How much real low-light advantage does an f/1.7 lens have over f/2.5?
Asked 7/14/2012
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I currently have a 14mm f/2.5 prime and am considering adding a 20mm f/1.7. I’m mainly interested in the practical low-light benefit of the faster aperture, not so much the focal-length difference. In real use, how noticeable is the speed difference between f/2.5 and f/1.7? For example, would it let me use a meaningfully lower ISO or faster shutter speed in dim light?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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It depends on what you're shooting.
if you're usually shooting towards to the end of your ISO range (for ex. at ISO 800 or higher in the case of your camera) then 1 stop extra will give you the difference between 800 and 1600 which is quite significant for your camera. The noise difference between the ISO stops is getting "bigger" (more noticeable) while the ISO increases so you will want to avoid that.
if you're usually shooting moving subjects you usually want to freeze them or to have them blurred just a little. For humans, this usually means that you must have 1/40 - 1/80 shutter speeds (very approximate values, don't flame me on this). In order to have such speeds in low/dim light you need a fast lens. See what shutter speeds your camera is able to obtain on a reasonable/usable (for your taste) ISO. It is that enough for you? If you have photos with a lot of noise because of iso and/or blurred subjects because of movement, then this is a sign that you need a faster lens.
if you're usually shooting handheld and your shoots are blurred because of handheld camera shake, this is another sign that you need a faster lens.
I don't know the lens, but usually the wide-open aperture is the "desperate" aperture. There are very very very few lenses which are at their best straight from their wide-open aperture. The wide-open is used usually when you are desperated to get the shoot. Usually stopping down will enhance the things considerably. Hence, having a f/1.7 lens stopped down at f/2.2 will likely deliver better results (wrt quality) compared with a f/2.5 lens wide-open. But again, in order to be sure here, we need to see some lab tests MTF charts etc. But, again, the vast majority of lenses behaves as I said.
So, if you don't have enough light to take good shots with your present lenses, I think that's better to invest in a faster one. Also, there are enough exposure calculators in order to see, based to your actual EXIF data of your problematic shots, if the new lens will really help you (ie. it will lower significantly the ISO, raise enough the speed etc.)
Originally by user952. Source · Licensed CC BY-SA 4.0
user952
14y ago
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Generated from our catalog & community — verify before relying on it.
f/1.7 is about 1 stop faster than f/2.5. In practice, that means under the same light you can usually do one of the following:
- use about half the ISO, or
- use about twice the shutter speed.
Example: if the f/2.5 lens needs ISO 1600, the f/1.7 lens could let you shoot around ISO 800; or if you were at 1/40s, you could go to about 1/80s.
That can be very noticeable in low light, especially if you’re already near the high-ISO limits of your camera or trying to avoid motion blur.
A couple of caveats:
- Lenses often aren’t at their absolute best wide open.
- The 20mm and 14mm do not frame the same way, so they’re not directly interchangeable.
- If you want the same depth of field, the longer 20mm lens may need to be stopped down more, which reduces some of the speed advantage.
So yes, the speed difference is real and useful, but whether it’s worth it depends on whether your current shots are limited by ISO noise, autofocus in dim light, or shutter speed.
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