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07-28-2009, 10:00 PM   #61
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QuoteOriginally posted by deludel Quote
... it seems like that's too much work. What am I missing?
You seem to be missing how the AF-select mode works. Once activated, you control which AF point to use by using the cursor keys (the four buttons around the "OK" button). This has become second nature to me and I find it much easier and quicker than panning back and forth while trying to focus and recompose. I do this without taking the eye from the viewfinder. You can assign the OK button to jump to the centre AF point from any other selected AF point.

You have the same control as with the centre AF point just the focus/recomposition movements with the camera are either eliminated (the AF point positions are chosen quite well) or significantly reduced.


Last edited by Class A; 07-28-2009 at 10:46 PM.
07-29-2009, 05:10 AM   #62
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QuoteOriginally posted by deludel Quote
A few of you mentioned these selective autofocus points. I guess I don't get the concept ... my Lori-Logic (which has led me astray many times!! ) says that with center-point, I have more control, 'cause it's the one point I can point my camera to and recompose. With selective focus ... it seems that your focal point can change with every single shot, so theoretically you are changing the point within the camera on every shot? Unless you are a landscape/portrait photographer, it seems like that's too much work. What am I missing?
You don't have to hand over control to the camera - I forget the details, but you can use centre, auto, or user-selected focus points. The auto setting is the one where the camera decides what ought to be in focus (which strikes me as total madness, but then I'm a manual focus gal). There's a menu setting somewhere that allows you to use the four-way controller to select the focus point.

edit: I just noticed that Class A already addressed this, probably more clearly because I rarely use it!

QuoteOriginally posted by Marc Sabatella Quote
You are assuming, as I do too, that "recomposing" for every shot is less work than selecting a different focus point for eveyr shot. That's largely because you and I are used to recomposing. People who are accustomed to selecting focus points on every shot often think of that as less work.
Mark makes an interesting point. I feel much the same way - I never had selectable focus points before, so I learned to focus-recompose reasonably well. Not to say I don't entirely fluff it once in a while, particularly with shallow dof, but I think I do just as well as I would if I were messing around selecting focus points all the time. Also my favourite lenses are entirely manual, so only the centre point works anyway! As long as you can make focus-recompose work for you (and you seem to have sorted that out), you shouldn't feel obliged to use the other focus points.

Happy shooting!

Julie
07-29-2009, 07:13 AM   #63
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The K100D has an auto-focus mode where AF is only active while the OK button is pressed. This makes focus and recompose easy.

Without using it I sometimes have a problem with the camera trying to re-focus while I'm recomposing.

Dave
07-29-2009, 09:47 AM   #64
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QuoteOriginally posted by foxglove Quote
Mark makes an interesting point. I feel much the same way - I never had selectable focus points before, so I learned to focus-recompose reasonably well. Not to say I don't entirely fluff it once in a while, particularly with shallow dof, but I think I do just as well as I would if I were messing around selecting focus points all the time. Also my favourite lenses are entirely manual, so only the centre point works anyway!
Of course, with a manual lens, there is no AF per se, but there is the AF confirmation light - just didn't want anyone to be confused by that. Anyhow, yeah, at some point I realized I hated having my camera work differently depending on what lens I had mounted (manual or automatic), and that's when I set my camera to center focus point, center-weighted metering, and M exposure mode, and just welded them there :-).

07-29-2009, 11:29 AM   #65
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QuoteOriginally posted by Class A Quote
You seem to be missing how the AF-select mode works. Once activated, you control which AF point to use by using the cursor keys (the four buttons around the "OK" button). This has become second nature to me and I find it much easier and quicker than panning back and forth while trying to focus and recompose. I do this without taking the eye from the viewfinder. You can assign the OK button to jump to the centre AF point from any other selected AF point.

You have the same control as with the centre AF point just the focus/recomposition movements with the camera are either eliminated (the AF point positions are chosen quite well) or significantly reduced.
It's because of tips like this that I read this Forum every day. Valuable post!
07-29-2009, 12:25 PM   #66
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QuoteOriginally posted by Marc Sabatella Quote
Of course, with a manual lens, there is no AF per se, but there is the AF confirmation light - just didn't want anyone to be confused by that.
Eh, yeah, that was unclear, wasn't it? Thanks for clarifying!

