What defines a "Fast" Lens?

Discussion in 'Digital Photography' started by Music_Producer, May 4, 2006.

  1. Music_Producer macrumors 68000

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    #1
    I see everyone comparing lenses and talking about how 'fast' a lens is.. this is in regards to be able to focus quickly on an object, correct? As in, for sports photography.

    I find my nikon 18-55 mm dx to be pretty 'fast' when taking sports pictures.. and my 85 mm f/1.8 lens seems to have a little 'lag' when focusing. Isn't my 85mm f/1.8 supposed to be 'faster'?

    Newb question.. :)
     
  2. Blong macrumors member

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    #2
    "Fast" in this situation refers to the aperture - not speed of focus. The 85mm 1.8 lens you refer to above would be considered a "fast" lens. i.e., wider aperture.
     
  3. davegoody macrumors 6502

    davegoody

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    #3
    Not Quite right !

    Actually, the term "fast" means the lenses ability to capture light. The larger the Maximum Aperture of the lens, the more light that gets to the sensor (or film) and therefore the camera has more latitude in the shutter speed range that is available to use. An example of this, though it may seem to be the wrong way round, is the smaller the rating (in f-Stops) the faster the lens. i.e. a lens that is f2.8 is far faster than one that is f5.6. The faster lenses are generally much more expensive. I am a Canon man myself, my lens of choice is the Canon 28-300 f3.5 - 5.6 L IS. This means that the focal length is 28mm to 300mm (though on most DSLR this is multiplied by 1.6) - the "speed" in this context is f3.5 at the lower end of the zoom and f5.6 at the top end of the zoom. The L designation means that it is a Pro Level lens (and it wants to be at £1700) and the IS means that it is Image Stabilised. This means that it has optical image stabilisation that cuts down camera shake at lower shutter speeds. I think the Nikon version of this is VR (Vibration Reduction). Hope this helps !
     
  4. Music_Producer thread starter macrumors 68000

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    #4
    Ahhh, thank you very much! I feel a little silly now.. :p I prefer getting answers here than reading a boring book!

    PS - Is there anything for the speed of focusing? Or something of that sort? Like "Hey my lens focuses much faster than the other one".. or do all lenses focus at the same speed?
     
  5. Abstract macrumors Penryn

    Abstract

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    #5
    No, lenses don't all focus at the same speed. Lens reviews will generally mention how fast and quiet a lens focuses. Nobody wants a slow lens, but some people don't want a fast lens that makes too much noise, especially when they're photographing something that may easily be disturbed.
     
  6. Chip NoVaMac macrumors G3

    Chip NoVaMac

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    #6
    Since you are using a Nikon, there are two methods that Nikon uses to drive the focusing; a gear driven method that was their first method (AF or AF-D). The second is what is called AF-S. This method uses a silent wave motor to drive the focusing. It can be quicker and quieter.

    Your 85mm 1.8 is an AF-D lens. BTW, the D indicates that distance information is communicated to some bodies for exposure and flash exposure calculations.

    Canon users have two focusing methods. Both have motors in the lens. First is a slower, nosier ring motor. While USM is the faster, quieter method .
     
  7. Blong macrumors member

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    #7
    Canon use the letters USM to distinguish their quieter and faster focusing lenses from others. USM means ultrasonic motor. These are generally more expensive than a non-USM lens with the same focal length.

    I think Nikon distinguish their faster focusing ones with AF-S.

    Edit: Chip - you got there first.
     
  8. Whiteapple macrumors regular

    Whiteapple

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    #8
    i was wondering, do you people use autofocus? I don't, and then there is no noise for the lense. Any advantages for autofocus? A "computer" focuses better than your eye? No offence there, just asking.
     
  9. iGary Guest

    iGary

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    #9
    *makes note to go hang out with Chip at work* :eek:
     
  10. jared_kipe macrumors 68030

    jared_kipe

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    #10
    Its not that a computer is doing it, it is using phase sensitive detection to determine focus. If calibrated correctly is usually much faster than you can focus, and usually better at finding focus than a human with a AF focusing screen. The old split prisms made focusing easier for humans, but still takes time.

