Testing

December 15, 2011

Auto Focus Testing – Sony Alpha 77 vs the Canon 5D Mark II in low light – Updated

Sony A77 w/ 85mm f/1.4 Carl Zeiss Lens


New Video with more detail AF Testing >>

In this video I’ll be testing the auto focus capabilities of the Sony Alpha 77 vs the Canon 5D mark II.  The lighting is extremely poor in this auto focus test which is by design. I wanted both cameras to be struggling with the focus, and the results are going definitely going to surprise you!!

Here’s what you need to know about the auto focus systems and aperture in order for this test to make sense. When slr’s or slt’s like the a77 are trying to focus, the aperture of the lens is always open to widest aperture until the shutter button is pressed. Then the camera then adjusts the aperture to whatever setting you have, and takes the picture.  This all happens in a micro-second as you would imagine. You may have noticed this about the aperture on some cameras when using the DOF (depth of field) preview button. What that button does is set the aperture to the actual value you have set, hence giving you an accurate depth of field preview.

In addition to this Sony a77 has a bright AF Assist beam that shoots out to “assist” the af system in focusing. The issue when in close range however, is the AF Beam doesn’t hit the subject where you need it.  So it’s basically focusing with no assist lamp, just like the canon.

Firmware Update fixed this issue considerable on the Sony A77!! Get it here >>

Sony:

For the Sony A77 I first used the 85mm f/1.4 Carl Zeiss lens, and I then switched to the Sony 16-50mm f/2.8 Lens.

Canon:

For the Canon 5D mark II I purposely used the 24-105mm f/4 L Lens.

Here is a closer look at the cardboard box used in the test so you can see the pencil lines.

Box used for AF Test

Box used for AF Test – Notice the pencil lines

New Video with Updated info etc.. >>

 

 

Interesting results right? Now if you a regular follower on SonyAlphaLab.com you will remember me complaining about the af system on the a77. I actually put the Sony A77 AF in the Cons section on my full review, because I thought it a bit weak in low light. This test proves it to me. At least against the Canon 5d Mark II much older af system.

Notice how well the Sony a77 worked when it had the f/1.4 lens mounted, but when the lens changed to f/2.8 it could no longer focus on just the cardboard.

The Canon was using a f/4 lens the entire time which also means it did not use it’s single cross type sensor. That only works with f/2.8 and faster lenses.

The Canon locked onto just the cardboard instantly with no problem at all.

I don’t know exactly how the sensors are orientated in relation to the light, but I believe the Canon’s sensors are getting a lot more light. I could be wrong though. Perhaps the sensors themselves are just better technology?

To be fair I do not know the physical differences between the actual focus sensors themselves, but looking at the Specs, the Sony a77 appears to have the advantage over the Canon in both sensor points and sensitivity. In the real world low light scenario however, the Canon out performs the A77 when not using the AF Assist lamp.

Why does this matter?

At close range like macro photography or product photography you may have questionable AF performance when shooting low contrast subjects.

The A77 needs the AF assist lamp in order to deliver the -1 ev sensitivity, and at close range that is not possible. The Assist beam doesn’t hit the subject where you need in close range due to the off-set placement on the camera. I ran into this issue when trying to take close-up pictures of the a580 using just the center af point on the a77. The low contrast smooth black body of the camera in low light was just to hard to focus on with no beam. I ended up manually focusing which is usually easier in these situations anyway.

Sony A77 Auto Focus System:

  • TTL phase detection AF (CCD line sensors)
  • 19 points (11 points cross type)
  • EV -1 to 18 EV (at ISO100 equivalent, with F2.8 lens attached)

Canon 5D mark II Auto Focus System:

  • TTL-CT-SIR AF-dedicated CMOS sensor
  • 9 AF Points (1 Cross Type) + 6 AF Assist Points
  • EV -0.5-18 (at 73°F/23°C, ISO 100)
 


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Thanks,
Jay

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About the Author

Jay
Hello, I'm the owner and operator of SonyAlphaLab.com. Please check out the About page for a full background on myself and the Lab ;) Google+ | Twitter | RSS-feed | Email Updates!!




