How lenses breathe

[bovk]

Some years ago, must be close to 10, I read a review of 24-70 f2.8G zoom by mr. Rørslett (at http://www.naturfotograf.com/lens_zoom_02.html#AFS24-70FX) and since then I remember one statement: "Unlike the 28-70, it has a fixed rear lens element so the tendency for the lens to act as a "suction device" is eliminated. In fact, the element is fixed both for focusing and zooming, very neat and clever." I have never saw this repeated in any other review of this lens. That surprised me because I consider it a big + for the lens. This brings me to my question - how does this lens breathe, and what is the breathing pattern of some other lenses for instance 14-24 and new line of f1.8 FX primes? Does this lens or some other lenses actually protect the mirror chamber from contamination?
I would be very interested hearing your opinions.
Bo

[pluton]

Note:  The term 'to breathe', in the context of photographic lenses, usually refers to focus breathing: the  change in apparent field of view when the lens' focusing mechanism is operated. 
As far as 'dust sucking/breathing', it does seem to be generally accepted wisdom that large/rapid changes in the rear lens element position act to suck air, airborne debris and dust into the interior of the lens and camera body.  I have not personally tested the idea or observed it in action.
It would be interesting to set up a comparative test for dust sucking, but it would be difficult to find an equipment donor for that experiment!

[bovk]

Pluton thanks, that test would be interesting (sorry for previous version of this message)

[richardHaw]

I call these kinds of lenses "pumpers" because the physical extension of the lens tend to make a suction effect and it sucks air in or out. 

in optical terms, "breathing" is used to describe "focus breathing" as was stated above so the correct term should be "suction" and I will use suction from now.

lenses that extend quite a bit (doesn't matter if it's manual or autofocus lenses) will create space inside the lens barrel as it extends and that space has to be filled by air. when a lens sucks in air, it brings dust and pollen into it (and fungus spores). this is inevitable. you can demonstrate this yourself by getting a lens that extends a lot and placing the end of it close to your eye. extend and retract the lens and you will feel a very slight air current. that current gets stronger as you pump the lens faster.

some lenses will have less tendency of doing this when they have a fixed rear element (but air still gets sucked in), or when the lens has a fixed dimension as you zoom in or focus such as the (70-200 family of lenses). since the lens is "sealed" like the case of the 70-200 lenses, not much air is getting sucked or pumped.

there are times when the current is so strong that you can feel it from the eyepiece of the camera. this happens with older or cheaper cameras and this is what ken rockwell calls "eyeblow".

if you really want to see this visually, blow some smoke from a cigarette and then pump your lens and you will see the smoke go in or out of it. I shoot clubs before and I see this sometimes. I know, this is not very smart but it can't be helped. there are lenses with gaskets and felt seals that help filter dirt and stuff but some junk still manage to get through.

hope I made sense.

[richardHaw]

http://richardhaw.com/2018/03/11/repair-af-s-dx-nikkor-18-200mm-f-3-5-5-6g-ed-vr-ii/

this lens is very typical of a pumper. thats the reason why these guys are usually dusty inside

many consumer zooms are like this to be honest.

[Les Olson]

Any lens that increases in length as it zooms or focuses closer draws air into the lens then expels the air when the zoom or focus is reversed.  If the rear element moves the volume of air inside the camera changes, so air has to move into and out of the camera as well as in and out of the lens.  In both cases the air carries dust and pollen, but dust inside the camera is generally thought to be more of a problem. 

I have never seen any measurements on the relative contribution to the accumulation of dust inside camera bodies of air sucked in during lens operation vs passive air movement when the body is open during lens changing. 

[MFloyd]

...

I have never seen any measurements on the relative contribution to the accumulation of dust inside camera bodies of air sucked in during lens operation vs passive air movement when the body is open during lens changing.

Mid-2016 I had a major dust problem with my new D4s. NPS representatives were finger-pointing at my frequently used 28-300mm f/3.5-5.6. I made the following test: 100 shots (of a white screen to note the dust spots) and between each shot 10 quick twists, from 28mm to 300mm, to suck in the dust; the overall "dust atmosphere" was one of a normal house, nothing exceptional. This resulted in no additional dust spots, as a result of pumping the lens for 1'000 iterations. This story ended by a replacement of the D4s shutter system.

