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Old Jan 3, 2010, 9:58 AM   #1
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Default Sony Alpha 200 autofocus issues...

(I noticed a similar thread, but as its not an exact match, I thought it best to start a new thread...)

While my A200 has some very good features, (I'm seriously impressed with the images it gets pointing near the Sun or other light source), I find the automatic focus to be very unreliable. Even in what I would call excellent conditions, (bright light, high contrast subject), it frequently gives a slightly soft focus, or worse.

As I like to take panormas and need a sequence of images in rapid succession, even one that is out of focus can ruin the sequence.

Perhaps I was spoilt with my previous camera - a Finepix S9600 super zoom, which pretty much always got the focus perfect, however fast I took took the shots...

The problem seems independent of which lens I use, (though I like to use extreme wide angle for the panos, a 10-20mm Tamrom most often).

Is it normal to need to be careful with AF on this camera?
Any tips on getting best focus results?
Is it possible to get the AF system checked over, in case it is faulty? (It clearly does something, as you can watch the lens turn to an approximate setting while pointing it at different subjects).

I checked 'Steves Reviews', which thought the AF excellent...

At the moment I am using the old one 9 times out of 10, which is a real shame.

All clues and suggestions very gratefully received!
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Old Jan 3, 2010, 10:20 AM   #2
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I don't have the A200, I use a KM 5D (slow focus) and a A700 (fast focus). It could be a slow focusing lens. I have a lens that will cycle through the whole close to far range if it misses locking focus on subject, before it tries to lock focus the second time around. Especially in dim light.

What do you have your AF area set ? For panorama sequence I set mine to spot (center sensor). Make sure it's not set to wide or local.

If it's dark bump up your ISO.
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Old Jan 3, 2010, 10:27 AM   #3
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Your camera's AF system is pretty decent for a camera in that market niche.

With a lens like that 10-20mm, you're going to need to be careful what you focus on, and what focus point is being used, due to the curvature of the focus plane with that type of lens. Otherwise, if you focus on something using an outside focus point at closer ranges, the center portion may exhibit backfocus.

You could also be seeing a depth of field issue (as you'll have a *much* shallower depth of field with a dSLR model than you're accustomed to using a point and shoot model like your Fuji, for any given subject framing and aperture. You may want to post a photo or two and let us see what it's doing, too.

Here's a copy of information from a post I made recently with some suggestions on how to check it (and I'd make sure to see if it's behaving differently with the Tamron versus a Minolta or Sony lens, as it could be a lens issue):


You see front focus (focusing in front of your intended subject), or back focus (focusing behind your intended subject) on a regular basis from a variety of camera models from virtually all manufacturers. When that happens, it's usually an alignment issue (AF sensor assembly, etc.). They can adjust that kind of thing using specialized test equipment.

Most of the time, it's something the user is doing wrong, though. For example, letting the camera select the focus point, versus selecting it yourself (and most cameras are going to default where the camera is selecting the focus point).

Keep in mind you've got 9 Autofocus points in the AF sensor your camera is using, and because the focus plane is not flat (it curves slightly inward as you go towards the edge of the frame with most lenses, especially at wider zoom settings), if you're using an outside focus point, the focus in the middle of the frame will be further away (resulting in backfocus for the portion of your subject in the center of the frame if you're close enough to your subject).

So, for starters, you want to make sure that you (and not the camera) are selecting the focus point being used. Just press the Fn button and look at the AF Area selection menu choice. It defaults to wide (where the camera decides the focus point to use). I'd try it using Spot (Center focus point), or Local (where you select from the 9 focus points) and see if you still see the same problem using the Center Focus point.

Here is an image showing the 3 different choices under AF Area, and how they differ (it defaults to wide, which can cause issues, depending on your subject type and distance if the camera decides to focus using something other than the focus point closest to your intended subject, or chooses an outside point):



If you still see an issue, here's a page showing you how to check it using a tripod and a chart you can download and print.

http://focustestchart.com/chart.html

If it's off with more than one lens, I'd send it back to Sony and let them calibrate it (and I'd send both the camera and lens in case it's a lens issue instead).
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Old Jan 3, 2010, 11:01 AM   #4
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If you're making panoramas, it would seem to me that a longer focal length would be better.

It would also seem to me that manual focus would be better too. Wouldn't you want a consistant focusing distance from one shot to the next? That would preclude having the camera refocus (correctly or not) for each shot.
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Old Jan 3, 2010, 1:19 PM   #5
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Thanks to all for the responses.

I don't really see how the area of AF is a factor as for the shots in question,everyhing is a long way off...

TCav, wider angle means fewr images to stitch, this makes life a LOT easier, particularly if there are large blank areas of sky... Manual focus is OK, if I set focus to infinity it does better.

I am using this as an example becuase I do these sorts of shots a lot...

JimC, thanks for such a detailed response.
It does not seem to be front or back focus, just out. The slight errors are the worse as they are not easy to spot until I get the images out.

