/2.8 (1.3 kg) and the 300mm f/2.8 (2.5 kg) : you can probably attach a 2 kg CCD on your lens (the best solution being of course attaching both lens+body on a specific support).