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Old Jul 8, 2006, 5:49 AM   #1
DMW
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Hi all,

Please could you offer some advise to a problem I am having. Since buying a Sigma 24-135mm 2.8-4.5 Lens I seem to be having a higher percentage of 'out of focus' shots.

This came to a head the other day when I was asked to take some shots of a friends Harley. To be sure I got the shot I wanted I set the camera on a tripod and using the timer I took two itentical shots changing only the Aperature. Below are the two shots and I have shown the focus points and Exif data.

Shot 1.



Code:
Shooting Mode
 Aperture-Priority AE
Tv( Shutter Speed )
 1/800
Av( Aperture Value )
 3.5
Metering Mode
 Evaluative Metering
Exposure Compensation
 0
Autoexposure Bracketing
 0
ISO Speed
 200
Lens
 24.0 - 135.0mm
Focal Length
 24.0mm
Image Size
 3456x2304
Image Quality
 RAW
Flash
 Off
White Balance Mode
 Daylight
AF Mode
 AI Focus AF
Parameters Settings
 Contrast Standard
 Sharpness Standard
 Color saturation Standard
 Color tone 0
Color Space
 sRGB
Noise Reduction
 Off
File Size
 6883KB
Custom Function
 C.Fn:01-0
 C.Fn:02-0
 C.Fn:03-0
 C.Fn:04-0
 C.Fn:05-0
 C.Fn:06-0
 C.Fn:07-0
 C.Fn:08-0
 C.Fn:09-0
Drive Mode
 Self-Timer Operation


Shot 2.



Code:
Shooting Mode
 Aperture-Priority AE
Tv( Shutter Speed )
 1/125
Av( Aperture Value )
 9.0
Metering Mode
 Evaluative Metering
Exposure Compensation
 0
Autoexposure Bracketing
 0
ISO Speed
 200
Lens
 24.0 - 135.0mm
Focal Length
 24.0mm
Image Size
 3456x2304
Image Quality
 RAW
Flash
 Off
White Balance Mode
 Daylight
AF Mode
 AI Focus AF
Parameters Settings
 Contrast Standard
 Sharpness Standard
 Color saturation Standard
 Color tone 0
Color Space
 sRGB
Noise Reduction
 Off
File Size
 7156KB
Custom Function
 C.Fn:01-0
 C.Fn:02-0
 C.Fn:03-0
 C.Fn:04-0
 C.Fn:05-0
 C.Fn:06-0
 C.Fn:07-0
 C.Fn:08-0
 C.Fn:09-0
Drive Mode
 Self-Timer Operation



The problem seems to happen when using the larger aperature, Any suggestions?

Many thanks.

DMW

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Old Jul 8, 2006, 7:08 AM   #2
NHL
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It seems like you have a field curvature with that lens...
-> you're too close with a 24mm and the object is flat!

Back out and use the tele end of the lens - this will also help to defocus the background and make the motorcycle stand out as the main object
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Old Jul 8, 2006, 7:42 AM   #3
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Thanks for the quick reply.

Your suggestion should cure this particular problem but in which type of situation would I use the 24mm 2.8 function of the lens?

I bought this lens because I thought a 2.8 aperature would give me a smaller DOF and produce the effect you desribed.

I have also found that this lens did not produce very good results when shooting indoors with my dedicated flash and dragging the shutter, same 'out of focus' results when using the larger aperatures.
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Old Jul 8, 2006, 9:12 AM   #4
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Mainly landscapes (far distance) when @ f/2.8 for sunrise/sunset since the lens curvature will be masked by the deeper DOF with the hyper focal distances
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Old Jul 8, 2006, 11:29 AM   #5
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also, i noticed you used the 9-point AF mode.... try using only the center focus point. the Canons seem to do better using that, since it apparentlyoffer more precise focusing than the rest of the focus point selections.

i can't say anything about the Sigma 24-135 lens, though i'd heard it was a bit soft, but i do know their 80-400 is razor sharp. or maybe i just got a good one? in any case, most lenses are not at their best wide open, and i expect your 24-135 is probably much sharper ifyou decrease the aperture a stop or two. and as NHL mentioned, if you want the shallower DOF, you can always back off a bit and use the 135mm end. remember, on an xxD body (you didn't specify which model camera you have), that equates to about a 215mm lens, and if you shoot that at f5.6 from a distance, it should still give you some nice background softening...




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Old Jul 9, 2006, 4:19 AM   #6
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Thank you for your replies.

I have a 350D and for this type shot I would normally use myCanon 75-300 USM and stood back like you said. I also normally use the centre focus point but I was trying to test this lens to see if I had a fault. I think what you are saying is that the lens is not faulty it's just not the highest quality.

I am still confused as to why the focus points are not even in focus, my understanding of field curvature is that the focus point of a flat object should be infocus but as you go further from the focus point the focus would decrease.

I have searched the entire picture and cannot see any part that is in focus. I have another photo taken at 2.8 where the camera reports to have focused on an object about 10 metres away but in fact the picture is betterfocused at about 5 metres. :?

I guess I will have to use this lens at nothing larger than 4.5 as it is very handy due to it's 24-135 range.


Here is another example. Taken at 24mm F3.2, the centre focus point was used but as you can see the object just in front of me is the best focused this in the picture.


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Old Jul 9, 2006, 6:04 AM   #7
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Have you try one-shot AF?
(or show us an example with one-shot AF...)
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Old Jul 9, 2006, 11:03 AM   #8
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I normally use AI-Focus. I will try it set on One Shot and let you know how I get on.
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Old Jul 9, 2006, 12:14 PM   #9
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DMW wrote:
Quote:
I normally use AI-Focus. I will try it set on One Shot and let you know how I get on.
you're not focusing and then shifting your aim point at all, are you? AI focus can mess things up when you're shooting stills. if the point of aim moves at all, so does the focus. for stills, you're best off using one-shot by far.

i don't know if you're looking for a new lens to replace the Sigma, but i have a Tamron SP-series 24-135 that is superb. it's f3.5 at the wide end, not as fast as your Sigma, but it is very sharp and crystal clear with my 30D, and in mybook, sharpness and clarity trump wide apertures every time. it also focuses down to about 10" (25cm) even at the 135mm end, which makes it very useful for close-up work. i can highly recommend it as a general-purpose "walkaround" lens. cost when i bought mine this past spring was under $400 US.
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Old Jul 9, 2006, 12:24 PM   #10
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In the case of the last shot I may have moved slightly after focusing as it was a hand held shot but the shutter speed was1/640 so I dont think that would explain why the object just infront of me seems to be in focus.

Also the shot of the Harley was on a tripod.

I just bought this Sigma lens as a "walkabout" lens hence why I need to find out if I can rely on it.

Maybe "One shot" will help.

:?
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