DOF problems

Discussion in 'Critique Forum Archives' started by rebecca the curious, Mar 18, 2007.

  1. rebecca the curious

    rebecca the curious TPF Noob!

    Joined:
    Mar 18, 2007
    Messages:
    22
    Likes Received:
    0
    hi, i have a canon rebel xti and i've started using the sigma 18-200mm f/3.5-6.3 lens.
    i've tried taking photos with a low fstop number to produce a shallow DOF. but it hasn't been working. the background is not blurred as i had hoped. i don't understand what's wrong.
    here are some examples of the photos i took recently with their respective fstop and shutterspeeds

    1) http://www.flickr.com/photos/rebeccathecurious/424669627/
    f/3.5, 1/1600 sec

    2) http://www.flickr.com/photos/rebeccathecurious/424636546/
    f/3.5, 1/3200 sec

    any ideas as to what i'm doing wrong would be appreciated. thanks!
     
  2. cosmonaut

    cosmonaut No longer a newbie, moving up!

    Joined:
    Oct 12, 2006
    Messages:
    1,122
    Likes Received:
    98
    Location:
    North Georgia
    Can others edit my Photos:
    Photos NOT OK to edit
    Every lens will produce a slightly different blurr, If you get to far away from your subject the camera will focus to infinity and the background will be less blurred. Try a couple of macro, close up shots with the same F/stop and see what happens.
    Cosmo
     
  3. markc

    markc TPF Noob!

    Joined:
    Mar 8, 2004
    Messages:
    4,237
    Likes Received:
    6
    Location:
    Rochester, NY Velocity: Unknown
    Can others edit my Photos:
    Photos NOT OK to edit
    f/6.3 on the long end is a pretty narrow max aperture. I'm thinking that you won't be able to get a shallow DOF out of that lens. One of the drawbacks to an inexpensive consumer zoom; especially one with such a long range. If you want to take short DOF shots, you might want to look at getting a 50mm/f1.8. They don't go for much.
     
  4. rebecca the curious

    rebecca the curious TPF Noob!

    Joined:
    Mar 18, 2007
    Messages:
    22
    Likes Received:
    0
    thanks for the reply cosmonaut, but i have tried some close ups and they're a little better, but not considerably so.

    i just find it really strange that the other shots have absolutely no blur even when i use aperture priority mode with the lowest possible fstop in excellent lighting conditions.
     
  5. rebecca the curious

    rebecca the curious TPF Noob!

    Joined:
    Mar 18, 2007
    Messages:
    22
    Likes Received:
    0
    markc: thanks for the help, and i understand what you're saying, but the shots i took were only at 18-60mm zoom and yet they still did not produce a shallow DOF. i've seen other peoples' photos using the same lens and they're able to produce a shallow DOF even at the 100mm length. so i'm still quite confused, but thanks for the input.

    and i already have the canon 50mm f/1.8 lens you're referring to. it's great, but i also need a lens with zoom capabilities. do you have any suggestions for better lenses that can produce shallow DOF's but have zoom? thanks
     
  6. markc

    markc TPF Noob!

    Joined:
    Mar 8, 2004
    Messages:
    4,237
    Likes Received:
    6
    Location:
    Rochester, NY Velocity: Unknown
    Can others edit my Photos:
    Photos NOT OK to edit
    What is the max f# at 100? I'm guessing around f/4.5 or f5. You'll get some blur, but not much. Do you have a link to the shots you saw, or were they prints?

    Also, the f# isn't absolute, it's a ratio to the focal length. F/3.5 will give you more DOF at 50mm than at 200mm, so even though the f-numbers are smaller at the wide-angle end than the telephoto on that lens, you are probably getting more DOF.

    This is math, so you might want to ignore it, but f/3.5 @ 18mm is an aperture opening of only 5.14mm (18mm/3.5). That's the same as f39 @ 200mm. Most of the image is going to be relatively sharp @ 18mm no matter where you focus.

    To maximize DOF, I would use the lens at 200mm and get as close to the subject as possible while having the background as far away as possible. It won't be great, but that's the trade-off between zooms and prime lenses. With a prime, you only have one focal length, but you can get a very wide aperture.
     
  7. markc

    markc TPF Noob!

    Joined:
    Mar 8, 2004
    Messages:
    4,237
    Likes Received:
    6
    Location:
    Rochester, NY Velocity: Unknown
    Can others edit my Photos:
    Photos NOT OK to edit
    Oh, as far as a recommendation, I'd say anything with a larger max aperture. Unfortunately you have to start paying for that on a zoom. Frustrating, I know. There are ones out there that are better than 6.3, but not by much. 5.6 is common for consumer lenses, and you have to pay to get down to f/2.8. The main thing is to stay on the telephoto side of whatever lens you are using if you want shallow DOF.
     
  8. rebecca the curious

    rebecca the curious TPF Noob!

    Joined:
    Mar 18, 2007
    Messages:
    22
    Likes Received:
    0
    oh. i seeeeeee. that's been very helpful. thanks, mark. i wasn't aware that the focal length divided by the fstop equals the diameter of the aperture (i'm very new to photography).

    the shots i looked at were prints, but i've found some other examples:
    1) http://www.flickr.com/photo_exif.gne?id=425886086
    is this because the focal length is 140 and fstop 10, therefore 14mm aperture diameter, which is wide, so shallow DOF?

    2) http://www.flickr.com/photo_exif.gne?id=409591014&context=pool-90562530@N00
    tamron equivalent of the same lens. this shot uses the 200mm like you recommended.

    ok, it makes sense to me now. but now i'm sad :( lol. i guess i'll just have to carry around both the zoom lens and the 50mm prime. thanks again for the help.
     
  9. rebecca the curious

    rebecca the curious TPF Noob!

    Joined:
    Mar 18, 2007
    Messages:
    22
    Likes Received:
    0
    what do you think of the sigma 50-150mm f/2.8 lens? i may trade the 18-200 for this one.
     
  10. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

    Joined:
    Dec 16, 2003
    Messages:
    33,817
    Likes Received:
    1,811
    Location:
    Edmonton
    Can others edit my Photos:
    Photos NOT OK to edit
    The wider aperture will surely help you to get a shallower DOF....especially at 150mm.

    This is a fairly new lens, and I haven't heard much about it...however, it's probably fairly similar to the Sigma 70-200 F2.8, which is a pretty good lens.
     
  11. rebecca the curious

    rebecca the curious TPF Noob!

    Joined:
    Mar 18, 2007
    Messages:
    22
    Likes Received:
    0
    thanks for the input, mike.

    do either of you know whether the 50-150mm f/2.8 lens can also produce a shallow DOF at 50mm?

    also, is the aperture fixed at 2.8 all the time on this lens or can the aperture be made narrower as well? that might be a stupid question, but i just want to make sure, lol. i'm very new to photography.
     
  12. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

    Joined:
    Dec 16, 2003
    Messages:
    33,817
    Likes Received:
    1,811
    Location:
    Edmonton
    Can others edit my Photos:
    Photos NOT OK to edit
    It's not fixed, F2.8 is the maximum.

    DOF is a result of focal length, aperture and the distance at which the lens is focused. You will be able to get a more shallow DOF at 50mm with this lens, than you could with that 18-200mm lens...because with this lens, you can use an aperture of F2.8.
     

Share This Page