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03-18-2011, 02:36 PM   #361
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QuoteOriginally posted by falconeye Quote
This isn't what data tells.
I was talking about a very subtle bias. Probably I saw just measurement noise or the result of you skipping the fine tuning but there seemed to be a very small trend which I saw by quickly replacing one chart with the other. I agree that it isn't terribly relevant for the overall result.

QuoteOriginally posted by falconeye Quote
The K-5 manages to focus dark enough for it's light meter to fail (with an f/5.6 lens). It leaves every other dSLR in the dust AFAIK.
You must be talking about the ability to lock in very low light independently where that lock is. It may be the case that the AF module is great in obtaining (potentially incorrect) lock in low light but then "for it's light meter to fail" doesn't seem to be a high bar to measure against. We agree that the colour calibration fails significantly too early, don't we? Or would the K-7 do no better in your tests?

I'm fairly certain that my even my lowly K100D would have obtained correct focus in the cockpit scene. It might have taken its time with several attempts but I've never seen such gross FF from it in any situation or had problems with it in studio light, etc.

Do I understand you correctly that you are saying "if the meter's sensitivity where up to scratch, the K-5's low-light AF performance would be spectacular", but we agree that the meter (probably by failing to provide information) manages to botch up the accuracy of the lock quite a bit earlier than this can be observed with other Pentax models.

Do you have a feeling as to whether the K-5's "low-light" (not really that low for slow lenses) performance is doomed to stay as it is because of the meter hardware used, or do you see a chance that another update might provide further help?

Or are you saying in the conditions under which you were measuring, the K-5 has been great even before the update and the front focus that users were reporting about earlier (and seems to have been addressed partially) took place in different conditions?

Frankly, I'm a bit confused.


Last edited by Class A; 03-18-2011 at 02:52 PM.
03-18-2011, 03:43 PM   #362
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QuoteOriginally posted by Class A Quote
I was talking about a very subtle bias. Probably I saw just measurement noise or the result of you skipping the fine tuning but there seemed to be a very small trend which I saw by quickly replacing one chart with the other. I agree that it isn't terribly relevant for the overall result.


You must be talking about the ability to lock in very low light independently where that lock is. It may be the case that the AF module is great in obtaining (potentially incorrect) lock in low light but then "for it's light meter to fail" doesn't seem to be a high bar to measure against. We agree that the colour calibration fails significantly too early, don't we? Or would the K-7 do no better in your tests?

I'm fairly certain that my even my lowly K100D would have obtained correct focus in the cockpit scene. It might have taken its time with several attempts but I've never seen such gross FF from it in any situation or had problems with it in studio light, etc.

Do I understand you correctly that you are saying "if the meter's sensitivity where up to scratch, the K-5's low-light AF performance would be spectacular", but we agree that the meter (probably by failing to provide information) manages to botch up the accuracy of the lock quite a bit earlier than this can be observed with other Pentax models.

Do you have a feeling as to whether the K-5's "low-light" (not really that low for slow lenses) performance is doomed to stay as it is because of the meter hardware used, or do you see a chance that another update might provide further help?

Or are you saying in the conditions under which you were measuring, the K-5 has been great even before the update and the front focus that users were reporting about earlier (and seems to have been addressed partially) took place in different conditions?

Frankly, I'm a bit confused.

Ditto

you lot lost me about 20 pages back...

I dont even know how to read Falks charts that hes put up...a bit of layman talk would help the less brighter of us..{ME}...
03-19-2011, 04:01 AM   #363
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QuoteOriginally posted by Smeggypants Quote
hey there planedriver, this pretty much
I found my K-5 was front focussing much more with more complex lighting. A combination of light from my computer monitors and tungsten light made it front focus much more than just tungsten light
thanks Smeggy it's good to know I'm not alone


pictures again..K-5 DA35 2.4

both iso1600 f2.4 1/40 EV-0.7 autoWB same lights

on this one focus on her right eye (left on the pic) spot on





this time focus on her face..still her shoulder, the letters on her t-shirt and the drawing on the table is in focus.





this is what I call unreliability

Andras
03-19-2011, 04:16 AM   #364
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Ouch ... number #2 is cruel in the sense that you see that if the focus was a bit to the back it would be just great and maybe unusual in that you can tell ... you are still getting this with the 1.03 fw, right? What order of magnitude would the hit/miss ratio seem to be?