11 Comments


  1. Nico

    What iso where you using for the test? What metering mode? What focus mode? A lot of variables you’re not listing for the test.


    • Jay

      Metering modes were both set to average, focus was set to center point only on the 5d Mark II. I tried all the focus modes on the a77 ( not all on video). None of them made a difference. I also tried different individual points with the same result. I also have face detection and smile shutter turned off just to let you know. ISO was set to auto on both cameras. Auto focus single shot was the mode I used. So once it focused it locked and you can then re-frame the scene.

      Best,
      Jay


  2. Mike sheppard

    I think the problem might be because of the Af assist lamp which works well in certain areas, but the beam in the video is at least half off the box the whole time!try again with a taller box. I will be getting my a77 in 10 days and will be checking these things myself also…I hope that my observations are helpful.


    • Jay

      Hi Mike,

      The AF assist beam is definitely not working in my test because I’m so close. When I move back to about 6 feet it focuses. The Canon was not using a beam so I figured it was a more even test that way. I’m still amazed that the Canon is so much better even with so much less light due to the f/4 lens vs the f/1.4 and f/2.8 lens. That in itself is incredible to me. I’m going to do more extensive testing on this so stay tuned and thanks for your comments/ Help Mike :)

      Best,
      Jay


  3. Matt

    1. The Canon has over 300% of the amount of light hitting the AF sensors on the Sony due to the translucent mirror setup. Only 30% of the total light is passed onto the AF sensor on A77.

    2. Further to this the AF sensor size is relative to the image sensor physically, so more total light passes through the lens onto the AF sensor (what it does with this light is up to the sensor obviously). Comparing full frame to APS-C isn’t really fair in this regard.

    3. The sheer number of focus points are not relative to how sensitive they are to light. The more sensors can mean more accurate focusing and focus tracking – but not low light capability.

    4. AF can be more difficult at lower apertures (due to shallow dof causing hunting) and thus why systems need those sub f2.8 sensors for low light. Try comparing them at f4. Also stopping the lens down will not help as the lens is left wide open until taking the shot. Further to this different lenses are better or worse at AF on the same camera – in this instance there are too many variables, the same lens should have been tested on both bodies to make a proper comparison between the AF systems of two very different cameras.

    This is enough to show that this is a rather silly test – unless someone was specifically considering either an APS-C A77 with 16-50 2.8 or a Full Frame 5D Mk II with a 24-105 f4 and wanted to know what the low light focus speed of each was like to make their final decision. Which seems rather specific.


    • Jay

      Matt, Thanks for taking the time to comment and explaining more about the AF Systems! I, and I’m sure many others, greatly appreciate it as I cannot seem to find detailed information anywhere about the af systems and how they work exactly:) What your saying makes a total sense, but I do still have a question if you don’t mind?

      On the Canon a prism is use to re-direct the light to the af sensor and the viewfinder from what I can tell? So how much light exactly is lost in that prism if any?

      I also totally agree that it’s a rather pointless test other than to simple compare the two cameras in a simple test. If you watched the part 2 video I went into a bit more detail.

      Thanks again,
      Jay


  4. Dennis

    The AF assist beam is definitely working! I can see it clearly :D! Open you eyes men !


    • Jay

      Yes its working, but the beam circle is not lighting up the box where any of the focus points are hitting, therefore “it’s not working” ;) Close range it doesn’t work.

      Best,
      Jay


  5. Michael

    note that on A77 you can group the 5 central AF points together which will definitely improve performance doing a test like this one.


    • Jay

      I wish it would have made a difference Michael, but it did not. It does not matter what you have enabled on the A77. If all it sees is the brown cardboard it won’t focus under these specific conditions. I tried everything it needs the dark contrast.

      This test is kind of silly as others have mentioned anyway on the web. It’s not even close and unfair to judge the A77 against a Full Frame camera like the 5d Mark II. The fact of the matter is that in real world situations you would have the A77 on the center group as you said, and it would focus 99% of the time. It’s only in these really rare low light situations where the A77 is super close to it’s subject. In that case the AF assist beam doesn’t help as the camera can’t see it.

      Best,
      Jay


  6. thanks a lot for review !



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