Focusing Issues: Me, Lens or Camera?

Discussion in 'Photography Beginners' Forum' started by K8-90, Nov 19, 2008.

  1. K8-90

    K8-90 TPF Noob!

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    Lately I've been having issues focusing properly, and I need help diagnosing the problem. I thought it was just me, but now I'm thinking my lens may be the culprit.

    I'm using a Canon XSi, and the problem-lens is a 50mm f/1.8.

    I am consistently missing the focus, both on auto and manual focus. It looks fine in the viewfinder, but then when I upload them, they are just off! I haven't had this problem with my other lens (Sigma 18-200mm f/3.5-6.3) and I thought that maybe this was due to the smaller aperture.

    Is my frustration making me jump to conclusions? How can I figure out where the problem lies?

    Oh, would pictures help?
     
  2. tenlientl

    tenlientl TPF Noob!

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    i find it hard to focus with the 50mm f/1.8 as well.. but thats because i dont wear my glasses/contacts.

    i actually find it hard to focus with the viewfinder and sometimes easier with Live View... have you tried shooting with a smaller aperture than the f/1.8?
     
  3. K8-90

    K8-90 TPF Noob!

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    Ok, here's a photo.
    I don't know if this even helps. I swear it looked right in my viewfinder!
    [​IMG]
     
  4. K8-90

    K8-90 TPF Noob!

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    Hmm. I haven't tried LiveView yet. I'll have to looksthrough my manual about that. Good suggestion!

    I have tried smaller apertures (under 3.5, though), but same thing.
     
  5. LaFoto

    LaFoto Just Corinna in real life Staff Member Supporting Member

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    I also find it is quite hard to auto focus the nifty-fifty. It sometimes gives me results such as the one you're showing as example, too. I really have to put the centre point (that's where I usually put my focus point) to the eye (for example), and recompose from there, else it will find itself other points (such as the pattern of the clothes, as has happened to you), throwing the face out of focus when wide open.

    I rarely use the widest open aperture of the nifty-fifty these days, but when light permits only start out at f2.5 or so. Seems to work a bit better.
     
  6. Jedo_03

    Jedo_03 TPF Noob!

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    If you look close at the 100%? of the girl's jacket you will see that the lower area is more in focus... and the higher you go the more OOF it becomes...
    So there's your clue...
    I'm not sure but I think the canon has 9 focus points... and what the focus engine does is to hunt around these points until it finds an edge with contrast... Looks like here it found one of the lower points..
    Remedy - choose centre point focus and focus on the canthus of the girl's eye...
    Jedo

    ETA - LaFoto beat me to it....
     
  7. jong

    jong TPF Noob!

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    i agree, and if you look closely , she is leaning backwards a bit , at f/1.8 , say 5 to 6 ft away your DOF is around .2 to .3 feet so thats one factor to.
     
  8. impressme

    impressme TPF Noob!

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    Question- are you watching which point you AF is selecting to focus? It may be as simple as selecting a specific point to use for your AF first, then AF and shoot.
     
  9. gsgary

    gsgary Been spending a lot of time on here!

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    Jedo and Jong have got it , i never had trouble focusing at F1.8 in very low light when i had mine, i always use center focus
     
  10. manaheim

    manaheim Jedi Bunnywabbit Staff Member Supporting Member

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    I keep re-reading this, and I'm not SURE but I don't think anyone has made this particular point yet...

    The 50mm 1.8 has EXTREME dropoff in focus at F1.8. Like, if you take a picture of someone from the side and get their nose in focus, their eye can be out of focus. This is obviously lessened somewhat if you are further away, but given the nature of the lens, you will tend to be fairly close (as I suspect you were with this shot).

    Parts of your shot are in focus, but literally an inch or two in front or in back of that point and you're out of focus. Looks like you focused on the plane of her jacket and maybe just missed the tip of her nose... I'm not surprised by your results here at all.

    What I've found is, yes, you have to be very sensitive to the focus point you are choosing with this lens, but you also have to be extremely sensitive to how wide open you are. If you are in a situation where you have so little light as to need F1.8 you have to REALLY watch to make sure you have the right elements as focus points or it will throw the whole thing totally out of whack.

    That 50mm lens is very neat but MAN is it a bugger. :)
     
  11. gsgary

    gsgary Been spending a lot of time on here!

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    That is called depth of field it is not just a 50mmF1.8 problem it is due to the size of aperture, in his shot it looks like he hade all focus point set and it focus on the jacket if the center focus point had been set there would not be a problem but shooting in that light at 1/3200 @ F1.8 he was asking for trouble

    This is with the 50mmF1.8@ F1.8 @ 1/15

    [​IMG]
     
  12. manaheim

    manaheim Jedi Bunnywabbit Staff Member Supporting Member

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    ^^^ well, I realize I didn't explicitly state that, but yes... I know... it's DOF.

    His moving his focus point would likely still have the same issue because of his closeness to his subject. Like, if he had focused on her eyes, her nose might have been out of focus (for example). In the shot you have there, you were further away, thus affording you more DOF.
     

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