Before the rush

Before the rush
by evan-pak

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I have observed that the wider the aperture, the more expensive the lenses are. However, I would like to know whether it really makes a difference in your photos or not?

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up vote 22 down vote accepted

Yes, there are several reasons for this.

  • Larger apertures allow for a smaller depth of field, and generally better bokeh.
  • Faster/more accurate auto focus, because more light is available to the focus system.
  • Much more versatility, because more light falls on the sensor at a wide aperture, which opens up your options in lower-light settings.
  • Better image quality. This is a little more complicated to explain, but imagine you have an option between an f/2.0 lens, or an f/8 lens. If you shoot the same scene with both set to f/8, the f/2.0 will almost always be sharper and have less vignetting. This is because lenses tend to get soft when they are wide open, and by stopping down partially you can improve both sharpness, as well as decrease the light fall-off that creates vignetting.
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Also, the main reason that they are more expensive, is because you need bigger lenses (more glass) to accommodate the wider opening. And glass (good glass), is the most expensive part. – ltn100 Nov 18 '10 at 14:50
Wouldn't the bokeh of a wide-open f/3.5 prime lens be better then a stepped-down f/1.4 prime thanks to a perfectly round aperture ? Just hit me.. – Berzemus Nov 18 '10 at 15:05
I would question the last point. That's certainly what I used to believe but the reality is a little different - for example the Canon EF 50 f/1.4 is no sharper stopped down to f/1.8 than the cheaper 50 f/1.8 wide open. There are also other examples of the opposite. When you get to a about f/2.8, primes stopped down don't seem to have that much of an advantage over slower lenses wide open. Sharpness at f/2.8 is dependant on other factors than dispersion due to a wide max aperture. I'd say sharpness is partially dependant on how stopped down you are and partly dependant on absolute aperture. – Matt Grum Nov 18 '10 at 18:38
Accurate AF is extremely hard with a narrow DoF. While it might focus faster, it will often focus on the wrong thing. I don't know if I’d consider this a benefit. – ieure Feb 18 '11 at 4:42
@ieure: except that the focus speed is based on the maximum aperture, not the aperture that is being used for the shot. The amount of light while focusing is the deciding factor. So, an f/1.4 lens stopped down to f/4 has 8 times as much light while focusing than a f/4 lens of the same focal length, but they would have the same DOF for the actual shot. – chills42 Feb 18 '11 at 12:28

Define better. ;-)

Professionals tend to want to use lenses with larger apertures so they can either limit depth of field or work in marginal lighting. Because these lenses are marketed towards professionals they usually have a higher build quality, and so are heavier and more expensive.

Also pros tend to want constant apertures in zoom lenses, which makes the zooms require more glass and more complicated lens groupings -- both of which increase the cost (along with the whole pro-build thing).

That said, there are plenty of medium and large format lenses that have a maximum apertures of no more than 5.6 -- and no one is complaining about the quality of those lenses. We aren't talking about that level of photography, but modern first party and major third party lenses should be of acceptable quality for most purposes.

At a personal level, one of my favorite lenses is an old Nikkon 50mm f1.4 -- not the new G model. That lens doesn't really look all that great in lab tests at, but I'm very happy with the actual pictures I've taken. I've also taken some good pictures with a Canon kit lens (variable aperture zoom).

Oh boy, so subjective ... does this count as an answer?

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Interestingly medium and large format lenses are often slower. In general in a MF camera a fast lens will be f2.8. And in Large format good luck finding anything faster than f5.6 (and in general that will shoot at about f11 or slower).

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