So dust sucked in by a "consumer zoom" ? Possible, but not from my experience.

[bovk]

 I expected some magic - like "some lenses do not suck/blow air in the mirror chamber" - but probably no such luck. From the replies it looks like they all do that to some extent depending on construction.
Quoting Richard "some lenses will have less tendency of doing this when they have a fixed rear element (but air still gets sucked in), or when the lens has a fixed dimension as you zoom in or focus such as the (70-200 family of lenses). since the lens is "sealed" like the case of the 70-200 lenses, not much air is getting sucked or pumped." - This is interesting, in case of fixed front and rear element the air could just move internally around the moving parts without an exchange with outside air!
Is this the case for 70-200 family of lenses or are there some other lenses of this type?

Thank you for all replies!

[Roland Vink]

This is interesting, in case of fixed front and rear element the air could just move internally around the moving parts without an exchange with outside air!
Is this the case for 70-200 family of lenses or are there some other lenses of this type?

Lenses with fixed front and rear elements include all primes with internal focus, and all zooms with internal zooming and internal focus.

You could also include all unit-focusing primes since air is not exchanged internally while focusing, but air will be pumped in and out of the mirror chamber when focusing.

I will add, none of this stops dust from getting into a lens, even the simplest unit-focusing prime appears to be well sealed, but air and dust can get in via the aperture linkage, and even more so in lenses with IF, zooming and VR. Also the black paint paint used internally to suppress reflections can peel and flake onto internal lens surfaces, oil from the focus ring can evaporate and deposit on lenses, and there is fungus also ...

[richardHaw]

dust can get in from anywhere. nothing frustrates me more than cleaning a lens thoroughly and then finding out that there's dust inside again after a few days. 

the older AF-D lenses are notorious for this because dust can come in from the aperture ring and the bayonet specially during lens changing and to make matters worse, some of them have an open-cell construction such as the 60/2.8 wherein dust can easily settle in the inner elements

[richardHaw]

Mid-2016 I had a major dust problem with my new D4s. NPS representatives were finger-pointing at my frequently used 28-300mm f/3.5-5.6. I made the following test: 100 shots (of a white screen to note the dust spots) and between each shot 10 quick twists, from 28mm to 300mm, to suck in the dust; the overall "dust atmosphere" was one of a normal house, nothing exceptional. This resulted in no additional dust spots, as a result of pumping the lens for 1'000 iterations. This story ended by a replacement of the D4s shutter system.

So dust sucked in by a "consumer zoom" ? Possible, but not from my experience.

that's probably oil spatter from the shutter 

[bovk]

I said to myself - OK, find out the IF lenses and see if if they do not blow into the mirror chamber. It looks like Nikon is confused too : quote
Internal Focusing (IF)
Imagine being able to focus a lens without it changing in size. Nikon's IF technology enables just that. All internal optical movement is limited to the interior of the non-extending lens barrel. This allows for a more compact, lightweight construction as well as a closer focusing distance. In addition, a smaller and lighter focusing lens group is employed to ensure faster focusing. The IF system is featured in most NIKKOR telephoto and selected NIKKOR zoom lenses.

As an example I looked up above mentioned 24-70 f2.8G at the same Nikon site quote :
Internal Focusing (IF) system; autofocus with a built-in SWM and manual focus

But we know this lens extends and sucks/blows in the mirror chamber.
Looks like there is no lens that does not exchange air with outside, getting dust in the lens. I expected that conclusion. Is the following true?
Maybe there is no lens that does not blow/suck the air via mirror chamber?

I do appreciate all your replies!

[richardHaw]

the 24-70 doesn't change its dimensions as you focus in or out but it does extend/retract when you zoom in or out. so technically, Nikon is correct. 

[Akira]

Unless a lens is of the latest "E" type with the electric aperture, the slit in which the mechanical aperture lever moves can let the air in and out when the IF elements travels...

[MFloyd]

that's probably oil spatter from the shutter 

No Richard, no oil splatter. Read here http://nikongear.net/revival/index.php/topic,4099.msg63098.html#msg63098