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DoF & Focus - Question

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DigiBot

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It is my understanding that the DoF increases with the amount of distance from the focus plane to the subject. For instance, f/5.6 at 6 ft. produces a shallower DoF than the same aperture but at 12 ft.

Assuming I am correct, why is my subject softer at 6 ft. than it is at 12 ft.? Is it because the auto-focus of the camera becomes less accurate as the distance increases? I can always re-produce such softness for portrait, and this even when I focus on the eye: the face is just soft.
--
Thanks,
http://stekar.smugmug.com
 
DigiBot said:
It is my understanding that the DoF increases with the amount of
distance from the focus plane to the subject. For instance, f/5.6
at 6 ft. produces a shallower DoF than the same aperture but at 12
ft.

Assuming I am correct, why is my subject softer at 6 ft. than it is
at 12 ft.?
Click to expand...
That is exactly why your distant subjects are sharper - the DoF is deeper.

At 6ft, depending on the lens and aperature, you may only be getting some of subject within your narrow DoF. For example, a person's nose will be sharp, but the ears not. At 12 ft, your DoF has increased so you are more likely to get all of your subject inside the acceptable range of focus.
 
Say you take the kit lens at 55mm f/5.6 and 6 feet. The total DOF you get at that setting is .76 feet.

At 55mm f/5.6 and 12 feet you get a larger DOF of 3.12 ft.

.76 feet of focus isn't much which means most of your photo will appear soft.

These numbers were taken from here if you want to look yourself:
http://www.dofmaster.com/dofjs.html
 
with kit lens and 50 f/1.8 but not an issue with my 70-400 f/4.

Do I need an expensive lens for my pics to be sharp when the distance increases then? While I have no complains about my $ 80.00 50 f/1.8 lens (great bang for the bucks) I am just curious as to what I need for lansdscape portraits shots for instance (i.e. 17-40 f/4 L)
--
Thanks,
http://stekar.smugmug.com
 
I was just about to ask what lenses you were referring to. It sounds like you're running into lens quality limitations with your kit and 1.8.

Can you post examples so we can take a look?
 


Although I've run USM and dized-down the pic, I think you can still see the softness and how it was originally.

Settings:
Focus: AF, center point, eye of the child
Focal len: 33mm
Av: f/4.5
Tv: 1/50
ISO: 100
--
Thanks,
http://stekar.smugmug.com
 
As far as I know, there's no technical reason why a shot at 12ft should be softer than the same one at 6ft. If it were the other way around perhaps, then focus and DOF could explain it, but it sounds like you understand all that.

Is it possible that it's just on a few photos only, and it's just a coincidence? That photo you posted is at 1/50 and 33mm. For a 1.6 crop that's right on the borderline where handshake blur often starts. It seems like possibly just a bit of handshake blur to me. Maybe you just need slightly higher shutter speeds.

Can you take the same photo with a tripod at 12ft and 6ft? That would tell you more accurately. I would be surpised if there was a difference then.
 
are like that with the 'cheap' lenses I own. However, headshots are really good because the distance is shorter. Basically, wide-angle portrait shots seem to produce very soft images with these lenses.

--
Thanks,
http://stekar.smugmug.com
 
I run into the problem with wide angle on my 17-85 IS USM kit lens I bought. Once I got above say 35mm, the sharpness was good, with the best being at the 85mm range. Below 35mm sharpness dropped off fast.

So my guess is the limitations of the cheaper kit lens, based on my own experiance.

With the 50mm 1.8, there is a recorded history of difficulty getting a sharp image untill you get to about f/2.8 or higher. So 1.8 produces a softer image than stopped down to say f/3.

Test this to see if my guess is true.

Place the kit lens on the camera and use a tripod.

Shoot at the widest MM setting with your kit lens, (not sure what that is for you, 17mm?) and then shoot the same subject at the longer 55mm, and see if it is not sharper.

Do the same thing with the 50mm. Shoot a subject at 1.8 and then at say 3.

For your target, tack a pice of white paper out side on the fence with a big black X created by a sharpie. (Auto focus uses contrasting points to determine when a subject is in focus)

If you don't see a noticable difference after downloading to your computer, then I don't know off hand. But this is my best guess.

I do not think it is really distance related.
 
The OP was only talking about subject distance.

You're talking about focal length, aperture and so many other variables, I'm not even sure what we're discussing anymore.

As far as I understood, the OP was saying (all other things being equal) his shots at 12ft were softer than at 6ft. I was assuming the focal length and aperture were the same on both shots?
 
Yes they're all important, but I'm not sure they're relevant to the original question, that's all.

If we don't keep some variables constant, how can we compare the effect of distance? All the OP said, was that shots at 12ft were sharper than at 6ft.

If you take one shot at 12ft with an aperture of F/5.6 and another shot at 6ft with an aperture of F/2.8, and the one at 12ft is sharper - is it sharper because you were at 6ft, or because you used a smaller aperture? You can't tell.
 
The previous repliers are trying to tell the OP how to test his equipment. Until we have more info, like if he gets the same results at different aperatures. We cant say, "oh, you just have a cheap lens" or "You have a defective camera".
 
Kit lens:
  • Headshot: around 50mm
  • Lansdcape portrait shot/full-body shot: around 30mm
50 f/1.8: only distance varies fro headshot vs. full-body shot

Aperture and shutter speed are the same between the type of shots.

I truly appreciate all the answers folks.

--
Thanks,
http://stekar.smugmug.com
 
Ok, fair enough. I guess I was interpreting the question a bit differently - more as a question about photography in general, and whether there was any technical reason why it might be so for all cameras, not whether there might be something wrong just with his camera.
 
If the aperture and shutter are the same, and you are using the same lens at the same focal length, then it sounds strange.

As I mentioned above, 1/50s for a 33mm shot is possibly a bit slow, depending on how steady your hands are. Plenty of people would get a blurred shot at that speed. But if it's like that for all your shots, then it does sound strange.

I think you need to post both a 6ft and 12ft example (pref. taken on a tripod or with much higher shutter speed) so we can see what you mean. I can't think of any photographic reason to explain it other than random AF or handshake blur on certain shots.
 
Hello,

I saw many ideas passed back and forth, but one idea I did not see posted:

In general, wide angle lenses are not as sharp as telephoto. So a picture taken with a wider angle lens say 18-28mm will be softer than a picture taken 70mm or up.

So if you take a wide angle shot from 10 feet it may be softer than a telephoto picture of the same thing taken from 20 feet.

I wish I could find the reference where I read that, but the general idea is that the longer focal lengths are considerably sharper.

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