QuoteQuote:
Anyhow, yeah, at some point I realized I hated having my camera work differently depending on what lens I had mounted (manual or automatic), and that's when I set my camera to center focus point, center-weighted metering, and M exposure mode, and just welded them there :-).
Ah! These cameras only allow centre-weighted metering with manual lenses, then. That would explain a few things... I think I should take a cue from the OP, and re-read the manual, clearly there's a lot I didn't properly digest!

As always, I've learned a thing or two from someone else's query. Thanks!

Julie
07-29-2009, 03:18 PM   #67
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QuoteOriginally posted by snofox Quote
It's because of tips like this that I read this Forum every day.
I'm glad the post was of value to you.
07-29-2009, 03:49 PM   #68
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QuoteOriginally posted by Class A Quote
A,
I have seen this link referenced several times. Did you ever wonder why the author does not show a single image that demonstrates this "suck?". My guess is that because it is a theoretical issue that does not show up in practice. He shows two images of perfectly focused images, and then says, "if I would have focus-recomposed it would have sucked." (My paraphrase.) Why not simply show this? All he would have had to do is focus again and press the shutter button? Again, it is clear to me that his straw man, has no straw.

07-29-2009, 05:03 PM   #69
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Analysis of the recompose error

This is a rewrite of my post after detecting errors in my argument. I shouldn't do "back of the envelope" calculations at 2 AM when I am supposed to sleep
QuoteOriginally posted by PentaxPoke Quote
My guess is that because it is a theoretical issue that does not show up in practice.
You are probably right in normal cases.

I am as much concerned about non-planar focal "planes" as I am about recompose DoF errors.

E.g., when I checked my DA* 16-50 for corner sharpness at 16mm fully open, I found MUCH better figures than any test, e.g., Photozone.de. But only if I manually focussed the corners using LiveView. Focussing with a corner AF point could help a bit here, too.

As for the recompose DoF error: you can [easily ] compute the effect. If you map the focal plane into subject space you see that recomposition moves the center of the focal plane on the surface of a sphere. E.g., 50mm on APS.C has 27 FoV. If you recompose by 2/3 of the distance from center to edge, this is a turn by 9. This moves the subject away from focal plane by
x = d (1 / cos (9) - 1) = ~0.012 d = ~ d/100 (or ~1% of d).
where d is the distance to the subject.
Now, this may induce a focus error if your DoF is smaller than 1% of the distance.


Let's check for a portrait situation:

d = 1.0m (50cm APS.C FoV) with 50mm and f/1.4: DoF extends 1.2cm in front and behind subject. The recompose error moves it by just 1.2cm, so DoF now extends from 0cm in front and 2.4cm behind subject.

So, you roughly entirely lost the DoF area in front of the subject in this rather extreme situation.

In a less extreme situation, e.g., d=3.2m or f/2.8, you would half the DoF area in front of the subject rather than entirely loose it.

In an even more extreme situation, e.g., d=0.7m or f/1.2, the subject would indeed move out of the DoF area and become unsharp even for non pixel peepers.

I guess you won't see it in a majority of your photography and personally, I happily recompose. But since you asked, I thought I'll do the math for you.

If this guy ever needs to provide a sample photo, it would have to be portrait shot from the side and focussing on the rear eye

QuoteOriginally posted by Original, erroneous version:
QuoteOriginally posted by PentaxPoke Quote
My guess is that because it is a theoretical issue that does not show up in practice.
You are probably right.

I am as much concerned about non-planar focal "planes" as I am about recompose DoF errors.

E.g., when I checked my DA* 16-50 for corner sharpness at 16mm fully open, I found MUCH better figures than any test, e.g., Photozone.de. But only if I manually focussed the corners using LiveView. Focussing with a corner AF point could help a bit here, too.

As for the recompose DoF error: you can easily compute the effect. If you map the focal plane into subject space you see that recomposition moves the center of the focal plane on the surface of a sphere. E.g., 50mm on APS.C has 27 FoV. If you recompose by 2/3 of the distance from center to edge, this is a turn by 9. This moves the subject away from focal plane by
x = 2 d sin^2(9/2) = ~0.012 d = ~ d/100 (or ~1% of d).
where d is the distance to the subject.
Now, this may induce a focus error if your DoF is smaller than 1% of the distance.


