Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Nikon dSLR

Reply
 
Thread Tools Search this Thread
Old Jan 16, 2007, 9:52 PM   #1
Member
 
Join Date: Apr 2006
Location: Montréal, Québec, Canada
Posts: 93
Default

Hi, can someone help me find what's wrong (or not) with this picture? I'm always afraid it's the D50 but this time, I don't rush to conclusion. This photo was taken with a "Sigma 18-125mm f3.5-5.6 DC" new lens that I bought recently. I know it's a rather cheap lens but I was after the telephoto thing. I did notice lot of distortion at 18mm and some pincushion at 125mm. But why is the left side of this photo blurred at only around 55mm? The center AF area was selected, I was on Manual. I read somewhere this very lens is made to be used solely with digital cameras cause it's a four third. Where's the deal for me aside the cost if the cam can't focus? Is there a Sigma lens forum where these things can be discussed? Thanks for your precious help and hints. Germain
Attached Images
 
Karmin is offline   Reply With Quote
Sponsored Links
Old Jan 17, 2007, 7:51 AM   #2
Senior Member
 
ReneB3's Avatar
 
Join Date: Jul 2005
Posts: 193
Default

The right side isn't all that sharp either, but better. It also looks like you were not square to the subjects. The left is closer to you than the right. Try setting the apreturre higher and getting all the subjects at the same distance from you and see if that is better.
ReneB3 is offline   Reply With Quote
Old Jan 17, 2007, 11:23 AM   #3
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

I think you're trying too hard to find something wrong. ;-)

As already mentioned, you were not square to your subjects, and you've got varying distances from the subjects in the image to your focus point.

You've got a limited Depth of Field. See this Depth of Field Calculator for more details:

http://www.dofmaster.com/dofjs.html

Some of the portions on the left side of the frame are much closer to the camera (and further away from your focus point). So, they're not supposed to be sharp. In addition, that photo was taken at an angle (from more than one direction, too). You were not square to your target.

If you want them sharper, use a smaller aperture (higher f/stop number), or don't fill the frame as much (shoot from further away or use less optical zoom).

Even if you were perfectly square to your target (and you weren't), you're still going to see a touch of corner softness at wide open apertures, especially with this type of zoom lens.

If you want to make sure your lens is OK (and I see no reason to doubt that it is), put your camera on a tripod and shoot something with lots of fine detail like a newspaper tacked to a wall, making sure the camera is *square* to your target (not tilted or shooting from an angle like you did with couch or chair at varying distances to your camera going from left to right and up/down)

IOW, don't test it using that type of scene (with some subjects in the scene closer to the camera's focus point compared to others). You were not square in more than one direction with that one (left side closer to camera and right side further from camera, with the same thing applying to the bottom versus top of the scene).

If you're square to a flat target like a newspaper, as long as the middle is in focus and all 4 corners are equally soft, the lens is fine (and it's unlikely that it has any problems).

Most lenses are not as sharp at wide open apertures (and this Sigma is not going to be an exception), and you've got a limited depth of field (see this DOF calculator I posted a link to see how the relationship between focal length, focus distance and aperture works for depth of field).

Being able to control Depth of Field your Aperture selection is one of the big advantages of a DSLR (so you can isolate your subjects from distractions by making them blurrier than your subject so that it stands out).


JimC is offline   Reply With Quote
Old Jan 18, 2007, 1:18 AM   #4
Member
 
Join Date: Apr 2006
Location: Montréal, Québec, Canada
Posts: 93
Default

Hello and thanks a million times for your hints. I guess, as usual, I red too many reviews on this Sigma 18-125mm !!! I did DL the DOF guide. Nice tool. Thanks JimC. I did the Newspaper test. Four or five tests. One test did not come out sharp in the center. Dunno what happened but the next one came cristal clear. I bought the lens from a guy who had it for a few months and wanted some cash for a NIkkor 18-70 instead. So I became suspicious. He sold it to me half the price he paid. Germain...have faith my boy. Thanks again forum friends.
Karmin is offline   Reply With Quote
Old Jan 18, 2007, 1:24 AM   #5
Member
 
Join Date: Apr 2006
Location: Montréal, Québec, Canada
Posts: 93
Default

I guess this one is more right on target. Taken in my "personal" studio. That is with the lighting sources of the poor man: small follow spot and two hand flashlights. That explains the very warm atmosphere. But it's to my liking for now since I'm working on my skills. And if it is not, point where I failed please. Again "merci beaucoup" mes amis. Germain.
Attached Images
 
Karmin is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 1:45 AM.