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Cameras, Lenses and Accessories: Discuss Front/Back Focusing issues? Images included......As many of you may know I've been pestering one or two of you on MSN About my recent purchase, ...
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Old 25-09-2005, 18:11   #1 (permalink)
Marcel
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Front/Back Focusing issues? Images included...

As many of you may know I've been pestering one or two of you on MSN About my recent purchase, the Sigma 70-300 APO DG Macro.

I've been reading (and thus expected) about this lens, and that anything greater than 200mm, shots are quite soft. Yet I have been quite disappointed with my lens. I'm not too sure what level of softness to expect, so started looking around.

Looking at EosD's shots (he has the same lens), Ive been thinking either I have a very very bad one, a faulty one, or I am doing something seriously wrong (which is more than likely )

Anyway, Ive been doing some more reading and I read mention of front/back focusing problems, so I did some more testing this afternoon.

I stood a DVD in the garden, mounted my camera on a tripod, changed to mirror lockup, plugged my cable release in, and took some shots, each time pressing the AutoFocus button before the shot (obviously giving the camera time to settle down before taking the shot).

Each time I pressed it, the focusing system twitched/moved a very little bit. Usually one press would knock it clockwise ever so slightly, the next press would turn it back a little, etc etc.

Anyway, I'm waffling, these are the three shots I took.







All taken f5.6, 1/125, ISO100.
As you can see, the point of focus is different on each one (more noticable on the third).

PS. I have the AF point manually set to the centre point only, which was square on the centre of the DVD box.

Thoughts? Faulty? User error? Am I barking?
 
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Old 25-09-2005, 18:23   #2 (permalink)
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If I remember correctly back/front focus issues tend to be related more to the camera body rather than the lens. Have you got the same problems with your other lenses?

There's a great explanation here along with a downloadable pdf test chart

http://md.co.za/d70/chart.html
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Old 25-09-2005, 19:14   #3 (permalink)
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Bod, the CMOS sensor used in your camera and my 20D tends to produce softish images out of the camera anyway. A lot of this is to do with the noise filter which is mounted right down on the sensor. I find all images benefit from some sharpening in post processing anyway. This isn't just my opinion it's the opinion of lots of people - Andy Rouse for one.

Your test shots were taken in what looks like a quite overcast light, so they lack a little punch anyway, but more importantly, they've been drastically down-sized for showing here. Any image which has been reduced in size loses a little critical definition and needs sharpening, so your test is pants.

Shots 1 and 2 definitely sharpen up acceptably in PSP and even Shot 3, given that it's the worst of the three still sharpens quite a bit, but the down sizing has had a detrimental affect on all three images anyway including sharpness. We need a 1:1 (unsharpened) crop of the box from those shots to be able to assess the images.

I accept that individual lenses can vary enormously in their ability to resolve fine detail, but having seen a number of your shots, including your squirrel shot, I'd take some convincing that there's anything seriously wrong with your lens. EosD may well have a great version of the lens, but it's also true that his post processing has improved dramatically recently.

The good thing about your test is that you've eliminated camera shake as a factor by using a tripod, but that leaves me thinking that it's your sharpening in post processing which is most likely to be your problem. You shouldn't expect pictures right out of your camera to be a done deal anyway as far as sharpness is concerned. Many pros prefer to disable sharpening in the camera altogether and deal with it in processing.
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Old 25-09-2005, 19:45   #4 (permalink)
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Excellent reply there CT. I've been contemplating whether my Sigma is as it should be. I think perhaps I expect too much of my shots out of the box as you say.

Matt
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Old 25-09-2005, 19:45   #5 (permalink)
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CT that's the most definitive answer I've found so far, cheers

I suppose I am guilty of what I would describe as 'bumbling along' with these tests, and also the same can be said with my use of my kit. It's all a big learning curve, especially for me

Originally I was concered about the images being very soft.
These are three shots I took a couple of weeks ago.

300mm @ f5.6
http://www.marcel.booth.dsl.pipex.co...4/01_300mm.jpg

238mm @ f5.6
http://www.marcel.booth.dsl.pipex.co...4/02_238mm.jpg

214mm @ f7.1
http://www.marcel.booth.dsl.pipex.co...4/03_214mm.jpg

All tripod mounted, remote release, mirror lockup, also waiting for the camera to 'settle' after using the AF.
Admittedly the third is much better, not only because it's at 214mm, but it's stopped down to 7.1. I did this test when I didn't realise that opening up the aperture had such an effect on sharpness.
That said, the first two shots are surely unacceptable? Mind you although I went to the lengths required (tripod, remote, mirror lockup), to eliminate camera shake, I can't guarantee there wasn't any. I do have a pretty crappy tripod after all, and there was a gentle breeze.
Anyway I'm waffling now. Onto the points you make.

