Spring 2012

Spring 2012
by ani

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I understand that back in the days when cameras captured images using photographic film instead of image sensors, the SLR design was a major innovation. It let you see through the viewfinder exactly the light that would be passed on to the film. Assuming you care about accurate photography, that's kind of a big deal.

Nowadays, however, cameras that use film are essentially specialist/niche products, and the vast majority of photography is done using digital cameras. And with a digital camera, you don't need a hinged mirror as you can show the user exactly what light will be captured by just routing the sensor output to an LCD display. Having a mechanical component that needs to be able to move around in a very precise manner and can break or fail seems like a very large liability. That leads me to some questions:

  • Why do manufacturers continue to build SLR mechanisms into their digital cameras, particularly at the top-end of their product lines?
  • Why do photographers seem to heavily prefer DSLR cameras over digital point-and-shoot models that offer the same features but without the SLR mechanism (there are, for instance, full-frame point-and-shoot interchangeable lens cameras available, though it's not clear if they're very popular among photographers), to the point where "DSLR" is almost synonymous with "serious photographer's camera"?
  • Is there any significant benefit to having an SLR mechanism in a digital camera? Particularly in terms of a benefit that's large enough to make up for the liability of adding a mechanical part into a design where a solid-state alternative is available?
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Duplicate of "Why the need for SLR in digital cameras?" photo.stackexchange.com/questions/26117/… – Mike Sowsun Feb 7 at 3:37
    
    
When you wrote "point-and-shoot interchangeable lens cameras", the current preferred term is "mirrorless interchangeable lens cameras". – Nayuki Feb 10 at 15:28

14 Answers 14

up vote 52 down vote accepted

And with a digital camera, you don't need a hinged mirror as you can show the user exactly what light will be captured by just routing the sensor output to an LCD display.

This is the reason for the rise in popularity of mirrorless interchangeable lens cameras (MILC). Without the mirror box, the camera can be smaller, lighter, less expensive, etc.

Having a mechanical component that needs to be able to move around in a very precise manner and can break or fail seems like a very large liability.

Probably not as much as you'd think. These same companies have been building SLR's with mirror boxes for decades, and they've gotten pretty good at it. There may be occasional mechanical failures, but at this point the mechanisms tend to last a lot longer than the useful lifetime of the camera. In other words, customers will want to replace the camera for other reasons (e.g. better sensors, more features, etc.) before the mirror mechanism fails.

Why do manufacturers continue to build SLR mechanisms into their digital cameras, particularly at the top-end of their product lines?

The main reason has to be that it's what customers want. DSLR's evolved from film SLR's, and photographers still want to buy cameras that let them see what they're shooting through the lens.

Why do photographers seem to heavily prefer DSLR cameras over digital point-and-shoot models that offer the same features but without the SLR mechanism

You answered this yourself pretty well: It let you see through the viewfinder exactly the light that would be passed on to the film. Assuming you care about accurate photography, that's kind of a big deal.

If you're not looking through the lens, you're seeing some digital interpretation of what the scene looks like. Electronic viewfinders (EVF) have improved tremendously in recent years, and they do have the potential to show you what the sensor will record, but that's not the same thing as seeing what's visible through the lens.

(there are, for instance, full-frame point-and-shoot interchangeable lens cameras available, though it's not clear if they're very popular among photographers), to the point where "DSLR" is almost synonymous with "serious photographer's camera"?

There are definitely "serious photographers" who have switched to MILC's. David Hobby and Zack Arias are two examples of well-known photographers that use Fuji mirrorless cameras. However...

There's a lot of inertia that will need to be overcome in order for DSLR's to really lose popularity among professional photographers. Lenses present a huge obstacle -- photographers already have large investments in lenses, and manufacturers have large existing lines of excellent (and profitable!) lenses for their DSLR lines. If photographers start jumping ship from Nikon or Canon to Fuji or Sony, and if Fuji and Sony can provide lenses that the pros need, then Nikon and Canon will surely start producing more top-end lenses designed for (and not merely adapted to) their mirrorless lines.

