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I already have a Sony SLT camera but I still wanted to understand the differences between DSLR and SLT. It's hard for me to compare because I don't have or ever owned a DSLR so the behaviour is very foreign to me.

From what I understand in theory, this is what I'm guessing:

SLT:

  • Live exposure preview (changes according to ISO, aperture, shutter speed, WB)
  • Full time PDAF both in viewfinder and in liveview (lcd)
  • DoF preview button can be used in viewfinder and in liveview

DSLR:

  • No live exposure preview with viewfinder but preview in liveview
  • Full time PDAF in liveview but not in viewfinder
  • DoF preview button can be used in viewfinder and in liveview

If I understood everything correctly, here are my followup questions:

1) How does a DSLR do continuous focusing (AI servo/AFC)? Does it literally flip the mirror up, take the shot, put the mirror down, refocuses and repeat?

2) How does a DSLR user account for the fact that the OVF has significantly greater dynamic range than the actual user? Do you just get used to it and compensate mentally?

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Best practice would be to ask one question per question title. Not sure if I even understand the question #2. On your two lists of features I would add differensies in manual focusing. –  Esa Paulasto Apr 29 '13 at 1:37

5 Answers 5

up vote 4 down vote accepted

Short answer: the difference is in an SLR the mirror flips up to take a photo, whereas the SLT employs a semi-transparent mirror so that the mirror never has to move.

Now to your questions:

1) How does a DSLR do continuous focusing (AI servo/AFC)? Does it literally flip the mirror up, take the shot, put the mirror down, refocuses and repeat?

That's correct, assuming the DSLR is not in live-view mode.

If the camera has a live-view mode and that is turned on, then the mirror will stay up the whole time (and you won't see anything through the optical viewfinder at any time) and it'll switch to CDAF which is usually slower in a DSLR.

Wheras in the SLT camera it should be able to continuously use PDAF auto-focus while taking a shot or not.

2) How does a DSLR user account for the fact that the OVF has significantly greater dynamic range than the actual user? Do you just get used to it and compensate mentally?

Yes, you get used to it, just as you get used to judging the contrast of a scene with just your eyes - no camera at all.

Besides, even with cameras that have a live-view or digital viewfinder, you can't rely on the depth of blacks or contrast shown there to be indicative of the recorded picture - often sunlight, reflections and the relative quality of the LCD display make that hard. You still have to do some level of mental correction. There's also histograms and things that you can turn on.

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Your understanding is accurate. There are variations but the most significant difference is what you see in the viewfinder. This answer goes into all the details.

The other significant difference is that the PDAF sensors are available during exposure and video-capture. This lets an SLT camera measure AF while an image is being recorded and then it adjusts the focus quickly between frames. It also does this during video.

Continuous focus works with both cameras by constantly measuring phase and calculating the lens movement necessary to make the subject in focus. When the shutter is released, the mirror has to go up with a DSLR and thus the Phase-Detect sensor gets blocked. During the time it takes between the mirror going up and the photo being taken, some DSLRs using predictive calculations based on subject direction, speed and acceleration to establish where focus should be. An SLT does not need to predict because it can still measure all the way to the time of the exposure.

Where it gets more complicated is when combining continuous AF with continuous shooting. It would be bad for focus to be adjusted during an exposure, causing blur, so neither type of camera focuses then, they both have to do it during between frames. Only the SLT can measure during an exposure too but only if the aperture is bright enough. For this reason, SLT cameras shoot wide-open at their maximum frame-rate.

DSLR users do not see exposure before hand. They learn, over time, to trust metering and to know when it would fail and by how much. They also chimp much more. I can tell you that within a few months of having a DSLR, I can predict with 95% accuracy when the meter will be off and by how-much. However, I do check because I have no idea when the 5% case will occur. This was one of the most pleasant thing to discover with an SLT, no chimping required. The instinct was still there but it took longer to overcome.

Modern DSLRs have Live-View but only Canon has implemented an exposure-priority view among DSLR makers. With all other brands, shooting in live-view is inaccurate.

