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Forum: Pentax News and Rumors
12-12-2012, 09:29 PM
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Forum: Pentax News and Rumors
12-12-2012, 06:58 PM
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wow wow here's all specialists.
I would like to see some autofocus performance tests ...
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Forum: Pentax News and Rumors
12-11-2012, 10:09 PM
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as i said before in this thread,
almost nothing is improved on sensor recent years. it is only upgrading the algorithm, firmware, and AF system.
now the sensor High ISO performance is roughly matter of sensor size.
today nikon's best aps-c sensor score is around 1200. according to FF sensor size is 1.5*1.5 times than the aps-c, the high ISO score would be 1200*2.25=2700. today nikon's best FF sensor score released in this year is around this figure....
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Forum: Pentax News and Rumors
12-11-2012, 07:18 AM
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by reading the DXOMARK results, the sensor actually improving very slowly, or even stopped improving recent years. now days the best sensor score is roughly matter of area of sensor.
best sensor aps-c sensor score is 1200. full frame sensor size is 2.25 times bigger than aps-c. so best FF sensor score would be around 1200*2.25=2700. actually by DXOmark the best FF sensor iso is around this number, best in 2012 is around 2900. which is lower than nikon d3s released in 2008.
btw, dxomark missing one big performance measurement, focus performances including accuracy, speed, sensitivity!
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Forum: Troubleshooting and Beginner Help
12-02-2012, 05:18 PM
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the LV focusing is good but it need to zoom, use tripod, along with keep your composition. Sony got a good idea on this, focus peaking sounds very handy in LV mode.
now I bought a split-impage rangefinder that is on the shipment. have any suggestion or hints to this method?
thank you jamesm007, very informative post. i'm reading the manual mostly for interest, as you say every lens differs on pentax camera some lens is lucky for some camera body some are not, so I'm not going to tune and try to get fine on every lens.
the 18 55's aperture is too small that in indoor activities, scene pictures, it is not obvious even if out of focus.
after previous tuning I've shot some scene picture with 55mmF/8 the picture is very sharp no sign of focus issue.
but when i look back indoor pictures taken before the tunings, actually many seems out of focus slightly, not that perfect . on indoor home shooting the 18 55 too can isolate subject from background forground slightly.
the 18 55's DoF is too wide, harder to determine back/front focus with a focus chart. maybe need to print a gigantic version of the chart
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Forum: Troubleshooting and Beginner Help
12-02-2012, 06:57 AM
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today i bought a focusing screen online. try after several days later, this is my last hope.
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Forum: Troubleshooting and Beginner Help
12-02-2012, 01:32 AM
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yesterday I calibrated on manual lenses and saw kit lens seems just fine, but when i test using focus chart find that already lose focus too much.
cost my whole day calibrating back on kit lens on 30/f4, 55/5.6 , so manual lenses are all front focus again. i'll never do again.
i've read on internet k20d can memorize idividual lens AF tuning, but k-r doesn't. its sad.
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Forum: Troubleshooting and Beginner Help
12-01-2012, 10:08 PM
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thanks for advice.
I don't judge focus with my eye on manual lenses.
instead watch the focus indicator hexagon light. the focus indicator is working with the PD AF system.
I'm using a tripod and 1m long metal ruler for focus chart.:lol:
even though I write a log every little change, it is somehow messed... I turn both screw back to 0, I assume it's original position, but My 18 55 already out of focus.
so i guess the focus problem have several causes.
- the max aperture. camera do light metering and autofocus on max aperture. aperture is another word for light ray angle toward camera. So I open the 50/1.7 lens, and removed the diaphragm actuator so the aperture size would be determined only by the aperture ring, not controlled by the camera. so on the same lens I tested on F/1.7, F/2.8, F/5.6, result shows on all stops behaves identical.
- focal length. maybe the autofocus behaves differently on different focal lengths. because on same focal length different max aperture lenses have no focus problems according to previous testing. but as i mentioned in original post, if calibrated on 50/1.7(with camera debug mode), the camera out of focus on the 18 55, including its 50mm. but I've also tested on the kit lens's different focal lengths It does have different focal behaviors. btw kit lens aperture is too small that too hard to do the test.
- lenses are defective. such as the lens mount is tilted, or internal parts are become inaccurate.
now i don't have many other lenses, and have no friends using pentax, brought this camera from other country, and no pentax service nearby so I've done those by myself.:mad:
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Forum: Troubleshooting and Beginner Help
12-01-2012, 04:45 AM
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I've bought my K-r 1 year ago, this is my first DSLR so at first I did not notice focus problem on manual lenses.
on the DA 18 55 II kit lens it seems no focus problem.
