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No, you wouldn't be right. Sorry.
The DOF preview stops down the lens to your chosen aperture to give you an indication of how much DOF you will see. When you focus the lens is wide open and so as bright as possible, when you press the DOF button the aperture narrows thus letting in less light which is why the image goes darker.
I must admit that it's a button I never really use.
Depth of Field preview is one of the most important functions on your camera - to not use it is lunacy and leaves getting the right shot to chance.
Many people don't fully unserstand how to get the most out of it ...
Set the camera into Aperture priority, with the lens wide open. Lets say f2.8 or f4 now focus on a specific point, quite close to you .... now press the button. You will notice very little change .. the background will remain out of focus and the viewfinder quite bright ...
Now stop the lens down by setting an aperture of say F16 or F19 ... focus on the same point again ... then press the depth of field preview button once more ...
The viewfinder will go very dark - give you eyes time to be adjust. You will notive that the areas in the background - which were out of focus before .. will now appear in focus (although dark) ..
This is your shot, as it will appear if you decide to take it ... you are getting to see, in real time, the actual depth of field of your shot at a specific aperture ... Play about with the settings changing aperture from F4 all the way to f22 or f32 ... you will be able to see the effect even a small change in aperture can have on depth of field ... this is especially true with longer lens's such as 200mm 300mm 400mm etc ...
This is crucial for macro and bird photography in particular as you want the subject in focus and the background thrown out of focus ... its also useful in every other area of photography.
If this explanation doesn't make sense - post again and i'll explain in more detail.
Dan
It's one of my camera's most used features. Boyd's decription is spot on. Try using it again. It takes a few seconds for your eye to get used to the lower light level caused by the aperture being smaller (especially when your using f/11 to f/22. When your eye is used to the darker viewfinder try press the button in and releasing a number of times to see what areas of the image look sharper as a cause of pressing the button in.
You use the DOF button to get exactly what levels of detail you need sharp in a shot. With fungi, for instance, which I was photographing yesterday, I'd start with the lens wide open to ensure a strong out of focus background, but I would then stop the lens down enough to get the front of the cap and the stalk sharp while picking a focus point some way between the two to allow the depth of field to cover the short range. This cannot be done easily without using the DOF preview.
EDIT: Quick on the draw Dan beat me too it 
Funghi are't my usual subject but I was out photographing such yesterday (will post one on here) the depth of field preview button is crucial ... without it I wouldn't have been able to find that F8 on my 70-200mm was the right aperture for the shot I wanted.
I actually got better focus on the funghi at F11, but the background wasn't really out of focus enough ... without using the depth of field preview to see how sharp my shot was in the crucial areas .. I would have gone home with a card full of wasted shots.
D
For the replies thank you.My understanding now is that the difference in light when pressing d.o.f.p isn`t the important bit it`s looking for the subtle changes in focus within the light/dark areas to gauge your focus points.Is this correct?Admittedly that is the 1st thing that jumps at you the light&dark . As pete & culture canvas say its the practice that makes perfect.So I`m going to play some more.
Cheers Darchy
Darchy
Essentially that is correct ... its the ability to check which part of your image stay in focus or become sharp through the stopping down of your lens. Make no mistake about it .. the DOF button is one of the most important funcations on your SLR and to not use it is lunacy !!
Dan
Come on then Pete ... are you posting one of your Funghi shots from yesterday ??
(Posted a dodgy funghi attempt myself !)
I'll go and take a look at a few of your funghi shots !! See if any are worthy of the coveted "Dan's Choice Award" lol
D
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