Tamron 17-50/2.8-Focus past Infinity?

I just picked up a Tamron 17-50/2.8 lens and I notice that the lens will focus

past infinity. Is that normal on auto focus lenses?

 

Specifically when I put it in manual focus on the close end it stops exactly at

the closest distance marked on the barrel. But on the long end it has the

Infinity symbol and a line that marks it. When I focus there at the line it is

at infinity, however it will go about 1/4" past there. When it does that

nothing appears in focus.

 

This is my first auto focus lens and I have never seen that on my manual focus

lenses for my Mamiya.

 

Is that normal for AF lenses? Is that normal for this model? Do I have a bad

lens?

 

Thanks for any insight.

Normal. The focusing algorithms of AF lenses sometimes require going go past infinity when they are hunting for focus in difficult scenes.

All my AF lenses go slightly past the infinity mark (and so does my Tamron 17-50/2.8) ... I believe that the ability to go past the infinity mark is speeding up the AF procedure.

Thanks! I was a bit worried that I had a lemon for a second. Glad to learn that is normal.

 

Guess I am still trying to figure out this auto focus stuff. Who knows this AF thing might just catch on ;-)

It may be as Evan said to allow "hunting for focus in difficult scenes". But I think that this was done because lenses change their geometrical (and therefore optical) configuration with temperature. Therefore, at different temperatures the focus will be achieved at a different position of the focus ring. So, the focus past infinity was designed to allow focusing at infinity at various temperatures.

The question then is why don't manual focus lenses allow focusing past infinity?

Last time I have used a manual focus lens was more than 8 years ago and as I remember it did go past infinity (but it was a russian lens). If your manual focus lenses do not focus past infinity then I wish I could take back my previous post. I was obviously wrong.

<p>Ok. Now, at least, I know where did I get this bad info from. It was from this website: <a href="http://photonotes.org/articles/beginner-faq/lenses.html#infinitymark">

photonotes.org</a>.</p>

My non-zoom manual lenses do not go past infinity.

 

My Tamrom 17-50 zoom does and I'm not worried about it at all.

I wonder if Tamron will be using the line "To Infinity, and Beyond!" in their promo material.

This is somewhat unusual on a short lens where focus isn't very critical, but perhaps the "facilitating AF" explanation makes sense (the AF system needs to look for maximum contrast at some point and maybe it would be faster if it could find the peak at infinity from either side).

 

All lenses that I have seen that have very shallow depth of field, e.g., 300/2.8 and 600/4, will focus beyond infinity to permit infinity focus after thermal expansion.

 

Another thing that distinguishes this lens, I am guessing, from your Mamiya lenses, is that it is a zoom. All zooms are slightly varifocal, even the ones that claim not to be varifocal. The sharpest infinity focus is going to vary a bit depending on zoom setting.

 

To your final question "Do I have a bad lens?" Compared to your Mamiya lenses... very likely :-)

Phillip- I think you are right, especially on your last comment.

 

This is my first auto focus lens and it is a still a frustrating learning curve. It does pretty good at closer range about focus but has a tough time with infinity. Sometimes it seems right on and I get a crisp image, but others it seems like alot of lens blur. Especially when there is alot of sky in the image-it does not like sky at all.

 

I have tried manual focus and at the wide end and distant subjects it is tricky since the lens does not stop at infinity. However I think I get better results that way more of the time than the AF does.

 

My AF results seem to lack alot of detail I was hoping would be there. I am using an XTI and have all 9 focus points on. Maybe I should cut back to just the center?

 

Is all this just the nature of AF or is it sign of a bad lens too?

 

Doing more testing tonight.

Ray,

 

I have found AF to be more accurate when I use only the center AF point. By more accurate, I mean, the focus was on what I wanted it to be, instead of the camera deciding. When I use all 9 AF points, the camera sometimes detected other things in the shot, and decided to focus on those things, instead of what I was intending to focus on. Since you are use to manually focusing and understand the Depth-of-field (and aperture settings), I would probably use only the center-AF point. I think, using 9 points is safer for people, who may want to shoot fast or moving objects, granted not all their shots may be focused, in the area they thought it would be. Stick with using one point... I sometimes focus and recompose. Or if I'm going to shoot several shots, I'll manually select one of the side or top focus points (so I don't have to recompose, as much)

 

Also, you mentioned difficulty with skies. AF sensor's try to find an area of "contrast", so, if the sky is uniform (all blue, no clouds) then it will have a harder time finding focus.

 

Good luck, (I have a very difficult time manually focusing, on my Rebel XT, FYI: they do make focusing screens, to help with manual focusing. Here is one company: http://www.katzeyeoptics.com/cat--Canon-DSLRs--cat_canon.html )