Canon 50mm f1.8???

Discussion in 'Canon' started by jazu, Dec 14, 2007.

  1. jazu

    acl Guest

    it just means you need to decrease the COC on the negative by the
    "crop factor", because that is how much more you enlarge. Had you not
    stipulated that the final print is the same (ie, had you kept the COC
    on the negative the same), then for the same lens, distance and f/stop
    you'd indeed get the same "DOF".

    For some reason, though, this seems to be a semi-religious topic.
     
    acl, Dec 17, 2007
    #41
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  2. jazu

    acl Guest

    Yes, but if you look at formulae for DOF, they specify it on the final
    print, not on the sensor. So to keep all parameters the same, you need
    to print to the same size, ie, you need to enlarge the small-sensor
    image more. So: same distance, same lens, same f/stop, same final
    magnification, the DOF is not the same (the smaller sensor has less,
    in fact).

    Of course if you prefer to work with DOF in the sensor itself, that's
    a different story (but that is not what is commonly done).

    Anyway there are subjective aspects to this too.
     
    acl, Dec 17, 2007
    #42
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  3. Feel free to think as you like, thought crimes are usually
    not punished (unless you voice them). :->
    DOF is a objective measure of sharpness, as defined by the
    CoC. Choosing the correct CoC may be subjective and ...
    .... subjective to enlargement (and, as you forgot to say, dependent
    on the viewing angle/viewing distance). But once a CoC is choosen,
    even arbitrarily, the DOF follows objectively.
    Not only almost. A 50mm does not "turns into an 80mm lense",
    not even on a crop camera.
    Exactly. A 50mm lens stays a 50mm lens. It does not become
    a 80mm lens.
    Ok. Let's see.

    I set up a good, stable tripod.

    I put the 50mm on a 20D, the 40D on the tripod and shoot an
    object 10 metres away with aperture f/4. => Image 1, 3504 x
    2336 pixel.

    I put the same 50mm on a 1Ds Mk III, put the camera on the
    very same tripod and shoot the very same object 10 metres
    away with f/4. => Image 2, 5616 x 3744 pixel.

    I trim 18.75% (of the original image size) from Image 2, from
    the top, bottom, left and right. I get a 3510 x 2340
    pixel image --- which looks like Image 1, BTW.
    I don't understand. Why does printing the 3510 x 2340 pixel image
    (Image 2, cropped) as 4x6 inch decrease the DOF versus the 3504
    x 2336 pixel image (Image 1, crop camera) as 4x6 inch?

    You surely won't say the 6 and 4 pixels difference change
    resolution of DOF, right?


    Can you explain?

    -Wolfgang
     
    Wolfgang Weisselberg, Dec 17, 2007
    #43
  4. [/QUOTE]
    No, because you would have to print the full size image at a
    larger size and then trim the print to get an identical
    image.

    You _could_ choose a longer focal length for the full frame
    image to get the same FOV on the full frame negative --- but
    changing the focal length changes the lens, and thus it is
    comparing different lenses on different sensors[a].

    Moving closer changes the perspective, hence is not
    comparable.
    It changes more with the final print size and viewing
    distance.

    -Wolfgang

    [a] You'll find that increasing the sensor size and
    increasing the focal length to keep the FOV identical
    also necessiates a smaller aperture to get visually
    identical results on the final print.

    See the DOF change of a P&S and of a DSLR and of a large format
    camera at identical object distances and identical FOV ...
     
    Wolfgang Weisselberg, Dec 17, 2007
    #44
  5. jazu

    acl Guest

    I think what I am saying is quite clear. If you switch the body but
    leave the lens the same,and keep the final print size the same, you'll
    end up having less DOF with the smaller sensor (this is what is
    assumed in the equations you'll find in most books, ie, it is assumed
    that the print size is the same). You didn't mention an identical
    image, just gave the mental experiment of cutting the centre of the
    negative, and I explained how that could go wrong. Experience, though,
    teaches that you're not about to say "why thank you for pointing this
    out" :)
    It changes in exactly the same way: a format double the size will
    necessitate a COC also double the size (so will have more DOF); or,
    doubling the size of the print will halve the COC; or, doubling the
    viewing distance will double the COC. So no, it doesn't change more
    with the final print size and viewing distance.
     
    acl, Dec 18, 2007
    #45
  6. You just decide to look differently on the same facts.
    That's OK.
    But you won't get the same picture, nor the same print, so
    it's some apples and some oranges.
    Thank you for pointing out that my mental experiment
    contained way to much unstated assumptions. :)

    "more" in the sense of "additional factors", not necessarily
    "stronger factors". English is a terribly unprecise language.

