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When I bought my Sony NEX-5R, I had a choice of an f/1.8 or an f/2.8 lens (both are prime, normal lenses). I was told the 1.8 will work better for low-light photography since it will let in lot more light. So I bought the 1.8.

But I now wanted to test out this hypothesis, so I took a photo at f/1.8:

f 1.8

And another with the same lens stopped down to f/2.8:

f 1.8

This was hand-held, and with the ISO set to Auto. But the camera ended up choosing ISO 3200 in both cases.

What, then, is the advantage of the more expensive f/1.8 lens, if it can't take better (lower noise) hand-held photos? Did I waste hundreds of dollars?

f/1.8 is more than one stop faster than f/2.8, so, if the camera chose ISO 3200 at f/2.8, shouldn't it choose ISO 1600 at f/1.8?

I do understand that the f/1.8 can use a shorter shutter speed, but I wouldn't spend hundreds of dollars to shave a fraction of a second or even a few seconds. If I'm going to use a tripod, I might as well wait a second for a longer exposure with the f/2.8 lens. What I care about is taking better low-light photos hand-held, and it seems like the f/1.8 fails at that task.

Is my conclusion correct?

Footnote: I understand the depth of field advantage, but I wouldn't pay hundreds of dollars more for it.

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    (1) To rephrase part of your question as an answer - "What is the advantage of an expensive quality camera if you use it in auto mode in demanding situations - you are wasting hundreds of dollars." (2) For equal exposure something has to change - if ISO did not then shutter speed must. Why did you not advise shutter speeds when they are an absolutely vital part of exposure calculations? || An f/1.8 lens allows (2.8/1.8)^2 =~ 2.4 x as much light in more light in than a f/2.8 lens does, all else being equal. Better than 1 stop. And that is not the sole reason to buy an f/1.8. – Russell McMahon Jun 1 '14 at 9:27
  • PLUS - your two images do NOT have identical exposures. This MAY be caused by condensing actions taken by you but, as presented, the light levels in the same locations in the two images vary eg around the street lights = and probably in many other places as well. Your comparisons must be more controlled to be useful. – Russell McMahon Jun 1 '14 at 9:36
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    Short answer: There was too little light in both situations; the ISO setting has bottomed out, and the camera uses an exposure time that is longer than ideal for both images. If you take images in better light you will see that f/1.8 gives lower ISO than f/2.8 in most low-light situations. – Guffa Jun 1 '14 at 23:52
  • Thanks, Guffa and Russell. Russell, I will try again with a more controlled comparison. The shutter speeds are 1/13 and 1/30 s. This is with a 35mm lens with optical stabilization, so I guess both 1/13 and 1/30 are good enough. When you say I'm using this lens in auto mode, do you include aperture priority as auto? Because I'm in aperture priority mode, which is what I use most of the time. – Vaddadi Kartick Jun 4 '14 at 4:23
  • Please don't put "I found the answer" information in the question. This isn't a discussion forum, and doing that confuses the Q and A separation. Comments like your addition work best as comments on the accepted answer. See meta.photo.stackexchange.com/questions/1601/… for discussion. – mattdm Jun 7 '14 at 12:57
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It sounds likely that you are taking your image in very low light, where the shutter speed is slow enough in either case that the camera's auto-exposure program is choosing the higher ISO in either case. There are probably some situations where it might leave the shutter alone and lower ISO first. It depends on the situation, your camera, and the settings you've chosen.

If you want the camera to do something outside of its program, you will need to take it out of auto mode, where, yes, you'll find that your faster lens lets you use about half the ISO at the same shutter speed. That's not a huge advantage — really, when you get to f/2.8 and faster, you kind of get diminishing returns for increasingly steep costs. If you are going to be using a tripod and don't care about exposure time, you're absolutely right that it isn't crucial to spend more for faster aperture alone.

If you care about taking photos hand-held in low light, though, it really may be worth it, and the camera is probably doing the right thing for you. It's impossible to hold completely still, and having the shutter speed can make a big difference.

But the faster lens may have some other advantages — it may have nicer rendering independent of exposure, be sharper stopped down, or have other features.

