How to photo edit underwater pictures - the basics

1b - Cropping to improve composition

usually your main subject ie fish or coral, looks boring when dead centre of the picture

christams tree worm centrally aligned
Christas tree worm - cropped to rule of thirds

How to crop - it is really simple

In the software package there will be a 'crop' tool to select.

Usually a rectangle will appear, super-imposed on your photo with 'expansion' or 'drag' handles to alter the size.

Sometimes you place your cursor on the picture to create the first point of a rectangle and drag it to create the entire rectangle.

Move the entire rectangle by selecting the univeral 'select and move' icon (middle of the rectangle).
how to crop

Post-processing - cropping - don't forget the final desired aspect ratio

When cropping think about the final ratio or shape you need for how you will usually view your picture ie usually on screen or on canvas?  

Vital, especially if you intend to print it to canvas.

Check with the print site to see what aspect ratios they have available first.
This aspect ratio stuff is pretty easy ...    Wiki describes this well.   It's the proportional ratio between the width and height of the photograph.

LCDs, tablets, smartphones vary so check yours out.   You don't want to hook your laptop via HDMI to the tele and have black areas above and below your picture.   Just doesn't look great.

1:1 aspect ratio
examples of 1:1 aspect ratio
4:3 aspect ratio
examples of 4:3 aspect ratio

Apply the rule of thirds more precisely when post-processing

There's a nice visual example of use of the rule of thirds to improve composition (with a definition) at Wiki.   There is no absolute 'right' or 'wrong' to the composition.

The picture above shows how photo-editing software divides your photo into 9 equal rectangles and creates indicator guide lines where it looks good to place main subjects.
Without the limitation of holding your breathe, use the software to precisly compose the photo using the grid-lines and horizons at leisure.   Try out various crops to see which intersection looks best.

Try crops to satisfy one or more of the rule of thirds, use of horizons and 'active space'.

Then, just play.   It's what looks nice to you.

How to photo edit
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Composition - a detailed illustration of cropping