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Old Oct 21, 2005, 10:30 AM   #1
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fozzyber's Avatar
Join Date: Nov 2004
Posts: 1

I just bought a used 50mm f1.4 minolta lens from KEH.
I seem to be having some issues with it soft focusing when stopped down. (f1.4)
My setup: I mount my 7D on a tripod w/ cable release stop down to f1.4
Autofocus on a high contrast lettering about 12-15 feet away.
And I get a good image, but if I zoom in on the LCD the high contrast
Lettering (lettering is about 3" tall) the fuzzyness is maybe ΒΌ" in real life.:?
If I stop up to about f2.8 or above the issues go away. Is this normal for this fast lens?
My 7D has also just come back from the factory for flash calibrate.
While it was there I told them to check for back focus so I don't think it is that.
Is this normal for this type lens or should I try to return it for a different one.

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Old Oct 21, 2005, 10:59 AM   #2
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Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378

What you're seeing is perfectly normal for this type of lens, but it looks like it should sharpenup a lot by about f/2 (comparable or better thanmost other lensesshooting wide open by that point). It's a very high quality lens.

Most lenses with apertures as large as f/1.4 are going to be a bit soft "wide open".

For that matter, most lenses are a bit softer wide open, even if they don't have larger apertures available. ;-) But, larger available apertures in a lens sometimes does force some design compromises. For example, it's not uncommon to see a lens with a smaller aperture that is much sharper wide open.

But, the larger apertures are nice to have for when you need them (lower light, or to minimize depth of field as much as possible (for example, portraits, where a slightly softer look is often preferred anyway).

If you look an MTF chart for this lens, you'll see that it is *quite* a bit softer wide open, compared to stopped down. It was tested as follows for weighted MTF (higher numbers are better):

f1,4 0,51, f2 0,75, f2,8 0,78, f4 0,83, f8 0,85

Click on this lens at photodo.com, and you'll see the MTF charts and numbers from testing. Scroll down a little after you click on a lens and you'll see the MTF tests. You'll also see a link on the page that comes up that explains what the numbers mean.

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