What you say about fresh out of the camera sharpening. This is something I have also been thinking about. I'm new to this DSLR lark and I have been thinking that maybe I'm too expectant of good results straight out of the camera, especially with me shooting in RAW.
I think this is partly the downfall of not just me, but also it's affecting my expectations somewhat.

This lens seems to be brilliant under 200mm from what I can see. (The squirrel shot was about 80mm IIRC). It's towards the higher end I'm worried about. With what I've been reading today I was wondering about the back/front focus problems, hence the above DVD shots. They were taken not to check the softness of the lens, but to test the AutoFocus. The 1st two the focus seems to have locked on very similar positions, yet the third, the focus is now to the rear of the box, which left me wondering.

I'm not sure why the focus sometimes twitches each time I refocus. It doesn't twitch and then move back, it moves slightly then stops. I refocus again, and it moves again (if this makes sense) Only very small movements. Maybe this is normal behaviour, I wouldn't know, as I say this is all a learning curve to me, so that's something I just don't know whether is normal or not.

I've just had a try of the above test linked to by dod, and the results seem fine.

I'll recrop the original three images above to 100% crops of half box, half grass, so you can see the detail clearer.

Regards
 
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Old 25-09-2005, 19:50   #6 (permalink)
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Quote:
Originally Posted by Marcel
I'm not sure why the focus sometimes twitches each time I refocus. It doesn't twitch and then move back, it moves slightly then stops. I refocus again, and it moves again (if this makes sense) Only very small movements. Maybe this is normal behaviour, I wouldn't know, as I say this is all a learning curve to me, so that's something I just don't know whether is normal or not.
Marcel, my 18-125mm does this. I would expect it to twitch 'out' and then back 'in'. However, it doesn't, and I have to twitch out then twitch in manually. I'd be interested to know if this is a trait of Sigma, or any other lenses.

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Old 25-09-2005, 20:14   #7 (permalink)
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Bod here's you first pic again.



If you were using the centre focusing LED square then it looks to me without measuring up that it's suspiciously close to the bottom right corner of that white facia board, but which shouts of two possible problems to me..
(1) the rectangle was completely filled by the white facia board which doesn't contain enough detail for the camera to focus on. Try this on a sheet of blank white paper if you need convincing.

(2) the rectangle was covering partly the facia board and partly the wall behind it which are on two different planes and will 'confuse' the focusing system causing it to hunt back and forth. It can only focus on one or the other - something you need to be aware of when you're choosing your point of focus in your shots. Again you can check this at home by simulating the situation with objects at two different planes in the rectangle.
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Old 25-09-2005, 20:28   #8 (permalink)
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CT I forgot to mention those are actually 100% crops of the original images, but as coincidence would have it you're not actually far wrong. The rectangle was covering the white board, just above the foremost drainpip. If anything the dot in the focus point would be just above the drainpipe before it tapers to the left to meet the gutter.
It is possible that it might have been to the left a little as you suggest, which would have caused the lens to focus on the rearmost brickwork, which to me, looks equally as unacceptable?
Also, if the rectangle was completely filled by the white facia bit, then surely if thre wasn't enough detail for it to focus, then it would 'hunt' before either giving up with an error (IE flashing light in the viewfinder), or hunt and then find something. It definately didn't have a problem locking focus from what I remember.
 
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Old 25-09-2005, 20:37   #9 (permalink)
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Here are links to fullsize uncropped versions of the above DVD's.

www.askbod.com/marcel/af_2236_full.jpg

www.askbod.com/marcel/af_2237_full.jpg

www.askbod.com/marcel/af_2238_full.jpg
 
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Old 25-09-2005, 20:46   #10 (permalink)
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Is it worth trying this test using manual focus to see if it is a trait of the lens or the AF system?
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Old 25-09-2005, 20:54   #11 (permalink)
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Quote:
Originally Posted by SammyC
Is it worth trying this test using manual focus to see if it is a trait of the lens or the AF system?
I tried manual focusing but unfortunately I become an even worse photographer as soon as my hand goes near the focusing ring. I think my shots would have been sharper if I'd rubbed half a kilo of lard all over the lens instead

This is another 'test' I did last week when I was looking for 'softness'.
Again, tripod mounted, all with mirror lockup, remote release, non windy day etc.
I metered for f5.6 in AV (the first one), then and set the shutter speed accordingly, then flicked to manual to take the shots, and with every step up in aperture, i stepped down in shutter speed, to keep the shots exposed the same.

