phase detection vs contrast detection

Started Dec 1, 2010 | Discussions
kumaiti
Junior MemberPosts: 45
Like?
phase detection vs contrast detection
Dec 1, 2010

I remember reading in one review something like this:

"Both systems have their own advantages..."

under what condition would it be better to use contrast detection?
(static or slow-moving subject is implied in the question...)

Also, if you were comparing 2 cameras and one of them had both systems while the other had only phase detection, would you say that having both systems would be a relevant factor in your decision?

thanks!

ps. the 2 cameras above are the EOS 550D and Sony SLT A55.

Bob from Plymouth
Contributing MemberPosts: 590Gear list
Like?
Re: phase detection vs contrast detection
In reply to kumaiti, Dec 1, 2010

I could attempt an explanation myself, or cut and paste other people's work but I won't do either.

Look here" http://www.cambridgeincolour.com/tutorials/camera-autofocus.htm

 Bob from Plymouth's gear list:Bob from Plymouth's gear list
Nikon Coolpix P300 Canon PowerShot S100 Nikon D700 Nikon D600 Nikon AF-S Nikkor 24-70mm f/2.8G ED +4 more
Reply   Reply with quote   Complain
HobbiesAreFun
Senior MemberPosts: 2,425Gear list
Like?
Re: phase detection vs contrast detection
In reply to kumaiti, Dec 1, 2010

kumaiti wrote:

Also, if you were comparing 2 cameras and one of them had both systems while the other had only phase detection, would you say that having both systems would be a relevant factor in your decision?

That would not be the biggest factor in my decision unless video was my primary concern. The phase detection allows the A55 to auto-focus during Video while the 550D cannot. It would only matter if the focusing speed was significantly improved from one camera to the next, which I believe it is not in this case.

ps. the 2 cameras above are the EOS 550D and Sony SLT A55.

The differences between these two cameras:

The A55 is Way beyond the 550D in feature set and frames per second ability. It also has a slight edge in high-ISO ability and actual Image Quality (According to Dxomark). The A55 has an Electronic Viewfinder, which is a plus for some, and a huge minus for others. For me it was a plus which is why I chose the A55.

At the same time, the 550D has smoother video (supposedly the smoothest video out of all entry level DSLRs according to some reviews) but cannot auto-focus like the A55 can. Its maximum frame-per-second rate is 3.7 unlike the A55's 10.

Blah blah blah blah blah.

I don't want to trash the 550D in comparison, but I easily could as I can't find any reason why I would want it except to upgrade to a higher level Canon body in the near future afterwards. I find all Canon bodies under the 7D incomparable with competitors in the same price range. And not in a good way either. Although the 550D has more variety in the lenses you can buy, the Sony also has a good set of lenses that only get outperformed (slightly) at higher levels and in build quality (And of course, in the Canon lenses being anything short of cheap).

All in all, the choice is up to you. If you want to go with Canon for the long run, buy the 550D. If you aren't planning on upgrading for a few years and aren't planning on being devoted to a single system, I would suggest looking into the Sony A55, Nikon D7000, Nikon D90, and Pentax K-r. Happy shooting!

-- hide signature --

A Beginning Amateur Photographer

 HobbiesAreFun's gear list:HobbiesAreFun's gear list
Nikon D700 Nikon AF-S Nikkor 28-70mm f/2.8 ED-IF +5 more
Reply   Reply with quote   Complain
kumaiti
Junior MemberPosts: 45
Like?
Re: phase detection vs contrast detection
In reply to HobbiesAreFun, Dec 1, 2010

This isn't a 550D VS A55 topic, please. I just mentioned which cameras I was talking about before someone come and throw some line like "no camera is like that, why are you even asking?". I think I know every theoretical difference between those models, I am trying to figure how the differences play out in the real world.

My question was, in other words: for PICTURES, is there any advantage in having a camera that is capable of PDAF and CDAF or is PDAF superior to CDAF in practically every case, thus rendering CDAF irrelevant if you already have PDAF.

Reply   Reply with quote   Complain
kumaiti
Junior MemberPosts: 45
Like?
Re: phase detection vs contrast detection
In reply to Bob from Plymouth, Dec 1, 2010

Bob from Plymouth wrote:

Look here" http://www.cambridgeincolour.com/tutorials/camera-autofocus.htm

Very interesting, thanks, but it doesn't really compare the 2 different AF modes I am curious about.

Reply   Reply with quote   Complain
JudyTee
Contributing MemberPosts: 564
Like?
Re: phase detection vs contrast detection
In reply to kumaiti, Dec 2, 2010

kumaiti wrote:

I remember reading in one review something like this:

"Both systems have their own advantages..."

under what condition would it be better to use contrast detection?
(static or slow-moving subject is implied in the question...)