Is there any significant benefit to having an SLR mechanism in a digital camera? Particularly in terms of a benefit that's large enough to make up for the liability of adding a mechanical part into a design where a solid-state alternative is available?

Again, I think you may be overstating the liability of the mechanical system. These things work really well. So, let's turn your question around and look at the other side: Is there any significant benefit to changing the reliable and well-understood DSLR design? Obviously, the answer is yes, because MILC's are getting some real traction in the marketplace, but at the same time the answer is not YES!!!, probably because the liability is not as great as you imagine.

My own feeling is that a much more interesting question is: Will DSLR's ever get electronic shutters that could give them much higher flash sync speeds, faster burst modes, and faster shutter speeds? I think you'll see that happen before MILC's unseat DSLR's.

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@aroth: It's funny that you bring up SSDs, because they have a liability similar to the mechanical shutters in SLRs: the storage inside has a more-limited lifetime than magnetic media. Neither SSDs or SLRs are good choices for high-rate, highly-repetitive write/capture applications. – Blrfl Feb 7 at 12:02
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@aroth: SLRs are no different; mine all have a predicted shutter life. Photographers needing their gear to be highly-available take steps to make that happen just like people with data: they carry low-frame-count spares, send high-count bodies in to have the mechanical parts replaced or buy new bodies. – Blrfl Feb 7 at 13:23
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A few Nikon DSLRs from ~10 years ago used electronic shutters and had high flash sync speeds. My old D40 can sync at 1/500, which is as good or better than any leaf shutter I've used. It was nice, but it wasn't a killer feature. – Dietrich Epp Feb 7 at 17:11
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@DietrichEpp The D40 is actually capable of syncing at much higher speeds (e.g. 1/4000 or 1/8000) if you go manual and don't use TTL. 1/500 was just the official "limit". I believe that was a benefit of the CCD sensor type and not available with the newer CMOS sensors. – requiem Feb 9 at 6:18
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@JDługosz sort-of; The high-speed sync feature (built into flashes) you are thinking of is a different feature designed to work around the rolling shutter effect from a mechanical shutter. The D40 used an electronic shutter for higher speeds, and the mechanical shutter remained fully open while that happened (so no front/rear curtain effects). The only real limitation is the time it takes for the flash to dump all the light, so a flash at full power might take 1/1,000th of a second to dump all its light, but dial it down to 1/16th power and it'd only take 1/10,000th. – requiem Feb 10 at 17:51

Is there any significant benefit to having an SLR mechanism in a digital camera? Particularly in terms of a benefit that's large enough to make up for the liability of adding a mechanical part into a design where a solid-state alternative is available?

Yes. Response speed for both autofocus and shutter release.

The mirrorbox has a number of side effects that aren't self-evident. Like the ability to use a completely separate autofocus sensor array. dSLRs, for the most part, do NOT use the main image sensor for autofocusing, the way mirrorless and compact digital cameras do. Phase detection autofocus sensors are in a completely separate array on the floor of the body and the mirrorbox is actually used to direct some light from the lens down to that array as well as up into the viewfinder.

Mirrorless and compact digital cameras tend to have additional shutter delay because composition must be done through liveview, and to avoid a ghosted image, all charge must be cleared from the sensor before the main exposure is made. A dSLR's optical viewfinder doesn't require this. With the mirrorbox and mechanical shutter in front of the sensor, the sensor itself does not need to clear any residual charge before taking an image unless liveview is used. This increases shutter responsiveness.

While there are strides being made in introducing phase detection from the main image sensor and shutter delays are reduced, dSLRs are still the tool for choice for fast action photography. Tracking autofocus performance, and autofocus speed are still better with dSLRs.

In addition, the use of older film-era technology also means compatibility with film-era gear. dSLR cameras can typically use film-era lenses in the same mount system with full native compatibility (including autofocus). Mirrorless cameras, while they can use adapted manual-only lenses with limited function, typically only have full autofocus function with lenses in systems that are only 5-8 years old. Nikon and Canon dSLRs are still part of the largest camera systems in existence, with the largest number of native-mount lens choices.

