50 mm tele prime, has anyone tried?

Discussion in 'Digital SLR' started by Bengt C, Jul 13, 2007.

  1. Bengt C

    Bengt C Guest

    I'm different. I have the opinion a "usable" picture has 1-2 MP
    resolution, I'd say "sufficient" resolution is 3-4 MP. I'd classify 10
    MP as "mostly non-achievable".

    I'm trying to decide what would be the next step after replacing the
    ridiculous Canon kit-lens with a decent 2.8 from Sigma or Tamron. I'd
    really would like to go wide angle, but that will be expensive and
    heavy, so lets look at the tele side for a moment instead. Something
    like 25-100 would provide a nice overlap, I frequently just want to
    bring one lens, but it would have mass, bulk and cost money. As said,
    I'm not really all that interested in tele. Besides, tele without image
    stabilization seems almost unusable. So, I got this idea:

    Buy a Canon 50/1.8 for almost no money and use it as a tele by cropping
    the 10 MP high quality picture it delivers. This will (surprisingly)
    give all these benefits:

    * Worlds cheapest tele lens $100 - On pair with my tele interest.
    * Worlds most lightweight tele 130 g - Perhaps it won't stay at home?
    * Has image stabilization - F/2 versus F/4 or F/5.6 for the real thing.

    Great when you think about it, right? Now, my question is: Will this
    work? Is a 50 mm prime so god, that a 2 MP crop actually has resolution
    close to 2 MP? Will a resampled (2 MP) full-frame image from a real
    tele be significantly sharper, or only slightly better? Can handheld
    (w/o IS) at F/4 really compete with a F/2 50 mm crop with respect to
    camera-shake?
    /Bengt
     
    Bengt C, Jul 13, 2007
    #1
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  2. Bengt C

    RichA Guest

    Keep dreaming. A 100mm or 200mm lens has to be pretty bad to have its
    image beaten by a hugely cropped one from a 50mm. Also, you use a
    Canon so how dark is the subject you intend shooting, if you can't use
    a 100-200mm lens and get a steady shot?
    Also, Canon is not know for superb wide and medium primes, unless you
    can afford the 50mm f1.2.
     
    RichA, Jul 13, 2007
    #2
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  3. Simpler to replace camera body and lens with a Nikon body, and their 18 -
    200mm image stabilised lens.

    <G>

    David
     
    David J Taylor, Jul 14, 2007
    #3
  4. Bengt C

    ben brugman Guest

    If you consider a 1-2 MP picture "usable", realise that a 1-2 MP
    picture has 1-2 M pixels with each three colors. If this picture is
    made with a 10 MP camera, these 1-2 MP pixels contain real
    information.

    Making the picture with a 1-2 MP camera gives you only
    1-2 M pixels with one color each, this is less information than
    a 1-2 Mp picture but reduced from a 'larger' format.

    a 50 mm lens will give you a 2 time fold zoom, but only with
    a quarter (1/4) of the pixels used. So 10 MP off one color
    pixels gets reduced to 2.5 MP off one colore pixels.

    a 50 mm lens will give you a 4 time fold zoom, but only with
    a sixteenth (1/16) of the pixels used. So the 10 Mp off one color
    pixes gets reduced to 0.625 one color pixels.

    So allthough you sceme is feasable, your quality will suffer
    even with moderate (digital) zoom.

    Digital zoom or cropping is a very valuable tool, but the
    range it can be used for is limited. A factor of 2 liniear is
    about the max in most situations.
    (Starting of with 6 MP crop of 2 times liniear leaves 1.5
    MP which is about the amount of most displays and still
    usable for a 4 inch x 6 inch print)

    ben
     
    ben brugman, Jul 14, 2007
    #4
  5. Everybods's entitled to one.
    May I introduce you to medium format digibacks?
    That will depend on what lens you replace it with, what you
    want, your budget, and so on and so on.
    Depends on your definition of heavy and expensive. Comparing
    the 17-40mm f/4 to the 70-200mm f/2.8 IS, I know which one of
    these is heavy and expensive, and it ain't the 17-40.
    Since you talk Canon, a 25mm lens has the same view angle as
    a 40mm lens --- not enough wide angle. The 17-40mm is
    probably a better match for you, especially if you don't mind
    cropping.
    Well, a lens without mass, bulk and costing money doesn't exist,
    at least not till anti-gravity lifters, inertia compensators and
    infinite wealth for everyone are available.