These are at 300mm, from 5.6 through to f16.

f5.6
www.askbod.com/marcel/300mm01_f5.6.jpg

f6.3
www.askbod.com/marcel/300mm02_f6.3.jpg

f7.1
www.askbod.com/marcel/300mm03_f7.1.jpg

f11
www.askbod.com/marcel/300mm07_f11.jpg

f16
www.askbod.com/marcel/300mm10_f16.jpg

These are 100% crops from the originals. the focal point was on the top right hand corner (ish) of the window frame
 
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Old 25-09-2005, 20:59   #12 (permalink)
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Sammy, it's a trait of any AF system I've ever used.

If we look at Bod's first house pic again with the point of focus measured accurately, you can see that it was probably the wall, drainpipe and or facia board causing the problem and confusing the AF system.



Going back to Bods video box pic you can see that the focus rectangle would have almost certainly been on the plain green background of the box. The printing on the box is a half tone image. (made up of a series of dots) At close distances the AF system would have no trouble keying and focusing on the dots, but at the distance the shot was taken, they'd simply appear as a plain green background and not have sufficient detail to focus accurately.



Bod would have done far better to focus on some graphic detail on the box containing lines to help the AF system lock on. Modern AF systems are fantastic but you do need to give them some detail to work on, even if you have to use the focus lock button and recompose afterwards.
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Old 25-09-2005, 21:35   #13 (permalink)
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Bodski

I have nowt constructive to add to the excellent comments Ct has posted.

Your images from the Sigma are no different to those of my Canon 75-300 Is above 200mm. I put it down to the glass tbh.
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Old 26-09-2005, 07:50   #14 (permalink)
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If its any comfort I have similar concerns on my Sigma lens. At the moment I'm putting it down to a combination of cheap lens, AF issues, and operator error.
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Old 26-09-2005, 16:42   #15 (permalink)
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ive got the 70-300 apo and i find it a little soft at 300mm f5.6 but fine at f8.

as many have said before sigma have quality control issues so some lenses are great, others are less so, maybe you got one that slipped through?
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Old 26-09-2005, 18:10   #16 (permalink)
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the shots with the numbers on, zoom in to the numbers, the one at 5.6 looks like movement blur to me, though the others are better, so i dont really know!
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Old 09-10-2005, 19:53   #17 (permalink)
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Quote:
Originally Posted by Marcel
As many of you may know I've been pestering one or two of you on MSN About my recent purchase, the Sigma 70-300 APO DG Macro.

I've been reading (and thus expected) about this lens, and that anything greater than 200mm, shots are quite soft. Yet I have been quite disappointed with my lens. I'm not too sure what level of softness to expect, so started looking around.

Looking at EosD's shots (he has the same lens), Ive been thinking either I have a very very bad one, a faulty one, or I am doing something seriously wrong (which is more than likely )

Anyway, Ive been doing some more reading and I read mention of front/back focusing problems, so I did some more testing this afternoon.

I stood a DVD in the garden, mounted my camera on a tripod, changed to mirror lockup, plugged my cable release in, and took some shots, each time pressing the AutoFocus button before the shot (obviously giving the camera time to settle down before taking the shot).

Each time I pressed it, the focusing system twitched/moved a very little bit. Usually one press would knock it clockwise ever so slightly, the next press would turn it back a little, etc etc.

Anyway, I'm waffling, these are the three shots I took.







All taken f5.6, 1/125, ISO100.
As you can see, the point of focus is different on each one (more noticable on the third).

PS. I have the AF point manually set to the centre point only, which was square on the centre of the DVD box.

Thoughts? Faulty? User error? Am I barking?

..haha! I tried a similar exercise at the weekend for my nightclass, tried four sets of five photos with my D70.. different focal length, focal point etc, and they all looked **exactly the same**.. I thought I was going potty!
 
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