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Why is f2.8 "better" than f4

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  • Why is f2.8 "better" than f4

    From what I understand, minimal F-number is determined as focal length divided by diameter of first lens element.

    But what happens, if I have same lens, like Canon 70-200mm, one f4 and the other one f2.8, and I set them both to f4.

    Do they capture the same amount of light, so the f2.8 really has no advantage over f4 if I set apperture higher than f4? (don't take into account that the f4 might have better image quality, because of the way it's built)

    If they capture the same amount, then they have equal speed. Why would then one bother with buying a f2.8 when you shoot at f8 to make the picture tack sharp?

    I know that there are some occasions, when you need to capture as much light as possible, but almost every lens produces better results at higher-than-minimal f-stop, doesn't it?

    Say that it's one f-stop higher, then the f2.8 would be better at f4, and the f4 would be better at f5.6. Is this the reason to buy a f2.8, because with f4, I would be forced to shoot at f5.6?
    Gear: Canon 1000D | kit 18-55mm f/4-5.6 | 50mm f/1.8 | 55-250mm IS f/4-5.6 | Battery grip BG-E5 | 430EX II

  • #2
    Well first off the faster lens will let you shoot in lower light or shoot with less depth of field. Second, the faster lens (within the same company line) will generally be of much better quality and provide sharper images. Most lenses are sharpest when stopped down some. So, if you shoot at f/4 on an f/2.8 lens you will get a better image than if you shoot at f/4 on an f/4 lens -- in general.
    Nikon D800e, D300, D5000, NIKON GLASS 85mm F/1.8 D, 105mm f/2.8 Micro AF-S VR, 70-200 AF-S VR f/2.8, 28-300 AF-S VRII,10.5mm Fisheye, 24-70 AF-S f/2.8, TC-20E III AF-S, Sigma 12-24 HSM, Sigma 30mm f/1.4 HSM, Sigma 150-500 OS, 2 SB-600 Speedlights, SB-900 speedlight, 4 YN-622N transceivers, Manfrotto 190MF3 tripod & 322RC2 ball grip head. - NJ, USA
    Flickr Photobucket
    Ok to edit and repost my shots on DPS forums


    • #3
      Yup. You've got an extra stop of "elbow room".

      The other reason, as Kirbinster mentioned, is that you'll have an advantage shooting in lower light. The camera's autofocus and autoexposure sensors "look" through the lens when it's wide open. The lens only stops down to the f-stop you set when it takes the picture. This is so that when you look through the viewfinder, you'll have the brightest possible scene to compose by, and so the sensors will receive the most light/information possible.

      With an f/4 lens, you're only getting half as much light to see by, wide open, as you would with the f/2.8 lens. So, the AF system has more of a fighting chance in lower light.
      I shoot with a Canon 5DmkII, 50D, and S90, and Pansonic GX-7. flickr stream and equipment list


      • #4
        A lot of it comes down to shutter speed and depth of field. Being able to blow out a background is important, though you don't always want to do it. That extra stop could mean the difference between a 1/30 shutter speed and 1/60, which is huge indoors.

        On the other hand, slower lenses are often smaller and lighter.

        Also, as Kirbinster said, many 2.8 lenses are better than their slower counterparts. If you look at the Canon 70-200s, however, they're all pretty evenly matched. +
        Nikon D300, D700, Sony NEX5n
        Zeiss 2/25; 1.4/50; 1.4/85

        Please read the rules before posting a critique thread. Rules here.