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What is Effective Aperture? And how it can affect my exposure? Also how does it relate to magnification?

A question that talks about effective aperture but without details

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Same basic question, but with slightly different follow-up questions: What's the difference between real and effective aperture? – mattdm Jul 23 '13 at 0:05

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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 retro-focal design means the actual opening must be larger than the effective aperture.

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