    However, there are bigger and smaller, lighter and heavier,
    expensive and cheap lenses.
    How did people do it 10 years ago, then?
    Ever heard of monopods, tripods, beanbags, bracing, fast exposure
    times and so on?
    Tamron's 55-200 is in the same price class and needs less
    cropping.
    If your problem is weight, use a compact camera, DSLRs ain't
    for you.
    I still prefer a 70-200 f/2.8 with IS. Sorry.
    More like "run off this roof, flapping your wings and you'll fly"
    --- sounds great, but only in theory.
    Oh, you can crop as much as you like. I doubt you'll like the
    results, but I don't know your standards.
    That depends a lot on the prime, doesn't it? And on aperture
    (you probably want f/5.6 --- here goes your image stabilisation).
    And on de-bayering. And so on and so on.
    In general, yes, a *lot*, unless you use a coke bottle bottom
    as lens.
    If you crop the f/2 50mm shot to the same view angle as the
    tele, and downsize the tele to your 2MPix, camera shake will be
    identically good or bad for both, but the tele image will have
    all information and less noise in the bargain.
    You gain 2 stops, but on the tele you can up the ISO 2 stops and
    still get no worse noise in the end result, so yes.

    Once you throw in a tripod, mirror up condition and a remote
    release, the 50mm is soundly beaten.


    You miss a few crucial facts.
    Let us assume you have a 1736 x 1160 pixels image (2 MPix).
    On your 50mm crop it'll be built out of 868x580 green and 434x290
    red and 434x290 blue measurements.

    On a tele it'll be composed out of ca. 3888 x 2592 measurements,
    1944 x 1296 green and 972 x 648 red and 972 x 648 blue
    measurements.

    You can do the math yourself, whether 868 measurements stretched to
    1736 points is gonna give a sharper image than 1944 measurements
    condensed to 1736 points. Yes, there are algorithms that can
    get a lot out of the bayerized data, but they can do that in
    both cases --- even though the tele + downsize probably won't
    profit just as much as the straight crop --- it'll not make up
    the differences all that much.

    -Wolfgang
     
    Wolfgang Weisselberg, Jul 14, 2007
    #5
  6. Bengt C

    Bengt C Guest

    Correct. But a F/2 50 mm prime can do other thing better. Things that I'll
    do more often than shooting at say 135 mm focal length.

    How can that be so? A (budget) tele will only provide say F/4 at best.
    I.e. in a situation where the prime will shoot at 1/250 s the tele will
    shoot at 1/60 s or even slower.
    Are you sure a 1600 ASA tele does outperform a 50 mm crop at 400 ASA?
    You may be right regarding normal dSLR's, but it certainly isn't self-
    evident on an Olympus (which I don't use for exactly that reason).

    Of course. But, as I said before, weight (and bulk) matters and the
    50 mm prime beats the tripod by an order of magnitude here. I can't
    carry (nor fiddle with) a tripod at those not so frequent occations
    when I want to take a tele shoot.

    You miss my inital point which is...
    ....I consider a 1-2 MP picture "usable" and 3-4 MP "sufficient". My
    first digicam, a 1.3 MP Canon A50, took lovely pics, but only under
    certain (rare) conditions. I wonder whether 2 MP crops from my current
    XTi with a F/2 prime will produce images comparable (better?) to that
    camera. The XTi has, for instance, much better dynamic range.

    /Bengt
     
    Bengt C, Jul 15, 2007
    #6
  7. Bengt C

    N Guest

    Think about perspective rather than angle of view. Perspective and focal
    length go hand in hand.
     
    N, Jul 15, 2007
    #7
  8. Bengt C

    Bengt C Guest

    I'was thinking they would be equivalent, as the view angle is the same for
    a crop and a true tele. But you're right. I forgott a true tele will let
    you shoot from a further distance, changing the perspective for the same
    "coverage". This also make this technique useless indoors. You can't come
    far enough from the subject. Guess a cheap Tamrom 55-200 is the answer
    to my pretty modest tele interest then.
    Cheers. /Bengt
     
    Bengt C, Jul 15, 2007
    #8
  9. Like low light shooting.
    Same view angle, same susceptibility to camera shake.
    Yes .... but let's have a look at
    http://www.photozone.de/8Reviews/lenses/canon_50_18/index.htm
    .. If you shoot wide open, you get a center resolution of
    1690 LW/PH on a 350D (that would be 845 black and 845 white
    lines still resolved at MTF50, i.e. contrast is halved)

    Let us assume that you get similar results from your 400D, so
    you get 1690 LW/PH on 2592 pixels, ergo 756 lines (378 line
    pairs) on your 1736 x 1160 image. That's not very much.

    With the 400D, cropping from 3888 x 2592 to 1736 x 1160, you
    get a crop factor of 2.23, i.e. a 50mm on your 400D would have
    the same view angle after cropping as a 112mm lens on your 400D
    (or said the other way: the 50mm on your 400D has natively the
    viewangle of an 80mm lens on a 35mm camera and after cropping a
    viewangle of a 179mm lens on a 35mm camera.)

