Help: Canon DoF Preview Button

[RobbieG]

Apologies in advance for asking what is probably a silly question, but I'm struggling to grasp the concept of the DoF preview button on my Canon 500D.
I understand depth of field and the concept of what is in sharp focus however, what I dont understand is how the DoF preview button shows me this.
Can anyone explain to me in simple terms how to use this button in a practical sense?
Whenever I press this button I can see the frame go dim but I don't see how this relates to depth of field.
Thanks

[RecurrentNerve]

DoF Preview buttons are leftovers from film cameras, where you obviously had to wait until the film was developed before you could see what the depth of field was like for any given shot.

They work by closing the aperture down to the current setting, which means that you can see the depth of field through your viewfinder. However, it also has the effect, as you noticed, of making the viewfinder dim, often rendering the exercise pointless; the dimness coupled with the relatively small viewfinder image means that often you can often hardly make out the DoF.

Nowadays it is easier simply to take the shot and look at it on your big, bright LCD than to try and make out what's going on through your viewfinder.

[zona5101]

the dof preview button stops down the lens to the chosen fstop. This, as you have noted, lets in less light...it will also alow you to see what areas will be in focus along with your subject. Find a subject like a fence line that is going away from you. focus on a spot, select a smaller apreture such as f8 then press your dof preview. even though it is dark, you should be able to detect the areas of the fence in front of and behind your focused point that are also in sharp focus (even though they may look OOF when you release the dof preview)

[carlyfan]

This confused me for the longest time, but I finally got it when I took a photography workshop. This is how it was explained to me.
When the lens is at rest, it is at it's widest aperture, let's say f 1.8. The camera does not adjust the aperture until you press the shutter button. So if you select a small aperture, say f 11, the viewfinder will still show the DoF of f 1.8 because you haven't taken the picture yet. The DoF preview button, as the others have said, stops down the aperture to the selected f stop. This causes the viewfinder to become dim because less light can come through a smaller hole, but it also shows your DoF. Don't pay attention to the fact that it gets darker, that doesn't mean anything. The best way to notice the difference is to select a high aperture, focus on something close to the camera, and hold down the DoF button and then release, hold and release, hold and release, each time looking at a different edge. I hope this helps!

[rudeboy]

I´m glad i have seen this post as i have never got the hang of the dof button,i will give it ago till i master it,watch this space.....or maybe not as it may take awhile

[sk66]

Originally Posted by RecurrentNerve

DoF Preview buttons are leftovers from film cameras, where you obviously had to wait until the film was developed before you could see what the depth of field was like for any given shot.

They work by closing the aperture down to the current setting, which means that you can see the depth of field through your viewfinder. However, it also has the effect, as you noticed, of making the viewfinder dim, often rendering the exercise pointless; the dimness coupled with the relatively small viewfinder image means that often you can often hardly make out the DoF.

Nowadays it is easier simply to take the shot and look at it on your big, bright LCD than to try and make out what's going on through your viewfinder.

I reassign my DOF button to something useful.

[RobbieG]

Thanks everyone for your responses. Seems to make sense now however as pointed out, it probably is of little use when I can just take the shot and then evaluate on screen. I will give it a try though and see if my eysight is good enough to pick the difference as I must admit it was the level of dimness that I thought was meant to be telling me something.

[inkista]

I actually prefer using the DoF preview button. Different tastes.

It's the secondary function of the DoF preview button that I find not only useless but dangerously useless, which is that if you have your flash on, the damn thing goes into modelling light mode, which if you hit too often to quickly can cause your flash to overheat. And it's not like it's even a useful modelling light.