Lens speed: Not always the best choice

Discussion in 'Digital Photography' started by RichA, Jul 10, 2009.

  1. RichA

    RichA Guest

    People buy fast lenses for 2 reasons:
    -The want the speed to capture action.
    -They want the shallow DOF afforded by such lenses.

    For for the first reason, speed is not always the best choice.
    Shooting at a higher ISO sometimes works out better than using a lens
    wide open and once you determine that it does for a specific lens,
    then there is generally no reason to shoot a specific subject at the
    wider aperture. The benefits are better image quality (fewer image
    aberrations), possibly more accurate focus and the potential to use a
    much cheaper lens. The downside is noise and giving up shallow DOF.

    Here is an example of a shot at 800 ISO with a lens wide open and 1600
    ISO with the lens stopped down. Shutter speeds were the same, so you
    aren't giving that up.

    The trick is to determine which lenses fall into this category.

    http://www.pbase.com/andersonrm/image/114808342/original
     
    RichA, Jul 10, 2009
    #1
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  2. Except that ISO 1600 at f/4 <> the same exposure as ISO 800 at f/2

    David
     
    David J Taylor, Jul 10, 2009
    #2
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  3. RichA

    Rich Guest

    A lens has to be pretty awful not to work well centrally at f8.0. But
    some lenses are amazing wide open. Nikon's 14-24mm f2.8. Check out
    the output from an Olympus 35-100mm f2.0, it's pretty amazing.
    Panasonic G1.
     
    Rich, Jul 11, 2009
    #3
  4. RichA

    Rich Guest

    True. Which means that the example is even more compelling because
    the 1600 ISO shot was underexposed but still looked better after NR.
    In that case, the lens wide open was no benefit at all, and you are
    better off going with a higher ISO.
     
    Rich, Jul 11, 2009
    #4
  5. RichA

    Ray Fischer Guest

    They need to take pictures in dim lighting.
     
    Ray Fischer, Jul 11, 2009
    #5
  6. RichA

    Don Stauffer Guest

    And which cameras. Cameras seem to vary quite a bit in their performance
    at higher ISOs. But it does seem like the more recent ones are getting
    better at that.
     
    Don Stauffer, Jul 11, 2009
    #6
  7. RichA

    PatM Guest

    For many people, the choice of a fast lens has little to do with the
    two reasons you mentioned. The real reason is that fast lenses
    focuses faster and better. That's a big advantage. If you're
    shooting at f8, a f2 lens will still usually outperform an f4 lens
    because of the difference in focusing. You'll miss the picture if
    your lens is searching for a focus point.

    In dim light you might have to switch over to manual focus, which
    really isn't a big deal, but many people don't like doing it.
     
    PatM, Jul 11, 2009
    #7
  8. RichA

    Paul Furman Guest

    What's the f/2 lens?

    --
    Paul Furman
    www.edgehill.net
    www.baynatives.com

    all google groups messages filtered due to spam
     
    Paul Furman, Jul 11, 2009
    #8
  9. RichA

    Ray Fischer Guest

    If somebody claims that a $400 camera has optics that are as good as
    those in a $2000 lens then they are a liar or a crackpot.
     
    Ray Fischer, Jul 11, 2009
    #9
  10. RichA

    Paul Furman Guest

    Specific examples... ?
     
    Paul Furman, Jul 11, 2009
    #10
  11. If the optical design playing field was level. Which it most certainly
    isn't in this case.

    Cost depends on the optical problems the lens has to solve. A DSLR
    lens has to deliver its intended quality while leaving enough room
    between sensor and rear lens element to accomodate the mirror. That
    adds a lot to the cost of lenses of shorter focal lengths.
     
    Chris Malcolm, Jul 11, 2009
    #11
  12. RichA

    Ray Fischer Guest

    Ray Fischer, Jul 11, 2009
    #12
  13. RichA

    Ray Fischer Guest

    And if you have a tiny sensor then you certainly can compromise the
    lens quality to match the sensor's abilities.
     
    Ray Fischer, Jul 11, 2009
    #13
  14. RichA

    Charles Guest

    For many people, the choice of a fast lens has little to do with the
    two reasons you mentioned. The real reason is that fast lenses
    focuses faster and better. That's a big advantage. If you're
    shooting at f8, a f2 lens will still usually outperform an f4 lens
    because of the difference in focusing. You'll miss the picture if
    your lens is searching for a focus point.

    In dim light you might have to switch over to manual focus, which
    really isn't a big deal, but many people don't like doing it.

    Good response.

    The autofocus system generally works much better (increased accuracy) and
    quicker with faster lenses .

    Some cameras cannot take advantage of their advanced autofocus
    sensors/systems, with slower lenses.
     
    Charles, Jul 11, 2009
    #14
  15. RichA

    Rich Guest

    You hope. Otherwise you end up with one of the innumerable cases of
    back or front focus we see people talking about. At least some added
    DOF might save a shot with a slower lens if that condition exists.
     
    Rich, Jul 12, 2009
    #15
  16. RichA

    Ray Fischer Guest

    In your case it may take 10 years.
    Nobody has said that it is, but lying bout other people is common with
    people who really have no way of justifying themselves.
    And much slower.
    More missed shots.
     
    Ray Fischer, Jul 12, 2009
    #16
  17. RichA

    TheRealSteve Guest

    Ahh, I see now. So if I took my cheap 50mm f/1.8 lens and glued a
    stop in the aperture lever channel so that it could never open any
    further than, say, f/2.8, I now have a diffraction limited lens that
    is so far superior to any P&S lens it'll make your heart spin.

    Of course, the same is true for *any* DSLR lens, i.e., I can turn them
    into a diffraction limited lens by removing the ability to open up
    wider and limit myself in the same way that P&S manufacturers have
    limited you. At least a DSLR gives me the *choice* to decide whether
    I want a faster lens at the expense of softness or whether I want to
    stop it down a little and have superior image quality and maybe bump
    up the ISO a little to deal with that. A P&S doesn't give you that
    choice. I guess they figure the typical P&S user isn't sophisticated
    enough to deal with options.

    You're stuck with a slow lens and a camera that doesn't have the
    ability to bump up the ISO to deal with a slow lens like a DSLR can.

    Steve
     
    TheRealSteve, Jul 12, 2009
    #17
  18. RichA

    SMS Guest

    And they are correct.
     
    SMS, Jul 12, 2009
    #18
  19. RichA

    Paul Furman Guest

    Plus if you stop down past f/5.6 on many of those, it's all downhill
    from there. DX will go to about f/11, FX to about f/16 with the same DOF
    as P&S at about f/5.6.

    --
    Paul Furman
    www.edgehill.net
    www.baynatives.com

    all google groups messages filtered due to spam
     
    Paul Furman, Jul 12, 2009
    #19
  20. RichA

    TheRealSteve Guest

    And with that, we now know you have no idea what you're talking about
    and are self inconsistent. If your magical diffraction limited P&S
    lens could open far enough, it would no longer be diffraction limited.
    So by your definition above, P&S lenses are not diffraction limited
    either, despite your claims otherwise elsewhere.
    Of course being limited to ASA25 and ASA64 stopped pros from getting
    shots they were after. That is unless you're so limited in your
    creative ability that you only ever want to take pictures of things
    sitting still or in bright sunlight or with a super flash. In that
    case, you're right. And the fact that you have no creativity is
    insight into why you don't feel limited by a P&S.

    Steve
     
    TheRealSteve, Jul 12, 2009
    #20
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