Question about lens F-rating on digital slr

Discussion in 'Digital SLR' started by Roger, Aug 22, 2006.

  1. Roger

    Roger Guest

    Take a full frame camera and a 1.6 crop digital.
    Put a F2.8L lens on both. Set both cameras to automatic and same ISO and
    take exactly the same shot. Will the shutter speed on the 1.6 crop camera
    be the same as the other, or do camera manufacturers "tune" the internal
    gain (effectively the ISO) to compensate for the different intensity of
    the light on the sensor?

    I've wondered about this since I read in an Australian lens review that
    putting a full frame lens (like the F2.8L) on a 1.6 crop digital can
    actually result in reduced performance over putting a lenses specifically
    designed for the smaller sensor.

    Any thoughts?
    Roger, Aug 22, 2006
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  2. Roger

    Todd H. Guest

    Not unless they want to break the entire notion of why some folks by
    an SLR they don't.

    Keep in mind that handheld external meters are sold and in wide use,
    and that external studio strobes are used with SLR's and if
    manufacturers start inventing their own scales and compensating in
    software for shutter speeds and such, they'd effectively break their
    cameras for use with all this fun external gear we SLR junkies like to
    I'm curious too what they meant by that. It would have to be if
    anything optical performance they're talking about in some respect, or
    the difference in depth of field in the two cases because of the
    perspective change.
    Todd H., Aug 22, 2006
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  3. Take a full frame camera and a 1.6 crop digital.
    I agree, but we should define some of the terms above more carefully.
    Two cameras on fully "automatic" settings might well choose different
    apertures, meaning they'd end up with different shutter speeds as well.
    Change "automatic" to "aperture priority" mode, using the same actual
    aperture setting on both (eg, 2.8), and then we're getting closer. But
    now we need to look at what we mean by taking "exactly the same shot".
    It is impossible to take the *same* shot with both cameras using the
    same lens. Presumably, we are actually talking about standing in the
    same place and aiming the camera at the same subject, but then, the 1.6
    crop camera is going to be looking at a different version of that scene
    than the full frame camera, so it may well still meter differently. But
    if both are set to spot metering, using the same metering point, or if
    the scene is something flat like a grey card, then I think we can say
    the results would be the same (and would, as observed, also be the same
    as on a film camera or as reported by a handheld light meter).
    I'm guessing it has to do with the issue of digital sensors being more
    sensitive to the angle of light striking them than film is. That's why
    you'll see "digitally optimized" lenses that supposedly try to overcome
    this, but that still work on 35mm film or full frame digital. A lens
    designed for film would in theory have more of these issues with
    digital, whether full frame or smaller. Although here, it would
    actually seem APS would be *better* than full frame.

    In any case, I would *never* assume "reduced performance" was meant to
    imply anything whatsoever about shutter speed. Definitely optical

    Marc Sabatella

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    Marc Sabatella, Aug 22, 2006
  4. Roger

    Jeremy Nixon Guest

    I'm seeing the question you asked, and then the question you probably mean.

    The answer to the question as you asked it is, no, unless by coincidence.
    The automatic exposure algorithms may be different, and one camera may
    choose an entirely different shutter speed *and* aperture from the other.

    The answer to the question I think you probably really meant to ask is,
    yes, f/2.8 is f/2.8 regardless and independent of the sensor size or the
    size of the image circle the lens projects. 1/125 at f/2.8 with ISO 100
    means the same thing no matter what camera you're using.
    There is a myth that lenses can be "optimized" for digital and then somehow
    offer improved performance, usually with an extra coating of unexplainium
    on the glass to reduce reflections off the sensor or somesuch bollocks.
    I've never seen any non-theoretical evidence that it's not just a way to
    convince you to buy more shiny new lenses.
    Jeremy Nixon, Aug 22, 2006
  5. Roger

    BobF Guest

    The shots will be the same, within the tolerances of the cameras. Canon cameras
    (and a few others) have higher sensitivities then rated, EG the 30D achieves
    almost ISO4000 when set to 3200... The ISO on a digital camera is "tuned" by
    the factory to match a standard, since it's just a gain setting, as opposed to a
    film emulsion.
    That's bull, it would be the other way around... you would actually take
    advantage of the less distorted center of the lens as well. Don't believe all
    you read...
    BobF, Aug 22, 2006
  6. Roger

    AaronW Guest

    It is more difficult (expensive) to make a lens covering a larger
    frame. So a lens covering a smaller frame can be sharper.
    AaronW, Aug 22, 2006
  7. Roger

    Bill Guest

    This comes up every once in a while.

