Focusing Issues: Me, Lens or Camera?

Dao

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And learn more about the depth of field.

Just play around with the DoF calculator. F1.8 is not always the problem. It is Aperture and the subject to camera distance.

For example: 50mm F/1.8 lens on a 1.6x crop camera

Subject is 4 ft away from the camera, the Dof is only 0.13 ft. And as manaheim said, if the nose is in focus, the eye may not be in focus.

Subject is 8 ft away from the camera, the Dof is 0.52 ft. So if the nose is in focus, the eyes are .. most likely in focus.

Subject is 11 ft away from the camera, the Dof is 0.99ft. Well, I am pretty sure the the subject should be in focus.

However, if you really need to frame the subject in the way that you need to have the camera closer to the subject, then you may need to change the aperture.

i.e.

Subject is 4 ft away from the camera, the Dof will be 0.52 ft when the aperture is f/7.1, just like F/1.8 with 8 ft away
 

manaheim

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No point having the eyes in focus if you can't see the rest of the face clearly...

I concur... unless that was what you were going for. I've found (through accidental experimentation) that generally pictures like that look kinda stuuuuuuuuuuuupid.
 
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K8-90

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OK, so lots of good info, thanks!

I understand DoF, and how it is determined by the distance to subject and the aperture, in the basic sense. But I think I'll look into it some more. Now that I consider it, f/1.8 was probably the wrong choice in this situation (as you guys explained about the whole face not being in focus and how it looks "kinda stuuuuuuuuuuuupid" :lol:).

However, my problem was more that I believed I had focused properly on the eyes. I think I'll need to refocus my viewfinder. And the the suggestion about which AF point to choose is also something I may have overlooked.

I appreciate the help, guys! I'm relatively new to this, so please excuse my beginner problems.

And just a note, I'm a girl ("Kate", as it says in my sig ;)). Haha, I got a little confused whether the posts were refering to me or not when they said "he".:mrgreen:
 

Drucifer

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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?
In my case it turned to be me. It was the first sign of my diabetes. Something about the pressure in my eyes would fluctuate. It's the reason why I dropped out of the hobby for so long. Only now, some twenty years later, that I'm picking up a SLR again.
 

weiser

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If any part of the shot is in focus (the clothing at the bottom of the frame) then nothing is likely wrong with the lens. As others noted the key is selecting the most important feature in the frame to set the focus. In most people pictures that point is the eyes of the subject. If you want greater depth of field (have a deeper range in focus) and have enough light to stop down the aperture, that will do it. The smaller the aperture and the greater the distance to the focal point from the camera the greater the depth of field. It's handy to remember that approximately one third of the depth of field is in front of the focal point and two thirds is behind it.
 
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Well, I'm reviving an old thread.

I'm still having issues - bad enough that I reverted back to my point and shoot on one occasion (school project)! I think I understand all the variables involved with getting a sharp and focused image, and I can rule out DoF as the culprit. I know about the focus points - I've tried them all. Even centered doesn't do the job.

Really frustrated...
 

fokker

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Viewfinder in focus?
Autofocus or manual? If AF, which point are you using and what is your method (focus recompose?)
 

Kristov

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Well, if it is sharp in the viewfinder, it should be sharp for the actual picture. This may seem strange, but do you wear glasses? If you do or do not, I would try switching eyes for a shot or two. Worth a shot.

Beyond that, hard to tell. If it is sharp with other lenses, AND sharp in the viewfinder, I'm not sure what to say. Mechanically speaking, it sounds like it is working fine.
 

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