Depth-of-field preview button

[freelanceshots]

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<p style="margin-bottom: 0in;"]Here's a question that I thought would
make for a good discussion where I've never seen much written about
it. How do you use the depth-of-field preview button on Canon's
DSLR's? I've owned a Rebel XT, 20D, 30D, 40D, and now the 5D2 where I've known
about the little button but I just don't understand how to use it
correctly. Is it worth using and will it really give you an accurate
view of the depth of field?

[neuroanatomist]

During most operations (looking through the viewfinder, focusing, etc.) the aperture remains at it widest setting, no matter what aperture is selected for the exposure. The aperture is stopped down just as the picture is actually taken. This means that even if you select a small aperture for your shot (e.g. f/22) to maximize your depth of field, you won't see that effect through the viewfinder. Instead, you'll get the bright view and corresponding shallow depth of field of the wide open aperture. When you press the DOF preview button, the aperture stops down to the selected setting - this means can look through the viewfinder (or use Live View on some recent Canon bodies) and get an accurate (but likely quite dark!) view of the depth of field your picture will have.


If you want to learn more about DOF, there are many tutorials. Here's one example:


http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml


The DOF preview button also has a couple of less commonly used/known functions:


1) With an external Speedlite flash attached, the DOF preview button activates a 'modeling flash' in addition to stopping down the aperture. Modeling flash is a high-speed series of bright flashes that illuminates area where the flash is aimed.


2) The DOF preview button has a 'hacker' function - for example, a lens can be reversed (using a threaded adpater) so the front of the lens is attached to the camera body, turning it into a makeshift macro lens. Obviously, no electronic aperture control is possible in that case, so the DOF preview is held down to stop down to the selected aperture, and then the lens is removed from the camera, leaving the aperture at the desired setting.

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I've used it just playing around where I noticed the
darkening/vignetting but I did not know how to read it/use it. Will
the darker areas displayed in the viewfinder mean that those areas
will be out of focus where the lighter areas expanding out from the
center will be in focus? I'm a little confused on how to read it.



Thanks for sharing your knowledge. I've seen the strobe effect
before but I did not know how to activate that feature. Where
would one use this strobe effect? Also when you talk about
turning your lens around and using the appropriate adapter to make a
makeshift macro lens have you done this before? What are some
of the lenses one could use to get the best results from the process
that you describe? Do the images look good?


-Bryan
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[musickna]

The darkening you see is simply the consequence of looking through a narrower aperture.


The DOF button is really useful. It gives you a preview of what your camera will actually record, and is invaluable for composing a picture with regard to what you want in focus and what you want blurred.

[peety3]

I've seen the strobe effect
before but I did not know how to activate that feature. Where
would one use this strobe effect?
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I find it incredibly useful when doing remote flash setups, particularly with multiple flashes. It allows me to visualize where light and shadows will fall, and fix issues before I get too wrapped up in shooting/chimping.


Studio flashes often have a modeling light (i.e. a 100W incandescent bulb) so you can see what you'll get. Many will automatically dim the modeling light proportionate to the brightness setting on the flash. The DOF preview/modeling light feature is a way to replicate the studio flash behavior (somewhat).


The beta firmware for the new PocketWizards enables modeling light functionality. I like it!

[wickerprints]

Certain lenses with a very large maximum aperture, such as the EF 50/1.2L, exhibit focus shift. One way to compensate for this phenomenon is to focus while activating DOF preview, from f/1.4 - f/2.8, and sometimes down to f/4.

[Colin]

Certain lenses with a very large maximum aperture, such as the EF 50/1.2L, exhibit focus shift. One way to compensate for this phenomenon is to focus while activating DOF preview, from f/1.4 - f/2.8, and sometimes down to f/4.
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What happend to my post? How do I use this!? Heck. If I was able to do that, I would have kept my EF 50/1.2L.... Cripes.... I've got 30d and 5d, so no preview available, but if I could view it with the actual aperture (and the autofocusing still worked), even if the light is lower, that would have been great...


Still would be handy, particularly for Macro stuff..

[wickerprints]

As far as I am aware, every EOS body ever made supports DOF preview.


However, when I mean focus while stopped down, you must do this manually. AF does not operate while DOF preview is enabled. The use of a split prism focusing screen, or a super-precision matte screen assists in this task.


The problem with focus shift is that it is independent of AF or MF. For example, if you manually focused at f/1.2, then took the photo at f/2, you would still observe focus shift. In other words, it is not due to the inability of the camera to acquire accurate focus, but rather it is the optical design of the lens (i.e., undercorrected spherical aberration) that causes the problem.


There are other ways of dealing with focus shift. See here:


http://toothwalker.org/optics/spherical.html

[neuroanatomist]

Also when you talk about
turning your lens around and using the appropriate adapter to make a
makeshift macro lens have you done this before? What are some
of the lenses one could use to get the best results from the process
that you describe? Do the images look good?

I have not done it with a Canon dSLR, although I did many years ago with a Pentax SLR (in the completely manual lens days, so aperture was set by a ring on the lens - remember that?). The basic adapters are simple, such as this one from Adorama, which couples a reversed lens directly to the camera body. There is also anupscale version from Novoflexfor connecting areversedlens, which has a cable and second ring to transfer the control functions. In either case, these are best used with fast wide-angle lenses, and can provide substantial magnification (a reversed 28mm lens gives about 3:1 magnification). Compared to a dedicated macro lens, this comes at the cost of convenience (both in getting a proper exposure and in a very, very short minimum working distance).


An alternate manual approach is to use a stacking ring or step up/down ring adapter (as you'd use with lens filters) to reverse a second lens mounted onto a lens mounted 'normally' to the body. That's often done with a 50mm lens reversed, mounted on a standard zoom (the 50mm lens is acting as a close-up lens, similar to theCanon 500D Close up Lens). This ends up being optically similar to an actual macro lens, but suffers from significant vignetting.


All things considered, although this approach is technically possible, it's really quite inconvenient unless you're using the Novoflex, and if you are going to pay $400 for that, you might as well just get a dedicated macro lens. If you'd rather try out macro photography without springing for a dedicated macro lens, consider adding extension tubes to short focal length lenses (very short minimum working distances) or close up (diopter) lenses to longer focal length lenses. You won't get true 1:1 macro with those accessories, but you'll get a taste - and if you do later get a dedicated macro lens, the tubes (and the close-up lenses, if you are lucky/careful in your lens filter size selection) can be used on the macro lens for &gt;1:1 magnification.

[apersson850]

On the 7D, the DOF button is included in the set of ten buttons that are programmable through C.Fn IV-1. Hence, it can have more uses than just DOF and flash preview.


Don't forget that you can use DOF with live view active. If you don't use exposure simulation, live view will increase the brightness of the rear LCD while stopping down. This makes the depth of field easier to see.