The measurement of dust quantity

Before cleaning the sensor is fundamental to verify if this operation is really necessary: some small piece of dust is normal and tolerable, and it isn't worth to clean the sensor.
To examine how much dust is present you'll need an objective with a very closed apertura: i.e. the Canon EFS 18-55mm, if used at 55mm, can be closed at f/36. And more you'll need a uniform light source: a very good source is a clear sky at zenith, but is't ok also a white sheet of paper, defocused, or a flat-field-ometer like the one shown here below, made for other purposes (obtain a flat-field for use with astronomical CCDs). The result of such a test is in this figure.

Here the sensor was not so dirty, but two small bristles are visible on the bottom. At left the raw image, at right the enhanced one.

My cleaning procedure

IMPORTANT NOTICE: all the operations here described has been tested and never damaged my cameras. However be aware that the author does not assume any responsability for eventual damages to the reader cameras using the procedure here described.

It's necessary to obtain the following items:

• ethyl alcohol 90% (the normal one, pink coloured), absolutely avoid the perfumed or diluted kind.
• a piece of absorbent paper (the kind found in kitchen), turned two times
• some sticks with cotton wool at the extremities

The necessary stuff for the cleaning, including my flat-field-ometer for capturing an uniform background.

Here is the procedure to clean the sensor:

• Open the shutter of DSLR camera with the sensor cleaning function, be aware to use a fully charged battery. If the battery is discharged you'll risk that the shutter closes when the cotton stick is inside....
• Observe visually if there are big particle of dust, be aware that almost all the one visible in the test shots will not be visible with unaided eye. I recommend to work under a strong light source and to avoid speaking, and possibly breathe, to avoid spraying even more particles on the sensor.
• Put a little alcool on the absorbent paper, not very much.
• Roll over the paper an extremity of the cotton stick, so that the cotton wool will be dampened. Be aware that the cotton must not be very much wet.
• Now clean the sensor with the wet cotton, with left-right and up-down movements, so as to clean uniformly all the surface.
• Immediately after the cleaning, and *before* that the alcool evaporates, turn the stick and repeat the movements with the dry cotton wool, so as to dry up the sensor without leaving halos. Be aware not to touch with the hands the dry cotton in the preceding operation, so as not to leave the greasy of the skin.
• Visually verify that no big dust particle is on the sensor, and if so, remove them gently with the dry cotton stick. If the sensor is still dirty, please repeat the cleaning with alcool and a new cotton stick.
• Turn the camera with the sensor toward the bottom, switch off to close the shutter and finally... restart to breathe... ;-)
  

All those operations have to be made very quickly, I usually need a minute or so. I recommend to try before the procedure on a less critical glass, like the front lens of an objective.

In the end you'll have to remount the objective on the DSLR and now verify the sensor cleanness. If the sensor was really dirty, maybe you'll need to repeat the procedure.

Here is the result of the cleaning (at left a raw image, at right a very enhanched one).

Conclusion

This page explain how to clean the DSLR cameras sensors with a very modest cost, and without so much effort. On the market there are many special liquids and specific kits, some of with very expensive and hard to import. My personal opinion, at the light of the results here shown, is that they are not necessary.

For any comment, send me an e-mail: comolli@libero.it

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