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| Rainbow Lorikeet photographed with a Canon SX60, a FZLC |
Last week someone posted a question on a photo user forum asking about the difference between cameras with and those without a mirror.
Old guys like me who have been using cameras for 60 years need to be reminded occasionally that not everybody has a long personal history of camera use and that many people wonder what all the mirrorless debate is about.
So I put together this brief summary of the characteristics of each type, with a third one thrown in for good measure.
Let’s start with a little acronym entrée:
ILC: Interchangeable Lens Camera
SLR: Single Lens Reflex (Camera)
DSLR: Digital Single Lens Reflex (Camera)
MILC: Mirrorless Interchangeable Lens Camera
FZLC: Fixed Zoom Lens Camera
PDAF: Phase Detect Autofocus
CDAF: Contrast Detect Autofocus
OVF: Optical Viewfinder
EVF: Electronic Viewfinder
The SLR Camera This is a type of ILC which arose in the mid part of the 20th Century. It was and to some extent still is, an elegant solution to several photographic problems:
* How to provide a range of lens focal lengths from wide to long, there being no zooms when the SLR was invented.
* How to preview the subject correctly oriented in a viewfinder then focus and frame accurately.
* How to achieve accurate exposure metering through any lens mounted.
* How to package all this in a compact device at a price point accessible to ordinary people.
The SLR is a quintessentially 20th Century invention. It relies on mechanical and optical solutions to the problems presented.
In due course camera makers replaced the film with a digital sensor and added electronic operation but otherwise kept the same basic design, thus creating the DSLR.
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| DSLR Schematic |
This is a very basic schematic of a DSLR.
When the flipping mirror is down, as shown here, light comes from the subject through the lens and strikes the mirror. Most of the light is reflected up to the focussing screen where it forms a visible image which is inverted laterally. Some light is passed through to the sub mirror and thence to the Phase Detect autofocus module.
Light passing upwards from the focussing screen bounces around the pentaprism (or mirrors in some models) and in the process the preview image becomes correctly oriented laterally and vertically.
Some of the light is picked up by another little mirror and deflected to the metering sensor.
The user looks in the eyepiece of the optical viewfinder to see a preview of the subject, with adjacent camera data which adds further complexity not shown in the diagram. The focal plane shutter is closed.
In this mirror position, image preview, exposure metering and (auto) focussing take place.
When the user presses the shutter button several things happen very quickly.
The flipping mirror lifts up, the sub mirror flips up with the main mirror and the operator’s view is blacked out for a short time.
The shutter opens, the imaging sensor is exposed to light.
Then the shutter closes to end the exposure and the flipping mirror drops down again, allowing the cycle to repeat.
A DSLR can also operate in monitor view mode with the mirror held up, the shutter open and light passing directly to the imaging sensor. In this mode a DSLR works very much like a MILC. The image preview, metering and focussing are all generated directly on the imaging sensor. Image preview data is fed electronically to the rear monitor.
You can think of a DSLR as in effect two camera types in one body, one with an optical viewfinder, the other with an electronic viewfinder (the monitor).
DSLR Variant: Sony SLT
Sony is the only manufacturer to offer this type of camera. It is a DSLR but some of the inner workings are different from the standard version.
The main mirror is a fixed pellicle type, not flipping. It transmits about 70% of the incoming light to the imaging sensor and reflects the remaining 30% up to a Phase Detect autofocus module located about where the front of the pentaprism would be on a conventional DSLR. There is no focussing screen. Autofocus can operate continuously with no interruption for image capture.
The viewfinder is Electronic, receiving data from the imaging sensor just like a MILC.
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| MILC Schematic |
The MILC is a creature of the 21st Century. It replaces the fixed and moving mirrors and other optical parts of a DSLR with electronic data processing and transfer.
You can see the MILC has fewer mechanical and optical parts than the DSLR, but it is more reliant on electronic data transfer and processing.
The body has less depth because the back focus distance is less, there being no need to accommodate the flipping mirror and focussing screen.
It may also be possible to reduce the height a bit as many EVFs are smaller than a pentaprism or mirror and there is no need to find room for the PDAF module in the base of the camera.
Image preview can be fed to the EVF and monitor simultaneously although the camera is usually set to display one or the other to save power.
All metering, CDAF and/or PDAF and image capture take place on the imaging sensor.