03-19-2011, 04:33 AM   #365
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QuoteOriginally posted by planedriver Quote
thanks Smeggy it's good to know I'm not alone


pictures again..K-5 DA35 2.4

both iso1600 f2.4 1/40 EV-0.7 autoWB same lights

on this one focus on her right eye (left on the pic) spot on





this time focus on her face..still her shoulder, the letters on her t-shirt and the drawing on the table is in focus.





this is what I call unreliability

Andras
Was the 2nd pic taken using a center AF point focus-recompose method?
03-19-2011, 05:37 AM   #366
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QuoteOriginally posted by jolepp Quote
Ouch ... number #2 is cruel in the sense that you see that if the focus was a bit to the back it would be just great and maybe unusual in that you can tell ... you are still getting this with the 1.03 fw, right? What order of magnitude would the hit/miss ratio seem to be?
I don't want to get into any guessing..I don't have a tripod to do more testing..some pictures are just not right. Still the 1.03 seems to be better than the previous version.

QuoteOriginally posted by GrinMode Quote
Was the 2nd pic taken using a center AF point focus-recompose method?
No I never use the recompose method except with my K-x..and that's why I know it has it's pitfalls. I like the selectable AF points very much.


Anyway sometimes I think I use my camera close to it limits...maybe? Or I expect too much?

Andras
03-19-2011, 07:42 AM   #367
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QuoteOriginally posted by planedriver Quote
thanks Smeggy it's good to know I'm not alone


pictures again..K-5 DA35 2.4

both iso1600 f2.4 1/40 EV-0.7 autoWB same lights

on this one focus on her right eye (left on the pic) spot on





this time focus on her face..still her shoulder, the letters on her t-shirt and the drawing on the table is in focus.





this is what I call unreliability

Andras
I agree that it is definitely OOF and to a certain extent the unreliability makes it worse. However I have one question; on the second shot where was the camera? Was it at subject eye level? Shirt level shooting upwards a bit?
It makes a difference, because I'm thinking that if the camera is at eye level, the shortest distance is between camera and eyes, it is a longer distance to shirt or drawing on table. Then the camera is back focusing. If the camera was lower, then the shortest distance is to the shirt/drawing and the camera is front focusing.

NaCl(simple geometry)H2O
03-19-2011, 09:14 AM   #368
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QuoteOriginally posted by jolepp Quote
Ouch ... number #2 is cruel in the sense that you see that if the focus was a bit to the back it would be just great and maybe unusual in that you can tell ... you are still getting this with the 1.03 fw, right? What order of magnitude would the hit/miss ratio seem to be?
Doing further rough real world tests I'm also finding a variability. My k-5 is always FFing at EV2 and below but the amount varies somewhat. This happens whether the focus barrel starts moving from infinity or nearest focus distance

03-19-2011, 09:22 AM   #369
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QuoteOriginally posted by NaClH2O Quote
However I have one question; on the second shot where was the camera? Was it at subject eye level? Shirt level shooting upwards a bit?
It makes a difference, because I'm thinking that if the camera is at eye level, the shortest distance is between camera and eyes, it is a longer distance to shirt or drawing on table. Then the camera is back focusing. If the camera was lower, then the shortest distance is to the shirt/drawing and the camera is front focusing.

NaCl(simple geometry)H2O
I think it was at eye level..however I'm still thinking about what you are talking about..if the camera focuses at a point..(at a distance..) than that "layer' at that distance should be in focus..I don't think it has to do anything with angles..am I wrong? Anybody?

Andras
03-19-2011, 09:58 AM   #370
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QuoteOriginally posted by planedriver Quote
I think it was at eye level..however I'm still thinking about what you are talking about..if the camera focuses at a point..(at a distance..) than that "layer' at that distance should be in focus..I don't think it has to do anything with angles..am I wrong? Anybody?

Andras
I think you are right. The focus plane should be a flat area with no curvature. Having said that, I know some lenses actually do have a curved focus plane (for example see Tamron AF 17-50mm f/2.8 SP XR Di II LD Aspherical [IF] (Canon) - Review / Test Report - Analysis).
03-19-2011, 02:59 PM   #371
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QuoteOriginally posted by starbase218 Quote
So you're basically saying that although the K-5 may under certain conditions frontfocus, other cameras are likely to not be able to focus at all in those situations?
now this is a situation i can certainly live with...... i don't take many shots in low lighting scenarios, but if my k-5 can do what canonikons can't, well that's just icing on the cake....
dave m
03-19-2011, 03:19 PM   #372
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QuoteOriginally posted by starbase218 Quote
I think you are right. The focus plane should be a flat area with no curvature. Having said that, I know some lenses actually do have a curved focus plane (for example see Tamron AF 17-50mm f/2.8 SP XR Di II LD Aspherical [IF] (Canon) - Review / Test Report - Analysis).
Curvature doesn't come into it I don't think. For the sake of argument, say that Andreas' camera was at the eye level of the subject's eyes and 50cm away. That is straight line distance. If you then measure the distance from the camera to the shirt it will be some measurment larger than 50 cm. Say again for the sake of argument 52 cm. That plane is 2 cm further away than the plane of the eyes. The illustration on the table is even further away, maybe 52.5 cm. So I would say that even tho the camera is telling Andreas that it is focused at 50 cm it is actually focused at 52 - 52.5 cm.
It makes sense to me but that really doesn't mean much. There's lots of stuff that makes sense to me that is false. If I'm wrong I hope someone can explain to me why.

