Depth of field

On a T2i what does the DOF button do.I have tried it a number of times in different settings and live view and I can not seem to see what it does or is supposed to do.

From your manual:

Press the depth-of-field preview button to stop down to the lens's current aperture setting. You can then check the depth of field (range of acceptable focus) through the viewfinder.

It's basically a WYSIWYG feature that lets you see how the picture will look using the aperture you (or the camera) has selected for that shot. Typically, cameras will focus and such with the lens at its largest aperture regardless of the setting you've selected. So if you lens is capable of f/4 at the focal length you're using, the camera will focus at that aperture and that is how you'll see the picture in the viewfinder. If you have the camera set to take a photo at f/11, the camera will stop down the lens diaphragm just before taking the picture. If you want to see how the depth of field looks at f/11, press the DOF preview button. Obviously, if you've set the camera to take the shot at f/4, you won't see a difference when you press the preview button. (Note, if you intend to use a smaller aperture, your viewfinder should get noticeably darker when you press the DOF preview button as less light will be available at that smaller setting.)

It tells the lens to stop down to the set F-stop so you can see what will and won't be in the depth of field of the shot at that setting. Put your camera in Aperture priority, stop it down to, say F8, look through the viewfinder and hit the DoF preview button. The image should get "dim" but more of it should appear in focus.

When you shoot in Av and you set the aperture to between f16 and f22 try pressing the DOF button then. You'll see the viewfinder darkens and shows the picture as it will be taken with the full DOF in focus. When you are shooting upto around f8 in sunlight you probably wont see much of a difference. Without pressing the DOF button you will see the view as it would be taken with the aperture wide open.

isle wrote in post #14758203
On a T2i what does the DOF button do.I have tried it a number of times in different settings and live view and I can not seem to see what it does or is supposed to do.

The thing to understand here is that if you close the aperture with the dial, or if some automatic mode does it for you the camera actually close it. In the Canon shooting model the aperture will always stay full open whole you are messing with the camera and only snap close to the desired aperture when you actually fire.

The button tells the camera to actually close to the desired setting. It does nothing if your selected aperture is the lens'es maximum aperture anyway.

Sometimes I wish Canon would communicate a little clearer instead of piling up "less confusing" terms and them spend 20 years making up more confusing terms just to be bug-compatible.

uOpt wrote in post #14758454
The thing to understand here is that if you close the aperture with the dial, or if some automatic mode does it for you the camera actually close it. In the Canon shooting model the aperture will always stay full open whole you are messing with the camera and only snap close to the desired aperture when you actually fire.

The button tells the camera to actually close to the desired setting. It does nothing if your selected aperture is the lens'es maximum aperture anyway.

Sometimes I wish Canon would communicate a little clearer instead of piling up "less confusing" terms and them spend 20 years making up more confusing terms just to be bug-compatible.

I don't see anything confusing about it. I had a DoF preview button on my Minolta SRT nearly 40 years ago, and it did the same thing. I used it religiously.

It's confusing to people without 40 years of photography behind them.

uOpt wrote in post #14768152
It's confusing to people without 40 years of photography behind them.

It was still confusing 32 years ago when I bought my first SLR since the viewfinder darkened enough that it was hard to see what was and what was not in focus.

Never used it then and still don't use it today.

jwp721 wrote in post #14768180
It was still confusing 32 years ago when I bought my first SLR since the viewfinder darkened enough that it was hard to see what was and what was not in focus.

Never used it then and still don't use it today.

Agreed, BUT today we have live view. Unfortunately, however, not DOF preview lock (maybe on 1-series though).

The main use I see for it with modern cameras is when on a tripod in LV mode where you can zoom in to selected parts of the frame and LV compensates for the reduced aperture. You can then use it to see if the entire (or at least most of it since you might not be able to zoom into the corners) is in focus when stopped down to your current aperture settings.

The viewfinders on modern AF SLR cameras don't shot DOF very accurately. So even when holding down the DOF preview button, the viewfinder isn't showing you very well what you are going to get. Where it does work well is in liveview.

Every time I press the DOF button my lens falls off

tkbslc wrote in post #14768423
The viewfinders on modern AF SLR cameras don't shot DOF very accurately. So even when holding down the DOF preview button, the viewfinder isn't showing you very well what you are going to get. Where it does work well is in liveview.

+1 Live view is much more useful with dof preview, maybe with the bigger cameras with a MF screen the DOF preview would be slightly more helpful. I find the viewfinder useful for rough "is it in focus" check but thats about it.

Not on the standard camera however Magic Lantern on the 600D will allow you to make the button sticky, I assume it functions the same for the T2i?

Nature Nut wrote in post #14768453
Every time I press the DOF button my lens falls off

Very droll!

But regarding the original question, I find depth of field tables more helpful. As already said, stopping down makes the image too dark to see anything useful.

But it becomes second nature. I know from experience that the DoF Is about 1/3rd in front of the subject and 2/3rds behind and also how deep it will be for various apertures. As I shoot landscapes I normally want the largest DoF I can get rather than the smallest. If I habitually wanted the opposite I'd soon get used to that.

Just a quick note... on my 20D and 50D, I seem to recall reading that the DOF preview button does nothing in the "picture" modes, it only works in the "creative" modes... P S A M. I assume it is the same with the Rebels.

And of course if the aperture is open as wide as it goes, there won't be a change. You need to stop down (set a higher f-number) to see a change when you push the button.