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I know the differences between a rangefinder and a SLR/DSLR but what is the real reason to put a mirror in front of a lens to reflect light into the viewfinder?

It raises lens prooduction price when you put extra distance between lens and sensor. So why do that? Does it improve image quality somehow? Isn't it logical to use rangefinders only?

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I fail to find a term Parallax error in any of the answers. Without it being mentioned I think they all are lacking something. –  Esa Paulasto Dec 10 '13 at 11:09
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3 Answers

up vote 11 down vote accepted

Simple, it allows you to see exactly what the camera will "see" when you expose the shot.

Nir has given you a part of the argument as well which is accuracy. In the "middle ground" of anything around mabye 20-100mm, building a rangefinder is not too difficult and Leica had adapters for longer and wider lenses if I am not mistaken. It takes some effort to calibrate but is doable.

However with an SLR you can use even more extreme focal lengths - try a 7mm or 15mm Fisheye lens, how do you get that into an extra viewfinder (which incidentally needs a similar lens system). Or maybe a 400mm, 800mm lens? Every system is a compromise somewhere - and using a mirror to reflect the light from the lens to the viewfinder (or focussing screen if one is finicky) allows the user to fully exploit the flexibility that is offered by the range of available lenses.

Coming back to rangefinders, you might have noticed that lenses typically span around 20-135mm and I think there are is at least one lens that offers 17mm on Leicas as well.

The extra distance between the sensor and the lens is itself also not disadvantageous given the way that sensors are designed. Leica's sensor uses a specially shifted micro-lens arrangement to improve the light gathering capability of the sensor. Now whether it works is another discussion, in theory it should. Moving the lens closer to the sensor means that the light hits the sensor at an oblique angle rather than a near right angle. Given the nature of "light wells" on a sensor, this increases light loss (hence the shifted microlenses on a Leica).

(This is actually the basis for this issue: http://www.luminous-landscape.com/essays/an_open_letter_to_the_major_camera_manufacturers.shtml )

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+1. However in case where modern cameras have live-view (LCD) display, is the lense reflecting kind of irrelevant? –  Dipan Mehta Jun 11 '13 at 19:11
    
Not really. A live view screen can be hard to see in bright sunlight) though they are getting better - OK, there is an electronic viewfinder for that. However electronic viewfinders (or LiveView) always induces some lag - not a problem for landscapes, but a problem for any fast paced motive (e.g. sports photography). In addition, one does not have a resolution limit in an optical viewfinder - or rather, the resolving power of the human eye is the limit. LiveView has its uses, but it is not as good as an optical viewfinder. Oh, and the "regular AF" is a lot faster than LiveView AF. –  DetlevCM Jun 11 '13 at 20:02
    
I might be mixing up the terminology, but I think it is phase detect without LiveView and contrast based with LiveView. –  DetlevCM Jun 11 '13 at 20:02
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Simple — you can't make an exchangeable lens rangefinder where the viewfinder is even remotely accurate. (Well, you can't without digital technology and live view — and then the mirrorless cameras makes more sense than rangefinders.)

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Its tradition. When we had film cameras, a mirror system was used to give the photographer an accurate image of what the lens was seeing irrespective of which lens was attached. Obviously one could have a system with a live viewer. Old time photographers are used to looking in a view finder and composing an image.

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Would you say there are no advantages, and that it's only tradition and the habits of "old time" photographers? –  mattdm Oct 2 '13 at 22:25
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@mattdm .. maybe the question today should be, now that we do digital, why not do live view on everything. Obviously some cameras do and some do not. I've yet to see one do live view through a classic viewfinder eye lens, though. That would be an inside LCD feeding the viewfinder mirror, perhaps. Still, I cannot pinpoint a specific advantage to using a fully optical path to the eye, anymore. –  Skaperen Oct 6 '13 at 2:23
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