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Not to mention lag in the viewfinder itself. That's gotten a lot better in recent years, but it's still the case that everything you see on the LCD screen is inevitably delayed from the real world. – mattdm Feb 7 at 6:19
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Yep, good answer here too. If I could accept both this answer and Caleb's, I would. – aroth Feb 7 at 12:33
    
To add to your paragraph about the mirror box, the specialized autofocus sensors on SLR cameras are much faster and simpler (line sensor) than the autofocus mechanism on mirrorless cameras. This is why sports and action photographers use SLRs. – Nayuki Feb 10 at 15:32

Don't forget a major drawback that EVF's require power to compose, and are much harder on batteries if you spend a lot of time with the screen on.

Also, as previously mentioned, because of delays, it is harder to follow moving objects with an EVF.

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+1. I don't think there's a single mirrorless camera out there that can survive 1000+ frames with a single battery charge. One of the reasons I'm hesitant to put my DSLR away... – progo Feb 7 at 8:34
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@progo: an extra battery is much lighter and smaller than the extra weight and space the mirror requires. – Ross Millikan Feb 8 at 4:42
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I am not sure aI buy that. I mean, seriously - ever tries using an optical fview finder under less than optimal light conditions? WHen you pump up the ISO a little (800) and use possibly some flashes and the OVF is DARK - but the EVF compensates? I can "see" In the EVF in near total darkness. I gladly carry one or two small batteres with me as change for that. And not talk about manual focus - with 10x magnification. – TomTom Feb 8 at 10:00
    
@RossMillikan: true, MILC/PnS with extra batteries is lighter to carry than a bulky DSLR, but then there's the circus of charging each battery at the end of the day. – progo Feb 8 at 10:32
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@TomTom To each his own. I'd rather let my eyes get acclimated to the dark and not have my night vision ruined every time I looked into the viewfinder. – Michael Clark Feb 8 at 23:31

Another disadvantage of using the sensor to generate a near real time preview in lieu of an optical viewfinder is the requirement to keep the sensor energized continuously. In addition to the increased battery usage, over extended periods of time this tends to build up heat which, as we all should already know, can affect read noise and thus the signal-to-noise ratio of a sensor. It's not much of an issue if you only shoot for a few minutes at a time. But it becomes a huge issue if you need to provide a continuous image preview for longer periods of time. This is particularly true when the shooting environment is already much warmer than during typical use. When television production companies first started using DSLRs to "tape" TV programming, they would keep multiple copies of each camera model on set. After each 10 minutes or so of use, the sensors heat up enough to affect sensor read noise, and the camera is rotated out and allowed to cool before being used again.

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Also there is problem, that the LCD on the back of camera does not have the resolution of main chip. So it shows you inaccurate image in much lower resolution, than would be then taken. Also the range of LCD values (from totally dark to totally light) is lower than the chip have - so another source of inaccuracy - naked eye is much better then cameras/LCDs in this way.

So the life view takes more energy, make more heat (and noise) and does not show accurately, what picture will be taken.

There are some situation, where it does not matter (and smaller and cheaper cameras are good enough) and there are some situation, where it matters.

As long, as there will be request for continuing this line and the line will be viewed as higher and better (and those, who think it matters would be ready to spend a lot more for their cameras and equipment), so long the manufactures will be making and selling it.

(I have smartphone with LV camera and it is good for everyday use, live remembering opening hour of near shops, making copy of paper prints and prices in shops, taking photo for position of flowers on garden and so. But I have also SLR camera which costs like 20x more (I know, cheap and easy model) and it have its use too - I am able to get details the smartphone is not just able to, make portraits, play with Depth of Field, get much higher resolution, where it matters, make much better macros set parameters of the picture much more accurate (and then postprocess it even more) so it have its value for me too. I Tried to use it in life-view mode, but it does not nearly so good as the SLR mode, when it came to details - and it is, what matters for me here)

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The point about the LCD image resolution is irrelevant, because on an SLR the optical focusing screen has a very limited resolution too. – Nayuki Feb 10 at 15:34
    
I think, you are mistaken - SLR have no optical resolution, there are no pixels. You are right, that the image is small, but that is something totally different. The first thing, that have there "something like" resolution in the sense X*Y pixels is the eye, everything other is just plain optic. Like glases or window - there is no resolution, just size. – gilhad Feb 11 at 19:02
    
You are only half correct - the mirror and pentaprism are full resolution, but the ground glass focusing screen has a limited resolution. This impacts me in practice because when I do manual focus, I actually get more accurate results when using the LCD (without even zooming in) than through though the optical viewfinder. – Nayuki Feb 11 at 20:59

Perhaps it's that I wear specs and can only see the end of my nose, but isn't one of the most obvious advantages of an optical view finder that you can actually see the image in daylight?