    -Wolfgang
     
    Wolfgang Weisselberg, Dec 20, 2007
    #46
  7. jazu

    acl Guest

    Experience, though,

    :)
     
    acl, Dec 20, 2007
    #47
  8. jazu

    Paul J Gans Guest

    I agree. What this discussion was lacking was a statement of
    what "same" means.
    Yup. I think folks forget that photography is an art form. What
    *is* good is what looks good to *you*. There will never be full
    agreement among everybody.
     
    Paul J Gans, Dec 20, 2007
    #48
  9. jazu

    Paul J Gans Guest

    I will try. Forget the pixels. That's an added complication.
    What we mean by "blur" is that sharp lines are no longer sharp.
    The circle of confusion measures that. Away from the point of
    focus all lines are a bit blurred. The blurring increases as
    you get further from the focus point.

    All that is elementary.

    Now blur means that what should be a point is spread out over
    a finite distance. Say you accept anything less than .001 inches
    thick as being "sharp". As you magnify the image, the 0.001 inches
    becomes larger. At 2X magnification it becomes 0.002. That means
    that a line considered sharp at 1X is not "blurred" at 2X.

    So blowing up the original image increases the blurring. That is,
    it decreases the sharpness and so decreases the depth of field.
     
    Paul J Gans, Dec 20, 2007
    #49
  10. jazu

    Doug Freese Guest

    Amen. I'd bet we would get 95+% agreement with Ansel Adams. ;)

    -DF
     
    Doug Freese, Dec 20, 2007
    #50
  11. jazu

    Doug Freese Guest


    It's bits of imformation like this that makes reading this group fun and
    putting up with dorks like RichA.

    For anyone that wants to some more of the COC gore take a look at:
    http://en.wikipedia.org/wiki/Circle_of_confusion

    Now I just need to absorb this and see how it will make take better
    pictures. ;)

    -Doug
     
    Doug Freese, Dec 20, 2007
    #51
  12. There are two points that people forget or don't realize about DOF.

    1. Since DOF is something that happens inside the viewers head, you have to
    compare the same print size and viewing distance. And you have to compare at
    some print size and some viewing distance.

    2. Since photography is about taking photographs, it makes no sense to
    compare DOF on different photographs, so comparing the DOF of the same lens
    on different formats makes no sense. You have to compare photographs taken
    with the same AOV from the same position.
    It's not the art bit that's a problem (DOF is an issue on snapshots as
    well), it's that people forget that photography is about taking photographs
    and that photographs are things that people look at.

    David J. Littleboy
    Tokyo, Japan
     
    David J. Littleboy, Dec 21, 2007
    #52
  13. jazu

    acl Guest

    This is discussed periodically here and in rpd, usually with people
    starting to insult each other after the first 30-40 posts or so. I
    could never understand why this DOF business gets people so
    passionate. To take an uncharitable view, probably because it's simple
    enough for anybody to understand but involves enough technical terms
    to sound impressive.
    Here's an idea: memorise the hyperfocal distance for a 10mm lens at f/
    10. You can use the approximate expression in wikipedia, H=f^2/Nc,
    with f the focal length, N the f/stop and c the circle of confusion
    (this you'll have to determine by trial and error for your printing
    size, or just believe the standard values given).

    It's easy to then work out the hyperfocal distance for any lens and f/
    stop, because the f and N for the H that you've memorised are round
    numbers; eg if you use a 20mm lens, multiply the number you remember
    by 4, etc. It sounds much harder than it is, try it!
     
    acl, Dec 21, 2007
    #53
  14. jazu

    Paul J Gans Guest

    I certainly would agree. But I know people who cannot stand
    his images.

    For myself, Dianne Arbus makes me crazy in my own head (which
    is impolite). Perhaps that's the point, but her images don't
    resonate with me at all.
     
