So how do i use it? Is it necessary? and When would one use it? Here is what i know: By pushing the button it is supposed to show me what area inside my frame is in focus and that the smaller my aperture the darker the screen gets. But that is all that i know. Whats the point of the screen getting darker other then it getting darker because less light is being let in? And how can i tell what part of the screen is in focus and what isn't? To me nothing changes when i push the button. I see the same thing that i see when i am not pushing the button. Now either that means i dont know what i am doing or i need to get my eyes checked =p Can anyone help me understand button is for and how to properly use it? Thanks a bunch in advance.
When you look through your camera, the aperture of your lens is typically wide open to allow as much light in as possible, hence the bright screen. It also means your depth of field is as shallow as it can be. You use the preview button to ensure that you have enough depth of field, because pressing it sets the aperture to what it will be when you press the shutter. Probably nothing new there - it seems likely you know all that already.
So, it gets darker because the aperture gets closed down to whatever your setting is. There is no point to it - that's just physics. Closing down your aperture, however, increases how much of your foreground and background are in focus. So, when you press it, unless your aperture is set to wide open anyway, more of your image will appear to be in focus (or, in acceptable focus, to be accurate) - that is to say, more in front of and behind the point at which you have focussed.
All that said, on a digital camera it is usually easier to take the shot, and then check it by zooming in on the screen. Assuming, of course, that you have a decent feel for how much aperture affects your DOF at a given focal length and camera-to-subject distance. Ironically, this is mostly because of the darkening that occurs when you press that DOF preview button (it's just easier to see when everything is bright). The button takes a bit of getting used-to, and is only moderately useful because most viewfinders make everything look so small. On a tripod it can be handy, with digital cameras, because you can use live-view and zoom in on your capture area to see exactly what is sharp and what isn't.
Jinx - great explanation. I also use the preview button to "lock in" the aperture when I use my reverse macro ring. I have to set the aperture in aperture priority mode, and hold the preview button while removing the lens - otherwise it stays wide open. Then I can use the reverse macro ring to use the lens backwards for awesome close-ups. But if I guess wrong on the aperture (usually too open for bight, sunny days), I have to put the lens back on and do the whole thing again with a different setting. But - other than that, I never use it for actually previewing - I just shoot and check the shot - then shoot again as necessary.
@jinximages Great explanation and of course the real "point" of the DoF preview button.
I've also used it to demonstrate the aperture to someone new to cameras. Use aperture priority mode and dial up a small aperture f/22 say and look down the lens from the front, initially of course, it will be wide open but then press and hold down the DoF preview button and you can see the aperture shutting down. A good way to tangibly demonstrate the difference between f/2.8 and f/32 for example :)
Well no it doesn't make things worse other than more questions, but then there are occasional things that i just am not going to understand. But yes, you did answer one question for me and that is I probably wont use the button. Never have found the need for it in the four years i have owned a camera. I am just reading thru my manual and trying to understand my camera more, that's all. Thank you very much.
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So, it gets darker because the aperture gets closed down to whatever your setting is. There is no point to it - that's just physics. Closing down your aperture, however, increases how much of your foreground and background are in focus. So, when you press it, unless your aperture is set to wide open anyway, more of your image will appear to be in focus (or, in acceptable focus, to be accurate) - that is to say, more in front of and behind the point at which you have focussed.
All that said, on a digital camera it is usually easier to take the shot, and then check it by zooming in on the screen. Assuming, of course, that you have a decent feel for how much aperture affects your DOF at a given focal length and camera-to-subject distance. Ironically, this is mostly because of the darkening that occurs when you press that DOF preview button (it's just easier to see when everything is bright). The button takes a bit of getting used-to, and is only moderately useful because most viewfinders make everything look so small. On a tripod it can be handy, with digital cameras, because you can use live-view and zoom in on your capture area to see exactly what is sharp and what isn't.
I hope I haven't made it worse for you!
I've also used it to demonstrate the aperture to someone new to cameras. Use aperture priority mode and dial up a small aperture f/22 say and look down the lens from the front, initially of course, it will be wide open but then press and hold down the DoF preview button and you can see the aperture shutting down. A good way to tangibly demonstrate the difference between f/2.8 and f/32 for example :)