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qarin Sep 30, 2006 4:25 PM

This is perhaps an odd question... I'm not asking what's great about dSLRs vs. compact (or not-compact) non-interchangeable lenses cameras, or why a bigger sensor is better, etc. So there's probably something sort of obvious I'm missing...What I want to know is, why the SLR design for these cameras? Is there an advantage to the mirror, to the optical through-the-lens viewfinder, instead of an LCD viewfinder (and live display on the back of the camera LCD)? This would, among other things, allow the camera to do digital movies (like the compacts!), and allow shooting without peering (not that I'm uncomfortable with the peering, but I admit I got used to it with my compact digital cameras, and it seems to be a dying art among The Masses, and there are times when it's nice, especially with a display that can be turned, for over head or at the waist shooting).I just got a D50, after coveting a dSLR since the D1 came out, but being financially limited to lower end digital cameras (Ricoh RDC2e, then a Nikon CP990, then a Casio EX-Z750), which managed to push my 35mm SLR (a Nikon F4s, which superseded my first SLR, a Pentax K1000) to almost zero use, even with their limitations. The D50 is wonderful, feels so good in my hands after all this time, but I'm just idly wondering why the mirror is still there.

rjseeney Sep 30, 2006 4:38 PM

One of the big reasons is so all the old slr lens collections that many working pros/amateurs own would not become obsolete. THe company's would have been asking all former SLR users to ditch their old lenses and buy into a new system, starting all over again. I'm sure this would have upset alot of people. It also allows everyone that owned an slr to transition fairly easily to digital without completely changing bodies and unlearning years of habits learned on slrs.

qarin Sep 30, 2006 4:40 PM

I don't think I understand- why would losing the mirror necessarily change the lens mount?

Bob Nichol Sep 30, 2006 6:59 PM

First, the optical viewfinder in DSLRs is still far superior to the electronic viewfinders at this time. (This has been discussed in many threads) The mirror is required to be able to view through the lens. Once the EVFs get as good as OVFs then you may see new paradigms in camera design.

Second, the presence of the mirror has imposed certain restrictions on the lens design such as the distance between the sensor and the lens. This has caused design problems getting good wide angle performance for example. (Look up retrofocus lens design) See any discussion of rangefinder and SLR/DSLR lens quality, particularly for wide angle lenses.

A camera without a mirror would be able to have the lens get closer to the sensor. (See a review of the Sony DSC-R1 for example.) This would permit better designs for many lenses.

Having the same mount would cause mechanical compatibility problems so a new mount would be required.

eric s Sep 30, 2006 7:50 PM

Maybe wrong, but I took the question to be "why build a camera that looks like an SLR?" (not why use the same lens mount.)

And to my mind, there isn't a great reason, other than there is a lot of momentum in that direction.

I believe there was a SLR produced that didn't use a mirror. It used a frosted prism (or maybe some kinda penta prism system.) It reduced the amount of light by around 1/3 a stop or so, but it was a near silent camera (not mirror noise at all, only film advance noise.) Loosing the light effects things, so the photographer would have to adjust. I don't believe it was very popular... I've only read about it.

One very big reason why SLR's don't have a live video mode is that their sensors are not designed for it. It reduce image quality (I'm sure technology will eventually fix that, but not yet.) The problem is at least partially related to heat... having the sensor "live" all the time - to update a EVF or live display or in a movie mode - causes the sensor to heat up. This reduces image quality. It's a classic problem in astro photography, where the telescope mounted cameras are kept cool with things like liquid nitrogen (I'm not talking store bought telescopes, I'm talking college or big science lab scopes.)


amazingthailand Sep 30, 2006 8:30 PM

The mirror is still in the dSLR because the SLR design has separate dedicated sensors for autofocus and exposure. Both of these are much more sophisticated than the software solution p&s cameras must use. The mirror is used to direct the incoming image to those sensors as well as to the OVF. The AF sensors are located in the bottom of the mirror chamber, at least in Nikon cameras.

P&S cameras use software and the image on the camera's sensor to determine autofocus and exposure.

And then there is the lens issue and clarity of the OVF already mentioned.

Widowmaker Sep 30, 2006 9:05 PM

Other than the obvious already stated... shooting via LCD is not ideal and is a pain under certain lighting conditions. Its also alot easier and faster to find and frame your subject looking through the viewfinder. Not really an answer to the mirror question but sums up the LCD as viewfinder option.

I started with digital P&S and then went to DLSR. The combination of the DSLR size and layout of the controls makes it easier to operate.

qarin Sep 30, 2006 9:23 PM

Interesting answers! So, it seems that it's a technology thing- I was thinking the technology was there but unused, but it sounds like an electronic viewfinder would simply not be fast enough to keep up with the large, fast sensors used in dSLRs, but that it's likely coming.I agree that using a good viewfinder is much nicer than using the back of the camera LCD in quite a few situations, but it would be nice to have the option.Thanks for the thoughts.

VTphotog Sep 30, 2006 9:32 PM

The main reason for digital SLR is that when the camera companies did the market research, this is what people (not knowing any better,) said they wanted. "Something just like my film camera, but digital." High quality cameras, to most, are SLRs, so that is what they asked for. People also tend to be conservative and more willing to adjust to incremental change than radical change.

It would be quite possible to build a camera like (for example) the Sony R1, with interchangeable lenses on the Sony/Minolta mount. Since those lenses are make with a focus distance which takes into account the mirror, there would be some extra space available which could be (possibly) used for an environmental seal which would prevent dirt/moisture from entering camera, even with lens removed.

Why doesn't someone make one? Someone (and possibly many) will, eventually. Probably part of long-term marketing strategy. Incremental change, not radical.


Bob Nichol Oct 1, 2006 11:12 AM

SLR cameras have always had mirrors. Some, such as the Canon Pellix and the Olympus E10/E20 used optical beam splitting rather than a swinging mirror but this would still occupy the same space in the optical path.

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