Tamron 28-75 vs. 18-250 AF performance

**wedge** · 08-29-2013, 11:09 AM

I recently got a used K-x (my K-r is in the shop for an extended holiday) and noticed that my two most-used lenses seem to exhibit very different AF performance (as in speed) on this camera. By this I mean different from each other.

On my K-r, the two lenses behaved a bit differently depending on the lighting. In brighter light (outdoors during the day, for example, the 18-250 would get a good focus lock very quickly, but indoors, it was sluggish. The 28-75 didn't seem to focus quite as quickly in good light as the 18-250 (but it wasn't far behind), and in poorer lighting, it was much quicker and more accurate.

On the K-x, it actually seems sort of opposite, at least in bad lighting. In daylight, both seem to behave as they did on the K-r, but when shooting indoors, it seems like the 28-75 will run back and forth through the adjustment range 2 or 3 times before getting a lock. I don't think it's a matter of the subject, either. For example, the image below was taken with the 18-250 because the 28-75 just wouldn't lock onto it, and this has some pretty decent contrast to it.

And because of the constant f2.8, the 28-75 has been my "go-to" indoors lens. With this type of AF lag, though, I may need to re-evaluate things.

**crewl1** · 08-29-2013, 11:32 AM

My K-x was a bit of a dog with AF indoors, even with fast lenses.

Did you test both lenses at the same focal range to see if it isn't just one was providing a more contrasty target?

**wedge** · 08-29-2013, 12:01 PM

Originally posted by crewl1

My K-x was a bit of a dog with AF indoors, even with fast lenses.

Did you test both lenses at the same focal range to see if it isn't just one was providing a more contrasty target?

The photo I posted was taken at 70mm. I tried getting the same shot, from the same position, using the 28-75 set to 75mm and it kept seeking back and forth trying to lock on the lampshade. Even on my K-r, I've had occasional issues locking at maximum FL on that lens, so I pulled it back a bit, to about 60mm and it still wouldn't lock. I should say also that the onboard flash was on for all of these shots. Maybe the K-r's focus assist lamp deserves the credit for this lens performing so well on that body?

**crewl1** · 08-29-2013, 01:02 PM

Yes the AF assist light could help but also the K-r and later bodies may have improved low light focusing due to newer AF systems.

(This feature is touted highly in the K-5II.)

When I had the K-x I would usually just revert to manual AF when indoors. I eventually upgraded it to a K-5.

I have both of your lenses and they perform really well on my K-5 indoors. The AF assist light does kick in occasionally when it is fairly dark.

08-29-2013, 11:09 AM	#1
wedge Site Supporter Join Date: Jul 2009 Location: Montco, PA Photos: Gallery \| Albums Posts: 154	Tamron 28-75 vs. 18-250 AF performance I recently got a used K-x (my K-r is in the shop for an extended holiday) and noticed that my two most-used lenses seem to exhibit very different AF performance (as in speed) on this camera. By this I mean different from each other. On my K-r, the two lenses behaved a bit differently depending on the lighting. In brighter light (outdoors during the day, for example, the 18-250 would get a good focus lock very quickly, but indoors, it was sluggish. The 28-75 didn't seem to focus quite as quickly in good light as the 18-250 (but it wasn't far behind), and in poorer lighting, it was much quicker and more accurate. On the K-x, it actually seems sort of opposite, at least in bad lighting. In daylight, both seem to behave as they did on the K-r, but when shooting indoors, it seems like the 28-75 will run back and forth through the adjustment range 2 or 3 times before getting a lock. I don't think it's a matter of the subject, either. For example, the image below was taken with the 18-250 because the 28-75 just wouldn't lock onto it, and this has some pretty decent contrast to it. And because of the constant f2.8, the 28-75 has been my "go-to" indoors lens. With this type of AF lag, though, I may need to re-evaluate things.

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08-29-2013, 11:32 AM	#2
crewl1 Site Supporter Join Date: Jul 2010 Location: Los Angeles, CA Photos: Gallery \| Albums Posts: 9,807	My K-x was a bit of a dog with AF indoors, even with fast lenses. Did you test both lenses at the same focal range to see if it isn't just one was providing a more contrasty target?

08-29-2013, 12:01 PM	#3
wedge Site Supporter Join Date: Jul 2009 Location: Montco, PA Photos: Gallery \| Albums Posts: 154 Original Poster	Originally posted by crewl1 My K-x was a bit of a dog with AF indoors, even with fast lenses. Did you test both lenses at the same focal range to see if it isn't just one was providing a more contrasty target? The photo I posted was taken at 70mm. I tried getting the same shot, from the same position, using the 28-75 set to 75mm and it kept seeking back and forth trying to lock on the lampshade. Even on my K-r, I've had occasional issues locking at maximum FL on that lens, so I pulled it back a bit, to about 60mm and it still wouldn't lock. I should say also that the onboard flash was on for all of these shots. Maybe the K-r's focus assist lamp deserves the credit for this lens performing so well on that body?

08-29-2013, 01:02 PM	#4
crewl1 Site Supporter Join Date: Jul 2010 Location: Los Angeles, CA Photos: Gallery \| Albums Posts: 9,807	Yes the AF assist light could help but also the K-r and later bodies may have improved low light focusing due to newer AF systems. (This feature is touted highly in the K-5II.) When I had the K-x I would usually just revert to manual AF when indoors. I eventually upgraded it to a K-5. I have both of your lenses and they perform really well on my K-5 indoors. The AF assist light does kick in occasionally when it is fairly dark.