    I think one of the common problems with people and autofocus is that they don't understand the system. So they're confused when the lens hunts when they point it at a pure black object. The system needs detail, aim the AF point at an edge or something it can use. When' you're focusing with your eyes you're not stareing at one specific spot, you're looking around the image for details to become crisp. So let the autofocus work the way its designed to.
     
  11. electronboy macrumors 6502

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    #11
    Without getting technical--the wider the opening or aperture of a lens means that it can let in more light. For night photography--were minimal light is the norm--if you have a lens with a larger aperture it will produce a similar exposure level on film in less time than a similar lens with a smaller aperture. Hence, a lens with a larger aperture seems to operate faster than a lens with a smaller aperture rating.

    What can be confusing is that larger apertures are represented by lower numbers.
     
  12. ChrisA macrumors G4

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    #12
    I generally use autofocus except in some situations, mostly when the camera is on a tripod.

    On a Landscape I can manually set the lens at the "parafocal" distance which is just short of infinity. For macro work manual allows me to better choose which part of the frame is in sharp focus. And of course with the manual lenes I have, AF is no possible.

    Many times the AF system will wrongly pick the closest thing in the frame to focus on and I'll have to manual "correct" it. For example you want a subject's _eye_ not nose to be sharp.

    But even with manual focus All nikon AF cameras have a focus indicator that helps those with not-perfect vision. I use this feature. So I gues you might call it "semi-automatic focus"

    Sometimes with action photos the AF system is to stupid and slow so I'll just turn the focu ring myself
     
  13. ChrisA macrumors G4

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    #13
    why they call it "fast"

    It's a funny slag term that predates the invention of autofocus by maybe a century. "Fast" means the lens opens to a wider aperture thereby allowing the use of a "fast" shutter speed.

    In modern SLR terms a "slow lens" is one of the low cost zooms that open up to only f/5.6 while a fast lens would do f/2.8 or even f/1.4. This allows shutter speed to be reduced by a factor of 4 to 16.
     
  14. jared_kipe macrumors 68030

    jared_kipe

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    #14
    Canon has this too, if youre on Manual focus, and are half depressing the shutter, as you focus it will blink and beep when its infocus.
     
  15. Whiteapple macrumors regular

    Whiteapple

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    #15
    thanks :)

    how does the focus work? ultrasonic ? light? other? i know on my sony camera it's laser but of course it's not invisible and it takes ages...
     
  16. jared_kipe macrumors 68030

    jared_kipe

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    #16
    As I said, Phase detection, its completely passive. http://en.wikipedia.org/wiki/Autofocus
     
  17. Chip NoVaMac macrumors G3

    Chip NoVaMac

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    #17
    Not quite how to take that. :)

    Sad though I answer all these same questions at work 5 days a week, and I come here and say the same things. :eek:
     
  18. Chip NoVaMac macrumors G3

    Chip NoVaMac

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    #18
    Amen. Screens on cameras are getting better, but no substitute for a micro-prism dot, or better yet a split-image rangefinder dot (these were used on SLRs till AF came along.

    The focus indicator dot is no better than what the camera would do with an AF lens.

    Now, not to confuse anyone - but here I go: AF sensors have a certain precision, meaning that the are "tuned" - if you will - for certain apertures. For example a consumer based SLR/DSLR may have an AF sensor that a precision matched to f5.6 or better. In upper level SLR/DSLR cameras they may have one or more AF sensors that have dual precision for both F5.6 and f2.8. The idea there is with less DOF at f2.8 a greater precision is needed in focusing lets say with 100mm lens at 15 feet at f2.8 than the same lens offering an f5.6 aperture. Tried for basic here, so be kind. :)
     
  19. Clix Pix macrumors demi-goddess

    Clix Pix

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    #19
    I am definitely a lover of "fast glass" because it offers more opportunities in a variety of shooting conditions, especially when the ambient light is not optimal. One can get some stunning photographs when using a fast lens in, say, a portrait situation where the subject is sitting by a window. No need for flash, which makes a huge difference. The problem, though, with using a fast lens "wide-open" at f/1.2, f/1.4, f/1.8 or f/2 is that the depth-of-field is affected. In portraits, it's best to focus directly on the eyes so that they will be sharp while everything else softens a bit....

