We have all been there. We have noticed a particularly stubborn sensor spot in our photographs that we just can not seem to remove. And sometimes there is a reason for that!
You know how it goes, you take your lens off, get the blower out and while your camera is upside down, you give it a good blow. If you are going in for a deep clean, you will then use a wet swab and run over the sensor, followed by a dry swab and then replace your lens. Once this is complete you will of course do your test shot.
Test shots are usually a quick shot you take of a clear blue sky, or an indoor shot of a white piece of A4 paper. The camera set to second or so exposure and an aperture of F/16-F/22. You then look at the back of your screen and zoom in, and dirty sensors become instantly visible (other techniques are available!)
And then, it is as if you have done nothing, and the spot (or spots) are still there! Although common sense tells us to check other variables, often many (including myself in the past), will repeat this process.
The next thing you do is get the glass cleaning equipment out (be it a cloth, or some spray and a cloth) and you target the front of your lens. These things always get dirty, so of course it is the front of the lens! It is as simple as that… or so you thought. You clean the front lens glass and take your test shot, and the results have not changed.
Now, for many people, this can be a really annoying experience, but there is one more place to clean, that almost everyone forgets about in times like this. The back lens glass! That’s right, the little bit of glass element on the back end of the lens that is connected to the mount of your camera.
Think about it, you usually just place your lens down on a surface and this part is often not covered. You think it is clean because you never touch it, and it is always safe inside the camera. And for the most part, it is safe. Occasionally though, it secretly collects dust, and from experience, bigger chunks of dust and hair than you normally see on the sensor.
You clean this part of the lens, put it back on the camera, do a test shot and just like that, your images are clean again!
How to notice this kind of dirt
Generally, at first glance, the dirt specks caused by a dirty back lens glass looks identical to sensor dust at first viewing. However, when you zoom in very close to the dirt in your camera or on your computer, you often see that the object causing the spots is more defined. It is subtle, but there is a difference between dirt on your sensor and dirt on the back-side of your lens.
Cleaning is super easy, just clean it as you would the front glass element, although on some lenses, this glass element may be a bit more recessed than your easy to reach front element.
Conslusion
Do not worry if you have been in this situation more than once. It is a little thing that we all forget about for some strange reason. Probably because that part of the glass is out of sight, and out of mind for the majority of the time.
Make it regular maintenance when doing a sensor clean, to clean your lens carefully too. It takes a few seconds and can cause you the heartache of looking for that stubborn spot!
If all else fails, repeat the process…
One Camera One Lens Photography 
Good reminder, Mark. Sensor spots are the first thing everyone suspects, but that rear lens element is easy to forget because it usually stays hidden inside the mount. I have run into the same situation more than once, chasing “sensor dust” that turned out to be somewhere else in the optical path.
It is one of those small maintenance habits that makes sense to add to the routine. When the camera is already open for a sensor clean, taking a quick look at the back element of the lens can save quite a bit of unnecessary cleaning and head scratching afterwards.
Great and very useful article !
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