Focus on a particular subject?

Try setting the camera to M manual focus and manually focusing on the subject. Some combination of focus size and focus type may not focus unless the sensor sees a subject and is in focus. So if the sensor cannot see and know it is in focus it will not fire. If you subject is not moving then focusing in manual will ensure the subject is in focus and the camera can shoot.

This depends on which Autofocus system that particular model uses.  Then manual should tell what is happening there- there is usually a more comprehensive manual on the supplied CD.
Failing that speak to the user support of you particular model

Try the FOCUS LOCK feature. This feature allows you to focus on a subject roughly the same distance away from you, then re-compose your picture without losing that focus. Open the lens barrier fully. Look through the viewfinder at the AF Target Mark (the 4 lines in the center of the viewfinder). Position the AF Target Mark on a subject roughly the same distance away from you. Press the shutter button halfway until the green lamp lights. While keeping the shutter button pressed halfway, re-compose your picture, then press the shutter fully.

Try the FOCUS LOCK feature. This feature allows you to focus on a subject roughly the same distance away from you, then re-compose your picture without losing that focus. Open the lens barrier fully. Look through the viewfinder at the AF Target Mark (the 4 lines in the center of the viewfinder). Position the AF Target Mark on a subject roughly the same distance away from you. Press the shutter button halfway until the green lamp lights. While keeping the shutter button pressed halfway, re-compose your picture, then press the shutter fully.

If the camera is having a difficult time auto-focusing, you can try the FOCUS LOCK feature. This feature allows you to focus on another subject roughly the same distance from you and then move the camera back to the intended subject and take the picture without losing focus. First, turn the camera on and locate the Autofocus Target Mark in the center of the LCD. The AF Target Mark resembles an open and close bracket [ ]. Position this AF Target Mark on a subject roughly the same distance away as the subject the camera is having trouble focusing on. Press the shutter button halfway enabling the lens to focus. While holding the shutter button halfway down, aim the camera at the original subject and press the shutter button all the way down.

Try the FOCUS LOCK feature. This feature allows you to focus on a subject roughly the same distance away from you, then re-compose your picture without losing that focus. Open the lens barrier fully. Look through the viewfinder at the AF Target Mark (the 4 lines in the center of the viewfinder). Position the AF Target Mark on a subject roughly the same distance away from you. Press the shutter button halfway until the green lamplights. While keeping the shutter button pressed halfway, re-compose your picture, then press the shutter fully.

If the camera is having a difficult time auto-focusing you can try the FOCUS LOCK feature. This feature allows you to focus on another subject roughly the same distance from you and then move the camera back to the intended subject and take the picture without losing focus. First, turn the camera on and locate the Autofocus Target Mark in the center of the LCD. The AF Target Mark resembles an open and close bracket [ ]. Position this AF Target Mark on a subject roughly the same distance away as the subject the camera is having trouble focusing on. Press the shutter button halfway enabling the lens to focus. While holding the shutter button halfway down, aim the camera at the original subject and press the shutter button all the way down.

If the camera is having trouble doing an auto focus you can try the FOCUS LOCK feature. This feature allows you to focus on another subject roughly the same distance away from you, and then move the camera back to your original subject and take your picture without losing that focus. First turn the camera on and locate the AutoFocus Target Mark in the center of the LCD. The AF Target Mark resembles an open and close bracket [ ]. Position this AF Target Mark on a subject roughly the same distance away from you as the subject that the camera is having trouble focusing on. Press the shutter button halfway enabling the lens to focus. While keeping the shutter button pressed halfway, move the camera back to include the subject you originally wanted in the image, then press the shutter fully. (Please see page 20 in the D-425 Advanced Manual in Section 2, under the heading titled “If Correct Focus Cannot Be Obtained”, for a more detailed explanation. This Advanced Manual is provided on a separate CD, in Adobe PDF format.)

1. Typical problem subjects for autofocus 1) Very low-contrast subjects 2) Overlapping nearby and distant objects 3) Very bright subjects in the center 4) Subjects moving very fast 5) Subjects through glass Focus on an object that is at the same distance as the desired subject, apply Focus Lock, and then recompose the picture. Or set the lens focus mode switch to (or), and focus manually. (Manual focus is only possible with cameras providing this feature.) 2. Attempting to take pictures out of the camera's shooting distance: When taking pictures out of the camera's shooting distance, the subject will be out of focus. The shooting distance differs from each camera model. Please check the specifications of your camera in the instruction manual to determine the shooting distance.

1. Typical problem subjects for autofocus 1) Very low-contrast subjects 2) Overlapping nearby and distant objects 3) Very bright subjects in the center 4) Subjects moving very fast 5) Subjects through glass Focus on an object that is at the same distance as the desired subject, apply Focus Lock, and then recompose the picture. Or set the lens focus mode switch to (or), and focus manually. (Manual focus is only possible with cameras providing this feature.) 2. Attempting to take pictures out of the camera's shooting distance: When taking pictures out of the camera's shooting distance, the subject will be out of focus. The shooting distance differs from each camera model. Please check the specifications of your camera in the instruction manual to determine the shooting distance.