The image size of the Sun when photographed depends on the focal length of your camera (or telescope) focal length. Longer focal lengths produce larger images. Typical focal lengths (say 50 to 100 mm) produce very small images compared to the picture frame. For example, the actual image size of the Sun for a 50 mm lens is 0.45 mm. Since a 35 mm frame is about 36 mm wide, the Sun's image would be about 13% of the width.

Small images may be acceptable if the photographer wants multiple images of the Sun in the same frame. However, producing images that show some detail requires much longer focal length.

The following figure shows the relative size of the Sun's image of a totally eclipsed Sun (with a hypothetical corona) compared to the frame of a 35 mm camera for a range of focal lengths.

Lens Focal Length vs. Image Size for Eclipse Photography
(Click image for larger size)

Figure from F. Espenak & J. Anderson, March 2008
Total Eclipse of 2009 July 22, NASA/TP—2008—214169, Fig. 24.

Therefore, the same lens produces an image that fills up more of the frame in cameras with a reduced sensor. This effectively makes a 35mm lens on a reduced sensor camera effectively equal to about 1.5x the lens focal length. (See box below.)