Landscape photography in particular, there is usually too much contrast
between the bright sky and the (usually darker) foreground. Cameras can't cope with this contrast
range like the human eye can. If you leave it to your camera's meter,
it will often expose correctly for the sky, but this means the foreground
will be too dark, or vice versa.
If there's nothing moving fast in the frame (grasses blowing in the wind etc.) you can use a digital camera on a tripod to get the same effect as graduated filters, just correctly expose one frame for the sky, then without moving the camera take another shot correctly exposed for the foreground. Best done with a cable shutter release so you don't move the camera by firing the shutter with your finger
1) Digital Blending Great technique for advanced digital camera users - see below. I used to use this technique for a lot of my images (nowadays I use a Sony A7R4 camera which has such good dynamic range that this technique is rarely needed), it saves carrying filters around but you need to use a tripod
2) Graduated Filters - The best ones are ND (Neutral Density) grads or Grey grads, which don't affect the natural colours in the image. Experiment with bracketing exposures (see below), so that you learn how the filter affects your images.
However, I never use grads anymore, now I just use digital blending, but if there are moving elements in a scene, or if you're hand-holding (tut-tut), graduated filters may be the best choice
Take two exposures
Take one image correctly exposed for the sky (e.g. 1/250th at f8) , and another correctly exposed for the foreground (e.g. 1/60th at f8) then combine two frames in Photoshop
Do the blending manually to get a professional look, as in the boat examples at the top. To do this, open both images, Ctrl A to select the bright sky image, Ctrl C to copy it. Then close this image. Ctrl V to paste the bright sky image onto the dark sky image. Then use the Eraser tool at low hardness to erase the bright sky image (Layer 1), revealing the darker sky underneath. Doing this properly can take an hour or more for a professional result that won't show on large prints!
More examples of images I created with Digital Blending: -
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