Pekka Buttler, May 2020
Have you ever looked at items on sale on eBay (or other platform) and encountered the phrase oily blades? In this post I will tell what oily blades are, what causes them, as well as discuss their ill effects, both immediate and long-term. Lastly, This post will mention why oily blades might not be the end of the world, and what can be done about them.
What are oily blades?
Photographic lenses (as well as enlarger lenses) have an adjustable aperture, which has various effects on the resulting image. That aperture is typically constructed using a set of thin metal blades which can move in a way which allows them to close down the aperture.
Today, these blades are typically painted matte black to minimise reflections, but especially in older lenses they might also have a metallic finish. Thanks to their thinness, smoothness and shape, these blades are able to slide against each other with barely any friction.
Oily blades refers to that some lubricant has managed to reach the blades, and has spread out to cover the blades partially or entirely.
Why are oily blades bad?
Oily blades are bad, as oil on the blades increases the friction of the aperture mechanism. As long as the issue is mild, it will only lead to a lame (sluggish) aperture, but it might progress to the point where the aperture becomes effectively stuck.
Let’s have a little more detailed look.
Since the advent of auto lenses (more than half a century ago), the idea has always been that the aperture is fully open to aid in focusing, and the aperture is closed down to whatever value the photographer has chosen, only a fraction of a second before the film (or today, sensor) will be exposed to the light passing through the lens.
In practice, this means that even though you set your nifty fifty to f/8, the aperture physically is held at fully open by a tug of war: A spring in the lens, which wants to close down the aperture as far as it goes (basically, to the selected aperture value), and a lever in the camera’s end of the lens mount which keeps the lens open to aid in focusing and composition. Of these, the camera is designed to be far stronger, thus negating the force of the spring inside the lens (n.B! With some camera mounts, the role is reversed, i.e. a spring is holding the aperture open, while the camera depresses a pin or lever to close down the aperture).
But when the photographer depresses the shutter, a lot of things start happening very fast: the camera raises its mirror, and releases the lever (thus allowing the spring in the lens to pull the aperture closed), and actuates the shutter. All these things have to be timed very precisely, because there’s no sense in exposing the film/sensor if the lens has not yet fully closed down, or if the mirror is not yet fully raised. While cameras can be constructed to make sure the mirror is raised before the shutter is actuated, most cameras are not designed to be able to check that the aperture to have closed – instead cameras tend to rely on timing: waiting a bit longer than what the manufacturer specifies as the time in which an aperture must be able to close from wide open to minimum aperture.
What then tends to happen when the aperture blades are oily is that the spring in the lens is not able to pull the aperture closed in time for the exposure. Say you have a lens with a maximum aperture of f/2, and you are intending to shoot at f/8, meaning 4 f-stops slower than the lens’ maximum. With most SLR body-lens combinations, your SLR will measure the amount of light at f/2, but knowing that you want to shoot at f/8, it will take that into account when metering, thus settling on a shutter speed suitable for f/8. You now press down the shutter button, and the body releases the “keep-it-open” -lever, and the spring starts pulling the aperture closed, but given the oil on the blades which make the lens sluggish, the aperture has not made it to f/8 when the exposure is taken. The result is an overexposure. Depending on how sluggish the blades are, it may be a mild overexposure, or a dramatic (e.g. 3-stop) overexposure.
But as mentioned, it can get worse from there: The blades can become totally stuck, or partially stuck so that they work (albeit sluggishly) in a range of apertures, but will not move beyond that.
Furthermore, oily blades can also contribute to haze forming on lens elements.
Obviously, the severity of oily blades depends very much not only on the severity of the issue, but also on the usage situation of the lens.
Oily blades – no discernible effect on motion of aperture or exposure:
This is not an issue which warrants an immediate intervention, but does indicate that something will have to be done sooner rather than later. Also, as lubricant on the blades is both very close to lens elements, and is furthermore in the light-path, the risk for lubricant evaporation and subsequent lens hazing should not be discounted.
Oily blades – aperture sluggish but working
At least as bad as the above-mentioned, but how much worse depends both on the type of lens and body.
If you’re using the SLR lens in its intended environment (a fully compatible SLR), your exposure’s correctness depends on that the aperture is not sluggish. In such cases, oily blades are bad, and should be fixed soonest. If you nevertheless have to shoot with the lens, use the DOF-preview-button to stop down the lens before pressing the shutter (you need to let the aperture enough time to close down).
If you’re using the SLR lens adapted on a mirrorless camera, a sluggish aperture is not a big issue, as metering and shooting is anyhow done closed down. Mind you, the issue can only get worse, and deserves addressing.
Finally, if your lens is designed to work with a direct coupling between aperture and ring (preset lenses, pre-auto lenses, many rangefinder lenses), oily blades is again not an issue which directly affects your photography, but is worth some professional attention at some stage.
Oily blades – stuck aperture
Well, unless the aperture is stuck at the very aperture you’re always using, the lens needs immediate attention.
Mind you, that an aperture being stuck may be caused also by other issues than oily blades, such as a broken coupling between aperture mechanism and aperture ring, or an aperture blade which has bent (thus, precluding normal movement).
Aperture sluggish, but no sign of oil on the blades
In this case, it might be that there is no oil (or other problem) on the blades, but that the misbehaviour is caused by a broken or lame spring (and is treated separately here)
What causes oily blades?
