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Keep Shooting! Photography Assignments (04 Feb 2013)

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Fast vs Slower lens, but at same Fstop-whats diff?

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5 years 5 months ago #1414 by Mitchell Graff
Hi guys - if I take two lenses (one fast, say an f2.8 17-50 and a kit lens (say 18-55 or whatever) and shoot both at say f8 at the same focal length... besides the obvious difference in glass quality, will there be any difference in LIGHT to the sensor? In other words, the faster lens is only truly faster at apertures that the slower lens just can't physically reach... otherwise its the same light to the sensor, and no real difference, right?

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5 years 5 months ago #1417 by Gregory Cazillo
It should be close...

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5 years 3 months ago #1614 by Dennis
the sharpest part of each lens is usually 2-3 stops below wide open


Therefore, a 2.8 is sharpest at 5.6 or 8 in most cases
an 4.5-5.6 lens is often sharpest at 8 or 11

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5 years 3 months ago #1624 by Roy van Ommen
It should be close on normal prints. But when you are going to pixel peep, i think the quality of the image of the 2.8 will be better than the kitlens 18-55.

Thats because the best sharp picture you will get around above 2 or 3 stops. So for the 2.8 that will be around F6 or F7, and for the f5.6 it will be around F8.

Good luck with it.

Greetings. Roy

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