Let's check for a portrait situation:

d = 3.2m (50cm APS.C FoV) with 50mm and f/1.4: DoF extends 10cm in front and 11cm behind subject. The recompose error moves it by just 4cm, so DoF now extends from 6cm in front and 15cm behind subject.

So, you roughly halved the DoF area in front of the subject in this rather extreme situation.

I guess you won't ever see it in your photography and personally, I happily recompose. But since you asked, I thought I'll do the math for you.

If this guy ever needs to provide a sample photo, it would have to be portrait shot from the side and focussing on the rear eye

Last edited by falconeye; 07-31-2009 at 02:50 AM. Reason: Correction of severe mistakes
07-29-2009, 06:47 PM   #70
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QuoteOriginally posted by foxglove Quote
I think I should take a cue from the OP, and re-read the manual, clearly there's a lot I didn't properly digest!

Julie
That manual has all sorts of hidden gems! When you first get your camera there is so much you don't know ... that you cannot possibly absorb it all at once. After using it for 1+ years, you are reading the manual from a completely different perspective. I am someone who does NOT learn well from books, but the manual is something that should not be ignored, no matter how long you've had the camera! An online (and very search-able) version is found here

QuoteOriginally posted by PentaxPoke Quote
A,
I have seen this link referenced several times. Did you ever wonder why the author does not show a single image that demonstrates this "suck?". My guess is that because it is a theoretical issue that does not show up in practice. He shows two images of perfectly focused images, and then says, "if I would have focus-recomposed it would have sucked." (My paraphrase.) Why not simply show this? All he would have had to do is focus again and press the shutter button? Again, it is clear to me that his straw man, has no straw.
YES! YES! YES! I felt so validated when I read your post! I had these same thoughts. THANK YOU!!
07-29-2009, 06:55 PM   #71
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QuoteOriginally posted by deludel Quote
I really, really need help with focus/sharpness. I did not post this in the "critique" forum, because I think it's due to overall technique.

Attached are three horrid examples which highlight my inability to focus.
My K10 was on "Center Focus" (the right-hand option, where it centers in the middle .. I started using that when I had issues with the auto-option) ... my style is to half-press to lock focus then recompose. This is CLEARLY not working.

First Image:
lens used: Tamron 28-75 /1:2.8
63mm focal length
1/180sec at f/2.8
ISO 200
Pattern Metering
EV 0

Second Image:
lens used: Tamron AD 70-300/1:4-5.6
110mm focal length
1/125 sec at f/4
ISO 200
Pattern
EV 0

Third Image:
lens used: Tamron AD 70-300/1:4-5.6
100mm focal length
1/125 sec at f/4
ISO 200
Pattern
EV 0


Overall, most of my images in all different settings suffer from terrible focus. Who can share insight as to what I can do differently?
I think others have mentioned. Take the no-name UV filter out.

Of the 3, the Tamron 28-75 /1:2.8 is of slightly higher calibre . You should get excellent result if you can step that down to F4 if not F5.6

You are off to a good start in asking the right question and exploring the potential.

Daniel
07-29-2009, 09:13 PM   #72
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QuoteOriginally posted by deludel Quote
YES! YES! YES! I felt so validated when I read your post! I had these same thoughts. THANK YOU!!
Thanks Lori, and thanks Falk for the mathematical proof. It is clear from your analysis why he didn't show an example of the "suck." Because he can't.

I personally don't like the multiple focus points in cameras, and I have always turned them off. I always try to assist AF speed by focussing on the highest contrast portion of the object I want to focus on, and then quickly recompose if necessary. When I try letting the camera select, I am often frustrated by the fact that it will focus on an object behind the subject because it has higher contrast. I have never found a use for it in any of my photography. Even sports and action photography. (That doesn't mean I am criticizing anyone else that finds it useful.)