    Paul J Gans, Dec 21, 2007
    #54
  15. jazu

    Paul J Gans Guest

    It gets worse. I passed a small street exhibition of photographs
    today (In Union Square, in New York). One exhibitor had a photo
    that stunned me. It was deliberately motion blurred. I normally
    hate that, but this one turned me on. I have no idea why.
     
    Paul J Gans, Dec 21, 2007
    #55
  16. jazu

    Paul J Gans Guest

    Back in the Good Old Days (tm) when folks used fixed focal length
    lenses, there was a distance scale engraved on the focus ring. And
    there were marks on the lens barrel indicating the depth of field.
    To hyperfocus a lens you put one mark on infinity and read off the
    close focus distance from the other mark.

    Of course you always allowed a bit because tastes vary.

    This was extraordinarily useful. Zoom lenses can't use such
    a simple mechanism as now hyperfocal distance depends both
    on aperture *and* focal length. But somebody could make some
    change by working this out for a family of lenses and a
    brand of camera (to take the sensor size into account).
    I'd love a small card I could carry. Of course I'd need one
    for each lens I have... ;-(
     
    Paul J Gans, Dec 21, 2007
    #56
  17. jazu

    Frank ess Guest

    et seq

    [ ... ]

    I certainly would agree. But I know people who cannot stand
    his images.

    For myself, Dianne Arbus makes me crazy in my own head (which
    is impolite). Perhaps that's the point, but her images don't
    resonate with me at all.

    --
    --- Paul J. Gans

    [ ... ]
    I think there is a key word in that last paragraph. A "work of art"
    sets something in the viewer into motion. A process, a recollection, a
    sublimated emotion, a compare-and-contrast session, something poignant
    to the state of the apprehender's interior landscape. Something gets
    its little clamfoot in a crack of meaning and levers it open. Cool.
     
    Frank ess, Dec 21, 2007
    #57
  18. jazu

    acl Guest

    That is one of the reasons I prefer MF lenses. It's very useful
    indeed. But what I describe really is not that hard, although the
    description I gave sounds clumsy.
    I have a a 35-70 zoom for my minolta xd7 somewhere; ok I found it, it
    has scales like this
    http://www.pbase.com/image/90592526
    (apologies for the reflection, I just got fed up tried to get my
    compact to focus). I don't know how usable it is as I practically
    didn't use the lens.
     
    acl, Dec 21, 2007
    #58
  19. I'd question anyone who expresses it in quite those terms...

    I recognize Adams as a great photographer, and do
    believe his landscapes are indeed great photographs, but
    it also happens that I have *never* had any desire to do
    that type of photography. I did study his techniques
    and did read all of his books 30-40 years ago, and it
    has influenced my enjoyment of photography greatly. But
    I still don't want to take pictures, beautiful or
    otherwise, of huge rocks...

    And while original "Moonrise over Hernandez" prints by
    Adams are supposedly worth $50,000 and called by some
    the best photograph ever made, Ansel Adams literally
    gave away the best 242 images he probably ever took, in
    my opinion. The collection used for his book on the
    Manzanar relocation center in California was given to
    the Library of Congress in 1965.

    http://memory.loc.gov/ammem/collections/anseladams/

    Today, however, I realize that an Adams' photograph of a
    rock is technically not different than the single most
    inspiring image that he ever took, for me. And he gave
    this one away, so we can down load a 137MB scan of the
    4x5 negative from the Library of Congress for free!

    http://memory.loc.gov/pnp/ppprs/00000/00007a.tif

    or just an 80KB JPEG

    http://memory.loc.gov/pnp/ppprs/00000/00007v.jpg

    The picture is of Aiko Hamaguchi, a nurse who must
    have caught Adams' eye to say the least, as there are
    at least twice as many images of her as there are of
    any other single individual (by my very rough count).
    Other than the name and Ansel Adams' pictures, I've
    not been able to find out anything else about her. If
    she is alive today, she must be perhaps close to 90
    years old.
     
    Floyd L. Davidson, Dec 21, 2007
    #59
  20. jazu

    Robert Coe Guest

    : I read that this is great lens for indoor use.
    : I don't understand something here. How 50mm could be be good to make i.e.
    : group of people around the table?
    : isn't 50mm to narrow for that?

    You could get the Sigma 30mm f/1.4. But even that's a bit narrow for groups
    around a table.

    Bob
     
    Robert Coe, Dec 21, 2007
    #60
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