Canon T4I - Wont take pictures of pure white paper

As others have mentioned, without contrast, the AF can't lock on. There is also a limit to how close you can auto focus. Are you within the lens' minimum focus distance to take the photo?

If you want to check how dirty the sensor is, just set the lens to F22, minimum focus distance and take a photo of the clear blue sky.

maybe take a few different shots of the white paper with a small X on the paper. now you have something to focus on, different positions if you are concerned about the X.

If you are looking for dust on the sensor then focus doesn't matter. Set the camera to M or AV. Set the lens to manual focus at the smallest aperture (f22) with a very slow shutter speed, position the camera pointing at a white sheet of paper so that it fills the image, press the shutter speed and move the camera whilst the shot is being taken.

The slow sutter speed and moving the camera will blurr out any marks or reflections on the paper, the small aperture will make any dust marks more obvious.

I will say it again "Focus doesn't matter" and the best results will be obtained when the lens is way out of focus.

Ray UK wrote:

If you are looking for dust on the sensor then focus doesn't matter. Set the camera to M or AV. Set the lens to manual focus at the smallest aperture (f22) with a very slow shutter speed, position the camera pointing at a white sheet of paper so that it fills the image, press the shutter speed and move the camera whilst the shot is being taken.

The slow sutter speed and moving the camera will blurr out any marks or reflections on the paper, the small aperture will make any dust marks more obvious.

I will say it again "Focus doesn't matter" and the best results will be obtained when the lens is way out of focus.

This depends on how you want to search for the dust. In my post I stated that focus to MFD and shoot the sky, aka something far away. This blurs out any objects that may appear to be dust, like aeroplanes, flies, birds etc. This makes it easier to see dust in this particular way. Setting focus at infinity may show these objects, which may appear as dust. Setting Shutterspeed in this case, does not matter, as long as it is properly exposed. Your last sentence is a bit odd. You say "Focus doesn't matter" and then state "best results will be obtained when the lens is way out of focus". Well than the focus does matter a little, doesn't it.

Aoi Usagi wrote:

This depends on how you want to search for the dust. In my post I stated that focus to MFD and shoot the sky, aka something far away. This blurs out any objects that may appear to be dust, like aeroplanes, flies, birds etc.

Yes that would work but it is still easier to use a clean sheet of paper especially if you live in a country where clear blue skies are a rarity.

Your last sentence is a bit odd. You say "Focus doesn't matter" and then state "best results will be obtained when the lens is way out of focus". Well than the focus does matter a little, doesn't it.

OK, so I should have said "correct focus doesn't matter"

Ray UK wrote:

Aoi Usagi wrote:

This depends on how you want to search for the dust. In my post I stated that focus to MFD and shoot the sky, aka something far away. This blurs out any objects that may appear to be dust, like aeroplanes, flies, birds etc.

Yes that would work but it is still easier to use a clean sheet of paper especially if you live in a country where clear blue skies are a rarity.

Gray, overcast skies work too.  Probably not at night though.

Your last sentence is a bit odd. You say "Focus doesn't matter" and then state "best results will be obtained when the lens is way out of focus". Well than the focus does matter a little, doesn't it.

OK, so I should have said "correct focus doesn't matter"

Understood.