The viewfinder will register no change in brightness or DOF until I hit f/2.8 on the aperture. 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.
#Slr final print install
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. I know from experience that the minimum aperture of my viewfinder ( Nikon D90 ) is around f/2.8. I believe the issue you're seeing is that the viewfinder cannot "stop down" as far as the lens installed on your camera. 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. 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. In both cases, I was using an f/2.8 lens at f/2.8.
#Slr final print manual
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. Here's what I got when I used a viewfinder screen with more diffusion (what Sony calls "type L"): Here's what I got from doing that with the stock ("type G") screen in the viewfinder: In both cases, I focused so that in the viewfinder, the 5 in 135 was somewhat blurry, but still readable.
Both are taken with an f/2.8 lens at f/2.8. These are taken with a Sony Alpha A900, but the principle is the same. Here's a demonstration of what I'm talking about. The downside is that the view through the viewfinder gets somewhat dimmer. For what it's worth, that also tends to make manual focusing easier (things that are out of focus look out of focus). 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. Yes, it's quite common for the picture to have substantially less depth of field than you see in the viewfinder. 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. On a Nikon D600, I can see differences down to F/2.8. 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. This affects the perception at large apertures only but how large is dependent on several factors. There is another factor which affects the difference in DOF between the OVF and final images and that is the focusing screen.
#Slr final print software
Some software let you specify your own parameter for this. Traditionally this has been an 8x10" print. The larger the print, the less apparent DOF.Įven DOF field tables and calculators use a built-in factor to determine what to consider too much out-of-focus. However, this is based on perception which is dependent on viewing size. At one point, they become so out-of-focus that we consider them outside the DOF. 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. You are right that the viewfinder shows the image at maximum aperture (unless you press the DOF Preview button). Viewing DOF on the LCD in Live-View gives a better approximation. If you look at your final image on a print or screen, it usually covers a larger field-of-view. Your T3i has a 0.85X magnification viewfinder which should cover about 27° diagonally. Since the image in your viewfinder covers a smaller angle of view as your final image, there will always be a discrepancy. Technically, the angle-of-view covered by the image when you see because visual acuity is determined by angular-resolution (Thanks to for pointing that out). Depth-of-field is dependent on viewing size.
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