To put this as simply as possible, both systems ultimately measure contrast in a part of the image. The "contrast detection system" is slower, hunts more, but may be more accurate, On the other hand, the "phase detection system"is faster, hunts less, but is more complex optically and mechanically and subject to being out of alignment. That is the reason we often read about "front focus" or "back focus" with DSLRs and never hear about such things with the P&S types.

-- hide signature --

Judy

Reply   Reply with quote   Complain
AnandaSim
Forum ProPosts: 13,422Gear list
Like?
Asking the right question...
In reply to kumaiti, Dec 2, 2010

kumaiti wrote:

This isn't a 550D VS A55 topic, please.

I know what you mean. But if the reality is that you are deciding between the two cameras, then AF method is lower in the priority of decision criteria. Those two cameras are VERY different and a choice on them can use criteria other than AF method.

My question was, in other words: for PICTURES, is there any advantage in having a camera that is capable of PDAF and CDAF or is PDAF superior to CDAF in practically every case, thus rendering CDAF irrelevant if you already have PDAF.

In STILL PICTURES and Interchangeable Lens, PDAF has been the classic way to go and is the fastest AF. CDAF is being improved and thrown in for Live View and VIDEO because PDAF does not use the shooting sensor so cannot AF during video and Live View without offbeat designs.

-- hide signature --

Ananda
http://anandasim.blogspot.com
https://sites.google.com/site/asphotokb

'There are a whole range of greys and colours - from
the photographer who shoots everything in iA / green
AUTO to the one who shoots Manual Everything. There
is no right or wrong - there are just instances of
individuality and individual choice.'

 AnandaSim's gear list:AnandaSim's gear list
Kodak EasyShare P880 Olympus E-510 Olympus E-620 Panasonic Lumix DMC-G2 Olympus PEN E-PM2 +15 more
Reply   Reply with quote   Complain
kumaiti
Junior MemberPosts: 45
Like?
Re: Asking the right question...
In reply to AnandaSim, Dec 2, 2010

AnandaSim wrote:

I know what you mean. But if the reality is that you are deciding between the two cameras, then AF method is lower in the priority of decision criteria. Those two cameras are VERY different and a choice on them can use criteria other than AF method.

Indeed I am deciding between them and for MY needs and considering the price where I live, they are direct competitors and very very similar.

Risking a complete thread derail, why would you say they are VERY different cameras?

Reply   Reply with quote   Complain
kumaiti
Junior MemberPosts: 45
Like?
Re: Asking the right question...
In reply to kumaiti, Dec 2, 2010

Let me try to rephrase my question again. I didn't expect that much misunderstanding...

I KNOW how each autofocus mode work, wikipedia is there for anyone to read. I also understand that DSLRs didn't have half-decent CDAF until recently with Live View becoming so important, that in video mode you can't use PDAF (unless using a A55/A33) and so on. But that is a lot of theory and not what I am trying to understand.

IN PRACTICE, given there ARE models capable of CHOOSING between CDAF and PDAF under certain conditions, UNDER THOSE CONDITIONS (slow moving object, taking a picture using live view), is there any reason why you would you use CDAF if you had PDAF in your camera already?

And given a previous answer that briefly touched what I am actually asking:

Is back focus and front focus a COMMON issue? Will I get a consistently better focus usind CDAF if the subject is still? If given the choice, should I use PDAF only when speed is important?

Reply   Reply with quote   Complain
AnandaSim
Forum ProPosts: 13,422Gear list
Like?
Re: Asking the right question...
In reply to kumaiti, Dec 2, 2010

kumaiti wrote:

Indeed I am deciding between them and for MY needs and considering the price where I live, they are direct competitors and very very similar.

The only thing similar about them is maybe price where you live. They are completely different cameras.

Risking a complete thread derail, why would you say they are VERY different cameras?

1. They grip in the hand differently. I have not spent a lot of time playing with these two brands or playing with these two models, but my perception is that the Canon is chunky and hollow feeling, the Sony is solid, small like a bridge ultrazoom camera and dense. I would not doubt that I can change EV on the Canon but with the Sony A33, A55, I would have to take the camera away from my eye, peer hold the EV button down and roll the dial, then put the camera back to my eye. Now, fit a long zoom lens on the cameras and the grip/balance will change considerably.

2. The Canon has a legacy of a brand that has lenses that go from cheap to expensive to super expensive. The Sony has access to Minolta old AF lenses and modern expensive Zeiss. However, if you go to any pro rental shop I'll bet inventory, servicing and support will be Canon.

