As an experiemnt, take a photo of a plain white wall (or a piece of open blue sky), but stick a tiny piece of "post it note" on the end of your lens (cut the post-it note down to about 1/4" square).
You'll notice that while there's this obvious square on the end of your lens... you don't actually see evidence of it in the photo (especially if you use a low Aperture value setting.)
This is because of the way light has to travel to get from the subject to the sensor. The ony place where things are truely in focus is at the subject itself... and on the sensor surface itself. The farther away a piece of dirt, hair, dust-ball, etc. is from a true focused plane, the less noticeable it will be. A camera cannot technically focus on a piece of dirt inside the lens.
But if you lay a hair across the imaging sensor (inside the camera) then the shadow of that hair will cast a very definite line in your photo. This is even more noticeable at high Aperture value settings.
To test for dust, dirt, oil spots, hairs, or other unwanted debris on the sensor, find a subject with as little contrast as possible (a plain white wall works well. A plain blue featureless sky (no clouds) also works well.) The camera doesn't even need to be focused (in fact, it's better if it isn't focused). Dial the camera to Av mode (aperture value) and dial in a very high Aperture number (e.g. f/22). Take a photograph of the featurless surface using those settings.
Now inspect the photo and any spots with dust, dirt, etc. should be very obvious.
Just remember tha tthe image projected into the camera is technically upside and backwards. That means if you see a dust spot in the upper right corner of your image... then the ACTUAL bit of dust is really in the lower left corner of your camera sensor.
BTW, when I write "sensor" I really mean "filter" because there are actually two transparent "filters" in front of the sensor. One is a UV/IR filter, the other is a "low pass" filter intended to reduce the effects of digital "moire" that can create wonky effects if you take photos of certain patterns. These are "glass" filters. Cleaning doesn't actually involve contact with the real sensor... you're atually cleaning the outermost of the two filters.
It is possible to clean the camera sensor yourself... but you'll need to get the proper cleaning supplies. You may wish to take the camera to a proper camera store (not a big box store that sells cameras) and they can clean it for you -- usually in just a few minutes. They may charge a nominal fee (they may also charge a steep fee -- depends on the store. Great if they'll clean the sensor for $10-20. But if they charge $50 it's time to learn how to clean a sensor on your own (or find another store.)
You can watch numerous tutorial videos on how to do this. I prefer to use a hand-squeezed air blower to see if a few puffs of air will clean it (e.g. a Giottos "Rocket" blower (hand-squeezed)). Do NOT use cans of compressed "air" because the propellant in the can can spit onto the sensor and fog it with a residue that does not evaporate (then the sensor has to be cleaned with special cleaning materials.) Hand-squeezed air... or nothing.
Sometimes a brush can be used to clean the sensor. This brush should be PRISTINELY CLEAN. I use a special brush with exceptionally soft bristles purchased expressely for this taks. This brush has never touched my hands fingers or any skin (lest oils collect on it which I do not want.) A very gently sweep may encourage the dust to leave the sensor.
If the air doesn't work and the brush doesn't work, then I resort to "wet" cleaning the sensor.
I use "Eclipse" cleaning solution (by Photographic Solutions -- available at most true camera stores (not big box stores that happen to sell cameras... along with washer/dryers, video games, cellphones and TV sets -- those aren't "camera" stores.) The solution is nearly pure methanol which means it evaporates VERY quickly and leaves no residue behind (which is exactly what we want.) The solution is applied with a special cleaning swap ("Sensor Swabs" made by the same company.) Again... numerous YouTube video tutorials will show you how this works. Use the swab ONCE and toss it in the trash. Do not attempt to save a few pennies by re-using a swab that may have dirt on it and scratch the filter.
It is nice to buy a special magnifier ("Sensor Scope") that rests on the front of your camera body (like a lens) to to inspect your work and verify that the sensor is, in fact, "clean". These usually don't cost too much.
Tim Campbell
5D III, 5D IV, 60Da
