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The effect of aperture size and visibility of specks:

You probably use your camera with the mode dial set to the green rectangle, (fully automatic).
The aperture size is therefore programmed by the camera, and the size isn’t an issue.

If you were to check what the actual aperture was, as selected by the camera, you would probably find that 5.6 and 8.0 would figure highly.

At aperture size of f:8 or greater, f:5.6 to f:2, dust specks are barely noticeable on the average image.

But at f:11 down to f:22 spots become increasingly more defined, especially against a bright background.

Rays of light passing through a smaller aperture helps to increase definition of the dust speck, as it’s shadow, cast on image sensor, becomes more defined.
This in turn makes particles easier to see.

Evaluating test images:

It is tempting to increase image magnification, in order to see the smallest specks.

But, if the image looks clean at 100% magnification, and certainly not greater than 200%, there is really no need to worry, and cleaning should never be done unnecessarily.

The same can be said about manipulating contrast and sharpness for the sake of showing up particles. It will just tempt you to try and clean, when it isn’t needed.

Artefacts, which are not necessarily dust, become more noticeable the more you increase magnification.

Some electrical aberrations may appear when the sensors sensitivity range is exceeded, especially at very low light levels, or when the exposure time is abnormally long.

You can’t ‘clean’ artefacts, so don’t worry about them.

How to expose test images for dust inspection before cleaning.

An essential part of sensor cleaning is to expose test images before, during and after cleaning!

Exposing test images will provide you with a reference that will help you determine:

You want a neutral, evenly lit background in order for specks to be clearly visible. The simplest, and the best, is a clear blue sky.

However, since the sky is not always cooperative, the next best is a piece of pale blue, non-glossy, cardboard as background (target).

The target is positioned so as to be evenly illuminated, but not directly in the sun. You now have an excellent controlled background.
The illustration shows the target I made up for my own use.

How to set the camera for best result:

If your camera has ‘Live View’ you can use this mode for a quick inspection, but it is no substitute for actual test images to provide a progress reference.

Dust particles and fibres will show up as fuzzy, irregular spots and marks, such as on the image below.

Dust check image from Pentax K DigitalLoose, static particles can be removed with a dry-cleaning method, such as the DUST-AID Platinum Silicone wand, which lifts away particles by a simple ‘stamping action’.

However, smearing and dense spots as shown below will only remove by wet-cleaning with sensor swabs and liquids.Sensor smears and spots

Viewing the test images at a magnification of 200% or greater will often show spot like features. If they cover an area less than about 12 pixels it is unlikely they will be noticeable on a standard size print.

… next a look at basic cleaning tools and techniques >