Effective aperture is the size, in terms of diameter, of the diaphragm opening as viewed through the front element of the lens. It is the apparent diameter that is used to calculate the f-number for a given aperture setting. For instance, if you have a 200mm lens and want an aperture of f/4, the effective aperture needs to appear to be 50mm wide, regardless of the fact that due to magnification between the front element and the diaphragm in conventional designs the actual width of the diaphragm is usually smaller. This is also why it is so difficult to make a large aperture ultra wide-angle lens: the retrofocus design means the actual opening must be larger than the effective aperture.

Entrance pupil (EP) is the more commonly accepted technical term. Effective aperture (EA) means the same thing when referring to the aperture opening used to divide the focal length by to figure the f-number. But EA is less accepted in technical circles because it is less specific in some other ways. It gets confusing because when we say we are choosing an aperture setting of, say f/5.6, what we are really doing is selecting an f-number of 5.6. The effective aperture we choose to get a specific f-number, such as 5.6, will vary based on the focal length of the lens.

If you have a non-circular opening, for example, you would need to measure the total area of the oddly shaped EP and then use the diameter of a circle that would yield the same area when dividing the focal length to derive the f-number of such a lens.