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07-01-2013, 11:58 AM   #1
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Maybe its a Focus Accuracy Issue

A month or so ago, I posted a topic regarding sharp focus issues

https://www.pentaxforums.com/forums/general-technical-troubleshooting/225673-me-camera-lens.html

there's the topic for reference.

So in prep for a week in Yellowstone I practiced on my technique and also shot focus charts, testing for front and back focus
I thought I had things down and found no front/back focus issues. so I went to Yellowstone and what did I discover?

My Sigma 100-300 f4 has a mind of it's own! now I'm looking for a work around

I now know it's not lens blur, because the issue showed up while on a tripod as well
I know its not front or back focus, because the testing eliminated that

It seems while shooting animals, the phase detect can't find enough edges to focus (especially even colored fur like a moose/wolf/bear in less than ideal light like shade or twilight) so instead it locks focus based on a tree or tall grass nearby even though I'm using center point focus.

What convinced me was an image I took of a wolf - with the camera on a tripod and the subject laying on the ground stationary! The FENCE in the background, about 6 feet behind the subject, was in perfect focus, but at f4, the subject was obviously not. (I was nearly head on to the wolf, so my focus point was between the eyes and I had enough time to carefully compose and verify "aim"). This happened again with some swallows perched on a branch about 20 feet from me. The tree BEHIND them was sharp and focused, the birds were mush. I've gone back and looked at several other images, including the ones that prompted my original thread, and it makes sense now. Foreground texture doesn't seem to affect the lens as much as background, or maybe I can tell when it locks on foreground and just refocus.

I managed enough "good" images with some in much tougher conditions to convince me I have a pretty good handle on what I'm doing mechanically and technically.

So if we eliminate operator error as the problem, how do I work around a lens that prefers grass, wood and metal to animal fur or feathers, since obviously my eyes aren't good enough to discern through the viewfinder when there's a disparity in focus?

07-01-2013, 12:20 PM   #2
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This somewhat of a problem, the focus system is a "dumb" system it does not know there is a bird a tree or a cloud so it focus on what has the greatest contrast.
Best way would be to focus on something with great contrast of first fully zoom in focus on the subject and then focus out and refocus again.
07-01-2013, 03:54 PM   #3
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dSLRs do not have screens that are good at judging focus, as they are just intended for composing while the camera takes care of mis-focusing. I changed the screen in my K-5, which helps me judge focus to use manual-focus lenses. I use only manual focus lenses, and just focus on what I choose. But then I've used only manual focus for over 50 years - it's much simpler.
If eyesight is really a problem a good rangefinder camera is much easier to focus exactly with normal to wide angle lenses. That's why my normal digital is a Leica M9.
07-01-2013, 04:17 PM   #4
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Unfortunately this is a common problem. On the K-5 series (you mentioned the K-5iis in the other thread), the AF points are quite large. For example, if you select the center AF point, you will find the sensor will grab anything in the area approximated by the little curved brackets in the viewfinder, the ones that look like a pair of parentheses. It's much wider than the focus confirmation LED would suggest. So if you have anything with fine texture in that area, or near the edge of that area, the center AF point will lock on to it.

There is a variation on the trick that Anvh mentioned, if you have a parfocal zoom. With a parfocal zoom, you can zoom in to fill the whole field of view, establish focus (either MF or AF), and then zoom back out without changing the focus to recompose. But if your lens is varifocal then that won't work, you will need to refocus.

I guess on a tripod, you could use live view and magnification to make sure your intended subject was in focus.

07-01-2013, 04:32 PM   #5
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QuoteOriginally posted by TomB_tx Quote
they are just intended for composing while the camera takes care of mis-focusing
Ain't it the truth. How I would love to have a dSLR with full-silvered mirror, a decent focus screen, and no AF. The OP's description of the wolf/fence situation is a textbook classic example of how AF can let you down.


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07-01-2013, 06:11 PM   #6
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There isn't a lot to add to Tanzer's post who hit the nail on the head.

Note that the camera is doing the focussing, not the lens.

Try to aim in such a way that the camera has minimal chance of being confused by background detail. This may mean that you don't have the woolf's eyes at the centre of the AF area, but perhaps a bit above it.

You can also try to take multiple shots, each time trying to latch on a different detail of the subject.
07-01-2013, 06:38 PM   #7
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QuoteOriginally posted by nomadkng Quote
the phase detect can't find enough edges to focus
That makes sense, especially when there is a lack of contrast in the subject. I get the same problem shooting birds with drab plumage in poor light sometimes, you can almost hear the AF thinking: 'where can I find something to lock onto?' Often boosting the DOF is the only reliable way to produce a result where I get decent focus.

