Most of today’s finest children’s portrait photographers use 35mm-format digital SLRs (DSLRs). The predominance of medium and large formats has gone away almost entirely, due to the popularity and flexibility of the DSLR.
The rule of thumb for choosing an adequate portrait focal length is to select one that is twice the diagonal of the format you are using. For instance, with a full-frame sensor (equal to a 35mm film frame), a 75 to 85mm lens is a good choice.With sensors smaller than 24x36mm, all lenses get effectively longer in focal length.But in telephotos and telephoto zooms, the maximum aperture of the lens doesn’t change, but when your wide-angles or wideangle zooms become less wide on the digital camera body, it can be frustrating. A 17mm lens, for example, with a 1.5X lens focal length factor becomes a 25mm lens. A 50mm lens on a camera with a 1.6X focal length factor becomes an 80mm lens.