Light, bokeh, sharpness, depth of field, there are so many things to think about when considering whether to buy a very low priced high-speed f/1.8 lens or a lens that costs three to four times as much for about half a stop more light at f/1.4. This guide will help you in determining which to choose.
Light: Honestly, I have had a lot of experience working with both types of lenses, and the f/1.4 makes a good bit of a difference when it comes to low light. However, if it just light that you are worried about, I wouldn't go out and spend the extra hundreds of dollars going to a 1.4 because the difference isn't big enough that it would be worth it. It might be better to buy a slightly higher grade Dslr that has a lower noise sensor, or (if you are old school with film) buying a more expensive high ISO film that has less grain.
Bokeh: Nikon and Canon lenses mostly have good bokeh, especially the newer CPU lenses. Everything is fairly round, and you will not be noticing much of a difference between a 1.4 and 1.8.
Sharpness: 1.4 lenses are typically a fair deal sharper than a 1.8, especially when stopped down to a 1.8. However, again this would not be something you should be looking at as a determining factor on which lens to buy (except for landscapes and portraits) because it isn't that big of a difference to warrant hundreds of dollars extra in price.
Depth of Field: in this realm there is also not a big difference. That 2/3rds stop larger aperture does not make the background any more blurry. What really makes the difference in the blurriness is the focal length because it magnifies the blur. Technically the depth of field is the same on all lenses of equal aperture, but it is more noticeable when you use a longer focal length, so it appears as though the depth of field is shallower. A high speed zoom f/2.8 at 100mm will look better than a 50mm f/1.4. for headshots and portraits, it's better to have a longer lens and more closed down aperture than a shorter lens and wider aperture. It will keep your entire subject sharp (you don't want blurry ears) but will make the background just a beautiful blurry wash.
So, here is my verdict:
Landscapes: 1.8 - save your money because you should be shooting at f4 anyway.
Portraits and Headshots: 1.4 - it will give you more control and is sharper for the eyes.
Weddings: 1.4 because at f/1.8 you don't have as much control and can't take in as much light.
Sports: 1.8 but not even... You should shoot mostly at f/4 anyway.
Night Shooting: f/1.4 just to have the option of the extra light. Plus having it will open up better portrait photography anyway.
I basically say have the 1.4 just if you are doing portraits, headshots, and weddings. Otherwise you could get away with the 1.8 for everything else, maybe spending your money on an external flash? Remember... the biggest difference in look is to buy different focal lengths, not different apertures once you are faster than a f/1.8.
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