The shortest possible way to explain oily blades, is that “oil” (lubricant in fluid state) has traveled from parts of the lens where it should be to the aperture mechanism (where it should not be).
In slightly more detail, there are parts of a lens which need lubrication, and there are parts which should not be lubricated. Most important among those parts needing lubrication is the focusing helicoid, which allows the moving of lenses to achieve focus, but also other places need lubrication (basically, wherever metal or plastic parts are ment to move respective to each other), either to guarantee a fluid motion, or simply to minimise wear and tear.
The problem with lubricants is, that in order to be able to work, they need to be fluid enough to be able to lubricate, and they need to retain that fluidity over a wide range of temperatures. Even though the lubricants generally recommended for optical assemblies are among the more thermally stable, the general rule is that viscosity increases with rising temperature (until the point where fluid starts to boil away, making the remaining lubricant stiffer).
While most lenses are designed so that the aperture mechanism is not in direct contact with lubricated parts, the degree of distance between lubricated parts and the aperture mechanism varies (which is why some lenses are more liable to get oil on the blades). Also, the more lubricant is originally used in the assembly or CLA (cleaning, lubrication, adjustment) of a lens, the shorter the time before it reaches the aperture (and other undesired areas).
Having cleaned the blades on a number of lenses, all those lenses (which I have encountered) suffering oily blades have either been seriously over-lubricated or have had a design feature significantly shortening the “jump” from lubricated areas to the aperture mechanism.
How do I know if my blades are oily?
The phenomenon is quite often clearly visible, as the oily film on the blades clearly changes the reflectivity of the aperture blades, making the blades look darker (or lighter, depending on blade coating) where there is oil on them.
Problematically – for the purpose of writing this piece – I currently have only one lens with oily blades, and due to the nature of that lens (and that the oil infestation is mild), I cannot offer spectacular pictures. You are welcome to contribute with bucketfulls of gruesome pictures, though.
The picture, however, illustrates one important aspect of oil on the aperture blades: It is not always easily visible. The phenomenon may not show up on all aperture settings (in the case of this lens, it is most visible between f/4 and f/8), and being able to spot it usually necessitates good light.
Also, the smaller the aperture (visibly) is, the harder the problem is to spot. Thus one should look closely with wide-angle lenses (as the lenses in front of the aperture make the aperture look smaller), and with long tele lenses, when the aperture is buried deep inside the barrel, a flashlight helps. I have also once encountered an aperture, where oil was only visible from behind, so look at the aperture from both ends.
If you’ve found oil on the blades, the next thing you should do is ascertain the seriousness of the issue. The simplest way is to ascertain the motion of the blades using the lens’ stop down lever/pin and look (or feel) for signs of sluggishness and resistance. If you find no sign of the oil having any ill effect on the lens, continue shooting, but have the lens cleaned sooner rather than later.
N.B! Especially with old non-auto and preset lenses, you may encounter visually oil-like patterns on your aperture blades. With these lenses, there are two things to consider. Firstly, it may not be oil at all (you need to break open the lens to make sure), but simply friction scarring due to age, less advance materials science, and increased friction due to (typically) higher blade-count. Secondly, given that the aperture in such lenses is always operated by human muscle (never a flimsy spring), the issue is never as bad (unless you think you can feel the oil increasing resistance) aswith any sort of auto lens. Again, if what you see really is oil, it can – given time – contribute to lens hazing, but (to be blunt) if your 50+ year old lens has not started hazing yet, there is little reason to think it will start doing so soon (and definitely no rush a repair).
When to address oily blades, and how?
Whether to hurry with having the lens cleaned depends on a combination of factors, especially: how much the oil affects your shooting; whether it’s a lens you depend on; and whether it simply might be more economical to buy a new lens. Considering that lens mechanics’ rates vary widely throughout the globe, I won’t hazard a guess on cost, suffice it to say that accessing the aperture typically necessitates a total strip-down of the lens (or the objective, if the lens is a two part construction). Thus, it will not be among the cheapest of operations. Luckily, it typically does not necessitate spare parts, and is by nature an operation any professional lens doctor can accomplish.
Alternatively, you may choose to treat the problem yourself.
WARNING: This is not a lens repair blog, so I will not give you details on the task. Also, unless you have some experience of lens repair and an inkling of what the task entails, either take the lens to a professional, sell it to someone more repair-adept (remember to disclose the lens’ failings), or relegate it to your display cabinet.
There are, however, two tips I will disclose about cleaning aperture blades:
Firstly, unless patience is your middle name, and you are known for a steady hand, try to avoid having to disassemble the aperture. Work with a suitable solvent and a Q-tip to remove oil of the blades. You can also drip solvent straight on the blades, and jiggle them back and forth, but remember that the solvent does not remove the contamination, it just makes it more easily removable, so you will need to brush off the gunk with something. You will need to be doing this for some time, but you’ll typically be able to remove enough oil to be able to claim success. Also – with some lenses the aperture mechanism is entirely removable – you may consider popping the entire mechanism into solvent, and finish off by polishing it with a Q-tip. Make sure no fibres remain on the aperture.
Second, before you put the lens back together, let it breathe and dry out. While solvents evaporate rapidly, solvents between blades take a while to evaporate entirely, and just as you do not want oil to evaporate between your lenses causing haze, you also do not want solvents to remain inside the lens.
Next article: Part II: Fungus