Do some AF lenses focus better than others?

justtakingpics · 04-27-2011, 06:41 PM

Was invited to a wedding and took some pictures. It was overcast, in the evening, not night but rain storm coming so dark light that.

Anyway, most pictures were in focus if they were further away from the camera. Others, closer to the lens, did not focus. I mean they were slightly out of focus. Of course these were the important shots. I don't even know if the distance had anything to do with it.

I am frustrated. I was frustrated with my mf lenses because I can't focus fast enough to catch anything. Now I'm no good with the AF.

I was using the k-x and the kit lens 18-55.

jellyfish26 · 04-27-2011, 07:09 PM

The K-x has a hard time focusing in dark situations. If you pop up the flash, it will flicker to assist in autofocus. Most of the times, objects further away are in focus because the focus is at/near infinity so everything far away is in focus. The camera relies on contrast to focus on.

Your close shots may be blurred rather than out of focus. In low light try using Tv with a shutter speed around 1/60. This may require a higher iso, but it would reduce the blur some what. Hold down the shutter button for 1-2 seconds until you see the "hand" before taking your shot.

**Lowell Goudge** · 04-27-2011, 07:12 PM

Note that when very close to a subject camera movement toward and away from the subject becomes critical. While SR can manage most issues of camera movement in the horizontal and vertical directions parallel to the focal plane it cannot manage and is not intended to manage motion toward and away from the subject. Before blaming the lens make sure it is not your technique

justtakingpics · 04-27-2011, 07:18 PM

Originally posted by Lowell Goudge

Note that when very close to a subject camera movement toward and away from the subject becomes critical. While SR can manage most issues of camera movement in the horizontal and vertical directions parallel to the focal plane it cannot manage and is not intended to manage motion toward and away from the subject. Before blaming the lens make sure it is not your technique

Well I was only hoping it was partly the lens. That's why I said I'm no good with either MF or AF.

Marc Sabatella · 04-27-2011, 07:19 PM

Another common issue is that DOF gets shallower when you are closer to the subject. t often happens that AF focuses on, say, an outstretched arm, and the face ends up being out of focus. You do have to *think* about focus and what the issues are likely to be; you can't just assume pointing your camera in the general direction of the subject will result in the specific *part* of the subject you care about being in focus. That's just a fact of life when dealing with cameras with sensors larger than typical P&S cameras - DOF is just a lot shallower, so "close enough" often isn't when it comes to focus.

justtakingpics · 04-27-2011, 07:24 PM

Originally posted by Marc Sabatella

Another common issue is that DOF gets shallower when you are closer to the subject. t often happens that AF focuses on, say, an outstretched arm, and the face ends up being out of focus. You do have to *think* about focus and what the issues are likely to be; you can't just assume pointing your camera in the general direction of the subject will result in the specific *part* of the subject you care about being in focus. That's just a fact of life when dealing with cameras with sensors larger than typical P&S cameras - DOF is just a lot shallower, so "close enough" often isn't when it comes to focus.

So how do I do it? Do you just point the center at whatever you want in focus (as obvious as it sounds)? I suppose I did just point it at the subject in general instead of focusing on the face. I need to be more careful.

**Ash** · 04-27-2011, 09:12 PM

If all else fails, centre point should be selected, point your subject right at the centre of the frame, shoot, then crop as necessary. But it will always be better to compose your image the way you want before the shot, so use AF point selection if you're comfortable using it.

Also, be weary of the moving subject in low light - it will cause you to tear your hair out trying to lock focus on it...

justtakingpics · 04-28-2011, 05:24 AM

Can you point at subject's face, focus then recompose and it still work?

dgaies · 04-28-2011, 05:48 AM

Originally posted by justtakingpics

Can you point at subject's face, focus then recompose and it still work?

It depends how much you recompose and how large of an aperture you're shooting with. If you're shooting with a very large aperture you won't have very much depth-of-field (DOF). So if you focus on someone's face and then recompose a significant amount, you may find the face no longer in focus. If this is the case, selecting the focus point would be a better option. That said, if you aren't shooting with very thin DOF, focusing and recomposing will probably work just fine (I do it quite often).

Marc Sabatella · 04-28-2011, 12:08 PM

Right - slight concern about slight shift in focus plane when recomposing, but insignificant in practice most of the time. There is a somewhat greater a risk when pointing at the face that the camera might focus on the background instead if the face doesn't take up enough room the completely cover the focus "point". Trial and error will show you how big the focus points are. Another trick is to aim direct at the stomach.

But the bottom line is, you do have to actually learn how your camera's AF system works in order to get best results.

04-27-2011, 06:41 PM	#1
justtakingpics Senior Member Join Date: Apr 2010 Posts: 288	Do some AF lenses focus better than others? Was invited to a wedding and took some pictures. It was overcast, in the evening, not night but rain storm coming so dark light that. Anyway, most pictures were in focus if they were further away from the camera. Others, closer to the lens, did not focus. I mean they were slightly out of focus. Of course these were the important shots. I don't even know if the distance had anything to do with it. I am frustrated. I was frustrated with my mf lenses because I can't focus fast enough to catch anything. Now I'm no good with the AF. I was using the k-x and the kit lens 18-55.

