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Old May 2, 2009, 1:34 AM   #1
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I just got this lens but it looks like it's defective. F1.4 is sharp. Then from from F2 to F5.6 it's blury and only sharpens up at F8.

Is there explanation for this? Or is it just defective?

Here is a sample (100% crop from center).


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Old May 2, 2009, 6:45 AM   #2
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There is probably another explanation for it (for example, blur from camera shake, blur from mirror slap if shutter speeds were too slow and mirror lockup wasn't used, focus error caused by using the wrong focus point or the camera set to fire without locking focus).

Were you using a tripod with a self timer or remote shutter release, and are you sure the camera was locking focus with the center focus point used, with it framed so that it was in the middle of your intended subject?

What were your shutter speeds at each aperture setting? Was stabilization on or off (with a tripod, I'd try it turned off). What camera model? Was mirror lockup used if you have it available?

Even if you were using a tripod with a remote shutter release and focus was locked correctly, blur from mirror slap will be worse at a some shutter speeds (if they're not slow enough so that the vibration has stopped for most of the exposure, or if they're not fast enough to offset any blur caused by vibration). So, that may have been what happened in this case (if everything else was done correctly).

It's better to do that kind of test in good light in very controlled conditions using a tripod with a remote shutter release or self timer, so that you're not running into potential problems with things like blur from camera shake or mirror slap; or focus problems due to moving the camera or not locking the center focus point on the center of your intended target without the need to reframe (since reframing after a focus lock can introduce focus errors, too).

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Old May 2, 2009, 10:08 AM   #3
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Another possibility might be Focus Shift.
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Old May 2, 2009, 12:49 PM   #4
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I was using tripod. Nothing to do with the camera since only this lens behaved this way.
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Old May 2, 2009, 1:49 PM   #5
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... 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.
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Old May 2, 2009, 2:18 PM   #6
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evilthought wrote:
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I was using tripod. Nothing to do with the camera since only this lens behaved this way.
Repeating my questions from the last post I made to this thread:

What were your shutter speeds at each aperture setting? Was stabilization on or off (with a tripod, I'd try it turned off)?

Look, the last time I saw "lens test" from you, the EXIF showed you were using shutter speeds of 1 second or longer using f/8 and ISO 100.

http://forums.steves-digicams.com/fo...mp;forum_id=84

That's no way to test lenses. Test them in better lighting with faster shutter speeds. Heck, at shutter speeds that slow, most cameras need to use a dark frame subtraction noise reduction system, decreasing detail during remapping of hot pixels, and you can't expect accurate exposure at shutter speeds that slow either.

If you were trying to do the same thing with this test in the same lighting, you'd be getting shutter speeds of around 1/30 second at f/1.4 and ISO 100, in the same conditions you'd get 1 second at f/8 using ISO 100.

So, that may be fast enough to reduce blur from things like mirror slap. But, as your shutter speeds get slower (for example, dropping to around 1/15 second at f/2, 1/8 second at f/2.8, etc.), then mirror vibration induced blur could be causing the problem, especially if you're using a cheap tripod.

As shutter speeds get slower (for example, when you're stopped down to around f/8 getting shutter speeds closer to 1 second), then mirror vibration becomes less of an issue, with the image getting sharper again since the time the vibration is worse is making up a smaller percentage of the total exposure time). You can't expect AF to be very accurate in light that dim either, if you were not using manual focus.

That's what I suspect is going on here, judging by your last lens test.

See how the lens behaves in better lighting using fast enough shutter speeds to take those types of issues out of the equation.

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Old May 2, 2009, 3:40 PM   #7
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"Repeating my questions from the last post I made to this thread:"

Repeating the answer from the last post, it has nothing to do with shutter speed or camera shake. This is the only lens that I have that consistently behave this way, even on tripod and fast shutter speed.

The shutter speed, by the way, was 1/60, on F2.










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Old May 2, 2009, 3:50 PM   #8
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And I see same problem with F2.8 and 1/15 shutter speed.
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Old May 2, 2009, 4:42 PM   #9
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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?
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Old May 2, 2009, 4:43 PM   #10
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Quote:
And I see same problem with F2.8 and 1/15 shutter speed.
Which may be too slow. You can get vibration causing blur from a variety of sources. ;-)

I still think it's highly unlikely you've got a lens that is sharper at f/1.4 compared to f/2 and f/2.8, then getting sharper again as you stop down the aperture more, unless you're doing something to cause it.

For example, camera movement, vibration, stabilization system induced blur (which is why I'd try with it turned off when using a tripod) or focus error.

Are those results repeatable?

What do you get when you try the tests in better light using faster shutter speeds?
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