I tried a rikoh 50/1.7 and another 135/2.8 on the camera both heavily lose focus.
both lenses can only focus on center point, on in focus the hex indicator lights, press the shutter the picture get always out of focus.
- so I adjusted AF fine adjustment from the menu got nothing improved, but got the kit-lens out of focus. so cancelled this method.
- after months, using Pentax's engineering mode software, focus calibration done on 135mm, the 50 lens and the 18 55 had failed. if calibrate on 50 the 135 would fail.
- and later one day i opened k-r's bottom cover, found two screw, which hiding deeply in a hole but can twist with a screwDriver, this must be for focus adjusting. I carefully turn both screw with same amount same direction, result is the same with engineering mode focus calibration. so i'm almost disappointed and gave up.
- today I thought what if turn focus screw independently? so i just tried, and got it fixed.
table in the attachment shows the operation and results. there are tools used to the procedure too.
Is there any service manual or something that telling how to calibrate? even though now my camera could focus on ricoh 50/1.7 and another maunal 135/2.8 lens roughly, without problem on kit lens. but the focus accuracy is not assured, and all have i done is just experiment, there is no theoretical instructions. seems that on my k-r left screw calibration effects longer focus range, and, right screw have no effect on this.
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Forum: Photographic Technique
01-31-2012, 12:36 AM
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to take the same subject with same lens same aperture, APS-C camera need to be further from subject than FF's.
if take picture from same distance, same aperture, same lens then obviously APS-C's background will look more blurry, but it looks cropped from FF's.
the 2 charts below shows blurriness with APS-C and full frame sensor camera.
first chart is area of CoC and area of sensor ratio, I think this is more close with viewer's feeling of blurriness.
second chart is diameter of CoC and sensor height(16mm or 24mm) ratio.
in both charts
F=85mm
f=F/1.4
Size of view(m):2.0*1.3
APS-C shooting distance 7.0m
Full Frame shooting distance 4.7m
so what do you think? :rolleyes:
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Forum: Photographic Technique
01-30-2012, 07:00 AM
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ideally, YES.
in fact there are slight complications, such as pupil shape is not always a circle, and varies between different lenses and apertures.
i did not take them in to consideration, but I think those do not effect too much to my calculation.
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Forum: Photographic Technique
01-30-2012, 05:57 AM
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thanks for your reply altopiet,
DOF is depth of field, the sharp and clear range in your photo, not directly mean blurriness.
"Bokeh size" in my table directly mean blurriness.
sometimes, different lens have same DOF but it is not true always have same blurring on background.
sometimes shallow DOF can't achieve a strong blurry background, as shown in table, the blurriness will significantly increase by the background getting farther, but if the background is too near to the subject to be taken, the background can't be very blurry.
to let the background be more blurry do:
1.make the subject close to camera;
2.choose farther background( as shown in table, blurriness of farther than 100m background is almost equal to infinity background);
3.use bigger aperture;
4.longer focal length can make background blurry easier.
the tables are showing, background blurriness for the same full body portrait(first table, 2*1.3m of view) or half body portrait(second table, 1.5*1m) with 50mm, 85mm and 135mm lenses at different aperture setting.
you can compare those lenses on blurriness of same distance of background:
such as in the first table, 50 F/1.4's background is slightly blurry than 85mm F/1.8 before the background is near than 5m, but 85 F/1.8's blurriness get far stronger as distance increases.
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Forum: Photographic Technique
01-30-2012, 02:26 AM
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It's a common mistake that the smaller DOF mean the more blurry background, actually DOF can't directly represent how blurry is the background in your photo, but Bokeh size does.
So I did some calculation, made this BOKEH SIZE TABLE to indicate some popular portrait lens's blurriness of various far of background.
In the table,
The first column mean distance between background and subject to be taken.
“Bokeh size” is the diameter of circle of confusion on a sensor.
“Vertical resolution on an APS-C sensor” mean how many circle can fit into image's height(shorter side, not width). This is not the exact resolution of the blurry background, but you can refer this value as a measurable number of blurriness of the background. The lesser resolution the more blurry bokeh.
for full body portrait
for half body portrait
The value is calculated using a simple formula, assuming the lens is a simple thin lens. May be not accurate enough for lens specific or industry level consideration.
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