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09-12-2013, 10:00 AM   #1
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Distance focus adjustment?

I have a lens that works great in close distances (example less than 30 feet) but has front focus on longer distances (example 50+feet)
I see lots of articles about using charts etc which do well with near focus but none to tune in at far distances.
Anyone know any articles or techniques that work at long distances?
Logically if focus is out even a hair which might not present it's self in regular focusing charts will be off the longer the distances are

Hope this makes sense
BTW please don't tell me to change F stop settings

Thanks

Randy

09-12-2013, 10:05 AM   #2
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Is this a zoom or a prime? I'm not sure if it matters but I'd think zooms may not always focus the same across the zoom range.
09-12-2013, 10:26 AM   #3
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QuoteOriginally posted by TER-OR Quote
Is this a zoom or a prime? I'm not sure if it matters but I'd think zooms may not always focus the same across the zoom range.
Oh yeah it's a zoom

Thanks

Randy
09-12-2013, 10:38 AM   #4
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Unless it's a parfocal zoom, you'd never get it to be accurate at every focal length.
It is a common problem with zoom lenses. They do not maintain focus as you change focal length.

09-12-2013, 11:41 AM   #5
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QuoteOriginally posted by slip Quote
I have a lens that works great in close distances (example less than 30 feet) but has front focus on longer distances (example 50+feet)
I see lots of articles about using charts etc which do well with near focus but none to tune in at far distances.
Anyone know any articles or techniques that work at long distances?
Logically if focus is out even a hair which might not present it's self in regular focusing charts will be off the longer the distances are

Hope this makes sense
BTW please don't tell me to change F stop settings

Thanks

Randy
I think the problem with distance is not so much having the right chart, but the fact that when you are shooting at a distance, and I find tele lenses are the worst, to a point, you also crop in more and magnify more in printing. The simple fact is that the theoretical DOF rules no linger apply, and the need for focus accuracy rapidly outstrips the focus adjustment capability. Remember the detail you are focusing on loses detail in the view of the focus sensor.

Also lack of in lens stabilization also causes focus errors because you do not present a stable image to the focusing sensor, this problem increases with distance in many cases, and since the focus sensor has a finite size, you never know whether it is focusing on foreground or background when shooting distant subjects

That's my $0.02
09-12-2013, 11:44 AM   #6
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Can you get good focus on distant objects using live view or MF?
And I dont think OP is talking about focusing, then zooming. But yes, zoom lenses change focus when you zoom. Except for parfocal zoom lenses (there is a thread with a list of these)
OH, and from my experience, some lenses are better at near ranges and others at far ranges. Its a bit odd, but with some lenses I am having a harder time getting sharp near-infinity and infinity shots. I assumed its just lens construction, a part of the design, and not an abnormality.

Keep in mind also that subjects that are far away might not appear as sharp, because there is more distance. That means less detail is visible, it has lower contrast, might be faded a little, there might be heat haze..

And I think focus charts should be used at a medium-far distance. I dont remember exactly, but there is a formula, where you multiply the focal length by a fixed number to get a good distance at which to place the focus chart.
09-12-2013, 12:04 PM   #7
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For a longer zoom like the 55-300 I usually spot-check infinity focus accuracy and sharpness by taking pictures of things - like a neighbor's brick chimney a few houses away, or a cell phone tower a half mile away. That's close enough to usually not have much haze or heat issues, and not moving around like treetops.
09-12-2013, 12:12 PM   #8
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I've given up on the focus charts. As you and others have noted, the focus targets are small, which forces you to use them at abnormally close distances.

I just use newsprint tacked up on a flat wall, a good average distance away. This pretty much guarantees that the focus sensors are not picking up anything closer or farther. Also, it is quite easy to compare successive shots. Just change the focus adjustment and make a series of shots.

There is a formula that one of the guys at CRIS told me about, some multiplier of the lens focal length, but I can't remember what is is. I just figure it is best to use a working distance that is somewhere in the middle of my average working range of the lens -- meaning somewhere between 6 feet and infinity. I guess the arithmetic average of 6 feet and infinity is, um, infinity. That is still kinda far away, so for practical purposes I just use the length of a conference room, maybe 18 feet or so. I figure this should be a good compromise between indoor/portrait shooting (where I am likely to use wider apertures) and outdoor/landscape shooting (where I am more likely to use narrower apertures anyhow).

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