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EOS 300 DOF preview? huh?

 
 
[blu|shark]
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      01-15-2004
i seem to fail to understand how _depth of field preview_ button on Canon
EOS 300D works. manual lacks a thorough explanation, and when i use the
button (in one of the creative modes), i don't notice any difference.
i use 3.5AV thinking it should maximize the effect, but nothing really
happens. (at least nothing i notice)

i'd appreciate a few words of explanation on this subject, and also a tip
on whether i should hold the button or just press it when i use it

thank you!
Matija

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Chris Brown
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      01-15-2004
In article <1017l0mjcbzff$.1q78q5ls7d9lo$(E-Mail Removed)>,
[blu|shark] <(E-Mail Removed)> wrote:
>i seem to fail to understand how _depth of field preview_ button on Canon
>EOS 300D works. manual lacks a thorough explanation, and when i use the
>button (in one of the creative modes), i don't notice any difference.
>i use 3.5AV thinking it should maximize the effect, but nothing really
>happens. (at least nothing i notice)


The DoF preview button simply closes the aperture to your selected value,
for as long as you hold it.

If the lens is set wide-ppen anyway, it won't do anything. Try setting the
aperture to f/16, press and hold the button, and notice what happens.
 
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cc
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      01-15-2004
This is a feature I have been hoping for ever since my Olympus SLR's
shutter broke (Olympus were the only brand to have DOF preview for a
while). I'm very glad to have it again on my 10D. What it does is allows
you to view what will be in focus or appear sharp when the lens is
stopped down to the aperture (f-stop) that you have selected. When you
are looking through the viewfinder and not pressing the DOF preview
button the lens is wide open and thus there is a very small depth of
field (only a thin plane is in focus). If you increase the f stop value
the DOF range will increase (more stuff will appear sharp), and pressing
the button will allow you to see this effect and see what will actually
be sharp in your picture. If you have set the aperture at its widest
(lowest number) the button will make no difference. If you press the
button when the f-stop is set to a higher number (smaller aperture) what
you see through the viewfinder will become darker and you will see that
more of a distance range is sharp.

[blu|shark] wrote:
> i seem to fail to understand how _depth of field preview_ button on Canon
> EOS 300D works. manual lacks a thorough explanation, and when i use the
> button (in one of the creative modes), i don't notice any difference.
> i use 3.5AV thinking it should maximize the effect, but nothing really
> happens. (at least nothing i notice)
>
> i'd appreciate a few words of explanation on this subject, and also a tip
> on whether i should hold the button or just press it when i use it
>
> thank you!
> Matija
>


 
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Mike Brodbelt
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      01-15-2004
On Thu, 15 Jan 2004 16:08:36 +0100, [blu|shark] wrote:

> i seem to fail to understand how _depth of field preview_ button on Canon
> EOS 300D works. manual lacks a thorough explanation, and when i use the
> button (in one of the creative modes), i don't notice any difference. i
> use 3.5AV thinking it should maximize the effect, but nothing really
> happens. (at least nothing i notice)
>
> i'd appreciate a few words of explanation on this subject, and also a tip
> on whether i should hold the button or just press it when i use it


Autofocus systems always operate with the lens aperture fully open. So,
when you focus on something, you will be using the lens at maximum
aperture, but when the shot is actually taken, the aperture stops down to
the selected value.

To see the effect of DOF preview clearly, set the camera to Av, choose an
aperture of f/16 or f/22, pick s subject to focus on, half press the
shutter, and then depress the DOF preview. By using f/3.5, which is the
max aperture on the 300D's kit lens, you won't see anything.

You will see the viewfinder image become substantially darker, as the
aperture stops down, and the depth of field visible in the viewfinder will
visibly enlarge.

HTH,

Mike.
 
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Jim Townsend
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      01-15-2004
[blu|shark] wrote:

> i seem to fail to understand how _depth of field preview_ button on Canon
> EOS 300D works. manual lacks a thorough explanation, and when i use the
> button (in one of the creative modes), i don't notice any difference.
> i use 3.5AV thinking it should maximize the effect, but nothing really
> happens. (at least nothing i notice)
>
> i'd appreciate a few words of explanation on this subject, and also a tip
> on whether i should hold the button or just press it when i use it


When you look through your viewfinder, you're looking through your lens.
(That's what SLR's are all about

EOS lenses have an iris made up of metal blades.. These blades work like
your eye and control the amount of light entering the lens by varying the
size of the opening. As you increase the f number, opening gets
smaller. If you reduce the f number the opening gets bigger.

DOF depends on the size of the iris opening.. A very large opening creates
a shallow depth of field.. (subject in focus, background out of focus).
A tiny opening provides a large depth of field. (subject and background
both in focus).

Here's the trick.. In EOS lenses, the iris doesn't shrink the opening *until
the shutter release is pressed*. It happens just before the mirror flips and
the shutter curtains move.

Because of this, you are always looking through the lens with the iris
wide open. If your lens is an f/3.5 lens, then you're seeing the scene
at f/3.5 no matter what you've set your f stop to. Because of that,
you can't see what in the image (besides your subject) will be in focus
or out of focus.

By pushing the DOF preview button, you cause the iris to do what it's
going to do when you press the shutter release. If you set the camera
to f/22 the iris will shrink the opening to a tiny circle. Now you
can see exactly what will be in focus or out of focus.. The only
drawback is that things get pretty dark because the opening is smaller.

Try this.. Put your camera in Av mode. Set the aperture to f/22
Now turn the camera towards you and look down the lens.. Press the
DOF preveiw button.. You'll see the iris open and close from wide
open to f/22 as you press the DOF button.. Look through the viewfinder..
Note objects in the far off distance as you press the button.

 
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jerry_tig2003
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      01-15-2004
"[blu|shark]" <(E-Mail Removed)> wrote in message news:<1017l0mjcbzff$.1q78q5ls7d9lo$(E-Mail Removed) >...
> i seem to fail to understand how _depth of field preview_ button on Canon
> EOS 300D works. manual lacks a thorough explanation, and when i use the
> button (in one of the creative modes), i don't notice any difference.
> i use 3.5AV thinking it should maximize the effect, but nothing really
> happens. (at least nothing i notice)
>
> i'd appreciate a few words of explanation on this subject, and also a tip
> on whether i should hold the button or just press it when i use it
>


When you press the button, the diaphragm of your lens will close to
its set number. If you set it to a high number, the image will be
darker, but the depth of field will be bigger. If you set it to a
small number, you may not notice the change.

To see the effect, use the Av mode, and try different diaphragm
settings.
 
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Tony Spadaro
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      01-15-2004
Try it at f16. There is an article here, on using the DOF preview button:
http://www.chapelhillnoir.com/mani/techs/mdofwalk.html
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"jerry_tig2003" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> "[blu|shark]" <(E-Mail Removed)> wrote in message

news:<1017l0mjcbzff$.1q78q5ls7d9lo$(E-Mail Removed) >...
> > i seem to fail to understand how _depth of field preview_ button on

Canon
> > EOS 300D works. manual lacks a thorough explanation, and when i use the
> > button (in one of the creative modes), i don't notice any difference.
> > i use 3.5AV thinking it should maximize the effect, but nothing really
> > happens. (at least nothing i notice)
> >
> > i'd appreciate a few words of explanation on this subject, and also a

tip
> > on whether i should hold the button or just press it when i use it
> >

>
> When you press the button, the diaphragm of your lens will close to
> its set number. If you set it to a high number, the image will be
> darker, but the depth of field will be bigger. If you set it to a
> small number, you may not notice the change.
>
> To see the effect, use the Av mode, and try different diaphragm
> settings.



 
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