NaCl(won't be the first time I've stepped in it, won't be the last)H2O

edit: BTW both are very good shots (well the second would be great if she was in perfect focus)
03-19-2011, 11:31 PM   #373
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Truly flat focal plane is the ideal goal, much like "ideal gas" or "frictionless surface". There's always some spherical aberration across the film plane. Lens design would become much simpler if we used hemispherical sensors - I've sometimes wondered why we don't, but I bet it has something to do with the difficulty of building a chip fab that produced hemispherical-section sensors.

That said, in an ideally designed lens, the focal distance at focus is measured at the axis of the lens. You're absolutely right that objects at the edges of the frame are further from the focal plane than objects in the immediate paraxial region. You can actually calculate how much farther away it is with the good ol' pythagorean theorum. For example, something in the focus plane and twenty centimeters from the focal point @50 cm would be sqrt(50^2+20^2) or ~53.8 cm from the film plane - but *should* be in focus with a properly designed lens.

Trivia - macro lenses used to be described (often) as flat-field, meaning they were optimized for copywork, having maximum flatness of field for highest edge-to-edge sharpness. /trivia
03-20-2011, 12:03 AM   #374
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QuoteOriginally posted by NaClH2O Quote
Curvature doesn't come into it I don't think. For the sake of argument, say that Andreas' camera was at the eye level of the subject's eyes and 50cm away. That is straight line distance. If you then measure the distance from the camera to the shirt it will be some measurment larger than 50 cm. Say again for the sake of argument 52 cm. That plane is 2 cm further away than the plane of the eyes. The illustration on the table is even further away, maybe 52.5 cm. So I would say that even tho the camera is telling Andreas that it is focused at 50 cm it is actually focused at 52 - 52.5 cm.
It makes sense to me but that really doesn't mean much. There's lots of stuff that makes sense to me that is false. If I'm wrong I hope someone can explain to me why.

NaCl(won't be the first time I've stepped in it, won't be the last)H2O

edit: BTW both are very good shots (well the second would be great if she was in perfect focus)
QuoteOriginally posted by jstevewhite Quote
Truly flat focal plane is the ideal goal, much like "ideal gas" or "frictionless surface". There's always some spherical aberration across the film plane. Lens design would become much simpler if we used hemispherical sensors - I've sometimes wondered why we don't, but I bet it has something to do with the difficulty of building a chip fab that produced hemispherical-section sensors.

That said, in an ideally designed lens, the focal distance at focus is measured at the axis of the lens. You're absolutely right that objects at the edges of the frame are further from the focal plane than objects in the immediate paraxial region. You can actually calculate how much farther away it is with the good ol' pythagorean theorum. For example, something in the focus plane and twenty centimeters from the focal point @50 cm would be sqrt(50^2+20^2) or ~53.8 cm from the film plane - but *should* be in focus with a properly designed lens.

Trivia - macro lenses used to be described (often) as flat-field, meaning they were optimized for copywork, having maximum flatness of field for highest edge-to-edge sharpness. /trivia

Ok I accept that...still I would expect that in a simple shooting situation like my second picture proper focus can be achived by pointing the focus point on her face.

If the above outlined theory had that much effect on photography I think there would be even more "out-of-focus issues".

Andras
03-20-2011, 01:22 AM - 1 Like   #375
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If anyone wants to compute the effect of "focus and recompose", I developed a formula in another thread about image sharpness. Plug in values for "d = distance to subject" and "alpha = angle from frame centre to point of focus" into
x = d (1/(cos alpha) - 1)
and "x" will amount to the additional distance introduced to the point of focus due to the rotation of the camera. A DOF calculator will then tell you whether the point of focus is still within the DOF. BTW, the effect is often quite small but big enough for Hasseblad to built in rotation sensors into some of their models so that they can readjust the focus accordingly. I didn't try to assess the respective values for the shot provided, but I very much doubt that the FF can be explained by a "focus and recompose" error, not only because a (pure) recompose rotation will always lead to back focus w.r.t. to the focus point.

Last edited by Class A; 03-20-2011 at 03:03 AM.
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