    As the others have mentioned, all things being equal they will produce
    the same results.

    The only difference between FF and cropped sensors is the field of view
    that you see and capture.
    Whoever posted that review should be taken out and beaten with a wet
    noodle until they submit to the error of their ways.
    There is no such thing as special "digital" glass, so no, the results
    will not be reduced at all.

    In fact, they got it backwards.

    Using a FF lense on a 1.6x cropped camera will use the "sweet spot" of
    the optics in the lense center, usually resulting in better sharpness
    and contrast across the 1.6x cropped frame.

    Lenses that were made 10+ years ago before DSLR cameras even existed
    will function perfectly on cropped sensor cameras and produce the same
    quality images.
    Bill, Aug 22, 2006
  8. Roger

    Sheldon Guest

    I have to agree. Optimizing a lens for digital would just mean not having
    to cover the size of a 35mm frame, which wouldn't make it any better or
    worse -- just cheaper. Just depends on the individual lens. A 35mm lens
    that gets good reviews should get better reviews (or at least the same) on a
    1.6x digital.
    Sheldon, Aug 23, 2006
  9. The easiest (and best) part of a lens is the center. So a lens
    covering a larger frame can be sharper, as it avoids all the
    problematic border areas.

    See? It cuts both ways.

    Wolfgang Weisselberg, Aug 23, 2006
  10. Roger

    AaronW Guest

    Is it easier (cheaper) to make a lens with soft corners, or a lens that
    does not need to cover the corners of the larger frame at all? (All
    other being equal, i.e., same sharp center).
    AaronW, Aug 23, 2006
  11. Ah, but there is:
    - lenses that don't fill out full frame, only the crop frame.
    Cheaper, lighter and --- if build well --- just as good.
    - lenses that (e.g. by multicoating) deal "better" with light
    entering from the sensor's end.[1] Where "better" is probably
    when compared to that manufacturers normal methods or marketing.
    Compared to that FF lens on a FF sensor. A bad FF lens will not
    magically become good on a crop sensor.

    Good lenses can beat a 8MP 1.6x crop sensors resolution (see>Reviews->Canon EF 35mm f/1.4 USM L) under good
    conditions. Not all lenses are good. Compare the 75-300mm
    f/4-5.6 IS lens at 300mm. (Of course, that lens probably
    would have even weaker borders on full frame.)
    The 75-300mm is from 1995 ... and most tests do not test
    things like "shooting into the sun", flaring, ...


    [1] digital sensors reflect much stronger than film (which rather
    scatters light than reflecting it, too). Which exacerbates the
    problem of in-lens reflections back to the sensor/film. That's
    the reason TTL flash metering is done with pre-flash(es),
    not with light reflected from the film/sensor.

    Of course, good lenses try hard to avoid reflections anyway ---
    you'll get bad results otherwise even with film when shooting
    e.g. into the sun. Thus these lens avoid having a flat,
    parallel pane of glass even as a protective first element,
    and for that reason 'protective' filters in front of a lens
    --- which are flat, parallel panes of glass perpendicular to
    the lens! --- can degrade the pictures with stray light and
    in-lens reflections.
    Wolfgang Weisselberg, Aug 23, 2006
  12. Roger

    Bill Guest

    I understand what you're trying to say, but I'm sorry that's a design
    issue, and has nothing to do with special glass properties.
    That one I will "agree" is marketing, nothing more.

    At first I thought there might be something to it - seemed somewhat
    logical. But after personally using over a dozen different lenses from
    both Canon and Nikon that were all designed, coated, and manufactured
    before digital SLR came into fruition, I believe it's a load of bunk.