The focal plane shutter is open when previewing the subject. If the shutter is of all mechanical type it closes when you press the shutter button then opens to make the exposure and closes to end the exposure. Some shutters have an electronic exposure commencement (a.k.a. electronic first curtain) and mechanical exposure termination. This is desirable to prevent shutter shock, see below.
Mirrorless variant: The FZLC
In this style of camera the zoom lens is not interchangeable. There is no need for a lens mount. The rear element of the lens can be quite large with a diameter equal to or greater than the imaging sensor, providing some optical advantages. This rear element can be very close to the sensor allowing further reduction in the overall depth of the camera.
The shutter is usually a diaphragm leaf type located inside the lens. This allows for further reduction in bulk of the unit.
Which is best ? Each type has advantages and disadvantages.
DSLR Advantages:
* For most non professional photographers I suspect the main advantage of the DSLR is incumbency. It is an established and widely recognised camera type. I suspect lots of people ‘know’ or think they do, that the DSLR is the camera type to get if good quality pictures are desired.
* Pro-Am and Pro level DSLRs can effectively follow focus on a moving subject with an appropriate lens fitted. Entry level DSLRs and many MILCs are less competent at follow focus, although MILCs are catching up with every new model generation. The situation is not static.
* The long established DSLR makers have a huge inventory of lenses and other accessories from which consumers can choose.
* Some users still insist that the OVF of a DSLR is preferable to the EVF of a MILC. That was true a few years ago but the new EVFs are now so good that in several respects the better ones have the advantage.
DSLR Disadvantages
* A DSLR requires extremely accurate location of several moving and still optical components. There is a manufacturing cost to this and also variation between samples. This leads for example, to each lens requiring AF micro adjustment on each separate body. This adjustment is not offered on entry/upper entry models.
This is a problem with accuracy which is the degree of closeness of a measurement to the true value, or in this case the closeness of the focus point to the correct one.
No such adjustment is required on a MILC or FZLC which measure focus directly on the imaging sensor.
* The PDAF system used on a DSLR is inherently faster but delivers more variable results than the CDAF used on MILCs.
This is a problem with precision which is the degree to which repeated measurements give the same result.
The CDAF system on MILCs and FZLCs is inherently more accurate and more precise than the PDAF system used on DSLRs.
That, by the way, does not mean every MILC and FZLC has more accurate and more precise autofocus than every DSLR. Some MILCs have CDAF implemented badly which is no use to anyone.
* Mirror slap. The flipping mirror can cause vibrations which can lead to blurred images at some shutter speeds, usually around 1/8 second and therefore not usually seen with hand held pictures.
* Cost: (soon) For the moment, MILCs are either equal in price to the consumer or even more expensive than DSLRs. Makers of MILCs are playing catch up in several areas of technology at present, adding to R&D costs but I expect that in due course it will be less expensive to make a MILC than an equivalent DSLR as there are fewer parts.
* Much has been made by manufacturers and users of the size/mass advantage which MILCs have over DSLRs. This is real but not as great as some MILC enthusiasts might have you believe. MILC bodies can definitely be smaller for any given sensor size, but the lenses are generally not and lenses make up the bulk of a multi lens kit.
Sony SLT advantages/disadvantages
* The main advantage would be the ability to perform continuous PDAF. This should benefit follow focus with continuous autofocus and high frame rates. There is no mirror slap.
There are also fewer moving parts than a DSLR.
* I can think of several disadvantages though.
That pellicle mirror is always between the subject and the sensor, collecting dust and potentially degrading image quality.
There is little if any size/mass advantage over a DSLR.
But perhaps the main one is ongoing uncertainty regarding Sony’s intentions about the SLT type and about the A mount in general.
MILC advantages
* I expect that in due course mirrorless cameras will be less expensive to make and buy.
* Size/mass is a minor advantage as pointed out above.
* Advantages of the EVF and the ability to configure the monitor and EVF to display the same information in the same way making for a seamless segue from one to the other.
* More accurate and precise single shot AF.
* In the end I suspect that mirrorless cameras will prevail at least for amateur buyers simply because gadgets in the 21stcentury are characteristically electronic and DSLRs are at the core mechanical devices with their roots in the 20th Century.
MILC disadvantages
* Mirrorless cameras are still playing catch up to pro level DSLRs for follow focus on moving subjects.
