budget (<$400) zoom lenses with fast autofocus for outdoor daytime use?

Started Nov 28, 2012 | Questions thread
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budget (<$400) zoom lenses with fast autofocus for outdoor daytime use?
Nov 28, 2012

Hi all,

I recently bought an A57 with the 18-135 kit lens, and am mostly very happy with it.  But I have been experiencing problems with autofocus speed at the long end of the zoom range -- it has the same slow hunting/clicking behavior as some other posters have described here , even under good lighting conditions.  This is a problem because some of my favorite subjects at the moment are our backyard hens, which move very quickly and are best shot from a distance.  (Unlike other wildlife, which probably runs/flies away if it notices you, the chickens run ​towards​ you, thinking you have food!)

So mainly I am wondering what you would recommend as a zoom lens (the most interesting focal length range being probably around 80-200mm) with ​fast and reliable autofocus​ for under $400.  This is for mostly outdoor daytime use, so large aperture is not as important as it would be for some other scenarios, except inasmuch as it is needed for PDAF.  I assume things like the Sony DT 18-250mm F3.5-6.3 are out (at least, I couldn't fully zoom them), since I heard the PDAF system doesn't work with max apertures smaller than F5.6.

My original theory with the 18-135 was that F5.6 was a sort of grey area, at the cusp of where PDAF would start to not work reliably.  But seeing that other thread about the 18-135 (and not seeing any other similar complaints anywhere else) makes me hope that it could be just a design flaw with that particular lens.  How about the Sony DT 55-200 F4-5.6 SAM?  Anyone who has that lens, what has been your experience using AF at full zoom?

Then there are various old Minolta Maxxum AF lenses, which look awfully tempting on eBay et al.  How quickly do those (e.g. the 70-210mm F4.0 "beercan", or the 100-200mm F4.5) autofocus compared to the more recent Sonys?  I have heard some comments that their AF is slow, but I don't know how slow is slow compared to the hunting I am getting right now from the 18-135, where it can take 1-2 seconds to focus!  And for any slow-focusing Minolta lenses, is that only when it has a large adjustment to make since the last focus?  e.g. let's say I focus on an object 100 feet away, take a shot, and then try to refocus on an object 103 feet away.  I am guessing that the camera's PDAF would determine the correct focus quickly, and even a relatively slow mechanism like the beercan would be able to adjust quickly since it would be a small change.  Is that sound reasoning, or am I lacking some understanding?



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