3. Stand in my town on a weekend and say "hey Canon shooter" and many heads will look at you because they are holding one. The Sony DSLR shooter is one in 10 or one in 20 or less.

4. The Sony has an EVF. The Canon has an OVF. That is night and day difference in what the scene looks like. Some people like one, some the other. But it is night and day difference. Try shooting at an open air event, near the axis into the sun and pan from sun to shade. Try following action, panning. Try doing a macro shot and manual focussing. Try viewing the shot at stop down and see whether you can check DOF...

etc... etc....

-- hide signature --

Ananda
http://anandasim.blogspot.com
https://sites.google.com/site/asphotokb

'There are a whole range of greys and colours - from
the photographer who shoots everything in iA / green
AUTO to the one who shoots Manual Everything. There
is no right or wrong - there are just instances of
individuality and individual choice.'

 AnandaSim's gear list:AnandaSim's gear list
Kodak EasyShare P880 Olympus E-510 Olympus E-620 Panasonic Lumix DMC-G2 Olympus PEN E-PM2 +15 more
Reply   Reply with quote   Complain
AnandaSim
Forum ProPosts: 13,422Gear list
Like?
Re: Asking the right question...
In reply to kumaiti, Dec 2, 2010

kumaiti wrote:

I KNOW how each autofocus mode work, wikipedia is there for anyone to read.

Data is not knowledge, Knowledge is not wisdom. Wikipedia by the way, is community edited and not vetted and may not be accurate. Even if it is accurate, it is just knowledge, not wisdom.

I also understand that DSLRs didn't have half-decent CDAF until recently with Live View becoming so important, that in video mode you can't use PDAF (unless using a A55/A33) and so on. But that is a lot of theory and not what I am trying to understand.

Most DSLRs didn't and don't have any CDAF.

IN PRACTICE, given there ARE models capable of CHOOSING between CDAF and PDAF under certain conditions, UNDER THOSE CONDITIONS (slow moving object, taking a picture using live view),

I don't have enough experience with the Sony. But all classic DSLR, for still photos, you would always use PDAF and OVF. Period.

is there any reason why you would you use CDAF if you had PDAF in your camera already?

You only use CDAF (if it exists) on a classic DSLR if you are using LV for movie shooting or macro shooting or special circumstance.

And given a previous answer that briefly touched what I am actually asking:
Is back focus and front focus a COMMON issue?

Yes.

1. The focus is ALWAYS WRONG. You calibrate a focussing system to an achievable and acceptable tolerance.

For most circumstances, your deeper DOF will cover the problem.

If it is very obvious and causing grief, you send both the body and the lenses to the service facility so that they can achieve better.

In many cases, volume sold lenses are of big enough f/no, f/4 for example, that the DOF is deep and the issue is not significant.

Will I get a consistently better focus usind CDAF if the subject is still? If given the choice, should I use PDAF only when speed is important?

You will get consistently better focus if the subject is still if you magnify the image 10x on the LCD screen, use manual focus. Why let an auto mechanism which has data but no wisdom control the success of your photo.

Understand DOF. I mean, don't read wikipedia or look at calculated DOF charts. Actually understand that real objects are 3D not 2D and the attractiveness of the photo depends on the photographer's wisdom and visual interpretation of which aspect to focus on.

-- hide signature --

Ananda
http://anandasim.blogspot.com
https://sites.google.com/site/asphotokb

'There are a whole range of greys and colours - from
the photographer who shoots everything in iA / green
AUTO to the one who shoots Manual Everything. There
is no right or wrong - there are just instances of
individuality and individual choice.'

 AnandaSim's gear list:AnandaSim's gear list
Kodak EasyShare P880 Olympus E-510 Olympus E-620 Panasonic Lumix DMC-G2 Olympus PEN E-PM2 +15 more
Reply   Reply with quote   Complain
Deleted1929
Forum ProPosts: 13,050
Like?
Re: phase detection vs contrast detection
In reply to kumaiti, Dec 2, 2010

"Both systems have their own advantages..."

under what condition would it be better to use contrast detection?
(static or slow-moving subject is implied in the question...)

DSLRs, which use phase detection, uses a mirror which flips out of the way when the shot is taken. While you are framing and focusing the mirror intercepts the light and some of it goes to the AF system that uses phase detection.

Cameras that don;t have a mirror can't use phase detect so they use contrast detect, which is sensor based. The problem is it is inherently slower than phase detect.

DSLRs that support contrast detection do so to support live view and video. So if you need live view or video you need contrast detection.

