Camera Labs

Just got hold of my first ever slr - a sony a580 and I'm in the phase of experimenting settings and functions.

As regards AF, I'm finding it difficult to grasp the concept of Auto focus and phase detect focus. Anyone can offer a simplified explanation, maybe suggest what is good for what?


ok..in the meantime did some more research beyond the manual, and found this:
http://www.cambridgeincolour.com/forums/thread104.htm

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Sony α580 dSLR
Tamron 17-50 2.8, Sony 18-55 kit lens, Minolta AF 50mm 1.7, Minolta 28-105mm xi zoom, Minolta 35-70mm 3.5-4.5, Minolta 75-300mm 4.5-5.6
Kata 3n1-22

My short photostream: ivandegi

Phase AF: usually fastest on DSLR. Works by calculating for a given point where the subject is relative to the current lens focus point, and also how far and in which direction. This means the AF system can quickly go the right amount in the right direction for correct focus. It also works well tracking subjects moving towards or away from you.

Drawbacks are the AF points work in contrast, so there needs to be some present and also in the suitable direction to be detected as the AF points only work on one axis. Depending on the system camera manufacturers often stack two at one point in a cross. It is also particularly sensitive to accurate calibration of the lens and body. If either of these vary, then you get offset errors in focus.

Contrast AF: works on the main sensor. No focus accuracy issues as it sees what it will capture, although it is still a contrast detector there needs to be some for it to look at. The main drawback here is speed. It can only look at changes in contrast by varying focus and has to guess direction and amount from that. In a good implementation it is as fast as phase for a static subject, but it doesn't work nearly as well for moving subjects.

On most DSLRs contrast AF only works in live view. You will be using phase AF through the viewfinder. Sony did add a 2nd sensor for fast (phase) AF in live view in some of their cameras although I'm not up to speed on which models have it nowadays.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.

The A580 has many varied modes, and the autofocus types are dependent on which mode you are in.

Using the optical viewfinder, you have phase detect autofocus. This mode is as fast as can be and like any normal DSLR.

Using the 'quick' live view mode, you have phase detect autofocus. This mode is equally fast as with OVF mode, something completely unique to Sony DSLRs - no other DSLR can do this.

Using the main sensor live view, you can use phase detect autofocus, or contrast-detect autofocus, depending on your preference. Phase detect will focus faster, but in this mode will have to flip the mirror down and up to focus then restore the live view...with contrast detect AF it will be slower, but tends to be very accurate and will not have to flip the mirror to achieve the autofocus.

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Justin Miller
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 200-500mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony NEX5N / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / Pentax K adapter / Konica K/AR adapter / bunches o' Konica & Pentax lenses!

Galleries:
http://www.pbase.com/zackiedawg

From my tests there is no contest with the focusing accuracy of phase vs contrast approaches. I am using a SONY 28 -70 f 2.8 Sam lens on the A 580.
The contrast focusing is excellent - phase always is slightly back focusing!

Question - how can I calibrate the camera to correct that issue. The back focusing is also an issue with other lenses that are not SONY. Tamron 20 - 40 and Minolta 70 - 210 beercan. What is worse is that these lenses can not be used in the contrast focus mode.

Any ideas?

Unfortunately many people are having A 580 back focusing issues identical to yours. I have the Sony 28 -75 f2.8 as well as a number of other lenses. I also went to the store I bought the camera at and we tried numerous other lenses. Bottom line is that Phase detection calibration is consistently poor.
There is a mechanical adjustment to correct this issue however if you as a user make the adjustment the folks at SONY services told me it would void the warranty. So I will have to send the camera back to SONY. There are a lot of web threads on this problem. A few folks have done the adjustment themselves and are thrilled with the results.
There is no need to go into high intellectual discussion about optics and phase detection technologies - it's much simpler than that - poor quality control in the production of the camera in setting up the phase detection component.
Having said all that when the camera is properly focused either manually or in contrast LV mode (only works with SAM and SSM lenses) the image quality is excellent. You will be amazed how much sharper your lenses will look - soft images will no longer be the norm.