Dust visibility at slower shutter speeds

[jani]

Was the shutter speed the only thing that changed, or did you change the aperture, too?

At smaller apertures (higher f/ number), dust near the focal plane is easier to see than at larger apertures. This is typically a problem at (my guesstimate) f/10 and smaller).

This effect is used to find dust specks on a sensor:

Take a shot of a uniformly lit, evenly colored surface (a clear blue daylight sky works fine), at minimum aperture.

Dust specks should be easily apparent, if they are there.

[mikecookson]

Thanks for the answers, I hadn't appreciated that using small apertures showed up dust on the sensor but I suppose that logically if d.o.f. within the image is increased, then there must be some corresponding, (but small), increase in the d.o.f. of the projected image on the sensor and so dust becomes more visible.

Thanks

Mike

[mikecookson]

Hi

I recently took some waterfall shots at varying shutter speeds on a tripod mounted camera. Shutter speeds of 1/30th resulted in hardly any visible dust whilst speeds of 1/2 sec. or longer produced images that contained 50 or more pronounced dust spots. The lens wasn't changed in between shots and so what caused this to happen? It's also worth noting that this happened  consistently on a number of different photography sessions.

One idea I have is that it might be to do with the additional time for which the sensor is electrically charged. One website suggested that small apertures increase the prominence of dust but I assume that this is incorrect as the dust would have to be on the lens not the sensor and it presumably would not be in focus either.  

Any suggestions to help me understand the cause of this phenomenon would be very welcome.

Thanks

Mike

[Jonathan Wienke]

It's your aperture setting; the smaller it is (larger f/numbers), the more pronounced the dust spots will be. Sensor static has absolutely no bearing, as the voltages in the sensor are less than 5VDC and are not strong enough to static attract anything.

[Jonathan Wienke]

The effect is easy to verify. Simply put the camera in Av mode, and shoot some frames of a piece of paper with the lens focused at infinity. Adjust the aperture from it's minimum to maximum value as you shoot, and then examine the resulting images. You'll see a direct correlation between aperture and dust visibility. BTW, shooting an OOF solid-color object at f/22 is one of the best ways to check for sensor dust.