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Oddly, were quite able to operate satisfactorily when "chimping" meant a several-hour wait at best and every click of the shutter cost actual money (sometimes tens of dollars per click). We didn't do that by "trusting the meter"; we did it by understanding what was being metered and how that translated to tone placement. Meters don't "fail", but scene-analytic heuristics may, so you need to be able to sanity-check them. –  user2719 Apr 29 '13 at 2:55
    
@StanRogers - Thanks, for making me look for chimping. Always nice to learn new words :) –  Esa Paulasto Apr 29 '13 at 5:09

I would like to add that unlike a traditional DSLR, the mirror in an SLT is not essentual to the operation of the camera. It can be removed in a few seconds and carefully stored in many small compact flash card cases. The camera will operate without a complaint in manual mode. The contrast based manual focus assist will still function. The preview in both the EVF and on the LCD will still function accurately as well.

Why would one do this and risk damaging a delicate and fairly expensive (around $100 US) part? In reality almost no one does. The amount of light the semi-transperant mirror reflects is not linear... in lower light it reflects a lesser amount of light than one might expect, so the difference this little "trick" makes in over all image quality in low light conditions is small. It is much easier to simply change to a "faster" lens or use one of the in-camera HDR modes that can compensate by taking multiple exposures and then filter and combine to reduce the noise. Numerous people have posted their test results over the past couple of years on various forums. It is an interesting experiment.

Their are rumors that the next generation of "Alpha" cameras will not have a mirror at all. They will use sensors similar to the NEX series that are designed to work without a seperate autofocus sensor. I wonder if this is being done for marketing reasons. I fear that the response and accuracy of the autofocus will most likely suffer under certain conditions.

I have been absolutely amazed at the quickness and accuracy of the autofocus system in my SLT when taking video. Most people who take video with reasonably priced DSLRs don't even bother with the autofocus system while taking video if they need good results. There is nothing more frustrating than finding your camera was hunting for focus during an important scene when you are editing later.

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It's worth pointing out that many expensive DSLRs don't even have autofocus while doing video as an option specifically because the focus hunting would be a problem. –  AJ Henderson May 14 '13 at 16:21

You covered most of the advantages that SLT cameras enjoy over DSLRs, but there are a few other differences that give the traditional DSLR an advantage as well.

  • With an SLT camera, 100% of the light entering the light box never reaches the sensor. In brighter conditions this loss is fairly minimal, but in very low light conditions it can mean the difference between a usable shot and an unusable one.

  • All of the light that reaches the sensor passes through the translucent mirror. Because the mirror is exposed in the front of the light box near the flange that interchangeable lenses are attached to, both DSLR and SLT cameras are more susceptible to picking up dust on the mirror than on the sensor at the back of the mirror box (especially in the case of a DSLR with a shutter curtain covering the sensor assembly). With a DSLR this is not a big issue since the mirror flips up out of the way when the exposure is actually made. With the SLT, the dust will probably not affect image quality either until it has accumulated quite a bit. The problem is that the mirror in neither the DSLR or SLT should be cleaned with anything other than a small air blower, as the coating on the mirror is probably the most fragile part of either camera. If something worse than dust gets on the mirror of a DSLR it is not the end of the world. It may be annoying, but it has zero effect on image quality. On the other hand, if a large blob of material or, heaven forbid, a scratch winds up on mirror of the SLT camera, image quality is affected until the mirror can be replaced.

  • A direct optical path from the front of the lens to the eye of the photographer. The DSLR has it, the SLT doesn't. This is most critical in low light situations, as often the nuances that the human eye can detect and adjust for are outside tha ability of the electronic viewfinder to discriminate. Even if the sensor can't record some things, being able to see them may help with composition and timing, especially if part of your scene is moving. The optical viewfinder also draws much less current from the battery than an electronic display, which means more shots for the same number of milliampere-hours (mAh).
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Regarding the point that 100% of light doesn't reach the sensor, it would be relevant to point out how much is lost. If it's 5%, it wouldn't matter in most cases. But if it's 50%, that's a big problem. –  Kartick Vaddadi Jun 6 at 5:40

1) How does a DSLR do continuous focusing (AI servo/AFC)? Does it literally flip the mirror up, take the shot, put the mirror down, refocuses and repeat?

Yes. Camera also predicts the correct focusing point accounting for the delays of the mirrors and object's movement.

However some DSLR also have a "live view" mode which allows it to detect focus using subject contrast and tracking algorithms.

2) How does a DSLR user account for the fact that the OVF has significantly greater dynamic range than the actual user? Do you just get used to it and compensate mentally?

The camera does not do any compensations.

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