Positioning DoF

[benlonghair]

Hi guys, this should be an easy question. I was playing with my 35 1.8 and a dog recently. I don't have any of the shots because I'm at work, but I can easily explain the problem.

I have understood that your DoF should be centered on the focal plane of the lens. In other words, if your DoF is 12", DoF should extend 6" to the front and beyond of your focus point.

Well it seems as though the DoF is actually 100% behind my focal point; that is to say the focal point is actually the back of the DoF. Everything beyond that point is OOF.

Is this normal? Do I just have to work around it?

 

[Petraio Prime]

Hi guys, this should be an easy question. I was playing with my 35 1.8 and a dog recently. I don't have any of the shots because I'm at work, but I can easily explain the problem.

I have understood that your DoF should be centered on the focal plane of the lens. In other words, if your DoF is 12", DoF should extend 6" to the front and beyond of your focus point.

Well it seems as though the DoF is actually 100% behind my focal point; that is to say the focal point is actually the back of the DoF. Everything beyond that point is OOF.

Is this normal? Do I just have to work around it?

Huh? Your post is unclear. Try again.

 

[shmne]

I understand what you're saying, sadly I have no experience having heard of something like this.

I've personally never used a camera or a lens where the focus is all behind the focal point. Could it possibly be a mode in your camera? I've used Nikons but never fooled around with the menu settings. Change anything lately?

Not the best person to answer this question, but I'll poke around a bit because I'm quite intrigued by this.

 

[Petraio Prime]

Hi guys, this should be an easy question. I was playing with my 35 1.8 and a dog recently. I don't have any of the shots because I'm at work, but I can easily explain the problem.

I have understood that your DoF should be centered on the focal plane of the lens. In other words, if your DoF is 12", DoF should extend 6" to the front and beyond of your focus point.

Well it seems as though the DoF is actually 100% behind my focal point; that is to say the focal point is actually the back of the DoF. Everything beyond that point is OOF.

Is this normal? Do I just have to work around it?

Depth of field is always in front of and back of the plane of focus. The depth varies with aperture and distance. The greater the distance, the more the DoF is behind the point of focus. At 1:1, it's equal on both sides of the focal plane.

On some lenses (normal and wide-angle) the DoF is very great except at extremely close distances, and almost all of the DoF is behind the plane of focus. You can get DoF to 'infinity' at relatively close distances with WA lenses. With macro lenses used close and telephoto lenses, the DoF is more evenly distributed.

 

[shmne]

In all my searching I can't find a situation where your dof can be shifted from your focal plane. Are you using a 70-300mm zoom by any chance? Or something where your lens has a switch with a 1 mode and a 2 mode.

I'm thinking that maybe if so, that you've accidentally switched it to the 2 possibly not know what it does. In this mode, the lens ignores nearer objects and focuses father away. Possibly making it appear as if all of your DOF is behind the subject rather than proportionally split up.

 

[benlonghair]

Here's the deal. I was focusing on the dogs eyes laying on the ground. The preview looked pretty well focused, but we know that isn't the best way to tell. Now, with the eyes in focus, f/1.8 and being about 10' away, my DoF should have about 1' in front and behind the focus plane.

Instead the dog's eyes and tip of his nose are in focus, plus about 2' of grass in front of him. Not the 1' of grass and the whole dog as I anticipated.

 

[benlonghair]

Are you going to post photos when you get home?

I will, but it just looks like I missed the focus, but I know I didn't because the whole point of the excercize was to move the DoF around. When I focused on the grass behind the dog, the dog came into perfect focus.

 

[Petraio Prime]

Here's the deal. I was focusing on the dogs eyes laying on the ground. The preview looked pretty well focused, but we know that isn't the best way to tell. Now, with the eyes in focus, f/1.8 and being about 10' away, my DoF should have about 1' in front and behind the focus plane.

Instead the dog's eyes and tip of his nose are in focus, plus about 2' of grass in front of him. Not the 1' of grass and the whole dog as I anticipated.

