I have decided to buy a d50 or rebel xt. My inclination is towards d50 but there is one small worry. d50 does not have mirror lock up whereas rebel xt has it. I know that it is useful only for long exposure or telephoto lens. Though initially I would be purchasing 18-70 lens only but I will buy a telephoto zoom lens after sometime - something like 135-400 from sigma. With this kind of lens, when I try to take pics at 400mm (eg bird pics), I will need to keep the shutter atless than 1/400 so that the mirror vibrations doesn't spoil the picture.Right? I will need to close down to f8 or lesser to have good enough resolution. Now my question is that if I want to click bird snaps in daytime(good light) or morning(less light) at 400mm and f8+ aperture andISO at 400 or 800(don't want grains)will I be able to keep the shutter at less than 1/400 so that the lack of mirror lockup doesn't spoil my pics.

I know I am getting very specific here, but I want to be sure that I am putting my money in the right thing, that will enable me to take the kind of pics I want to.

Well, I'm sorry to disappoint you but you knowledge is wrong.

Mirror lock-up is useful when you're using tripod.

When you're shooting with long focal lenses anti-shake is useful. Nikon and Canon have their VR and IS lenses for that, they are really expensive though.

Check out the wildlife forums to see what people actually use (download some and check the exif data):

http://www.stevesforums.com/forums/view_forum.php?id=11

From what I've seen, with f8, in very bright light you'll often be able to either dial the ISO down lower, or maybe increase the shutter to 1/1200 or more. On the other hand, 1/400 and f8 at ISO 400 would still be pretty bright daylight. Many times you will have to lower the shutter speed, kick up the ISO, or open up the f-stop a bit. And in some cases you migh have to use a bit less zoom to keep your hold steady. Or maybe all of the above, especially in early mornning or cloudy conditions.

The point is to use those recommended settings whenever conditions allow it. When conditions don't allow it, you start to make tradeoffs. I see plenty of good bird shots at 1/60 sec and f5.6.

Being able to lock the mirror up also helps with macro work.

I had a jeweler complaining about the soft/blury pictures he was getting with his 350d canon slr. After letting him know because he was using low iso and shutter speed, that the mirror was probably jolting the camera slightly when he took the picture, to dig into the menu and lock the mirror up to prevent this movement. In a couple of hours he phoned me back up and couldn't/wouldn't stop talking about how sharp and clear his photo's are now.

Roopang - Mirror lockup is only useful for very limited range of shutter speeds (1sec to say 1/15sec). You use it to damper the vibrations caused by the mirror moving out before the shutter opens. Here is an articles by Bob Atkins.

http://www.photo.net/learn/nature/mlu

In regards to telephoto, you would not need mirror lock unless you shooting at very slow ss, like 1/15 or so. In normal shooting your ss would be much higher. Don't know where you got that you need to keep ss less than 1/400sec while at f8. you need to read up on basic exposure.

MLU is not a big deal just like DOF preview button on crop bodies.

You may not need miror lock up per se. My Pentax *ist DL does not have it, BUT i can use the 2 second time delay trigger. Then it moves the lens, waits 2 seconds and THEN exposes. I think most DSLRs can do that.

Rick