Related to issues of AF I always like to post this image from the manual for the Nikon D4, illustrating that even with 'A 91,000-Pixel RGB 3D Color Matrix Metering III sensor partnered with Nikon’s Advanced SRS to deliver unmatched accuracy in every frame. Add a faster, more responsive 51-point AF system.. ' Nikkon still sees the need to mention potential problems with AF:


QuoteOriginally posted by Tanzer Quote
the focus confirmation LED
There is no such thing in the viewfinder. The only actual focus confirmation item in the viewfinder is the green hexagon at the bottom of the viewfinder. The little red AF LEDs in the viewfinder are just guides to which AF area(s) may be active. This accords with your general point that the AF points used in the camera, even the centre-point, operate across a larger area than indicated by the little pin-point red LED.

FWIW, this is how Pentax visualize the AF areas (and the AF sensor type) in the K-5 II:


Last edited by rawr; 07-01-2013 at 07:23 PM.
07-02-2013, 04:20 AM   #8
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Right, I stand corrected. Should have said focus selection LED.

Might be interesting/different if they made them approximately as large as the actual area, maybe like a fuzzy blob that would glow. That might reduce the misunderstandings, or it might just be annoying. I use center-point so I try to mentally concentrate on the parenthesis area anyhow.

07-02-2013, 08:06 AM   #9
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So what we need is Pentax to develop much smaller AF points, or allow the User to minimize or maximize the size of the CENTER point?

I'll deal with lens hunting a little more if I don't have to worry about my once in a lifetime, two grizzly cub, award winning shot being useless because the %^*$% camera locked focus on the side lit tall grass behind them!

I'd rather KNOW there's no focus than the camera giving me false hope.

I've made the comment a couple times now when culling through my photos, if I had been using a gun, the critter would be dead, but I'll be damned if I could get a good photo
07-02-2013, 08:22 AM   #10
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What was the saying, A Good Craftsman Never Blames His Tools

You need to learn how it works and the quirks, nothing is perfect you know so i rather they make it simple and stupid so you know what's going on.
07-02-2013, 10:39 AM   #11
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Yeah, smaller points is probably the most needed improvement. In fairness, this kind of AF issue isn't really exclusively a Pentax issue.

You might greatly appreciate an aftermarket/retrofitted manual focus screen, particularly if your lenses are f/4 and faster. I plan to get one myself, for these type of situations. Problem is, I have a couple slow-ish lenses in my collection that I like. But the pros probably outweigh the cons.
07-02-2013, 11:25 AM   #12
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+11 on smaller focus points. The large focus points can be a real PITA especially with wide angle lenses (larger DOF is no substitute for correct focusing).
07-02-2013, 06:21 PM   #13
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55 or whatever and smaller AF points won't necessarily make the AF more intelligent or always useful (except most likely for AF-C tracking purposes). An AF system with 100 small AF points could still be pretty dumb or insensitive. The important thing is for the camera to focus on the 'right' or intended thing, which is not just a function of the number or size of the AF points, but often seems to require some machine intelligence (and some mind-reading perhaps ).

Aside from perhaps throwing in more and smaller AF points, Pentax could do what others have done and bring a greater range of other sensor information to bear on the AF decision making process - eg more subject colour information and scene selection intelligence. They might also make it easier/more ergonomic for people to choose AF points and AF modes, I guess, to make working the AF more fluid, natural and precise.

But, as the Nikon D4 image reminds us, even amazing AF will still face challenges out in the field, and then it is up to user experience and camera craft.
07-03-2013, 08:05 AM   #14
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ok, so given the limitations of AF "intelligence" and now knowing that the camera can be fooled by sharp edged objects in the vicinity of the real subject, do any of you have a "technique" to compensate?

I don't have the luxury of sitting in a blind for hours, so most of my wildlife encounters are spontaneous. This newfound distrust of AF is a source of frustration, especially when I look at 5 or 6 "coulda been awesome" shots from a bucket list trip that are now in the recycling bin because AF locked onto wrong thing.

given a typical field scenario where you are out hiking (or driving) and come upon a subject, and the limited time you have to focus and snap off some pictures, how do you convince the camera to lock on your target? Pick a rock or branch in front and "walk" the focus to your subject? Stop aiming for head and aim for body so that the full center AF point is filled by the subject?
07-03-2013, 08:12 AM   #15
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Well, depends on how much of the animal fills your frame. If you can get the little circle in the middle of the viewfinder on the face of the animal, then that's going to be good enough. If the face of the animal only fits half the circle, I'd suggest hitting the body - aim for a contrasty part, like where fur changes color (usually neckline, belly, shoulders). If most of the animal is inside the circle, then you might be a bit far.

For a bit of information, I've used my k-x with my Tamron 70-200 for almost a year now. In the beginning, it was a pain in the butt shooting random animals or faraway objects. Nowadays, I'm so used to the combo that I usually hit my focus on non-moving subjects (or slowly moving). It's just time and effort.
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