    A lot of times when shooting macro it is necessary to go into manual focus as opposed to auto-focus in order to get exactly the image you want. As mentioned, auto-focus does have the issue of needing some kind of contrasting line or area upon which to focus. With manual focus you can get exactly what you want.

    Never heard it called anything other than "hyperfocal distance" before....but, yes, this is something which can make a huge difference when shooting landscapes. It is easier to do the hyperfocal distance thing with the older lenses which have all the markings on them, as opposed to the newer lenses which don't.... There is definitely still a place for those older lenses in photography! Sometimes automation can carry things too far. With Nikon, being able to use older lenses on their modern cameras really is a benefit.
     
  20. ChrisA macrumors G4

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    #20
    not even a typo

    Wow, I can't even blame that one on a typo. How about "the brain just don't work like it used to". "parafocal" has more to do with how optics are mechanically mounted... I think two lenses are parafocal if the image is the same distance from their mount flange. So all Nikon lenses would be parafocal. but note that if you used a view camera those lenses are typicaly not. . My Mamiys RB67 was not that way either. the lens mount to film plane distance changes for each lens Anyway, yes a completely unrelated term.

    ABout hyperfocal: If a lens if focused at infinity about one half of it the available depth of field is "wasted" because it is "in back of infinity". Hyperfocal places infinity at the far edge of the zone of sharpness.
     
  21. Chip NoVaMac macrumors G3

    Chip NoVaMac

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    #21
    This one is even new to me (parafocal). I think I understand the concept. But not the practical application of it.

    Now as a RF user, I fully understand the use of "decent" DOF scales (on most modern AF lenses, they are mostly useless IMO) in order to assure proper "focus" of the background and foreground, as well as the subject. Is this what you mean?
     
  22. cookie1105 macrumors 6502

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    #22
    Inspired by a this post and a similar post on another forum, I made a "mini" spreadsheet for comparing light gain between two lenses. I am sure this sort of thing exists already.

    You can download it here.

    Just enter Focal length & Aperture in the blue fields.

    Yes, I lead a very boring life.......:)
    Thank you to Stan Walker for his explanation.
     
  23. Music_Producer thread starter macrumors 68000

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    #23
    When I was taking a few sports shots.. I tried shooting with manual focus and autofocus. The autofocus takes a little more time, than manual focus (obviously) but some pictures taken with manual focusing were totally soft.. made me wonder if I was blind or something!

    So I stick to autofocus.. and yes, the first time I got an SLR.. I tried and tried to focus on a glass surface or a black surface.. and was wondering why the hell it wouldn't focus :rolleyes:

    This one's for Chip.. my Nikon D50 has 3 AF systems - AF-A,S and C. It was selected to AF-A by default.. is that the same as AF-D? And if I select the AF-S mode, will it work with the 85 mm f/1.8 lens (work as in, it does work.. but does it do what its supposed to do?) Thanks!
     
  24. mrichmon macrumors 6502a

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    #24
    An experienced photographer can often manually focus and track focus faster than an autofocus system. However, for non-professional photographers, an auto-focus system is going to focus faster.
     
  25. Chip NoVaMac macrumors G3

    Chip NoVaMac

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    #25
    When I mentioned AF, AF-D and AF-S; I was referring to the different Nikon AF lens mounts and what it can mean in terms of performance of focus speed and such.

    In the Nikon D50, what you are asking about is how the camera will focus regardless of what AF lens that is attached. AF-A is where the camera can select the proper focusing mode itself, based on the subject. AF-S in this case means that it is best suited for stationary subjects. With AF-C being for moving subjects.

    A neat thing on the D50, D70s, and D200 (will have to check the other Nikon DSLR models) is the help function in the menu. The WB button has a question mark on it. When in a menu selection, press the WB button for brief help on that menu item.
     

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