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04-27-2011, 07:09 PM	#2
jellyfish26 Forum Member Join Date: Apr 2011 Location: Texas Posts: 87	The K-x has a hard time focusing in dark situations. If you pop up the flash, it will flicker to assist in autofocus. Most of the times, objects further away are in focus because the focus is at/near infinity so everything far away is in focus. The camera relies on contrast to focus on. Your close shots may be blurred rather than out of focus. In low light try using Tv with a shutter speed around 1/60. This may require a higher iso, but it would reduce the blur some what. Hold down the shutter button for 1-2 seconds until you see the "hand" before taking your shot.

04-27-2011, 07:12 PM	#3
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,888	Note that when very close to a subject camera movement toward and away from the subject becomes critical. While SR can manage most issues of camera movement in the horizontal and vertical directions parallel to the focal plane it cannot manage and is not intended to manage motion toward and away from the subject. Before blaming the lens make sure it is not your technique

04-27-2011, 07:18 PM	#4
justtakingpics Senior Member Join Date: Apr 2010 Posts: 288 Original Poster	Originally posted by Lowell Goudge Note that when very close to a subject camera movement toward and away from the subject becomes critical. While SR can manage most issues of camera movement in the horizontal and vertical directions parallel to the focal plane it cannot manage and is not intended to manage motion toward and away from the subject. Before blaming the lens make sure it is not your technique Well I was only hoping it was partly the lens. That's why I said I'm no good with either MF or AF.

04-27-2011, 07:19 PM	#5
Marc Sabatella Veteran Member Join Date: Sep 2006 Location: Denver, CO Photos: Gallery Posts: 10,685	Another common issue is that DOF gets shallower when you are closer to the subject. t often happens that AF focuses on, say, an outstretched arm, and the face ends up being out of focus. You do have to think about focus and what the issues are likely to be; you can't just assume pointing your camera in the general direction of the subject will result in the specific part of the subject you care about being in focus. That's just a fact of life when dealing with cameras with sensors larger than typical P&S cameras - DOF is just a lot shallower, so "close enough" often isn't when it comes to focus.

04-27-2011, 07:24 PM	#6
justtakingpics Senior Member Join Date: Apr 2010 Posts: 288 Original Poster	Originally posted by Marc Sabatella Another common issue is that DOF gets shallower when you are closer to the subject. t often happens that AF focuses on, say, an outstretched arm, and the face ends up being out of focus. You do have to think about focus and what the issues are likely to be; you can't just assume pointing your camera in the general direction of the subject will result in the specific part of the subject you care about being in focus. That's just a fact of life when dealing with cameras with sensors larger than typical P&S cameras - DOF is just a lot shallower, so "close enough" often isn't when it comes to focus. So how do I do it? Do you just point the center at whatever you want in focus (as obvious as it sounds)? I suppose I did just point it at the subject in general instead of focusing on the face. I need to be more careful.

04-27-2011, 09:12 PM	#7
Ash Loyal Site Supporter Join Date: Mar 2007 Location: Toowoomba, Queensland Photos: Gallery \| Albums Posts: 23,920	If all else fails, centre point should be selected, point your subject right at the centre of the frame, shoot, then crop as necessary. But it will always be better to compose your image the way you want before the shot, so use AF point selection if you're comfortable using it. Also, be weary of the moving subject in low light - it will cause you to tear your hair out trying to lock focus on it...

04-28-2011, 05:24 AM	#8
justtakingpics Senior Member Join Date: Apr 2010 Posts: 288 Original Poster	Can you point at subject's face, focus then recompose and it still work?

04-28-2011, 05:48 AM	#9
dgaies Veteran Member Join Date: Oct 2009 Location: Maryland / Washington DC Posts: 3,917	Originally posted by justtakingpics Can you point at subject's face, focus then recompose and it still work? It depends how much you recompose and how large of an aperture you're shooting with. If you're shooting with a very large aperture you won't have very much depth-of-field (DOF). So if you focus on someone's face and then recompose a significant amount, you may find the face no longer in focus. If this is the case, selecting the focus point would be a better option. That said, if you aren't shooting with very thin DOF, focusing and recomposing will probably work just fine (I do it quite often).

04-28-2011, 12:08 PM	#10
Marc Sabatella Veteran Member Join Date: Sep 2006 Location: Denver, CO Photos: Gallery Posts: 10,685	Right - slight concern about slight shift in focus plane when recomposing, but insignificant in practice most of the time. There is a somewhat greater a risk when pointing at the face that the camera might focus on the background instead if the face doesn't take up enough room the completely cover the focus "point". Trial and error will show you how big the focus points are. Another trick is to aim direct at the stomach. But the bottom line is, you do have to actually learn how your camera's AF system works in order to get best results.