I'm new to photography, and I don't know why the pictures I'm taking are coming out blurred. Here's a sample image, taken with the Sigma 18-50mm f/2.8 on my Nikon D5200 at 18mm, 1/60s, f/2.8 and ISO 1400. However, what seemed to be in focus when I took the picture looks blurred when I look at the photo on my PC:
This photo is severely back focused. Areas about 15-20 feet behind the main subjects are in focus.
Some things to keep in mind:
When using wider apertures, especially combined with larger sensors, Depth of Field becomes shallower. So any focusing errors are more apparent. This is especially a problem in low light when most camera's auto focusing systems are not as accurate as when in brighter surroundings.
When using higher ISO (combined with the lower exposure values allowed by the higher ISO) the image will have more noise in it. To combat the noise of the higher ISO setting the camera will use more aggressive noise reduction at the expense of detail.
The LCD screen on the back of your camera Lies like a politician! They are designed to make every shot look as good as possible so when you look at them at the store you will say, "This camera takes really good pictures!" Remember, typical DSLRs now have resolutions of around 20MP, give or take a few. The LCD screen on the back of most DSLRs is about 1MP, give or take a few. What this means is that blur that is less than about 5 pixels wide (or high) will be combined into a single pixel on the LCD preview screen and will appear to be sharp!
The picture was taken in low light conditions and then the following issues will start to conspire against the sharpness of the picture:
- The autofocus is not very accurate in low light conditions
- The small 18mm focal length instead of, say, 50 mm also makes the autofocus less reliable
- The exposure time of 1/60s seconds is not fast enough to prevent blurring due to camera shaking when looking at a magnified version of your picture on your PC.
- At F/2.8 the depth of field is very shallow and any inaccuracy of the focus will show up much more than at a smaller aperture.
Using a higher ISO allows you to reduce the exposure time, shoot at a larger zoom and/or choose a smaller aperture. But this may come at the price of more noise and noise suppression comes at the expense of resolution.
I suspect what you're seeing here is that the depth of field when using a large sensor camera like the D5200 is much smaller than that from something like a smartphone or a typical compact camera, especially when you're using a relatively fast lens like your Sigma. It's a little hard to tell on the image you've posted, but it looks to me like the gentleman in the dark blue shirt is reasonably sharp, but the other people are a bit out of focus, which does sound like it's a depth of field issue.
If your issue is more than everybody in the picture is out of focus, then it's probably a case of having focused on (slightly) the wrong point. The other thing to remember if you're looking at your photo at full zoom ("100% crop") on your PC is that that's equivalent to a print around 1 metre wide - unless you're actually going to be printing at that size, don't worry too much if it's perfectly sharp at that zoom level or not.