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

Fast vs Slower lens, but at same Fstop-whats diff? 4 years 7 months ago #1414

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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Fast vs Slower lens, but at same Fstop-whats diff? 4 years 7 months ago #1417

  • Gregory Cazillo
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It should be close...
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Re: Fast vs Slower lens, but at same Fstop-whats diff? 4 years 4 months ago #1614

  • Dennis
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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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Re: Fast vs Slower lens, but at same Fstop-whats diff? 4 years 4 months ago #1624

  • Roy van Ommen
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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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