Der Crop beeinflußt nicht die Brennweite sondern die nötige Bildweite, Es kommt ein wenig darauf an, wie groß jetzt das Motiv ist, was du abbilden möchtest. Das Verhältnis von Sensorgröße zu Motivgröße ist der ABbildungsmaßstab. Hier mal einfach für deine angegebenen Brennweiten die Werte für Gegenstandsweite, Bildwinkel und der entsprechenden KB-Brennweite bei durchlaufendem Abbildungsmaßstab (der Sensor der 300D/100D ist 22,7mm -> bei 1:1 wird also ein 22,7mm großes Objekt formatfüllend abgebildet, bei 1:10 ein Motiv von 22,7cm usw.):
f = 200mm; g = 400mm -> 1/1; Winkel a = 3,25° -> Fäqui = 246,2mm
f = 200mm; g = 600mm -> 1/2; Winkel a = 4,33° -> Fäqui = 266,7mm
f = 200mm; g = 1200mm -> 1/5; Winkel a = 5,42° -> Fäqui = 290,9mm
f = 200mm; g = 2200mm -> 1/10; Winkel a = 5,91° -> Fäqui = 303,4mm
f = 200mm; g = 4200mm -> 1/20; Winkel a = 6,19° -> Fäqui = 311,1mm
f = 200mm; g = 10200mm -> 1/50; Winkel a = 6,37° -> Fäqui = 316,3mm
f = 200mm; g = 20200mm -> 1/100; Winkel a = 6,43° -> Fäqui = 318,1mm
f = 200mm; g = 40200mm -> 1/200; Winkel a = 6,46° -> Fäqui = 319,0mm
f = 200mm; g = 100200mm -> 1/500; Winkel a = 6,48° -> Fäqui = 319,6mm
f = 300mm; g = 600mm -> 1/1; Winkel a = 2,17° -> Fäqui = 369,2mm
f = 300mm; g = 900mm -> 1/2; Winkel a = 2,89° -> Fäqui = 400,0mm
f = 300mm; g = 1800mm -> 1/5; Winkel a = 3,61° -> Fäqui = 436,4mm
f = 300mm; g = 3300mm -> 1/10; Winkel a = 3,94° -> Fäqui = 455,2mm
f = 300mm; g = 6300mm -> 1/20; Winkel a = 4,13° -> Fäqui = 466,7mm
f = 300mm; g = 15300mm -> 1/50; Winkel a = 4,25° -> Fäqui = 474,4mm
f = 300mm; g = 30300mm -> 1/100; Winkel a = 4,29° -> Fäqui = 477,2mm
f = 300mm; g = 60300mm -> 1/200; Winkel a = 4,31° -> Fäqui = 478,6mm
f = 300mm; g = 150300mm -> 1/500; Winkel a = 4,32° -> Fäqui = 479,4mm
f = 400mm; g = 800mm -> 1/1; Winkel a = 1,63° -> Fäqui = 492,3mm
f = 400mm; g = 1200mm -> 1/2; Winkel a = 2,17° -> Fäqui = 533,3mm
f = 400mm; g = 2400mm -> 1/5; Winkel a = 2,71° -> Fäqui = 581,8mm
f = 400mm; g = 4400mm -> 1/10; Winkel a = 2,96° -> Fäqui = 606,9mm
f = 400mm; g = 8400mm -> 1/20; Winkel a = 3,10° -> Fäqui = 622,2mm
f = 400mm; g = 20400mm -> 1/50; Winkel a = 3,19° -> Fäqui = 632,6mm
f = 400mm; g = 40400mm -> 1/100; Winkel a = 3,22° -> Fäqui = 636,2mm
f = 400mm; g = 80400mm -> 1/200; Winkel a = 3,23° -> Fäqui = 638,1mm
f = 400mm; g = 200400mm -> 1/500; Winkel a = 3,24° -> Fäqui = 639,2mm