|
Forum: Pentax DSLR Discussion
08-16-2007, 02:00 PM
|
| |
See my first post.
Aha. This I've never tried yet. Thanks for the suggestion.
It's a bit difficult to shoot that fence again, since it's prob about 10000km from where I am now, but I tried shooting my computer screen again, and it's difficult to see differences due to SR off or the 2 second delay. Perhaps with other lenses the difference will be bigger.
Well, I knew about that, but I wasn't interested in the noise from the high ISO. You're right that it improves the picture, but it wasn't the primary problem of the blurry picture.
The polarizer I used was a cheap Targus 62mm C-PL.
Thanks,
Erik.
|
|
Forum: Pentax DSLR Discussion
08-14-2007, 03:05 PM
|
| |
Some more experimenting showed that it was the polarizer which caused the focussing problems !
I removed the polarizer and now get sharp pictures, even handheld (with SR on).
Thanks you all for the helpful hints !
Erik.
|
|
Forum: Pentax DSLR Discussion
08-14-2007, 01:26 PM
|
| |
YES ! This fixed it.
And even more: when I switch back to AF.s the blur reappears.
I used F/6.7 to verify this and the DOF isn't that big, so I had to adjust only a very tiny amount to get the better focus. In the viewfinder the difference is very small.
Unfortunately I have a 1.3 firmware, so I can't start experimenting with focussing parameters (I thought this was possible with 1.1 firmware using some debug settings).
Is this now the end of the road for me ?
Thanks,
Erik.
|
|
Forum: Pentax DSLR Discussion
08-13-2007, 03:58 PM
|
| |
This is interesting:
A picture take from the computer screen
Again Pentax K10D, Sigma 18-200
Common settings: 200mm, 1600 ISO.
Camera stabilized on a tower of CD boxes :-)
Attached picture is a 100% crop; the "GHz" was more or less in the center of the original picture.
First with F/5.6, 1/90sec
Attachment 4304
Then with F/38, 1/2 sec
Attachment 4305
I guess my problems are now reduced to either:
a) the lens (bad resolution at large apertures)
or
b) focussing problems (the F/38 increases the DOF, which masks the focus error ? )
I still don't understand why the blur is more pronounced on the vertical lines...
Erik.
Forgot to mention: SR was off in both cases.
|
|
Forum: Pentax DSLR Discussion
08-12-2007, 03:06 AM
|
| |
If it's a focus error, it must be a pretty big one, since you can see that neither the background nor the foreground is in better focus then the center.
Interesting... Can you give some more info how you use the camera ? What focus method (AF.c or AF.c, focus selection: center/auto/manual, ISOs ? etc...) Clearly you have a different lens. Which one ? Have you ever tried the Sigma 18-200 ?
Now about the Sigma: I have some doubts about the indicated 200mm focal length. When the lens is turned to 135mm on the zoom ring, the LCD dispays 133mm (that's not far off). The if you turn the zoom ring till then end, I notice that the lens barely extends any further, and also the framing of the picture changes very little. The difference 200mm/135mm should give me roughly a 1.5 magnification right ? That's not what I see in the viewer: that looks more like 1.1 factor of zoom.
Oh one more thing: the picture shown above was taken with a circular polarizer on the lens. I guess this shouldn't have a big impact since I focussed manually. I remember also checking the focus ring, and it was set to 'infinity'. That should have been a good approximation for 800m.
Thanks for your feedback,
Erik.
|
|
Forum: Pentax SLR Lens Discussion
08-11-2007, 05:00 PM
|
| |
| |
|
Forum: Pentax DSLR Discussion
08-11-2007, 03:33 PM
|
| |
Mostly I'm using center focus (including the pictures above), but as mentioned my AF switch was stuck at that time and I had to use MF.
Let me explain my theory why SR can never be perfect on the K10D.
Cameras get the focus length from the lens via a few contacts on the lens mount. That information however is not very accurate since the numbers increase in large steps. To be more specific: I set the camera to wide angle and zoom in very slowly, while frequently pressing the info button to read the focus length on the LCD.
So it starts at 18mm. It remains the same number while zooming in untill suddenly the camera reports 22mm.
Again in other words: the information which the camera has on the focus length increases in discrete steps. For my Sigma these are: 18, 22, 26, 30, 34, 40, 45, 53, 63, 75, 93, 113, 133, 155, 170, 200.
This info is critical for the SR: when you tilt the camera up by 1 degree at 18mm the sensor needs to shift down by a small amount. When you tilt it by 1 degree at 200mm, the sensor must shift by much more.
Based on these two facts, I conclude that SR can never compensate your movement (even if it knows exactly how much you moved) exactly because it doesn't know whether the actual focus length was 177 or 199 mm.
Let's say that a 1 degree shift needs to be compensated by a 1mm movement of the sensor at 200mm. But maybe the actual length was 171mm. Now the sensor moves by 1mm, whereas it should have moved by something in the range of 0.85mm. The result is a significant amount of blur remains.
Does anyone see any holes in this theory ?
Note: maybe this limitation doesn't exist on in-lens stabilization systems: they might have more accurate focus length info.
Erik.
|
|
Forum: Pentax DSLR Discussion
08-11-2007, 03:36 AM
|
| |
Here's my problem: I have a Sigma 18-200, which I use on my K10D and when using the lens at 200mm, I very often have blurry pictures. What troubles me is that I don't know exactly what I'm doing wrong. Some of the reason I have in mind are:
- SR is unable to compensate my shaking hands when the lens set to 200mm.
- no good focus ( the auto-focus was not usable for several weeks because of a bad contact in the MF/AFc/AFs switch, so I've had to use MF a lot during my holidays)
- The sigma just isn't sharp enough at 200mm
It cannot be only the SR, since I've tried shooting at 1/1000sec and still have problems.
I doubt it's only the focussing, since I rely on the 'in-focus' beep when manually focussing.
I can't just put the blame on the sigma, since I _do_ have some pictures which are sharp at 200mm.
So I guess it's just a combination off all of the above ???
What are your tricks to have sharp pictures at such focus lengths ?
Here's an example of a really bad one: it's the impact hole in the center of the "Meteor Crater" in Arizona. It's was taken at a distance of about 800 meter on a very windy day, ISO 400, 1/1000sec, 200mm, F1/6.7.
Original:
Attachment 4237
100% crop:
Attachment 4238
Thanks,
Erik.
|
|
Forum: Pentax SLR Lens Discussion
08-11-2007, 03:03 AM
|
| |
Yes, that's what I meant. The reason why I ask this, is because I have a lot of trouble getting sharp pictures with the Sigma at 200mm. I first suspected that even the SR may not be good enough to stabilize my shaking hands, but I've had blurry pictures even with shutter speeds at 1/1000 sec (ISO 400).
|
|
Forum: Pentax SLR Lens Discussion
08-10-2007, 03:52 PM
|
| |
I have a Sigma 18-200 and am dissapointed when using it at 200mm.
Can you guys post a few 100% crops of the Tamron at 200 and 250 mm ?
Does anyone have both the Tamron and the Sigma to do a direct comparison at 200mm ?
Thanks,
Erik.
|
