Dirt on your lens matters ... but maybe not for the reason you think.
Many people believe that a spot of dust on the lens will cause a spot on the photograph.
It won’t. You can have a spot of dust on your lens the size of a small coin and you might not notice it.
Don’t believe me? Here’s the proof ...
These two photographs were taken one after the other, with my camera lying on its back pointing up at the ceiling in the hallway of my house.
I didn’t move the camera between shots. I didn’t change the exposure. Both were taken with the diaphragm fully open, at ƒ1.4. But in one of those photos there is a small Swiss 5 cent coin lying on the lens front element ...
Which photo was taken with the coin on the lens? (Answer in the comments section).
Even if you got it right, I think you’ll agree that it was a tough one to spot.
So, what’s the point of that demonstration? That it’s okay to let your lens get dirty? Even huge chunks of grot won’t make any difference?
No. The point is that dust on your lens has a much more insidious effect which may not be easy to see. A dirty lens lacks contrast and colours in the image are washed-out. The effects occur across the whole image and are impossible to correct with image editing software. Here’s a demonstration ...
So, if you want the crispest, sharpest photographs that your lens can give, with vibrant colours, keep it clean.
Step 1. Make sure that you keep a cap on your lens whenever it is not in use. One will have been supplied with the lens when you bought it. Don’t throw the thing away. And if you have lost it, buy another. Most camera shops carry a selection.
Step 2. Blow any dust particles off with that big fat rubber blower that you used to clean the camera’s exterior. Don’t forget to blow out the inside the lens cap. Dust easily gathers there, too.
Step 3. Gently wipe the surface of the lens. No, not with your shirt-tail, handkerchief or a piece of toilet paper. (You’d be surprised how many people use these things.) With your microfibre cloth. But make sure that the cloth is spotlessly clean. A tiny piece of grit trapped in its folds will play havoc with your coated lens surface. If you want to be really particular, have one cloth for the camera body and another entirely different on for the lens. Or buy some lens-cleaning tissue. Packs of it are very cheap and, unlike toilet paper or paper handkerchiefs, lens-cleaning tissue does not shed fibres. You can also use a drop of optical cleaning fluid to help the process.
Step 4. Finally, give the lens a few hefty puffs with the blower to evaporate any cleaning fluid left or remove any stubborn specks of dust.
Next on 'Grime Doesn't Pay' - Cleaning your Sensor
Incidentally, I didn’t actually let my lens get dirty for the photo above. I used an old skylight filter – one of those clear ones – and coated it with fine dust. The effect is the same. Then I took one photo with the filter off and one with it on and split them down the middle to make a comparison.