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Thread: Focal Length of DSLR and PNS

  1. #1

    Default Focal Length of DSLR and PNS

    I understand when a PNS camera says 50x zoom. It is the max focal length divided by min focal length.
    So in theory, at 28mm wide angle, the max focal length is 1400mm. This would be sufficient to shoot a person standing 50m away, to be filled up a photo.

    When it comes to DSLR (esp FF), 28-1400 mm zoom is not possible (I could be wrong ?). So what would take a tele-zoom to take similar picture ?
    Would a 70-300mm sufficient ? Or at least a 200-400mm ? Why do you advise if I need to shoot animals in the zoo / safari or corner of a temple roof ?

    Why is it possible for the PNS (compact) to do so ? Is it because of the small sensor, which is good in long disctant cover, but bad in wide angle ? Which I read some article the difference of a FF and APS-C ?

  2. #2

    Default Re: Focal Length of DSLR and PNS

    Most focal lengths are referenced to the 35mm film format now, which is what we normally call Full-frame.

    Take for example the Canon SX50. If you look at this review page, Canon PowerShot SX50 HS: Digital Photography Review, you'll see the focal length range listed as 24-1200 (50x zoom), but the lens is marked 4.3 - 215mm. The latter is the true optical focal length of the lens. 24-1200 is the equivalent on 35mm/full-frame.
    So in your example, it would take a 1400mm lens on a FF camera to achieve the same framing. A shorter focal length lens with a teleconverter would work, but there are trade-offs.
    For a APS-C sensor DSLR, divide the 1400mm by the crop-factor. For Canon (crop factor 1.6), you'd need a 875mm lens.
    These are mostly theoretical focal lengths of course. You'd have to find the nearest equivalents.

    The physical length of the lens is related to its optical focal length. There are ways to make it shorter e.g. mirror lenses, but generally the longer the focal length, the longer the lens. So a 215mm lens on the SX50 is physically much shorter (and lighter and less expensive!) than an equivalent 1200mm lens on a FF DSLR.

    The trade-off? Of course the small sensor compact has considerably lower image quality and high ISO performance. You'll have to determine if it's good enough for you. For normal holiday snaps, compacts are fine.

    With long teles, you'll have to watch out for camera shake. Atmospheric haze can also lower image quality.
    Last edited by Edwin Francis; 25th October 2012 at 02:58 PM.

  3. #3

    Default Re: Focal Length of DSLR and PNS

    Thanks for your reply. So in short, the equavalent focal length of the Canon SX50 of 1200mm on FF is only 215mm.

    For my illustration of 1400mm of APS-c canon (crop factor 1.6) is 875mm; while that of Nikon (crop factor 1.5) will be 933mm !!
    To achieve the same focal length result, looks like a 400mm lens with a 2x tele-converter will be the closest match.

    What are the short falls of a tele-converter ? IQ ? Noise at high ISO ? What is the acceptable value (<1600 or 800) in general for bright, clear day ?

  4. #4

    Default Re: Focal Length of DSLR and PNS

    Quote Originally Posted by blueblood View Post
    ngth result, looks like a 400mm lens with a 2x tele-converter will be the closest match.

    What are the short falls of a tele-converter ? IQ ? Noise at high ISO ? What is the acceptable value (<1600 or 800) in general for bright, clear day ?
    Smaller equivalent light (aperture value), softer IQ (usually not bad with 1.4x, almost always noticeable with 2x), ISO will be better than a compact, and acceptable noise will be up to you and your gear. What one person finds acceptable may not be the same for you.
    Alpha

  5. #5

    Default Re: Focal Length of DSLR and PNS

    Also bear in mind that since the equivalent max aperture drops with a teleconverter (1 stop for 1.4x; 2 stops for 2x), your camera may not be able to autofocus. Most AF sensors are usable down to f5.6 (this depends on the model).

    If you start with a 400/5.6 and add a 2x teleconverter, you end up with a 800/11.

  6. #6

    Default

    Just think of a picture taken with a long zoom compact as one taken with a FF dslr and then cropping the centre n magnify it.

    Result? A longer reach, but lower quality (cropping from FF reduces resolution, while long zoom compact suffer from dynamic range and low light performance.

    In any case, you would probably find not much practical use in 1400mm. U will need at least 1/2000 shutter speed to shoot handheld, which is only possible in midday. Unless u r really going into a particular genre that requires such focal length, I really see no need for that.
    hi

  7. #7

    Default Re: Focal Length of DSLR and PNS

    Quote Originally Posted by Edwin Francis View Post
    Also bear in mind that since the equivalent max aperture drops with a teleconverter (1 stop for 1.4x; 2 stops for 2x), your camera may not be able to autofocus. Most AF sensors are usable down to f5.6 (this depends on the model)
    .
    So does it mean that AF lens will not work on down from f/5.6 for most cameras? How do I know what is the AF workable f stop? What would be written on the lens specs?

    In this case, for long distant focal length, so we need to use MF with the focal length at infinity in order to get a sharp picture?

  8. #8

    Default

    Quote Originally Posted by blueblood

    So does it mean that AF lens will not work on down from f/5.6 for most cameras? How do I know what is the AF workable f stop? What would be written on the lens specs?

    In this case, for long distant focal length, so we need to use MF with the focal length at infinity in order to get a sharp picture?
    This is a spec of the body, not lens. That's why lenses always af with a wide open aperture and only close down the aperture when the shot is taken.
    Alpha

  9. #9
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    Default Re: Focal Length of DSLR and PNS

    Quote Originally Posted by blueblood View Post
    In this case, for long distant focal length, so we need to use MF with the focal length at infinity in order to get a sharp picture?
    There is no such thing as "long distant focal length" and focal length is not infinite, but rather well defined, finite and written on the lens.
    Let the camera do the focusing. If the result is not what you want, check yourself first: using the right AF point (they can be selected), having a target with a certain contrast (dark / bright, patterns etc), sufficient light. If all fails, use Live View and / or manual focus. Another option is to use hyperfocal distance settings. If this all does not make sense to you best you read the sticky threads here and start learning. Despite all advanced technology there is still one thing that cameras cannot do: to read your mind what you actually want. So it remains your task to manage the camera, regardless of the amount of Auto functions and features.
    EOS

  10. #10

    Default Re: Focal Length of DSLR and PNS

    Thank you for all the well responses and enlightenment. Well, I've not got my DSLR yet, but already starting to read its manual and the forum threads. Still awaiting for the price to drop further, and planning to buy before the Xmas. Hope my preparation can help in my practical session.

    This forum will definitely be my learning ground, and will continue to bother all seniors and gurus in the future.

  11. #11

    Default Re: Focal Length of DSLR and PNS

    Taken with a small sized sensor (but with pretty good lenses)




    more here...
    http://www.clubsnap.com/forums/world...portraits.html

    The catch is this.
    How much you want to pixel peep and how large you want to print?

    This is better than most DSLRs cropped to the same crop factor.
    But its still the same lens trying to resolve onto a small sensor with very packed pixel density.
    At such crop factors, any misfocus, movement blur, lens imperfections is easily picked up when you delve into 50% or more of the image.
    Only when all factors 'fall into place' will you get perfect images even at 100% crop.

    However, as the images above show, it can be good enough or even pretty good.
    You'd also pay the heavens for a 1100/2.8 if it was on a DSLR (200/2.8 on a 5.5x crop camera).
    So you decide what you need/pay.

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