Should I clean the sensor before mounting the new lens?

Question

Hello guys! Sorry for a silly question, but I'm not sure I know the correct answer. One of the cameras I'm using is Canon 5d classic, and it has some dust on the sensor and viewfinder (not unusual for 5di). I just got a new lens, and I'm wondering if there's a possibility for the dust on the sensor/viewfinder get inside the lens while shooting? Or I should clean the sensor/viewfinder before mounting? Or just forget about dust as dust will evertually get inside the lens? Thanks! I appreciate your answers! 
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Answer 1

Good advice so far....
1. You don't have to worry about using your lens on the camera. The lens is pretty well sealed against dust, so any dust that's in the camera's mirror box is unlikely to migrate into the lens (it's more likely to migrate onto the sensor, but even that is low risk). There's no such thing as a completely dust free lens, anyway. A few specks of dust inside a lens have virtually no effect on images.
2. You probably should take advantage of your CPS membership to have the camera cleaned and checked.
But you also might want to learn to clean it yourself. Some call the 5D classic a "dust magnet"... I just think it's got a large sensor for dust to get onto and has not got a self-cleaning feature as is found on later models. It's going to get dusty again, you can bet. So at the very least you might want to get a bulb blower ("Rocket Blower" is one brand) and learn how to use it.  
You probably already know, dust seen in the viewfinder is generally harmless. It's usually on the mirror or on the focus screen and won't show up in images made with the camera (unless it manages to migrate onto the sensor itself, which is hidden and protected behind the shutter except for the brief instant when an image is being made).
If dust seen in the viewfinder appears sharp and in focus, it's on the focus screen. If it's blurred and out of focus, it's on the mirror. Unless you know how and have the right tools for the job, it's not a good idea to give the focus screen and mirror anything more aggressive than a puff of air from a bulb blower. These can be cleaned more thoroughly, but both are very easily damaged: The focus screen is optical plastic and can be ruined by using the wrong type of cleaning fluids on it. The mirror is front-surfaced with vaporized aluminum, so is very easily damaged. Aside from an occasional puff of air from a bulb blower, I recommend leaving the mirror and focus screen alone until you have a professional cleaning done.
A more serious problem to watch for is failing light seals. There are foam seals around the rim of the focus screen, which the mirror presses up against when it's raised, to keep stray light out of the mirror box during exposure. Over time those foam seals begin to degrade and can shed particles or even smear the glue holding them in place onto the mirror and focus screen. In general, cleaning up the mess, removing the old and installing new foam seals is a bit tricky, so  I'd recommend you have any foam seal replacement done by a professional. (Also, there are similar foam seals  elsewhere inside the camera that are likely to need replacement too, if the seals in the mirror box are going bad. A pro will know what's needed and have the skills to work deeper inside the camera.)
Time doesn't always allow to send a camera in for a simply sensor cleaning, so it's something you might want to learn to do yourself. There are two websites that discuss it in good detail: www.cleaningdigitalcameras.com and www.copperhillimages.com. The first is maintained by professional, independent camera repairers who also are suppliers to the repair industry and sell most of the specialized tools and supplies needed for sensor cleaning. The second is part of the commercial site of one of the particular cleaning procedures ("Copper Hill Method"). Both sites have good information. Read them thoroughly before attempting any cleaning yourself.
If you do decide to learn to clean the sensor yourself, some things you need to know...
Never use a common cotton bud (i.e., "Q-Tip") inside a camera. They shed fine cotton fibers that can get into and jam delicate mechanisms.
Check the manufacturer's website that any sensor cleaning fluid you are considering using is formulated and approved for your particular camera.
Any digital camera that's never had the sensor cleaned will almost always need a "wet cleaning" the first time. This is because there are nearly always oils on the sensor (actually in most cameras the surface being cleaned is a filter that covers and protects the sensor). These oils are excess thrown off from the shutter mechanism. Any "dry cleaning" method that touches the sensor will smear the oils and make matters worse. So a "wet cleaning" is always called for, first time around.
It's important to have the camera's  battery(ies) fully charged prior to cleaning and to work promptly, so that power to hold the mirror up and the shutter open doesn't run low allowing the shutter to close and the mirror to drop onto a tool being used for the cleaning. Professional cleaners often use a continuous power supply instead of a battery, to be safer.
It's handy to have a magnifying loupe and light of some sort to assist with the cleaning. When viewing the sensor from the front, keep in mind that the image is upside down and reversed. So, for example, if you saw a spot in the upper lefhand corner of your images, when viewing the sensor directly you'll find the offending speck in the lower righthand corner.  
Usually you will need a variety of tools to do a multistep cleaning....
"Bulb blower cleaning" to puff away loose particles ("Rocket Blower").
"Wet cleaning" with approved sensor cleaning fluid and optical cleaning swabs to remove oils and more strongly adhered particles ("Copper Hill Method kit", "Pecs Pads", "Pecs Swabs", "Eclipse" solution).
"Adhesive cleaning" using a lightly sticky surface to lift off lightly adhered particles ("Dust Aid").
"Anti-static brush cleaning" to clear stubborn, remaining lightly adhered particles and to reduce any static charge that's attracting dust ("Arctic Butterfly").
"Individual speck removal" with a reusable, precision tool ("Speck Grabber").
"Final polishing" is always a last step to remove haze left behind by cleaning fluids  ("Sensor Pen by LensPen"). This also helps discourage dust from adhering to the surface again. 
I've included some product brand names in the parentheses just as examples. There may be other equally effective products.
Most of the time I find myself using several or even all of these when I do a sensor cleaning.
Cheers!
Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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