Few generic photography questions

Discussion in 'Digital Photography' started by grahamtearne, Feb 19, 2009.

  1. grahamtearne macrumors regular

    Jun 23, 2006
    Hey guys, really enjoying my first DSLR (Sony a200) and I have taken some great pictures with it. However in order to improve my knowledge I'd like some clarification on several terms I have seen around the net and some personal questions.

    OK I had the Panasonic Lumix LX2 before I stepped up 2 months ago to an SLR. The LX2 when focusing would show what points have been focused on with a red light on the screen and would often show more than one section. On the a200 only 1 of the 9 points ever shows up as the autofocus point. Is this normal, is it a limitation of the camera (after all its entry level) or is it actually focusing on more but just showing the 'main' focus point? It never crossed my mind as I used my friends d40 that only has 3 focus points and only one would light up. It wasn't until I used my friends 450d last week where several points would actually light up (again I understand that the 450d is a more expensive model).

    Secondly, Prime lenses. I see a lot of talk about prime lenses. Do these offer higher image quality and a fixed focal length and aperture?

    Thirdly, I guess its 'framing'? Like a full frame camera view finder see's 100% of the image and has a 1.0 crop ratio so 18mm is effectively 18mm in the final image. Well mine is 95% finder which means i don't see 5% of the image that will actually turn out once I take it (so for example i could think I have missed out a certain obstacle but could still end up in the image?) and it also has a 1.5x crop factor. Does this mean it doesn't take everything the lens see's? so for example at 18mm x 1.5 = 27mm - Are my shots actually 27mm instead of 18mm? Ie the same lens on a full frame camera would result in an image more zoomed out at the same location (18mm)?

    Sorry I know its probably a confusing post but if anyone has the knowledge to help me out it is much appreciated.
  2. mrkgoo macrumors 65816

    Aug 18, 2005
    I would imagine that your camera is capable of more than one autofocus point, but that you have only set it to one. I am unfamiliar with your camera model, so be sure to check your manual.

    Prime lenses are fixed focal length. Because of this they tend to offer much higher quality than regular zoom lenses that cover the same focal length. They tend to be sharper, for one. They also tend to be lighter and often cheaper. Furthermore, they tend to offer much wider apertures than zoom lenses, and thus lets in more light (and thus faster shutterspeeds), and narrower depth of field.

    Your understand of crop factor is more less ok from a working perspective. The crop factor is in relation to a fullframe sensor or 35mm film. Your sensor is 1.5x smaller, thus captures a field of view that is narrower than that captured by the same lens on a full frame camera.

    Note, the focal lengths never changes, regardless of what sensor you place behind it. An 18mm is an 18mm. What changes is the field of view - that is what is captured from that lens. An 18mm lens on your 1.5x crop camera has the same field of view as a 27mm lens on a fullframe, but is still an 18mm lens. It is a common mistake to think that focal length changes - it's due to marketing where people relate the focal length back to field of view, and people then get one confused with the other. For sure, they are related, but only as far as what sensor size is being used.
  3. Cliff3 macrumors 68000


    Nov 2, 2007
    SF Bay Area
    Prime are also known as fixed focal length lenses. The early zooms were of inferior optical quality compared to prime lenses, but this is no longer true. Some current zoom lenses are superior to their prime counterparts, for example Nikon's 14-24 is considered to be superior to the equivalent prime lenses at any focal length. When comparing professional zoom lenses with their fixed focal length counterparts, the difference is mainly size and heft. Professional zooms are big and heavy beasts. A photographer may not need to use the whole range of focal lengths covered by the zoom, in which case carrying a couple of primes may make more sense. At the telephoto end, physical size and maximum aperture compromises cause zooms to be less desirable than primes.

    Framing is the image that is displayed in the viewfinder. A 100% viewfinder provides the photographer with an exact view of what will be captured by the camera. A 95% viewfinder crops the edges of what will be captured, hopefully evenly on each side. A 100% viewfinder takes any guesswork out of framing an image. Less than 100% coverage means you will probably not realize the full potential of the sensor when framing an image.

    The other notion is that of field of view cropping. DSLR lens focal lengths are usually evaluated in the context of their 35mm film equivalents. Sensors smaller than a 35mm film frame (24x36mm) cover a smaller portion of the image circle transmitted by a lens. The ratio of coverage is the crop factor, and it varies from sensor to sensor. Canon has 3 sensor sizes - full-frame with a crop factor of 1, aps-h with a crop factor of 1.3, and aps-c with a crop factor of 1.6. Nikon and Sony have 2 - full-frame or FX with a crop factor of 1, and DX or aps-c with a crop factor of 1.5. Olympus's 4/3rds system has a crop factor of 2. Multiply the lens' focal length by the sensor's crop factor to arrive at the equivalent 35mm focal length
  4. pprior macrumors 65816

    Aug 1, 2007
    I would disagree with the last poster, at least from a canon experience. I own most of the canon "L" zooms and also several L primes. The image quality of the former (though light years ahead of the consumer level zoom lenses) is nowhere near the latter.

    A good prime will be faster (I used to think 2.8 aperture was good enough, now anything slower than F2 is disappointing), lighter, and better image quality.

    Now the world of Nikon may be different, but the best canon primes (35L, 85L, 135L, 200L, 300L) just cannot be touched by ANY zoom lens in my experience.

    the other questions are well answered. Crop factor basically makes your tele lenses better and your wide angles worse :)
  5. grahamtearne thread starter macrumors regular

    Jun 23, 2006
    Cheers guys, thanks for taking the time to reply to my questions, its much appreciated.
  6. techie4life macrumors 6502

    Jul 19, 2007
    Regarding your autofocus question:

    Chances are, you can easily change which one of your autofocus points you want to use. I have a Nikon D80, and you can change the autofocus points with the "joystick" control. However, with your camera, check pages 67-70 of your manual, particularly page 70. The "Local" autofocus option is probably what you want. Hope it helps.
  7. dllavaneras macrumors 68000


    Feb 12, 2005
    Caracas, Venezuela
    Hehe, I thought the same thing when I got my 100mm macro lens. F2.8 was mighty fast compared to the kit lens' f5.6. Now I have a Sigma 50mm f1.4 and it's simply awesome. I now find the f2.8 aperture slow as molasses. :p

    Basically, primes give better quality images, are lighter and smaller than zoom lenses because there are less glass elements. Less glass = less chance of CA, purple fringing, image softness, etc.
  8. Cliff3 macrumors 68000


    Nov 2, 2007
    SF Bay Area
    If the difference is that pronounced, it makes me wonder why Canon shooters aren't complaining about a lack of quality zoom offerings.

    I am a Nikon shooter so I can't comment specifically on Canon offerings. Nikon's various professional-level wide to medium telephoto zooms range from superb to very good. At 300mm or longer, as I noted in my previous post, primes have an edge. For special applications such as large apertures or perspective control, design constraints make primes the only reasonable option. I can't make the generalization that Nikon primes are categorically superior to Nikon zooms because that is just not a true statement.

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