    So let's compare it to the Sigma AF 18-125mm f/3.5-5.6 DC
    http://www.photozone.de/8Reviews/lenses/sigma_18125_3556/index.htm
    (note the "Verdict" at the end about AF failing below 50mm before
    rushing to buy one). At 125mm, f/5.6, the border only has 1437,5
    LW/PH. Scaling down to 2MPix, you of course retain the resolution
    until you have less pixels than resolution, hence you get at least
    the maximum of 1160 lines (580 line pairs) after scaling down.

    Now compare vertically 378 line pairs to 580 line pairs. Over 50%
    more.

    Ok, so you stop your 50mm down. The best LW/PH reached is
    2069 at the center at f/5.6. (Oops, here goes your "faster
    lens" argument!) --- and you get 926 LW/PH (463 line pairs).
    Still 25% advantage to the mediocre border resolution of the
    18-125mm.


    So basically, cropping even from a very good lens does not get
    better results than downsampling from a mediocre lens that's got
    the right focal length. I would advise against it.

    Quite sure, yes. Even a mediocre tele, see above, outdoes
    cropping, and if you don't stop down the 50mm, you'll be in
    even worse problems. And you don't gain much by it regarding
    focal length.
    As a 50mm prime, sure, as a tele lens, nope.
    1.2 Mio effective pixels, 1/3" sensor, CYGM-Bayer on CCD.
    Can do RAW.
    The same is true for all cameras, though they have been working
    on broadening these conditions for a century or more.
    It'll deliver better crops, especially regarding noise etc.

    "much better" depends a lot on post processing, for example.

    -Wolfgang
     
    Wolfgang Weisselberg, Jul 15, 2007
    #9
  10. Bengt C

    G.T. Guest

    Sufficient for what, printing nothing larger than 4x6s and/or posting
    cheesy pictures to the web?

    Greg
     
    G.T., Jul 16, 2007
    #10
  11. Bengt C

    Bengt C Guest

    Thank you for your explanation Wolfgang. It basically confirms what I
    suspected; Tele by cropping from a 50 mm prime will produce similar image
    quality as my first 1.3 MP digicam.

    As it happened I bought a Sigma 18-125 DC the other day. Now, I didn't
    "rush"
    to buy cause you wrote about it. I just went to the shop in order to get a
    decent lens to my new XTi. (Rarely can I remeber buying something that made
    me disappointed within only a few minutes of use - the kit lens). I had
    problems chosing light or reach. I was considering Sigma 17-70 or
    18-50/2.8.
    Due to limied supply, I couldn't try either at this particular visit.
    Instead
    I spent a few hours testing Sigma 18-125 and Canon EF-S 17-85. The Sigma
    was
    better than the 3x more expensive Canon, except it didn't have
    stabilisation.
    It just couldn't justify the extra money at this stage, nor was the size of
    the Canon appealing. Further, I tried the autofocus in dim conditions under
    a table. The Sigma "worked" at all focal lengths (with my XTi) and it was
    no
    worse than the Canon for that matter. Canon "hunted" just as much with low
    contrast subjects, it only did it more silently and possibly somewhat
    faster.

    As the Sigma 18-125 DC is going out of production right now, I decided to
    buy
    while still available. The 18-200/250 lenses will be available in the
    future,
    but as far as I can tell, there are more opticaly compromised. They have
    massive amount of distorsion at wide angle and Sigma 18-200 seem
    particulary
    week in the borders at 35mm. A poor compromise I'd say. As my tele-interest
    vanish at ~100mm, the Sigma 18-125 is perfect for me. What's pussling me
    though is the 18-200 lenses get higher user reviews than the 18-125. As I
    see
    it, it should clearly(?) be the other way around.

    So now I have all the tele coverage I'll ever need and also much improved
    normal-zoom performance at large appertures. Should I want even more
    quality
    or light in the future, the 18-50/2.8 EX (and also maybe a 10-22 EX) can be
    considered. At this stage I must say I'm reluctant to buy an expensive 2.8
    though. First, I'm not sure I actually need it. Second, should I decide to
    buy in the future, I surely will wait for an ultrasonic version to arrive.
    So
    I'll use the 18-125 for now, and aquire some first hand experience with it.

    I have however experienced one limitation of the lens. It doesn't seem to
    be
    sharp at the extreme right border at 18mm, large appertures. I have to go
    all
    the way up to F/11 for consistent performance all over the frame (at 18mm).
    There are no such problems at other focal lengths. Maybe flaws like these
    are
    expected when buying glass in this price level?

    Cheers. /Bengt
     
    Bengt C, Jul 22, 2007
    #11
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