Last edited by PentaxPoke; 07-29-2009 at 09:19 PM.
07-29-2009, 09:26 PM   #73
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QuoteOriginally posted by PentaxPoke Quote
Did you ever wonder why the author does not show a single image that demonstrates this "suck?".
No, I didn't, but thanks for making us dig deeper.

QuoteOriginally posted by deludel Quote
I felt so validated when I read your post!
I would have understood this, if it had been a reply to Falks's analysis. All credit to PentaxPoke for not simply believing the "recomposition sucks" story but he did not really contribute anything that would help to validate either hypothesis.

QuoteOriginally posted by falconeye Quote
This moves the subject away from focal plane by
x = 2 d sin^2(9/2) = ~0.012 d = ~ d/100 (or ~1% of d).
Is this an approximation for small angles?
When I derive the formula (with simple trigonometry) I get
x = d (1/(cos 9) - 1)
which results in larger values for x.
So for 45 I get 41.4% of d rather than 29.3% of d with your formula. For 9, I get 1.25% (rather than 1.23% with your formula). Not much of a difference in this case.

QuoteOriginally posted by falconeye Quote
Now, this may induce a focus error if your DoF is smaller than 1% of the distance.
1.25%.
I got to give you that, your 4cm approximation is spot on!

QuoteOriginally posted by falconeye Quote
Let's check for a portrait situation:

d = 3.2m
Come on, what are you portraying here? A bus?
Just joking.

QuoteOriginally posted by falconeye Quote
DoF extends 10cm in front and 11cm behind subject. The recompose error moves it by just 4cm, so DoF now extends from 6cm in front and 15cm behind subject.
This assumes perfectly balanced AF. Unless you have calibrated your lens for that specific distance (and focal length if it is a zoom) you will have less of a margin to still be within the DOF after recomposition. Also, there is considerable variance in AF performance.
We are also assuming a perfect rotation of the camera around the sensor plane. If you do this hand-held the recomposition error will be larger (as it also will on a typical tripod) since the rotation point will be some distance away from the camera, introducing spatial translation.

In your analysis you did not specify what output size you are assuming. If we assume a 11.7 inch wide print and a distance of 1m, plus 20/20 vision, we get a DOF of 1.6cm (4.9cm for "standard' vision). With a recomposition error of 1.24cm (for 1m) we now have a real effect.

I fully admit that often times situations in which recomposition occurs are not as demanding as the one I describe. I'm thankful to PentaxPoke for stirring this up and Falk for doing the analysis. I would have thought the error is bigger. But I also believe that I've shown that there can be situations where recomposition can be problematic.

EDIT: Also note that if you are using a focal length higher than 50mm, the DOF will become shorter (when you don't change the distance). For f=85mm you'll have ~ +/- 3.5cm DOF (instead of ~ +/- 10cm DOF for f=50mm, both for d=3.2m), for instance. Granted, the recomposition error one may be able to introduce also reduces (because of the reducing FOV).

Playing the devil's advocate against myself: For the AF point selection method to provide improved results we must rely on all the AF points to be at least as precise as the centre on.

BTW, for shots involving movement, recomposition still sucks.

Last edited by Class A; 07-29-2009 at 10:51 PM.
07-29-2009, 09:59 PM   #74
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QuoteOriginally posted by PentaxPoke Quote
It is clear from your analysis why he didn't show an example of the "suck." Because he can't.
While I am less concerned about recomposition now than I was before, I wouldn't go as far as saying "he can't". I'm sure with the situation I described you'll get less keepers with a recomposition method than with a properly calibrated out-of-centre AF point (due to AF variance).


QuoteOriginally posted by PentaxPoke Quote
When I try letting the camera select, ...
Note that "AF Auto" and "AF select" are two different modes. You can use AF points other than the centre one without letting the camera chose which one. I thought this was clear but you seem to make a connection again where there isn't one.
07-29-2009, 10:09 PM   #75
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QuoteOriginally posted by Class A Quote
Note that "AF Auto" and "AF select" are two different modes. You can use AF points other than the centre one without letting the camera chose which one. I thought this was clear but you seem to make a connection again where there isn't one.
I have never talked about AF select. That is no different than what I do which is to always "select" the center. When I talk about "letting the camera choose" I thought it was obvious that it means the "Auto" setting.
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