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Old Feb 6, 2009, 8:25 AM   #1
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Ihear how some shots are "soft around the edges" or soft at a certain focal length. Can someone give me a link or post some shotsand point out exactly what it is to be soft? I assume it just means notsharp, but how do you tell the difference between soft and maybe just blurry from shake or something.I just purchased the tamron 70-300 di ld. How canI find out the best focal points to avoid a soft shot?
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Old Feb 6, 2009, 9:17 AM   #2
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All the focus points are in the central part of the frame, and are not likely to be influenced by edge or corner softness. And the autofocus system looks for contrast, and settles on the focus setting that gives the greatest contrast, so even if that portion of the image is soft, you'll still get the best the lens has to offer (barring, of course, Focus Shift, Frontfocus, Backfocus, or really large pieces of dirt on the lens.)

You can look at http://en.wikipedia.org/wiki/Soft_focus

Sometimes soft focus is intentional, sometimes it is a flaw that you can capitalize on, but mostly it is a flaw that interferes with a desired result.

As inexpensive telephoto zoom lenses go, the Tamron 70-300 Di LD is one of the best. [See http://www.slrgear.com/reviews/showp...uct/455/cat/23] It is especially sharp when stopped down even just one f-stop.
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Old Feb 6, 2009, 10:01 AM   #3
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Thank you for the quick reply. One more question. can you please explain
Quote:
It is especially sharp when stopped down even just one f-stop.
Does that mean going from f6-f5? Thanks again.


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Old Feb 6, 2009, 10:17 AM   #4
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antonlmo wrote:
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Thank you for the quick reply. One more question. can you please explain
Quote:
It is especially sharp when stopped down even just one f-stop.
Does that mean going from f6-f5? Thanks again.
The F-Number [See http://en.wikipedia.org/wiki/F-number] is the ratio of the focal length to the pupil diameter. The smaller F-Numbers are for larger openings. When you use a larger F-Number, the pupil diameter is smaller, so most lenses will become sharper. Some lenses actually need two or more stops to reach an acceptable level of sharpness, but the Tamron 70-300 Di LD gets very sharp after just one stop.

At a focal length of 70mm, the maximumaperture of the Tamron 70-300 Di LD is f/4.0, and it is reasonably sharp at that aperture. But when stopped down to f/5.6, it becomes quite sharp. At 300mm, it would be soft at it's maximum aperture of f/5.6, but at f/8.0 it becomes reasonably sharp.

So "stopped down" means using a smaller aperture, which is represented bya numerically larger F-Number.
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Old Feb 6, 2009, 11:15 AM   #5
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Thanks again,I really blundered with the f6-f5 comment! lol Can anyone suggest some simple tests thatI can perform to see thedifferences,since It should be a pretty weekend here, maybeI can try shooting at the different apertures at something like a coke bottle. Any suggestions as to how far awayI should be from the subject?I hope this makes sense
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Old Feb 6, 2009, 12:28 PM   #6
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I like brick walls. They will show both sharpness and optical distortion. Make sure it is well lit by direct sunlight. If a cloud passes by, wait until it has passed before continuing.

Use a tripod (and turn off SSS.) Pick a focal length (minimum, maximum, and whatever otherincrements are on the lens barrel.) Use Aperture Priority. Vary each shot by a full f-stop (f/4.0. f/5.6, f/8.0, f/11, f/16, f/22, f/32.) (You can probably skip f/22 and F/32, since those aresmaller thanthe Diffraction-Limited F-Number for Alpha dSLRs. (That's where thephysical properties of light start making the image soft again.))

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Old Feb 7, 2009, 9:22 AM   #7
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OK I will try that. Just take some pics of a brick wall, I don't think I can mess that up. lol
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Old Feb 7, 2009, 10:16 AM   #8
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Be sure to use a tripod. That way each shot at the same focal length will be of the same bricks, so you can 'pixel peep' from one shot to the next, and the only difference will be the aperture.

Also make sure you are shooting straight at the wall, so that the bricks in the upper left corner of the image will be just as far away as the bricks in the lower right corner.
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Old Feb 7, 2009, 10:39 AM   #9
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TCav's suggestion of a brick wall for a target is a good one. Another good one is a page of want ads stuck up on a wall. Makes for something to do on one of those rainy/damp days when you don't want to go outdoors. Make it as flat as possible and follow all the points TCav made about shooting a brick wall.
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Old Feb 9, 2009, 6:35 PM   #10
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Ok. I did it with the brick wall and did begin to see the differences. I just got my Maxxum 28-85mm. Since it's dark outside I will try want ads in the house. I think am through with buying lens until I can learn to actually use them properly. lol I just ordered a camera backpack from amazon to hold everything in. Then I will be all set.
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