Are you using a manual-focus camera?

 

[benlonghair]

Here's the deal. I was focusing on the dogs eyes laying on the ground. The preview looked pretty well focused, but we know that isn't the best way to tell. Now, with the eyes in focus, f/1.8 and being about 10' away, my DoF should have about 1' in front and behind the focus plane.

Instead the dog's eyes and tip of his nose are in focus, plus about 2' of grass in front of him. Not the 1' of grass and the whole dog as I anticipated.

Are you using a manual-focus camera?

I AF/MF pretty much everything because the AF on the D60 leaves a ****load to be desired. At 10' with my 35 I'd be lucky to get it to AF on a 5 gallon bucket much less eyes. So I get close with the AF and MF the rest.

 

[Petraio Prime]

Here's the deal. I was focusing on the dogs eyes laying on the ground. The preview looked pretty well focused, but we know that isn't the best way to tell. Now, with the eyes in focus, f/1.8 and being about 10' away, my DoF should have about 1' in front and behind the focus plane.

Instead the dog's eyes and tip of his nose are in focus, plus about 2' of grass in front of him. Not the 1' of grass and the whole dog as I anticipated.

Are you using a manual-focus camera?

I AF/MF pretty much everything because the AF on the D60 leaves a ****load to be desired. At 10' with my 35 I'd be lucky to get it to AF on a 5 gallon bucket much less eyes. So I get close with the AF and MF the rest.

You cannot 'position' DoF, only the plane of focus. The DoF is determined by aperture and distance, and always extends both in front of and behind the plane of focus.

 

[benlonghair]

You cannot 'position' DoF, only the plane of focus. The DoF is determined by aperture and distance, and always extends both in front of and behind the plane of focus.

You're nitpicking. I know basically how big the DoF is supposed to be and I use the focus point to position it on the subject as I want. So, yes, technically I'm using the focal point, but in doing so, I position the DoF in the image where I want it.

And if what you say is true then my lens is not showing an accurate focus.

 

[shmne]

Hyperfocal Distance and Depth of Field Calculator - DOFMaster

Input your equipment if you haven't used this already just to double check. Honestly I think that your auto focus is messing up. If you manually focus only does this same problem persist?

My only thought is that it is the camera, as the lens could not be doing this.

 

[benlonghair]

Hyperfocal Distance and Depth of Field Calculator - DOFMaster

Input your equipment if you haven't used this already just to double check. Honestly I think that your auto focus is messing up. If you manually focus only does this same problem persist?

My only thought is that it is the camera, as the lens could not be doing this.

Both AF and MF have this problem. It's just with AF I tend to miss the focus completely fairly regularly.

As I said above, I rarely let the AF do all the work because of the massive size of the AF area. It gets me close and I MF the finished focus. My eyesight is pretty good so I know I'm not seeing it wrong.

Maybe it is the camera. Higher end cameras have focus correction that you calibrate to each lens, so maybe that's the problem.

 

[shmne]

No after hearing that I don't know if it really is the camera. Which lens are you having the problem with? Do you have a buddy that you can try out their camera with the same lens?

Is it only one lens giving you this problem? I think you can see where I'm going here, I just assumed that it would be more likely the camera messing up rather than the lens, however if it happens with manual focus you would be able to see the plane of focus and therefore you can tell that it is out of whack.

 

[Petraio Prime]

You cannot 'position' DoF, only the plane of focus. The DoF is determined by aperture and distance, and always extends both in front of and behind the plane of focus.

You're nitpicking. I know basically how big the DoF is supposed to be and I use the focus point to position it on the subject as I want. So, yes, technically I'm using the focal point, but in doing so, I position the DoF in the image where I want it.

And if what you say is true then my lens is not showing an accurate focus.

You won't see the DoF change with the aperture selection unless the lens is actually stopped down. Many cameras no longer allow this.