Live view is used for stills photography when you cannot, due to the position of the camera, use the viewfinder. In ye olde days people used right angle viewers for this ( which let you use phase detection because it was an extension of the viewfinder ).

The Sony A55 and A33 uses a system where the mirror does not flip up during exposure. This (old) idea enables it to use phase detection all the time. The downside is that you have a mirror in the optical path to the shutter taking about 30% of the light and potentially resulting in occasional unwanted optical defects. I think the jury is still out on whether that's a problem or not. At this stage I wouldn't want one for professional work. The up side, apart from having phase detection all the time, is that the mechanical mirror flipping is no longer needed and the fewer moving parts the better.

Fujifilm recently released a P&S with phase detection built into the sensor. This would be very interesting in a large sensor camera like a DSLR or a MILC, but we've heard nothing from Fuji or anyone else about implementing such a system. Fujifilm's P&S seems to be a disaster as far as image quality is concerned so it's rather useless innovation in the only camera it's in. In principle it could be a game changer for MILCs.

Hopefully one or both of these technologies will be developed more.

Also, if you were comparing 2 cameras and one of them had both systems while the other had only phase detection, would you say that having both systems would be a relevant factor in your decision?

You need to think in terms of tools for the job. If you need live view or video then you need contrast detection. If you don't then I think contrast detection is of no interest.

Having a reasonably fast and reliable AF system is the only concern. Panasonic produced a very good contrast based AF system on the Panasonic G1 and that I'd be OK with. However if I was shooting sports professionally I'd be paying for a top of the range DSLR with a phase based AF system designed for that function. Right tool for the job, you see ?

-- hide signature --

StephenG

Reply   Reply with quote   Complain
Leon Wittwer
Forum ProPosts: 12,868
Like?
Re: phase detection vs contrast detection
In reply to JudyTee, Dec 2, 2010

JudyTee wrote:

To put this as simply as possible, both systems ultimately measure contrast in a part of the image. The "contrast detection system" is slower, hunts more, but may be more accurate, On the other hand, the "phase detection system"is faster, hunts less, but is more complex optically and mechanically and subject to being out of alignment. That is the reason we often read about "front focus" or "back focus" with DSLRs and never hear about such things with the P&S types.

Phase contrast systems have the potential to be more accurate than contrast detection systems. The latter are limited by the sensor resolution. Phase detection systems use a different sensor which can be more highly resolved than the image sensor and thus have the potential for a more accurate focus solution. Of course, I am assuming a properly operating system.

Reply   Reply with quote   Complain
Skipper494
Veteran MemberPosts: 8,195
Like?
Re: phase detection vs contrast detection
In reply to Bob from Plymouth, Feb 13, 2011

This link appears to mostly avoid the differences ans certainly avoids how well various manufacturers address the problem, as in contrast detection, Nikon does it much better (my P7000 and L22), Fuji does it poorly (the HS10 I tried.

Neither does it attempt to explain why DSLRS are so much faster and more reliable using phase detection, although they say it amounts to the same thing, which, of course, it doesn't. For a simple explanation, check Wikipedia or Stanford Ed.

Reply   Reply with quote   Complain
Jim Cockfield
Forum ProPosts: 14,903
Like?
changes easily without moving camera from eye
In reply to AnandaSim, Feb 14, 2011

AnandaSim wrote:

1. They grip in the hand differently. I have not spent a lot of time playing with these two brands or playing with these two models, but my perception is that the Canon is chunky and hollow feeling, the Sony is solid, small like a bridge ultrazoom camera and dense. I would not doubt that I can change EV on the Canon but with the Sony A33, A55, I would have to take the camera away from my eye, peer hold the EV button down and roll the dial, then put the camera back to my eye. Now, fit a long zoom lens on the cameras and the grip/balance will change considerably.

That's not accurate. I'm currently testing a Sony A33 and it's very simple to change Exposure Compensation without moving the camera from your eye. Just use your right thumb to press the +- button while spinning the control wheel (in front of shutter button) with your right forefinger.

The ergonomics are well designed for that purpose (the area of the camera with the +- button is even slanted towards you some to make it easier to change, and it feels quite natural to use one that way (thumb on +- button, spin control wheel while watching the meter display in the EVF).

So far, I've been impressed with the ergonomics, especially when using Live View. Using Live View with one is far easier than most people give it credit for, thanks in no small part to the tilting LCD. You can brace your elbows against your side and peer down into a tilted LCD. IMO, it's even easier (and makes for a more stable platform) using one that way than it is to hold the camera up to your eye.

-- hide signature --
Reply   Reply with quote   Complain
Keyboard shortcuts:
FForum MMy threads