LCD Question about DSLRs

I don't own a DSLR currently, but was talking with some people on another online forum for general discussion. One guy posted a few pictures he took with his new Digital Rebel, and soon after it was mentioned that previewing your photo on the LCD before you take it is not possible on a DSLR, and you must use the viewfinder to take the photos, but can later review them as normal.

Can anyone shed some light on this for me?

You cannot use the LCD to compose the scene, but once you actually take the picture it can be viewed on the LCD screen. You need to look through the viewfinder to compose.

Thanks... any idea why this is so?
Maybe it's just my lack of knowledge showing and speaking for me, but I would think that the LCD would still be able to provide the image... maybe there's something about the way they're designed that makes this not so. The viewfinder uses mirrors so that you're actually viewing through the lenses you put on the cam as opposed to the way others (for instance my G3) are made, where it's actually looking above it, and a bigger lens added on can get in the way?

With an SLR, you have to utilize (look through) the through the lens viewfinder, actually looking through the lens. I have both, and still prefer looking through the lens as opposed to composing my shot on the LCD, though it can come in handy at times.

In DSLR, the Sensor is blocked by the mirror and shutter curtain.

Try this:

Remove the lens, Switch to M, Set to Bulb, Press and hold the shutter release button. You will see a dark green sensor.

Let go the button, the shutter will close and the mirror will go down.
Sensor is only expose when the button is pressed.

Therefore DSLR can not use LCD as a viewfinder.

Reminder:
DON'T TOUCH the sensor.

In DSLR, the Sensor is blocked by the mirror and shutter curtain.

Try this:

Remove the lens, Switch to M, Set to Bulb, Press and hold the shutter release button. You will see a dark green sensor.

Let go the button, the shutter will close and the mirror will go down.
Sensor is only expose when the button is pressed.

Therefore DSLR can not use LCD as a viewfinder.

Reminder:
DON'T TOUCH the Sensor, Mirror and Shutter.

Thanks Liang, but as I stated, I don't current own a DSLR yet....

something else, though- some SLRs such as the 10D have a prism instead of a mirror.... is it true that the ones that have prisms reflect to both the viewfinder and LCD rather than only being able to use the viewfinder?

By the way, while someone such as my self am Familiar with out the SLR part of the camera works,I've been wondering how the Autofocus system works thru the viewfinder.

The prism is above the mirror and is used to split the light to the viewfinder and the AF sensors (I believe). The 10D does have the mirror below the prism though.

There is one special film SLR by Canon, the EOS 1something (RS?) which uses a prism in place of the mirror. This makes it possible to look through the viewfinder at the same time as the camera is exposing an image (and, more importantly for users of the camera, shoot more film faster since the camera doesn't need to wait for the mirror to come up), but does come at a price: since the prism splits the light between the film and the viewfinder/AF, the camera loses one stop of light from the film.

The reason you can't use the LCD to compose scenes with DSLRs is related to the sensor, which works differently than in P&S digicams. The benefit in the DSLR sensors is in the noise, of which there is much less than in P&S sensors. I don't know the exact differences though, but they have been discussed both here and on other forums in the past.

P&S cameras are fairly similar to video cameras, and most can capture video. The sensor can capture frames at a fairly high frame rate, and the camera "looks" at the frames to do auto-focus. The viewfinder isn't "through the lens", it's a view through a separate, dedicated, viewfinder lens.

This is an inexpensive arrangement, but AF response is slower and less accurate, and the viewfinder can suffer from parallax and doesn't demonstrate the effect of lens filters. But you do get "live preview".

On a DSLR, the image from the lens is split by a mirror/prism combo (or mirror/mirror combo on the 300D), delivering some of the light to the viewfinder and some to dedicated AF and exposure sensors. This gives you a "through the lens" viewfinder (no parallax, you see the effects of lens filters) and faster and more accurate AF sensing. But the sensor doesn't see the image until you press the shutter button, at which point that mirror flips up out of the way, the viewfinder goes dark, and the lens image is projected towards the sensor, then the shutter is opened/closed.

Most DSLRs have the ability to "lock up" the mirror, so you could move the mirror out of the way (losing the viewfinder and exposure/AF sensors), and lock open the shutter, but then again, most DSLRs don't have sensors that can provide a frame rate high enough to give a good "live preview".

Two different designs. The DSLR sacrifices live preview to gain better exposure/AF sensors and through-the-lens optical viewfinder.
-harry

In May, a friend of mine borrowed my 10D to try to shoot some pictures of his newborn. Not knowing about the lack of the preview feature through the LCD, he called me up and ranted at me about what a PoS my camera was. "I wasn't able to take a single picture with that thing! It doesn't work!" I suppose that he was looking at the LCD while trying to compose his picture ... LMAO.

Last week, I bought him a 99 EUR HP digital P&S camera for his birthday. :P

The point of all this is that once you have the ability to preview your image by looking through the lens as opposed to looking at an LCD screen, you'll instantly recognize one of the more significant superiorities of the SLR design. In fact, the only time you might ever find yourself missing the LCD screen is on those rare occasions when you have to hold the camera at arms-length, like over a crowd. Then the swivel display can be pretty handy.

I remember using the wide-angle lens on my G2 and to a lesser degree, the telephoto; they both really screwed up the viewfinder image. That can't happen on a DSLR.

Thanks for all the great responses!
I can definitely see why it's not possible to compose the photo in the LCD on an SLR, and when you get used to using it, I'm sure it's much preferred in most instances.

Thanks again all!

I'm a little late on this, but here goes...

http://entertainment.h​owstuffworks.com/camer​a.htm

Craig

Is he one of those people who would say that your camera sux because you can buy 8mpixel point and shoots for less?