  • Regarding your comment about things depending on the settings I've chosen, I'm in aperture priority mode, with ISO set to auto, and optical stabilization enabled. At f/2.8, the camera chooses a 1/13 s shutter speed. At f/1.8, rather than keeping the shutter speed the same and using a lower ISO, the camera uses a shorter shutter speed: 1/30 s, presumably to reduce blur caused by camera or subject motion. I guess I have to use the fully manual mode (rather than aperture or shutter-speed priority) to get it to do something different, but I don't want to manually meter the scene :( – Vaddadi Kartick Jun 1 '14 at 4:17
  • I do see your point about the camera potentially making different choices to deal with this situation, and that it's a judgement call. Thanks. – Vaddadi Kartick Jun 1 '14 at 4:19
  • You may be able to leave the aperture and shutter speed set programmatically and turn off auto ISO. I'm not sure for the Sony NEX offhand. In general, both 1/13th and 1/30th are too slow to get sharp images hand held unless your camera or lens features image stabilization and you are showing at a relatively wide focal length. It's just impossible to hold that steady. (The traditional rule of thumb for slowest shutter speed handheld is one over focal length — so, for a 35mm lens, you want to be at 1/35s or faster.) – mattdm Jun 1 '14 at 4:25
  • Does the 1/focal length rule apply to the FF-equivalent focal length? If I have a 35mm APS-C lens, is the max shutter speed 1/35 s or 1/50 s? I'd assume it's the latter, since a 35mm lens on an APS-C camera results in a greater magnification of the image than on a FF camera, and so needs a correspondingly shorter shutter speed. But I wanted to check. BTW, this is a 35mm optically stabilized lens, so I assume both 1/13 and 1/30 s are fine. And, yes, setting the ISO manually and leaving everything else to the camera may be the best strategy for handheld low-light photography. – Vaddadi Kartick Jun 1 '14 at 4:55
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    As suggested, I tried taking pictures during dusk rather than waiting till it's dark, and I found that f/1.8 lets the camera use ISO 160, as compared to ISO 400 with f/2.8. – Vaddadi Kartick Jun 7 '14 at 13:01
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I'm on my third nex now (NEX5, 5R and now NEX 6). when you are in aperture priority mode, some functions are automatic. Generally the camera would try to take pictures with a decent shutter speed to allow hand held pictures, which is how more than 90% of the pictures are taken. When you increase the aperture to more than double, your camera would have reduced the ISO if your shutter speed was roughly 1/60 or faster. Since it wasn't, it kept the ISO at the same level to give you a speed advantage. The assumption is that if slower speed didn't matter to you, you are likely using a tripod and would use manual mode where you control every thing yourself. I think I've also learned it the hard way. Thanks

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If shutter speed does not change, then going from f/2.8 and 3200 ISO to f/1.8, you should in fact be at 1600 ISO. But the auto feature may have opted instead for a faster (shorter) shutter speed. Either way, one of the reasons people pay for a "faster" lens (1.8 is faster than 2.8) is for those times times when you want a faster shutter speed without the added noise of higher ISO settings. Other reasons include out of focus backgrounds, and the tendency that a 1.8 lens stopped down to 2.8 will generally be sharper than a 2.8 lens used at its max aperture.

  • Yes, the camera does clip ISO to 3200 (unless you manually set it to a higher value). As I said, DoF is not why I chose to spend hundreds of dollars more on the f/1.8. Your point about sharpness is taken. Thanks. – Vaddadi Kartick Jun 1 '14 at 6:37
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    Why will increasing the amount of light passing through the Lens (f/2.8 to f/1.8) will you want to increase the sensitivity of the sensor (ISO 3200 to ISO 6400)? Increasing the size of the aperture will decrease the ISO (assuming a constant shutter speed). – damned truths Jun 1 '14 at 8:12
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    You've got the effect on ISO backwards. If you increase the aperture by 1.3 stops, then to get the same exposure value with the same shutter speed would mean a decrease of 1.3 stops ISO. So the expected change would be from ISO 3200 at f/2.8 to ISO 1250 at f/1.8. – Michael C Jun 1 '14 at 18:44
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    The iso is backwards, going from f2.8 and 3200 iso, to f1.8 the iso should be 1600 not 6400 – laurencemadill Jun 2 '14 at 16:16

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