I'm a D80 user, and I've stood behind others with their smart phones and point & press cameras and looked at their screens. It's just a black shiny square. You might glimpse some highlights (which you're over exposing anyway but what smart phone photographer cares?) I'm not sure which is worse, the sun in front or behind the screen. Either way, it's the primary reason I've not switched to a smaller camera.

When I've gone all the way to Giza, I'd like to be certain that I've got the pyramid in the shot. The viewfinder allows me to frame the shot as I'd like.

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That's an issue with the big screens that you view from a distance, not so much of an issue with an electronic viewfinder that you hold up to your eye. – Peter Green Feb 8 at 9:00
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The converse is also true. Shooting long exposures in the dark, your eye may be able to see some of the bright spots (street lamps, brightest stars, horizon...) for composing; but with an electronic viewfinder you are composing in the blind. – Davidmh Feb 9 at 11:13

Also consider low light and long exposure scenarios - at ISO 100 f8, the live preview is going to show you a pitch black screen - which makes it pretty difficult to frame and focus! Your eye through a viewfinder of a SLR or SLT is still going to be able to see the scene, even if you are relying on distant street-lights to accurately focus

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When you look at technology you can not decide better by just taking into account what is newer.

New != Better

What you need to do is take a look at the overall architecture. In the case of the SSD versus the harddrive its undeniable that a SSD has a better overal architectural design. Its not a question of mechanical versus electronic components per se but a paralell design versus a serial design. The SSD architecture is ultimately more flexible thus can offer more growth.

Now the reflex mirror is not so clearly a worse design. The mirror can be turned away thus anything you can do without a mirror you could do with a mirror. The mirror uses up some space and add cost true, but it has some benefits as you can reroute light to different destinations giving you architecture fexibility (as noted it can be used for things like autofocus, but in furure for other things too). As with SSD we know that cost is not nesseserily a determining factor. Size and weight might be a big reason for some usecases.

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Neither SLR nor SSD technology is exactly new. SLR cameras started gaining real momentum in the 1960s, and it can be argued that early digital computers had only "SSDs" for internal storage (or, well, maybe excepting those that didn't use delay-line memory and things like that, but if we use SSD to mean "digital storage with no inherent moving parts"). – Michael Kjörling Feb 9 at 19:29
    
@MichaelKjörling nothing is truly new often the technological ideas existed really long time before final conception and breakthough. – joojaa Feb 9 at 19:53

In short, current sensors and lens-systems are very close to being up to the task of replacing the film-era mirror & prism, but have a few remaining limitations that, for some shooting conditions, make the SLR worth the extra size of both the body and the lenses.

I'll add that I thought up-to-the-eye viewfinders were a film-era throwback, until my eyes got older and I noticed that the back now LCD requires reading glasses.

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I leave my mirrorless on peephole mode and don't show the image on the back. That way when I get someone to take a picture of me, he'll naturally use a more stable posture! Arms out holding up a largish camera is not stable. Holding it against your face, elbows tucked in, is much more stable. – JDługosz Feb 10 at 17:01

DSLRs ... having worn out 5 Nikons, 2 D100s 1 D5100 2 D7100 I can tell you it is the shutter that fails and not the mirrors. With no exceptions. For many reasons I feel that the interchangeable lenses are the real advantage to the DSLR, and to see exactly what the image will look like. For those who understand depth of field this is critical. Stopping down is the added bonus. The viewfinder uses no power. The LCD display however uses most of the power, and is difficult to see in bright conditions, unless you crank up the power for the backlight.

The real difference is the quality of the lens and lack of the many abberations associated with a snaphot selfie box.

