Cant get sharp enough photos

Hi, I have a Sigma 583306 17-50mm f2.8 EX DC HSM and use it on a Nikon D7000. I have also used a couple of Nikon lenses and are fine on this camera.
No matter what I do I feel I can never get the photos very sharp with this lens; the photos are in photos but they always seem to lack any substantial sharpness. I have no issue with the focus points using the lens. I use Aperture priority mostly and I do have it set mostly to full aperture of F2.8 (I bought this lens for low light), and I have noticed that it is slightly sharper when set to F4 +. My iso is usually pretty low unless the light is bad, and my shutter speed always fast – at least 300/1 sec + minimum mostly.
Its just bugging me so much that it never seems crisp, or have I been spoilt with the Nikon lenses? Help

No matter what I do I feel I can never get the photos very sharp with this lens; the photos are in photos but they always seem to lack any substantial sharpness. I have no issue with the focus points using the lens. I use Aperture priority mostly and I do have it set mostly to full aperture of F2.8 (I bought this lens for low light), and I have noticed that it is slightly sharper when set to F4 +. My iso is usually pretty low unless the light is bad, and my shutter speed always fast – at least 300/1 sec + minimum mostly.
Its just bugging me so much that it never seems crisp, or have I been spoilt with the Nikon lenses? Help


My first thoughts are whether you're standing properly, whether you're focusing in the right place and whether the lens has been dropped.
Correct stance, with the camera pressed lightly to the face, left hand under the lens and elbows tucked in to the body, makes a huge difference.
One of my mentees didn't know about focus lock and many of his shots showed a sharp background between the subject heads.
Was the lens bought new? Has anybody else used it during your ownership? The only Sigma lens that I ever owned, the 15-30mm zoom, was OK-ish when new but it didn't keep its sharpness during professional use. I always did wonder whether my assistant had dropped it and kept quiet.
Nikon lenses, or rather the professional versions, are designed to withstand a hard life!
Correct stance, with the camera pressed lightly to the face, left hand under the lens and elbows tucked in to the body, makes a huge difference.
One of my mentees didn't know about focus lock and many of his shots showed a sharp background between the subject heads.
Was the lens bought new? Has anybody else used it during your ownership? The only Sigma lens that I ever owned, the 15-30mm zoom, was OK-ish when new but it didn't keep its sharpness during professional use. I always did wonder whether my assistant had dropped it and kept quiet.
Nikon lenses, or rather the professional versions, are designed to withstand a hard life!

If I were taking this shot, I would use Aperture Priority but would set f5.6 or even f8. I would set the ISO to suit the light so that I am using a reasonable hand hold speed (almost all my lenses have IS anyway). For focussing I would use a single centre spot and focus on the eyes of one of the people (centre one in this case). Having locked in the focus, I can recompose and take the shot.
If you are using a multiple detection auto-focus, it will focus on the nearest object which would be the front of the table, tent pole or guide rope, all of which would be wrong.
Dave
If you are using a multiple detection auto-focus, it will focus on the nearest object which would be the front of the table, tent pole or guide rope, all of which would be wrong.
Dave

Without sounding elitist you are asking an awful lot of a mid priced wide aperture extreme wide angle zoom if you shoot at f2.8 and expect sharp results. With the restrictions in upload file sizes added to full aperture fall off you will still struggle to maintain sharpness in your finished images. A little tweak in contrast/clarity in a photo processing prog may help but its an old rule....you can soften an image but sharpening?....im not sure. I have an F2.8 17-35 ed Nikkor which despite its squeak in the focus motor I use as my motorbike pannier standard lens on a battered D700 Nikon. I would never use it a f2.8 unless absolutely forced or unless I was looking for a soft image for Adobe CC to work with. Sods law guarantees that pushing your iso settings too far whilst giving you the extra stop down dividend will push you into the limits of sensor quality. My answer would be to look for a used high quality 20mm prime. I have just had a second look at your posted air force image and a vignette on a sepia b&w would perhaps go some way to rescuing the whole thing.