Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Sony Alpha dSLR / Konica Minolta dSLR, Sony SLT

Reply
 
Thread Tools Search this Thread
Old May 2, 2009, 4:06 PM   #11
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

You're shooting with an A200, right?

You don't have mirror lockup using the self timer with that model, and shutter speeds of around 1/15 second are about where I'd expect to see mirror vibration induced blur start showing up, especially if the tripod isn't very sturdy (and even if it is, the stabilization system could be allowing some CCD movement from vibration transmitted through the camera body).

When you start slowing down shutter speeds a lot more (as you would when using smaller apertures, represented by higher f/stop numbers) then the blur will start to diminish, because any vibration induced blur makes up a smaller percentage of the total exposure time at slower shutter speeds.

I'd rule out something you may be doing wrong (mirror slap induced vibration, focus errors, stabilization system induced ccd movement, etc.) before assuming anything is wrong with the lens with the symptoms you're showing us.

Are the test results even repeatable? Have you tried it in better lighting with faster shutter speeds?

JimC is offline   Reply With Quote
Old May 2, 2009, 4:15 PM   #12
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,544
Default

GBynum wrote:
Quote:
TCav wrote:
Quote:
... which increases the possibility that Focus Shiftmight be the problem. Focus Shift is what can happen when a camera's autofocus system focuses at a large maximum aperture, but when the photo is taken, the aperture stops down, causing that which was in focus at f/1.4 to be out of focus at f/4.
I'm neither the OP nor contributing, rather asking a followup optics question relating to the OP's question.

To check for focus shift, I'd guess a range of objects in front of and behind the intended plane of (auto) focus would be appropriate?

ASS_U_MING focus shift to be the problem, are levels of this magnitude common in lenses? Is there a way to correct for it? Is it a lens defect? Are there tests to quantify it?
Read the article I mentioned: http://diglloyd.com/diglloyd/free/FocusShift/index.html
TCav is offline   Reply With Quote
Old May 2, 2009, 4:19 PM   #13
Member
 
Join Date: Apr 2009
Posts: 35
Default

Why do you say it's unlikely?

According to the link posted above about "Focus Shift."

http://diglloyd.com/diglloyd/free/FocusShift/index.html

that's exactly what it should be.

Also, note only this lens does this.

Here is 50mm F1.7



and

Minolta 24-105





The F1.4 lens is worse under F5.6 consistently (see first post).




evilthought is offline   Reply With Quote
Old May 2, 2009, 4:48 PM   #14
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

Now you're comparing some of the images from one set of test images to some of the images in another set of test images. ;-)

You've got different exposures and different results in the tests from the 50mm f/1.7 above, especially since it appears to be a lot blurrier at f/4 in the first set of images compared to the f/4 shot in the ones you just posted (where f/4 looks like the sharpest one from the 50mm).

Everything you're showing me so far leads me to believe something else is causing the results you're getting and and it's probably not the lens, especially with the shutter speeds I see in your samples that still have EXIF data in them.

I'd repeat the tests in better lighting using faster shutter speeds (bumping up ISO speed if you need to), making sure you've got accurate focus. I'd try several sets of shots and see if there is any variation, too.

JimC is offline   Reply With Quote
Old May 2, 2009, 6:43 PM   #15
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,544
Default

I agree that your test methods have not been good, and that you should use a subject in better lighting. It's possible that Focus Shift is the cause of what you're seeing, but I'd prefer to see the results when the controls aremore rigid.
TCav is offline   Reply With Quote
Old May 3, 2009, 3:05 AM   #16
Member
 
Join Date: Apr 2009
Posts: 35
Default

JimC wrote:
Quote:
Now you're comparing some of the images from one set of test images to some of the images in another set of test images.


I bumped the ISO to 1600. You were partially correct. I don't get the same result as

last time but I still get random result. Only this lens is doing randomness. I still say

the lens is defective.



ISO is 1600 and the pictures were taken at the same time on tripod.




Minolta 50mm 1.4







Huh? That's random. I tried it again this time with 10sec self-timer delay and on

tripod which is pretty steady..







Still random. Why the other lenses don't behave this way if something was just

wrong with my testing method?

Here is F1.7 lens tested at the same time with ISO 1600





And Minolta 24-105 right after that



And Sony 18-70



I was wrong in earlier post though. Minolta 50mm 1.7 is sharpest. (however it does have bad lens flare).


