Should the depth of field in the viewfinder be the same as the final images with the aperture set to wide open?

Question

When I take pictures of flowers, I often try to get the classic soft background around the in focus flower.

I have a canon t3i, and a lens that goes to f/2.8. I set that to wide open, take my photos, and get results I like.

However, whenever I am looking through the viewfinder, it always seems to appear with a much wider depth of field than the final images. Is that true, or am I just crazy? I thought that the whole point of the SLR type camera was so that what you see in the viewfinder is what you get. Is that just for framing and composition, or should it reflect the DOF accurately as well? Since I set the aperture to its max f/2.8, it shouldn't be any previewing behavior.

So am I really crazy, or is there a difference in DOF between the viewfinder and final image?

Answer 1

As Mike Johnston explains the viewfinders, the focusing screens used in modern cameras tend to be made super-bright in order to accommodate the slow zoom lenses that often get connected, but this brighter construction (like an array of miniature lenses, or a bunch of very short fiber-optic cables) also makes much of the image to appear in focus, much more so than on final image. So the apparent depth of field is larger than on resulting photo by design.

If you want a truer representation, use Live View or a custom focusing screen (tuned for manual focusing, usually with split prisms).

Answer 2

No. Depth-of-field is dependent on viewing size. Technically, the angle-of-view covered by the image when you see because visual acuity is determined by angular-resolution (Thanks to @Imre for pointing that out). Since the image in your viewfinder covers a smaller angle of view as your final image, there will always be a discrepancy.

Your T3i has a 0.85X magnification viewfinder which should cover about 27° diagonally. If you look at your final image on a print or screen, it usually covers a larger field-of-view.

Viewing DOF on the LCD in Live-View gives a better approximation. You are right that the viewfinder shows the image at maximum aperture (unless you press the DOF Preview button).

What is important to understand is that DOF is not a hard limit where things are in focus on one side and not on the other Things get gradually out of focus away from the focus plane. At one point, they become so out-of-focus that we consider them outside the DOF. However, this is based on perception which is dependent on viewing size. The larger the print, the less apparent DOF.

Even DOF field tables and calculators use a built-in factor to determine what to consider too much out-of-focus. Traditionally this has been an 8x10" print. Some software let you specify your own parameter for this.

EDIT For Completeness:

There is another factor which affects the difference in DOF between the OVF and final images and that is the focusing screen. This affects the perception at large apertures only but how large is dependent on several factors.

On a Pentax K-5 with default focusing screen, I can see differences in DOF down to F/2 and they correspond to my expectations when I see the image large. On a Nikon D600, I can see differences down to F/2.8. Those are just the two cameras on my desk right now based on experience, most DSLRs should fall withing those limits with their stock focus-screens.

Answer 3

Yes, it's quite common for the picture to have substantially less depth of field than you see in the viewfinder. I'm not sure specifically about the T3i, but for at least some cameras you can get a different viewfinder screen that diffuses the light more, so it shows depth of field more accurately. For what it's worth, that also tends to make manual focusing easier (things that are out of focus look out of focus). The downside is that the view through the viewfinder gets somewhat dimmer.

Edit:

Here's a demonstration of what I'm talking about. These are taken with a Sony Alpha A900, but the principle is the same. Both are taken with an f/2.8 lens at f/2.8. In both cases, I focused so that in the viewfinder, the 5 in 135 was somewhat blurry, but still readable. Here's what I got from doing that with the stock ("type G") screen in the viewfinder:

Here's what I got when I used a viewfinder screen with more diffusion (what Sony calls "type L"):

I'll repeat: In both cases, I used manual focusing, adjusted so that the 5 in 135 (just before the big 36) was (somewhat) blurred but still readable when viewed in the viewfinder. In both cases, I was using an f/2.8 lens at f/2.8. In both cases, we're dealing with what I saw through the viewfinder, with an f/2.8 lens at f/2.8, not a faster lens, nor using DoF preview, nor comparing the view through the viewfinder to a (larger) print.

So, one more time, all together on the count of three now: yes, the viewfinder screen does affect the apparent depth of field, even with an f/2.8 lens, even when it's wide open.

Answer 4

I believe the issue you're seeing is that the viewfinder cannot "stop down" as far as the lens installed on your camera. I know from experience that the minimum aperture of my viewfinder ( Nikon D90 ) is around f/2.8. Therefore, any time I install a lens with a minimum aperture of less than f/2.8, I'm going to have to guess the DOF. This can be easily determined by installing a fast lens on your camera, and progressively stopping down the aperture and hitting the DOF preview button, while framing something in the viewfinder. The viewfinder will register no change in brightness or DOF until I hit f/2.8 on the aperture. If your camera has the same limitation, then you will never see any DOF preview changes until you hit at least f/2.8 on the lens. As mentioned in other answers, your aperture is wide open until you release the shutter. The light is simply being reduced through your viewfinder, with the same effect as stopping down the aperture. @Ital and @mattdm's link explains this much more succinctly.