How do I use canon dof preview button?

From what I know about it, it will show how the shot will expose with the current Aperture and Shutter speed. If its going dark then I believe thats underexposed.


However Im not 100% on this, so if anyone can either clarify or correct :) please do!
 
Pressing the DOF preview button will will manually stop the lens down to the set aperture value, which enables you to look throught the viewfinder and see the extents of what will be in focus.
However at small apertures the viewfinder will apear dark as less light is being let through the lens.
 
The DoF button shows how much of your shot is in focus, basicly if you set your aperture at say 2.8 and press the DoF button it will only show a small area in focus.....as you decrease the aperture to say 8 pressing the button will show a greater area in focus due to the amount of depth of field. As you decrease the aperture so more of the shot comes in focus (though the shot also gets a tad softer at the smallest apertures). The reason the screen gets dark is because the aperture is being stopped down and so less light is coming in...don't worry your shot won't be like that if you expose right. Bear in mind that if you choose a smaller aperture you will need to allow a longer shutter speed so as to let in the right amount of light to expose correctly.

HTH though its probably confused yo more :D

Or what Namllihs said in a few words lol
 
it does exactly as it says on the tin.

Depth of Field Preview Button, it gives you a preview of the Depth of Field......
 
Lens stop-down button is used to check depth of field, to check the aperture operation, to lock the aperture down and remove the lens eg for manual macro tubes, and it also fires a stream of flash pulses to check flash light modelling (on Canons at least).

I used to use it a lot with film as that was pretty much all you had to go on for DoF, but with digital a) you've got an instant LCD image, and b) on crop cameras the viewfinder is so small and dark you can see FA anyway. It's much more useful for DoF checking on full frame as you can see a lot bit more.
 
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