The intensity of the dust spots will depend on the aperture setting used for the particular exposure.
At low focal ratio values (f-stops) the dust spots will appear weaker. At high focal ratios they'll be well-defined.
To clear these, you can try a few tings...
1) Indoors (no wind), remove the lens. Point the camera body at the floor and switch the camera off... wait a moment, then swtich it back on. You can also use the menu system on your camera and navigate to the menu item that lets you manual invoke a sensor cleaning cycle.
Technically the dust isn't on the camera "sensor" per se... there's a couple layers of filters (glass) directly in front of the sensor. These do things such as block IR and UV light. There's also a "low pass" filter (reduces the moiré effect) that can show up when shooting certain patterns. But the front-most filter is attached to a piezoelectric charge device which causes the filter to vibrate quickly. This can often "shake" the dust off the camera.
But the reason for removing the lens and pointing the camera opening at the floor is so that when the filter does vibrate to shake the dust loose... the dust can exit the camera (otherwise it's just going to settle somewhere else and it's just a matter of time before it's right back on the sensor again. The "problem" is every time you take a shot, the reflex mirror "wooshes" up and down and that blows the air around inside the camera so the dust can migrate.
2) Hand squeezed air blower
Sometimes just a couple of sensor cleaning cycles is enough. But if that doesn't work... then use a HAND-SQUEEZED air blow (emphasis on HAND SQEEZED). Do NOT use cans of compressed air. Do NOT use an air compressor. Both are very bad.
As air rapidly decompresses from an air-compressor it will splatter moisture and air compressors are lubricated with oil and the "air" will put a residue on your filter. The same is true of cans of compressed air. The propellent leaves a "film" residue if the can is not held upright (and not shaken). So the probability is that if you use canned air or an air compressor you'll actually make things worse.
If you use a hand-squeezed bulb, the puffs of air don't have any contaminants, no oils, no films, etc. and it's not rapidly decompressing air that will fog the sensor by chilling it. It usually doesn't take much air to clear the sensor.
The Giottos "rocket" blower is a popular hand-squeezed blower.
If it isn't clean at this point, you have a decision to make. You can either (a) take the camera to a camera store that cleans sensors (they usually don't charge very much and it can be done quickly), or (b) decide to clean it yourself.
If you choose path (b) you'll need some proper cleaning supplies (more on that later) but the MAIN thing you have to realize is that you can damage things if you aren't careful. So don't go rushing into anything further without reading and following the cleaning instructions, watching instructional videos, etc.
3) Sometimes a few puffs of air isn't enough. If it's not enough... I tend to grab a magnifying loupe to inspect the sensor to see what I'm working with... and then I use two more tools.
a) You can use a sensor brush. This brush should be pristinely clean and free of oils (resist the temptation to feel the brush with your fingers and get skin oils all over them.) Give the brush a firm "whack" on the edge of a desk to knock loose any dust that might be clinging to the bristles.
Put the camera into "manual sensor cleaning" mode. This causes the reflex mirror to swing up and it opens the shutter curtain so you can access the "sensor" (again... it's really a glass filter in front of the sensor... the actualy sensor is a couple layers below the surface you can touch). Gently (with virtually no pressure) see if the brush can wipe the dust away. If it cannot wipe it away with gentle (virtually no pressure) swipes then DO NOT use more pressure... that can scratch things. Time to go to a wet cleaning method.
b) The "wet" cleaning method involves using sensor cleaning solution and a swab. The popular solution is called "Eclipse" by Photographic Solutions, Inc. The same company makes something called "sensor swabs" and these swabs come in sizes which are designed to match the height of the sensor. The Canon T3 has an APS-C size sensor so you'd get the "Type 2" size swabs.
Solution is almost pure methanol. The advantage is it works as a solvent sto clean whatever is on the sensor but methanol evaporates VERY quickly and leaves virtually no residue of it's own behind. So you get a very clean surface.
You can find instructional videos on how to use the product. But basically you follow the instructions which is to use just a few drops (maybe 3 drops) on the swap. You then do one clean pass across the sensor from one edge to the other. Then you immediately toss that swab in the trash (do not reuse a swap. yes, the swabs are "expensive" for what they are ... each swab is probably about $3. But a $3 swap is a LOT cheaper than sending the camera in for service to have the filter replaced because you scratched it. This is NOT the time to try to save a few bucks by reusing swaps.
4) Verify your work
It's hard to "see" dust on the sensor -- especially if it's small.
Turn the camera off. Attach a lens. Put the camera into Av mode and set the HIGHEST possible aperture your lens will allow (f/22 or f/32 depending on the lens). Put the lens in manual focus mode. Deliberately de-focus the lens (so everything is blurry). Point the lens at a plain white (but well lit) wall ... and snap a photo.
Now inspect that photo very closely. If there are any spots or dust bunnies remaining on the sensor, their shadows will appear on the image you just shot.
LASTLY... do not clean anything else inside the camera. For example, I don't recommend attempting to clean the reflex mirror or the viewfinder screen. If there's dust on those... just let it be (if a few puffs of air don't remove it, then ignore it.) The mirror and viewfinder are very easy to damage AND since the mirror swings clear (and covers the viewfinder) when you take a shot, any dust or spots on those surfaces can't effect your images anyway.
Tim Campbell
5D III, 5D IV, 60Da
