Size and weight are big decision factors when you are trying to find the ideal camera for your needs.
In this section, we are going to illustrate the Canon ELPH 500 HS and Canon SD780 IS side-by-side from the front, back and top in their relative dimensions. Canon ELPH 500 HS has external dimensions of 101 x 56 x 25 mm (3.98 x 2.2 x 0.98″) and weighs 185 g (0.41 lb / 6.53 oz) (including batteries). Canon SD780 IS has external dimensions of 87 x 56 x 18 mm (3.43 x 2.2 x 0.71″) and weighs 155 g (0.34 lb / 5.47 oz) (including batteries).
Below you can see the front-view size comparison of the Canon ELPH 500 HS and the Canon SD780 IS. Canon ELPH 500 HS has the same height with Canon SD780 IS. On the other hand, with a width of 101mm, it is 14 wider and also with a thickness of 25mm, it is 7mm thicker than Canon SD780 IS.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Canon SD780 IS is significantly lighter (30g ) than the Canon ELPH 500 HS which may become a big advantage especially on long walking trips.
Weather Sealing
Unfortunately neither the Canon ELPH 500 HS (IXUS 310 HS / IXY 31S) nor Canon PowerShot SD780 IS (Digital IXUS 100 IS) provides any type of weather sealing in their bodies, so you have to give extra care especially when you are shooting outdoors. If weather sealing is a must-have feature for you, check these sections: Top Compact Cameras with Weather Sealing
LCD Screen Size and Features
Canon ELPH 500 HS's 3.20" LCD screen is slightly larger than Canon SD780 IS's 2.5 screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.