Why SLR for digital camera?

Question

Why we need a mirror in DSLR camera to block the light from entering to CCD sensor as in film camera?

The first generation DSLR does not have LCD preview ( live view) due to this reason and now with additional sensor for live view.

Since the CCD sensor will not be damaged by light, why not simply use CCD sensor for previewing and also for taking picture as in digital compact cameras.

Is there any good reason for blocking light in DSLR cameras?

Answer 1

Most photographers experienced with SLR prefer to use the viewfinder to compose the images. Even with best live view mode, you can still have better control, better see your focus and depth of field (in DOF preview mode, if camera has it), and have instant feedback to zoom or moving the camera. Also you see better in low light, etc.

So why mirror? So that you can use the viewfinder!

Do you understand how SLR works? When light enters the lens it hits the mirror positioned at exactly 45 degrees, which reflects the image up into the viewfinder, where it is flipped (the lens produces image upside down) using either pentaprism or a cheaper but less bright pentamirror, and reflected into actual viewfinder eyepiece, where you can see it.

When you press the shutter button - the mirror flips up closing the viewfinder momentarily, but opening the path for light to hit the sensor, at which time the shutter is open and picture is taken.

This is the same exact technique that is used on film SLR's. There's only one way a light can go. When you're composing the image, it goes to eye piece, when shooting - to sensor.

Also auto-focus and auto exposure sensors work better when focusing screen is hit by light directly. This is what makes auto focus work a lot faster in SLR cameras than in point and shoot ones, where auto focusing is done through CCD contrast detection. But you can't put a focusing screen in front of the main CCD, so the cameras that have to CCD's have the focusing screen in front of the second smaller CCD, which is not used for taking the pictures,

There is an alternative solution. A prism, or a half-mirror, that both reflects about 50% of light into the viewfinder and lets the rest pass through. But with that you automatically lose half the light, or one stop of exposure. Making it both harder to compose, because the image is dimmer in viewfinder, and require slower shutter speed or wider aperture, or higher ISO to shoot.  I used to have a camera that used this technique - Olympus E-10 utilized a prism to allow both live view and to use a viewfinder at the same time. In all honesty, I very rarely resorted to LCD, because composing using eyepiece works a lot better!

LEM.

---Re: additional info---

I think you are confusing the mirror with shutter. Look, in film point and shoot cameras there are no mirrors. Nor are there in digital point and shoot. The shutter is what protects the film from being prematurely exposed. The only use for mirror is to direct the light into the viewfinder piece. Remember, you are looking at the image projected by the lens, not, like in p&s camera an auxiliary image made by a small lens imitation within the viewfinder itself, that is often not an accurate representation of even the coverage area, leave alone focus, and the rest which are not even visible through these things at all...

So as I said, mirror reflects the light, that's it's purpose. You are also not correct about focusing. When the light goes to your viewfinder, it also passes a focusing screen on which an image forms. This is the image you see in viewfinder. It is very clear and you get to see all of it, not just the limited part that is presented by usually low-resolution LCD (while your sensor may be 10 megapixels, LCD screens rarely have even 1 million pixels on them). So you end up seeing a very washed down version of what you are about to photograph. Can't manual focus that one. But the nice and clear image from focusing screen can be manually focused day or night.

Also there's an array of auto focusing and metering devices relying on the mirror being there. You can't stick them in front of the sensor, because they'd take away part of the pixels. And no, focusing and metering using actual CCD is not as accurate, it uses a slower and less reliable contrast detection method, when adjacent pixels are checked for the highest possible contrast and when that achieved focus is said to be reached. SLR's system is usually phase detection, which actually steals a bit of your light from the main mirror by making it semi-transparent in auto focus spots (but at just the tiny little spots you don't get to see in viewfinder, because they're really tiny) and letting light into autofocus sensor through a secondary mirror. Those sensors split the image in two and detect a focusing position when the two images match up. It is a lot faster and far more accurate.

Look at the diagram at the bottom of this page:

http://www.sensorcleaning.com/whatisansl...

It shows where everything is in the SLR system. If you remove all the mirrors, you'll end up with the camera performing like point and shoot for focusing. And that's not fast enough for most pros. Nor will you be able to manual focus, etc... So you will basically reduce the performance of your great camera to that of point & shoot... Thanks, but not with my SLR! I'm keeping the mirror!

Answer 2

It's not there to block the light--it's there to, as you know, send the image to the viewfinder above.

If you flip the mirror to 'up' position, you should not be able to see naked sensor (behind a protective glass of some sort). There should be a focal shutter thingie that covers it.

I don't think 100% of light is sent up to the optical viewfinder with a camera with live view (wonder if it makes the viewfinder dimmer?).

I suppose I am not answering your question at all.

Answer 3

It's a mirror, or pentaprism and it does not block the light, but splits it into different directions. The sole purpose of this is to direct the image to the view finder. In this way, what you see in the view finder is what you get for your image. "This is a camera where the photographer sees exactly the same image that is exposed".

I think they are right, and that you are confusing the shutter with the mirror. The shutter blocks the light to prevent capturing images prematurely. You are partially correct that sensors in digital cameras don't require this protection. But it's not to protect them. It's to keep the camera from capturing unwanted images. Also, even modern sensors can be overloaded and suffer failure from that. It is an extreme, but can and does happen. That is one reason for keeping a shutter. Another is in the way photography works. Controlling how much light and how long the light is allowed composes the picture. Not using a shutter would mean that only a limited range of photography would exist, as it did in the beginning when shutters were just caps people removed by hand.

Photography is actually quite complicated. It's made much easier by modern technology and it becomes easy to underestimate what really goes into it. Take a look at the link below, it may help you.