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Excuse my ignorance but as far as I know, SLR was invented in order to get the photographer to see (through the viewfinder) exactly what image will fall on to the film. In digital cameras the image falls on the CCD (or whatever type the censor is) and afterwards the image is transferred on the LCD in real time. In other words, you don't actually need a viewfinder since there is the LCD screen of your camera and you see what image will exactly fall on the CCD. Am I right in my thinking?

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4 Answers 4

up vote 8 down vote accepted

I suspect the major reason this is true for DSLRs is to get the lightning-fast focus times that point-and-shoot cameras don't have. The autofocus mechanism is not actually part of the CCD/CMOS, but a separate device in the camera body, and the mirror splits the light coming through your lens so that half goes to the viewfinder and half goes to the autofocus sensor. See, for example, this site describing the differences between a mirrorless camera and a DSLR; note in Figure 1 the autofocus module below the mirror.

This autofocus module is doing a phase detect autofocus, which is extremely fast. Without a mirror, you have to do contrast-detect autofocus, which is slower. Recent cameras (e.g., the Sony a55), have gotten their autofocus fast enough that they no longer need a mirror, but it's taken quite a long time for the technology to get there. So I suspect the trend will be towards cameras with DSLR quality and focusing speeds, and no mirror (perhaps with an electronic viewfinder, instead). But it's only been recently that such things have become possible.

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1  
Also, EVFs are not good enough compared to OVFs in low-light / don't always show the scene as it actually is, only as the sensor is recording it. Also, in keeping the sensor continuously active, it gets hot and noise from hot pixels could happen during a long days shoot. –  camflan Aug 28 '12 at 18:32

The best answer I can provide is MILC article on Wikipedia. I'd add power consumption to the list of drawbacks - using an active viewfinder/LCD screen requires current delivered to the sensor and of course to the display.

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yeah but isn't the LCD screen always on providing real time image in every digital camera as you turn the camera on? i've never seen a digital camera that has the option to turn the lcd screen off to save power. –  sterz Aug 10 '12 at 23:02
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Nearly every DSLR in existence at least has the option of doing this, and on most it is enabled by default. In fact, it wasn't until relatively recently that so-called "live-view" modes started appearing on DSLRs that allow you to even use the LCD as a real-time image display. –  David Aug 10 '12 at 23:18

You have just invented the mirrorless inter-changable lens camera. SLRs were needed because back then, in the 60s and early 70s, there were no electronic viewfinders. Things like the Nikon F were amazing and advanced.

Some people prefer the pentaprism viewfinder, but that is just a personal choice. With a good electronic viewfinder, cameras can be lighter, smaller, and perhaps even cheaper.

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I guess it is about the "true-ness" of the captured photos, I.e. whether a photo is exactly like one that captured in film. Although it's true, like you said, that all digital cameras offers the SLR viewfinder's effect, only DSLR's give you the best images thanks to the use of much much bigger sensors than a casual point and shoot.

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yes but this still does not justify the usage of single lens reflex method. a manufacturer can install a bigger and better sensor to a non DSLR camera. –  sterz Aug 10 '12 at 23:03
    
1) "true-ness" ? This doesn't make sense. You need to qualify what you're talking about here. Are you saying DSLRs are truer? They aren't always -- low-to-midgrade dslrs don't have 100% viewfinder coverage. 2) you can't put a much bigger sensor on your compact MILC cameras because that would make them bigger. Look at a camera optics diagram. The bigger the sensor, the farther away your lens has to be, so it will have to grow not only in the 2 dimensions of the sensor, to accomodate the additional area, but also in depth. –  Kir Sep 20 '13 at 17:11

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