depth of field preview button

Discussion in 'Photography Forums' started by shaolin101, Mar 27, 2006.

  1. shaolin101

    shaolin101
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    is it needed? why.

    I thought you would look through the camera viewfinder and it would show you what it will take a picture of - or am i missing something?
     
  2. midtones

    midtones
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    It's wide open for viewing, when you actually take the shot then it sets the desired aperature. Press the button and you will notice the viewfinder go dark(er) as it adjusts to your aperature, and you have to look very carefully to see what will be in focus when you take the shot.

    K.
     
  3. barongreenback

    barongreenback
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    It takes a bit of practise and getting used to, but it really does work. If you look through the lens on its widest aperture you'll see pretty much what your photo will come out like. Stop down to f/22 and you'll see no change until you use the depth of field preview. If you hold this down and look carefully, all the frame will now be in focus (and a fair bit darker).
     
  4. scotty38

    scotty38
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    Not being funny but do you know what Depth of Field is? If not then it's the amount of the scene (that will end up being in focus) in front of and behind the point on which you focussed.

    This depth of field will vary depending where you focus, the focal length of the lens and the aperture used.

    Pressing the depth of field button will allow you to see this result as these days metering is carried out at full aperture so you don't always see exactly what you'll get.
     
  5. shaolin101

    shaolin101
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    i do know what DOF is - just didn't realise that you dont see it when you look through the viewfinder. My SLR knowledge dates back to a Zenit-E!

    Might just be my basic photographic understanding but i persumed that you look and twist the lens to focus (which you can see through the lens) and take picture!

    The things you can do now-a-days!
     
  6. scotty38

    scotty38
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    If your knowledge dates back to a Zenit E then you'd be used to seeing the DoF every time you either metered (if it had stop-down metering) or set the aperture in accordance with another metering method.

    Most (if not all) modern slr's use full aperture metering so until you actually press the shutter release the lens is always at maximum apeture so the only way of seeing what it really looks like is by closing the aperture down manually hence the button. Not all slr's have this feature although I could never imagine buying one without it myself :)
     
  7. Peakoverload

    Peakoverload
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    As has been explained before, what you see through the viewfinder is with the iris wide open therefore you dont see the actual depth of field that the aperture you set would give you. The reason for this is actually simpler than it is to explain.

    As you know exposure is controlled by three things, ISO, aperture and shutter speed. The thing is that until you actually press the shutter release only one of those things has any effect. This means that at larger apertures the image in the viewfinder would be to very dark as the shutter isnt there to allow the correct amount of light to pass. This is why what you see is what you would get with the lens wide open as it allows the maximum amount of light into the lens making it similar to what the human eye would see.

    Note though that it is similar NOT identical to what the human eye see's as there is one element of dof that does still work with the lens wide open albeit to a lesser degree and that is focal length. A 300mm lens fully open will produce less dof than a 17mm lens fully open so you will may see the background being out of focus but you should not think that this is exactly what you will see in the finished photo as 300mm @f/2.8 will produce a different dof to 300mm @f/11 or f/22 etc.

    All of this can be seen (if you are quick anyway or use a long shutter speed) by looking into the front of the lens when you press the shutter release where you will see the iris contract. This is exactly what the dof preview button does, it simply closes the iris to the aperture selected which therefore limits the amount of light going into the lens and being reflected into the viewfinder which is why it sometimes looks very dark. As long as it's not too dark you can usually work out how shallow or deep your dof is and adjust accordingly.

    As for is it an essential feature of a DSLR? It all depends on the type of photography you do. For macro I would say it's a very useful tool, and for portraits it's also handy to have, for landscapes it's very useful in some situations but not at all in others as for landscapes (generally speaking) you are talking about capturing a large scene which makes dof preview very hard to judge (as everything is so small in the viewfinder) so you are probably better off using hyper focal focusing or just setting the smallest aperture you have or that the conditions allow if you dont have a tripod or support.

    All of that said, without looking, I'm not aware of any DSLR's that dont have this feature, are there any?

    I dont often use my dof preview button but that said when I have I found it very useful.

    HTH
     
  8. iandrews

    iandrews
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    Speaking of DOF and aperture what kind of effect can you get say with the Canon A620 (or A700) as I see it has a mode where you can set the aperture? I suspect to get a very narrow field that you need a DSLR, but can you still get the effect where say someone is in focus, and the background behind them is out of focus.

    Ian.
     
  9. Peakoverload

    Peakoverload
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    The problem with compact cameras is the size of their sensors, the smaller the sensor the harder it is to knock the background out of focus due to the circle of confusion. To show the differences between them here is an example:

    A compact camera with a 1/1.8" sensor shot at f/2.8 @ 100mm would produce the same depth of field as a DSLR with a 1.6x sensor @ f/8.4 @ 300mm. Now that is a BIG difference!

    So can you knock the background out of focus with a compact? Yes......and No.

    Basically yes you can but not in all the situations that you could if you had a DSLR, in other words you have to manipulate your surroundings and technique a little to allow you to knock the background out. So how?

    Two things spring to mind that will help you achieve a soft background.

    1. Dont position your subject close to the background. Compacts have a much shallower dof so you need to increase the distance between your subject and the background as well as choosing the widest aperture you can.

    2. Use as long a focal length as you can. The longer the focal length you use the more the background will appear out of focus so for portraits, for example, you may be better off standing back and zooming to approximately 200mm and setting the largest aperture you can. This will cause you to achieve a different field of view but that's the trade off I'm afraid to having a small sensor.

    Ultimately though you really need a larger sensor as otherwise you are always going to have to compromise.

    HTH
     
  10. iandrews

    iandrews
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    Thanks for that. So what's the point in having a mode where you can set the aperture then if it doesn't make too much difference?
     
  11. midtones

    midtones
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    It'll make a difference, on that kind of camera. Just compared to a dslr you see the stark difference :)

    K.
     
  12. Peakoverload

    Peakoverload
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    Because its still relative. f/2.8 on a compact will produce a shallower dof than f/5.6 on a compact. Also many compact cameras have a macro mode and dof is also affected by how close a subject is to the lens which is why on a DSLR and a 100mm macro lens you may need to set an aperture of f/16 to achieve a dof of just a few mm yet on a subject that was a few meters away this would give you a dof of several meters. If you didnt have an aperture control on a compact it would make creative macro photography harder.
     

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