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The root of the issue is the assumption that the LCD display is as good as the SLR display. That assumption is incorrect.

The many reasons for this are listed in other posts.

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Here's a good article on the question asked. Not so much the 'why not' (mirrorless) part, because the CSC (compact system cameras) is closing the gap fast.

I read the article, and the bottom line is, that the pros and cons of both types are not substantial. They are comparable in price, quality and versatility (features), a bit less in weight (though large lenses make that a smaller factor), and that the only really substantial difference is in battery life (advantage for DSLR).

Having said that it is a good article, two things:

  1. Nothing found about the sensor noise, for perfectionists a reason to stick to DSLR.

  2. There are very high quality mirrorless system cameras with an optical viewfinder. They were around before the digital era, the Leica M series for example. Of course those cameras don't have the quick battery draining when only switched on, nor do they heat up the image sensor all the time. The digital Leica M has a basic model (still $5k+) without live view, even as an option, though it has a back display. Modern optical viewfinders can automatically adjust to the lens mounted.

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I've been using a "mirrorless" α 6000 which is considered a high-end among things that aren't SLR, in addition to my dSLRs. I find it's not quite there yet.

The EVF could be higher quality. They actualky reduced the pixels in the 6000 compared to the previous. If you weren't trying to make the product as small as possible, you could design a product with a killer display, both eye-piece and screen, and key off the advantages of a EVF like fancy heads-up display, zoom-in focusing, and light boosting in dim light, and not really miss the optical display.

I think a live optical view gives more subtle tonal response. But comparing my tablet screen to the clunky things used in EVFs, I dare say the technology is available to do far better.

The main issue with the Sony mirrorless is that it takes a moment to come on. The dSLR "boots up" instantly when the power switch is worked or it wakes up from autosleep.

Why would the mirrorless be slow to turn on when it shoots faster than a dSLR? I don't know.

In short, the products have not been developed along those lines. It would be a novel design and new product line to invent a system that was not based on old SLR (I.e. "mirrorless") but not trying to make focus the product goal on being as small as possible.

If you didn't care about being super-small, to the extent of being the same size as the dSLR; well, that's the "live view" mode. If you got rid of the mirror and used a high-quality EVF eyepiece (as well as screen) that might be too much like a dSLR to bother. Just use a loupe stuck on the screen instead.

If you designed a new lens system to be for mirrorless, but left the camera body the same size as a crop-sensor body in the other dimensions, you'd have room for the dedicated buttons and knobs and room inside for better EVF and bigger battery; it's awkward to hold a flat box anyway so add a grip...which is the depth of the box on a normal dSLR now!

Saving 2cm thickness of the body when a lens is larger than that anyway doesn't sound like a different enough product to matter.

Some people have mentioned the autofocus: note that "live view" (and video) focusing has improved anyway. It's gotten to the point that using touch-screen select to focus and face tracking is a trade off from the still-better dedicated focusing sensors, and live focusing is generally good enough. But as noted earlier, it's nice to have both the live-view mode and the optical mode available and this is another advantage to keeping the mirror.

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Is there any significant benefit to having an SLR mechanism in a digital camera? Particularly in terms of a benefit that's large enough to make up for the liability of adding a mechanical part into a design where a solid-state alternative is available?

Ultra-fast auto-focus and EVF battery usage. Lastly, many lenses are designed around an SLR mount and the distance away from the film/sensor. When sony went full frame/EVF thus abandoning the SLR format in it's professional cams, it had to develop an entirely new lens line up from scratch. So far the high prices and poor selection of lenses means many professionals will stay away for many years. Not to mention the pissed off pros who invested in the sony alpha SLR line up.

Many pros have $5-10k+ worth of lenses....if Canon/Nikon/Pentax did what sony did then everybody would either have to use an adapter(poor AF performance, less IQ) or drop tons of cash on new lenses. Even worse, the wait for new lenses takes years....Sony released 15 FE lens in the past 3 years. Canon and Nikon have hundreds of lenses available used and new at all price points.

In 10 years, I suspect EVF and sony's non slr format to be the new norm. To early to drop $5k switching over for less performance per dollar.

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