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I have noticed. Instead of twisting the zoom ring on my 18-135 DX F3.5/ F5.6 G ED. I can instead push it to the camera to retract the lens using the lens hood as a "handle"

How safe is this "technique"? Its not like I HK Slap it (Smash it with power) but instead I push it gently but how safe is it?

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    In engineering it's called 'back driving', often not the best idea mechanically. – Lamar Latrell Dec 2 '19 at 4:14
  • @LamarLatrell It depends on the mechanism. Some mechansims can be backdriven without problems (or even by design), for others it can be critically damaging. It depends entirely on the design. – J... Dec 2 '19 at 18:43
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We are presumably talking about a purely mechanically linked zoom here. The lens groups and shafts run in threaded gear tracks. What you are doing here is reversing the operation, pushing a shaft to make its gear track turn and move all of the other groups/gears in turn. The respective mechanical elements and tracks are not designed to work this way round: it's a bit like trying to turn a screw by pushing a nut on it. In theory, that's in some way an invertible operation but in practice you are not going to do your threads a favor.

In the best case, you'll put much more of a load than usual on the respective pins moving the lens shaft, making the positioning of it and its affixed elements/groups lose precision in a smaller amount of time than the lens has been designed for.

Few compact cameras contain mechanically linked zoom: making the comparatively small components reliably survive regular human operation and irregular human operation comes at a cost in complexity and materials that may be more than a zoom motor actually weighs in at.

In short: just seems like a bad idea asking for trouble along the road.

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    "like trying to turn a screw by pushing a nut on it" – xiota Dec 2 '19 at 0:40
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    @xiota FWIW: Yes. Just like that, but less severe than with most nuts. A nut or a lens retraction system is a spiral wound inclined plane system and there are standard expressions for what happens as the plane angle varies. Below a certain steepness the force which is imparted vertically. Below a certain incline angle (too fine a thread) the mechanics are such that it's impossible to overcome the coefficient of friction on the thread and the system is said to "not overhaul". For nuts with VERY coarse threads it IS possible to rotate the thread, by pushing on the but, but this is exceptional. – Russell McMahon Dec 2 '19 at 5:35
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It certainly depends on whether or not your lens manufacturer supports this officially or has planned for people using it. Some mechanisms won’t be pushable, which means you might break them when attempting to do this.

You could always send a mail to the specific lens manufacturer and ask them about it. I would not recommend doing it for any lens that’s worth a lot of money or that you can’t afford breaking right now. I’d also not suggest doing this if the manufacturer advises you not do to it. They know best if the mechanism inside your lens is able to handle it or not.

In addition to that, this might cause undesired effects on lenses that do not use a direct manual zoom, but instead use a sensor with a servo motor to automatically adjust the zoom when you turn the non-mechanically connected ring. These might start being miscalibrated or might actually create a current that may or may not damage the lens or the camera, as motors are usually the same as generators. Turning a motor usually creates electric current and if your lens/camera isn’t able to handle it, the current may or may not damage it. (This is also why people advice holding your computer fans still when cleaning dust off of them, it may break sensitive components)

Also, think about why you want to do this in the first place? Is it really worth potentially risking that the lens mechanism can’t handle it just so you can avoid twisting the zoom ring?

TL;DR: Ask your manufacturer. If they advise against it, don’t ever do it. If the lens is automatic, don’t do it.

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  • Any circuit capable of driving a consumer electronics DC motor should be resilient to stray voltage. Electronics damage is extremely unlikely. – MooseBoys Dec 2 '19 at 5:10
  • I agree that it’s unlikely, @MooseBoys. I personally wouldn’t want to risk it though, especially because I wouldn’t trust cheap lenses to be protected against this and because I wouldn’t do anything like this to expensive lenses in fear of potentially ruining them due to unsupported ways of operation. – Florian Wendelborn Dec 3 '19 at 14:25
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I do something similar with my Sigma 100-400 zoom, it is normally a manual ring zoom but it's slow to operate that way in some cases, so I sometimes use it as a push-pull zoom when tracking fast moving subjects.

What makes me think that it won't suffer from this is that it extends by itself fairly quickly when pointed downwards, and folds when pointed upwards (and comes with a lock for transport), so I'm not pushing against some gear.

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It's not very safe. The gears form a kind of a worm gear and you're putting the mechanism under much more stress when you push on the lens tube.

This particular lens does not have tight tolerances to begin with and has some play in the mechanism, so it might not cause too much damage, but in the end, it will damage the lens. You will get wobble towards the ends of the zoom scale and in some cases, you might end up being unable to zoom in or out all the way.

J... mentioned that some lenses are design to operate this way. Emphatically, not this one. Nikon's old 80-200 was a push-pull design, Canon's 100-400, 35-350 and 28-300 are other examples of this design. Those lenses are designed to be pushed back to retract it.

While your 18-135 will probably not fail outright from pushing the lens shaft into the body, there are lenses that will fail. However, macro lenses with precise manual focus will fail from pushing their lens shaft into the body.

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