* Refresh rates on even the best EVFs have still not quite caught up to the DSLR OVF.
These issues still make Pro and Pro-Am DSLRs preferred for sport/action/wildlife/bird photography.
The gap is closing however as rates of electronic data processing and transfer increase with each new generation of mirrorless camera.
* ‘Shutter Shock’. Some cameras with an all mechanical shutter display blurring of the image, sometimes with double imaging, with some lenses at some focal lengths and some shutter speeds, usually in the range 1/20 to 1/200 second. This is caused by the first ‘shutter close’ action vibrating the camera/lens unit.
What about the FZLC ?
With recent developments in the technology of small imaging sensors and high range zoom lenses using aspheric elements some of the latest FZLCs are starting to look like an attractive alternative to an entry/upper entry DSLR.
The main advantage of this type is that it represents an ‘all-in-one’ solution to many users’ desire for a go-anywhere-do-anything camera with no need to carry or change lenses.
FZLCs typically have a greater zoom range in a more compact, lighter, less expensive package than any superzoom lens on a DSLR or MILC.
The leaf shutter is compatible with flash at all speeds, is usually very quiet (most of the ‘shutter sound’ you hear is electronic and artificial) and does not cause shutter shock.
The disadvantages are that there is no option to change lenses and high ISO image quality of FZLCs is not as good as that available from many ILCs, due to the smaller sensors required.
But FZLCs are improving with every generation.
What about lenses ?
In the ‘good old days’ lenses were focussed manually by turning a ring on the lens barrel. This moved the whole optical group back and forth on a helical mount.
When autofocus was invented, lens makers kept the same basic helical mount but the user’s fingers were replaced by a little motor. Some lenses appeared using just the inner/rear group of elements for focussing but still using a helical action.
This worked (and still does) just fine with PDAF.
But cameras with CDAF are a very poor match for lenses with a helical focus action.
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| PDAF vs CDAF Schematic |
The diagram above relates to the discussion below.
With PDAF the AF module measures some light rays and figures out:
a) which direction the focus elements in the lens need to move
b) how far the focus group needs to move in order to achieve correct focus.
Having done that the AF system says to the lens “go there” and it does. With a basic PDAF system the camera doesn’t check whether the lens is actually in focus at the ‘go to’ point. I believe the more sophisticated (=expensive) systems may have a final ‘are we really there’ check before the shutter fires.
A basic CDAF system cannot tell if the lens needs to move one way or the other to achieve focus.
So it moves the lens and repeatedly asks ‘are we getting closer’ until the focus action overshoots the mark and the camera detects the focus action has to reverse. As indicated in the diagram above the process of finding correct focus involves several back and forth movements of the focus elements.
This back and forth movement requires the focussing parts of the lens to accelerate, stop then reverse extremely fast.
A helical mount is simply too slow even with a powerful motor.
So most lenses particularly zooms, designed for CDAF have a different design. They
a) utilise a few small optical elements in the interior of the lens for focussing and
b) those elements, in a lightweight housing, are driven directly back and forth on rails.
This is much faster and has allowed cameras with CDAF to match or better those with PDAF for single shot autofocus speed.
But basic CDAF is still not good for follow focus on moving subjects as it has no predictive capability.
Camera makers have tried to deal with this problem in two ways.
Most have converted some pixels (about 10,000 or thereabouts) on the imaging sensor to PDAF sensors. The idea is to use the PDAF function to drive the lens to where it estimates the correct focus point to be then fine tune with the CDAF.
MILCs from Canon, Nikon, Sony, Olympus, Samsung, Fujifilm and Olympus have this technology.
Panasonic uses a technology which it calls DFD (Depth from defocus) which uses data about the optical characteristics of out of focus images to estimate the direction and amount by which the focus lens must be moved. It is a way by which CDAF might emulate the best feature of PDAF.
For a first generation new technology it seems to work pretty well. My GH4 and FZ1000 with DFD are better at follow focus than my GH3 (without DFD), but still not up to the standard of a high end DSLR. Maybe the second generation of DFD will boost follow focus even further.
Which system will prevail ?
In a recent interview a very senior Canon executive said he did not know whether DSLR, MILC or ‘Compact’ by which I think he probably meant FZLC, would the dominant technology for most camera buyers in the future.
If he doesn’t know then I guess nobody does. The buyers will decide, as usual.
We live in interesting times.
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