Sony 18-70 is worst. Minolta 24-105 is not bad, given I got it new last week for only

$160.....

I will get rid of Minolta 50mm 1.4 though...


evilthought is offline   Reply With Quote
Old May 3, 2009, 4:41 AM   #17
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

Look at your last 50mm f/1.7 test. You're getting a sharper image at f/1.7 in that last set of samples, compared to the f/4 shot. That's not the way that lens behaves, and with random results from the 50mm f/1.4, you've got to have a problem that's in common with them.

If you're sure the tripod is steady (and most cheap tripods are not) with the camera mounted securely, something else is causing the issue. That's what you need to determine.

It's either going to be focus error (not using the correct focus point, camera set to fire without a good lock, something moving after focus lock, not enough light for accurate focus), or movement related blur (either from the camera, one of it's components like the CCD, the subject, the tripod, or lens mount issues).

Was stabilization on or off? Try it off, as you could be seeing SSS induced CCD movement when using a tripod.

Is your test target steady (i.e., not being blown by an air duct, etc.)?

How are you focusing (focus mode, focus point? Are you locking Focus for the entire series, or are you refocusing between shots? Are you sure the center focus point is locked on your target in an area that has good contrast? The camera isn't set to fire without a focus lock, is it?

How about your camera's lens mount (check it for loose screws, etc.)?

Any filters that could be causing optical degradation? Remove any you have installed.

Any bright light sources in the frame that may be causing focus errors from flare or focus problems from reflections from something in the frame?

Have you tried it in good lighting (i.e., open a window in the daytime by your tests if you need to)? Is the light source incandescent (versus any fluorescent lighting that may be cycling)?

You should not see those types of symptoms (like your 50mm f/1.7 sharper wide open versus stopped down to f/4) unless something is wrong that is in common with more than one lens.

JimC is offline   Reply With Quote
Old May 3, 2009, 5:51 AM   #18
Member
 
Join Date: Apr 2009
Posts: 35
Default

Quote:
Look at your last 50mm f/1.7 test. You're getting a sharper image at f/1.7 in that last set of samples, compared to the f/4 shot.
Yes, but the overall result was not as random as the one lens...


Quote:
Was stabilization on or off? Try it off, as you could be seeing SSS induced CCD movement when using a tripod.
Stabilization was on. I will turn it off next time...


Quote:
Is your test target steady (i.e., not being blown by an air duct, etc.)?
Yes steady.


Quote:
How are you focusing (focus mode, focus point? Are you locking Focus for the entire series, or are you refocusing between shots? Are you sure the center focus point is locked on your target in an area that has good contrast? The camera isn't set to fire without a focus lock, is it?
Spot focusing. Refocusing between shots. The camera doesn't fire without focus lock. It's black text on white paper. That's good contrast.

Quote:
How about your camera's lens mount (check it for loose screws, etc.)?
Looks fine...


Quote:
Any bright light sources in the frame that may be causing focus errors from flare or focus problems from reflections from something in the frame?

Have you tried it in good lighting (i.e., open a window in the daytime by your tests if you need to)? Is the light source incandescent (versus any fluorescent lighting that may be cycling)?
It's fluorescent plus incandescent.

I will try it outdoors today if weather is good.


evilthought is offline   Reply With Quote
Old May 3, 2009, 8:51 AM   #19
Member
 
Join Date: Apr 2009
Posts: 35
Default

I went out and tested again outdoor. I really shouldn't have wasted the time. It was

pretty clear from the first post that the lens is defective.



Full image...



Minolta 50mm 1.4





Minolta 50mm 1.7







Minolta 24-105







Sony 1870



evilthought is offline   Reply With Quote
Old May 3, 2009, 9:17 AM   #20
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

Now, the results are more what I'd expect from a test, with the lens softer at f/1.4, getting sharper as you stop it down more (versus the results you were getting indoors).

But, it appears that it's probably front focusing or back focusing, *if* you're sure you were using the Center Focus point for those shots.

I'd still use better test conditions so that you don't have any potential AF problems from flare related issues if you already have filters removed (for example, light from the shiny portion of the bottle or from shooting with a bright sky in the background).

I'd also try it on a different body and see if you have the same symptoms, and you may want to check your body for front or back focus using your 50mm f/1.7 (since those types of focus errors tend to be more problematic with an f/1.4 lens). Here's one way:

http://focustestchart.com/chart.html

JimC is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 2:38 AM.