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Message: <blockquote data-quote="Geoffc" data-source="post: 254411" data-attributes="member: 8705">The Sigma is 1.7 T-Stop whereas the Nikon is 1.9. The lower the number the better and it indicates how well the light passes through the lens. This would explain the lower ISO on the Sigma. The bottom line is it sounds like the Sigma is optically superior at passing light at the same apertures as the Nikon. The Nikon 1.4 is also 1.7 T-stop. It is not inconceivable that I've taken a small amount of knowledge and made it sound like an answer, when in reality it has nothing to do with your observation Glenn <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile    :)" loading="lazy" data-shortname=":)" /></blockquote>