    As I'm sure you remember, some years ago the digital craze really took
    off, and anything that had the word "digital" slapped on the side of it
    was a hot seller due to brilliant marketing in the new digital age.

    It still shows today, but to a lesser degree, when buying something that
    is not at all digital, like home theater speakers.
    I didn't say it would.

    Obviously they will not become great lenses. But they do produce better
    results at the sensor edge than they do at the film edge.
    That has little to do with digital, and a lot to do with lense coatings
    and quality of design.

    Quite simply, a well designed and high performing lense made years ago
    will work just fine on a digital body.

    I've read reports of people using Nikon F-mount lenses from the 60-70's
    on their digital bodies, and they all work just fine.

    A good lense is a good lense.
    I'm sorry, but pre-flash TTL came into existence prior to digitals
    proliferation, back in 1995 for example for Canons E-TTL. Digital had
    nothing to do with it.

    From what I understand, because the AA filter over the sensor is flat,
    any light it reflects goes straight back instead of scattering light the
    way film does. So bouncing light to the TTL flash sensor inside the
    mirror box was no longer practical with digital bodies.

    That's the reason why we can't use older TTL metering methods. The
    sensor was moved into the light path prior to exposing the sensor, where
    it measures the pre-flash and then continues the rest of the capture
    Bill, Aug 23, 2006
  13. Depends on the lens. Making a low end lens with vignetting and
    soft borders on a FF sensor (but a good center) is probably
    cheaper than a lens with a good center, good border and little
    vignetting, especially as the borders are _used_ by and the
    vignetting _matters_ on an crop sensor.

    Not to forget: a FF lens you an sell to everybody, a crop-lens only
    to those who have crop sensors and don't worry about upgrading.
    Unless it's an extreme wideangle lens (say 10mm) for said crop
    cameras, you'll have to compete with all the full frame lenses,

    Of course, if you already have a world class fast telezoom with
    stabilizer and all the fun, it's not going to be easy to construct
    a crop-frame lens with the same capability, though building it
    may be cheaper (less glass) and lighter.

    Wolfgang Weisselberg, Aug 24, 2006
  14. "glass" as in "lens", yes, that's a digital lens, since the
    APS format is dead.
    "glass" as in "UD elements" or "flourite elements", no,
    there's no special "digital elements" stuff.
    Ah, but did you sample good lenses? These would be designed right
    (instead of cutting corners).

    Have you also tested them in high contrast situations, e.g.
    shots where the sun is near or in the frame, possibly without
    using a lens shade?
    Naah, there are many 'digital' thingies that didn't make it.
    I didn't say you did.
    True, unless they were completely broken by design (e.g.
    being even worse halfway to the border than at the border).

    However, that only matters if the lens itself is good enough
    for the pixel peeping possible with todays digital cameras.
    Ay, and there's the rub: does the lens coating handle
    light reflected from the film/sensor well?
    *IF* used so that it's strengths come into play, not
    it's weaknesses. (Every lens has both!) As an example, I
    wouldn't like trying to focus a MF lens on today's amateur DSLR
    viewfinders, with just a poor excuse for ground glass and no split
    screen nor microprism ring --- at least in most circumstances.
    (Small apertures and hyperfocal distances come to mind.)

    I also wouldn't like a 28mm for WA on a crop sensor.

    I also wouldn't like provoking flare and stray light with some
    lenses (just look at 30 odd elements of some IS zooms of today),
    even if they are near perfect otherwise.
    And why shouldn't they?
    True --- look up Canon's A-TTL for even earlier use of

    But that doesn't invalidate my statement (ok, I was unclear:
    "digital sensors reflect much stronger [and don't scatter
    light]. That's the reason [for] preflashes]."
    So is the CCD or CMOS chip. It has to be flat and exactly
    perpendicular to the optical axis, or you'll have problems with
    varying sharpness especially wide open.
    Yes. Now, the light is reflected, straight back. If the lens was
    only coated and designed for light entering from the other side,
    that might give problems, like stray light being reflected back
    to the sensor, and not necessarily on the same pixel ...

    Wolfgang Weisselberg, Aug 25, 2006
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