While I accept that the settings may help, it does not seem reasonabe to me that a digital camera in this price range should perform so much worse than every cheap digital autofocus I have ever used...

Can other Alpha users tell me if they can trust the AF?
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Old Jan 3, 2010, 1:27 PM   #6
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I'd post some images so that we can see what you're talking about.

Chances are, you're not realizing the differences between a point and shoot model like your Fuji (which has a very tiny sensor) and a dSLR (with a much larger sensor). That impact Depth of Field (how much of the image is in focus as you get further away from your focus point, which you can control via your aperture settings).

IOW, you're going to have a *lot* more depth of field using a model like your Fuji compared to a dSLR for any given subject framing and aperture. So, that takes some getting used to. On the plus side, that means it's far easier to isolate your subject from distracting backgrounds by blurring them. On the downside, you'll need to learn how important it is to insure you're focusing on your intended subject, choosing a more appropriate aperture setting if you want more depth of field.

Or, you may not realize that you'll have less sharpening by default with most dSLR models like yours (which you can change under your Creative Style choices).

You could also have other issues you're not realizing (for example, shutter speed is too slow for the settings you're using).

I'd post some downsized samples with the EXIF intact (which is a header in your images), so that we can see the camera settings being used.

The free Irfanview is a good tool for that purpose (look under Image>Resize/resample; then downsize it to around 1024 pixels on the longest side, saving it at around 80% JPEG quality using the File>Save As menu choice, giving it a new filename so that you don't overwrite the original).

See this post for how to attach an image here:

How I post a photo to the forum
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Old Jan 3, 2010, 3:02 PM   #7
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Thanks again Jim, as per your suggestion here are a couple of typical soft focus shots, resized down.The EXIF data seems to be intact. (I used Photoshop Elements 6).
Attached Images
  
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Old Jan 4, 2010, 4:55 AM   #8
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Thinking a bit more, I can see that it could be unrealistic to expect the same autofocus effectiveness on the A200 that I get with the Finepix S9600...

But think back quite a bit, I also got much better AF performance from my old Canon film SLR. I can't recall the exact model number, but it was about 250, and a long way from the top of the range.

The AF system on that was extremely reliable, except in very dim light conditions.

Is that a fair comparison?
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Old Jan 4, 2010, 5:15 AM   #9
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Hey Nick.

I don't see any EXIF in those images. Their file sizes are also very small. My guess is that you either used "Saved for Web" with elements (which strips out the EXIF), or your file sizes exceeded what we allow here. When that happens (either the dimensions or file sizes are not within limits), the forums software will automatically downsize the images and recompress them using much lower JPEG Quality, and strip out the EXIF at the same time.

Note the comment this post in the thread I linked to earlier:

http://forums.steves-digicams.com/ge...ml#post1026659

Quote:
Note from JimC:

One other thing I'd like to point out to members, is that you need to make sure both the dimensions and file sizes for images you upload are within the specified limits you'll see shown in the box that pops up that allows you to manage and upload attachments. Otherwise, the forum software is going to resize and recompress the images using a lower JPEG Quality (which may make the images appear softer than they would have been if they were not recompressed).
Using something like the free Irfanview I linked to, around 80% JPEG quality is usually going to let an image stay within the limits. If not, you'll have to go lower if you don't want the forums software messing them up.

Now... exactly what lens is that Tamron supposed to be? I can't find any 10-20mm lens that Tamron has made in the past.

They do make a 10-24mm. But, from user reviews I see for it from KM and Sony owners, it's soft at all apertures on these camera bodies (and you need to make sure a given lens works properly on the type of camera you're using, as they don't always work the same in different camera mounts), especially on it's wider end. See some of them here:

http://www.dyxum.com/reviews/lenses/...asp?IDLens=483

Some quotes:

Top review in list:
Quote:
soft at all apertures
Next review in list:
Quote:
You can stop it down to the level where diffraction is a real issue and it still isn't sharp
Bottom review in list:
Quote:
-not exceptionally sharp on the wider end even at f11...
Getting the picture?

If that's a Tamron 10-24mm lens you're using, the lens is probably your problem.
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Old Jan 4, 2010, 5:29 AM   #10
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IOW, your issue probably has nothing to do with the camera's AF accuracy if you're using a Tamron 10-24mm lens. That lens (at least the one they offer for Minolta AF mount) is soft based on KM and Sony user reviews I see for it.

Also, sometimes a lens can have compatibility issues causing AF problems and the lens manufacturer can "rechip" it so that it communicates properly with a newer camera (and you may want to contact Tamron to find out), especially if it's an older model lens (or one that was manufactured early in a lens production cycle, before the manufacturer realized there was a problem).

That may or may not be the case with that lens model. But, from user reviews I see from KM and Sony users, it doesn't look like a promising lens to use on your camera. You'll find large differences between lenses, and you really need to take them on a case by case basis when comparing optical quality.
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