Weird lens performance

futureal33

futureal33

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Hi all,

I cant quite get my head around this.

What do others think, I have a feeling it might be a lens centering issue... but really not sure.

The focus and DoF seem completely random on this lens (Sigma 70-200 f2.8)
I had diagnosed it as being a front focus issue, but now I look closer I am not so sure!

I am focused on the centre of the "Magnolia White" circle - which is in focus - great. But if you look at the 100% crop you'll notice that a non-uniform selection of other areas of the image are also sharp, and others arent.

At f2.8 the DoF should be pretty small, meaning that areas above and behind the focus area should be soft...but either side of it (ie, on the same focal plane) should also be sharp - which they are not.

Focus was bang on the middle of the magnolia white circle.

FWIW - camera was tripod mounted, AV mode, f2.8, Auto Focus etc etc

What are your thoughts?


IMG_6748 by futureal33, on Flickr


100% by futureal33, on Flickr
 
I'm not sure what the problem is here? Looks fine to me :shrug:

If you focussed on the centre of the magnolia circle (which is a bit risky anyway, as the lens needs contrast to focus), and it's sharp, surely it isn't front or back focussing? It's quite hard to work out what should and shouldn't be sharp, given that the book is curved in just about every direction though!

Chris
 
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If we go off the colour names:

Mangolia white = focus point, in focus - great.
Jet Grey & Smokey Blue - same focal plane - should be in focus, but are slightly soft.
Iceland green - "Iceland" is almost on the same focal plane and should be in focus - but isnt
Alabaster White - should be completely OOF, but isnt! This is possibly the sharpest area of the lot
Havanna Brown - why is this OOF if Alabaster White isnt?
Chestnut brown - should be OOF and is - great
Crimson Red - should be OOF and is - great
Cloudy Grey - should be OOF but is sharp as hell

It just seems to me that the sharp and OOF areas dont follow suit at all
 
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Ah right, I see what you're thinking. I'd say the focal plane is running from top left to bottom right, through alabaster white down to the top part of the cloudy grey circle.

The book is tilted, the 'jet grey' circle is closer to the lens than magnolia, and the smokey grey one is further away I'd say. If you don't mind me drawing on 2 simple lines I'll show you want I mean (your edit box says no, so I didn't do it originally :) )
 
Your test is flawed from the outset, far too many different planes due to curvature of the page, so it is impossible to accurately say what should be in focus.

Try the test again, but a bit more scientifically - here's a site that may help, with a chart:
http://focustestchart.com/chart.html
 
futureal33 said:
Fire away :) I dont mind you editing this, but I see what you mean regarding the focal plane. Its like the lens is twisted almost 90 degrees!!

Funny, but on other pictures Im getting different results, 100% inconsistent
Click to expand...

Here you go:

unled1kk.jpg


In fact, I'd say it's more complex that this, as what's in focus is probably curved, but it gives you a general idea. Remember, it's whatever is the same distance as the focal point that will be sharp. The book is sloping both from left to right, and background to foreground, making it a nightmare to work out what's what.


daugirdas said:
It is probably a good old centering defect.
Click to expand...

Is that assumption based on anything other than the fact it's made by Sigma?!
 
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david357 said:
Your test is flawed from the outset, far too many different planes due to curvature of the page, so it is impossible to accurately say what should be in focus.

Try the test again, but a bit more scientifically - here's a site that may help, with a chart:
http://focustestchart.com/chart.html
Click to expand...

Already done that, see below.

Test chart results indicate it to be fine.... but then look at the 2nd pic where I am focused straight on at the sign, yet the wood behind the sign is in focus and the sign soft.


IMG_6777 by futureal33, on Flickr

Sign

IMG_6754 by futureal33, on Flickr

100% of the sign - showing the wood behind is in focus, not the sign itself.

100 by futureal33, on Flickr
 
I don't put much faith in the focus test charts either to be honest (and they need to be perfectly flat too, being crinkled with some pebbles on doesn't help matters :D ).

The shot of the sign though is a much, much better test! What were the focussing settings, which AF points, which mode etc? I notice that on your other thread (the one comparing DOF at f/2.8 and f/4) there didn't seem to be this sort of error though.
 
chris321 said:
I don't put much faith in the focus test charts either to be honest (and they need to be perfectly flat too, being crinkled with some pebbles on doesn't help matters :D ).

The shot of the sign though is a much, much better test! What were the focussing settings, which AF points, which mode etc? I notice that on your other thread (the one comparing DOF at f/2.8 and f/4) there didn't seem to be this sort of error though.
Click to expand...

F2.8, manual focus selection point (on the 2nd L of "Willy's") - AV mode.

Ps - the one of my parrot on the other thread was Manaul Focus, and still wasnt 100% right lol :)
 
futureal33 said:
F2.8, manual focus selection point (on the 2nd L of "Willy's") - AV mode.

Ps - the one of my parrot on the other thread was Manaul Focus, and still wasnt 100% right lol :)
Click to expand...

Ah, got it. Was the first image (of the paint colour book) manually focussed, or autofocussed?

And does it do this every time? If it needs calibrating, it should be focussing behind the subject consistently, every time. Is there a chance it just threw a wobbly this time? Every lens will be prone to the occasional misfocus afterall...

Chris
 
First shot is a very deceptive subject. I would dismiss it on that evidence.

If you have real concerns based on consistently observed behavior, then do some proper tests. Try a sheet of newspaper on the wall (not anything small like a £20 note) but you must make sure you are absolutely perfectly exactly dead square to it, and that it is smooth flat.

Focus on the middle and check the edges and corners at lowest f/number. Don't look for ultimate sharpness because you probably won't see it in a tough test like that; what you are looking for is consistency - equal levels sharpness in all four corners.

If there is decentering you'll see it immediately; if you have to look twice and three times, there's probably nothing to worry about.

Checking AF accuracy is another matter, best done not with an angled target shot at very close range!
 
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