Positioning DoF

[benlonghair]

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.

As far as I can tell it's just the 35 1.8. My 18-55 and 70-300 behave exactly as I would expect them to, focus point being the middle of the DoF.

I don't have anybody around that I know that has a nikon system to check it out on, unfortunately.

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

I know that I don't have DoF preview. Do they even make cameras that have a DoF preview anymore? I also know, given a distance and aperture and lens length, approximately how much DoF I should see and where I should see once the image is made.

I am not talking about that, I'm talking about the DoF not being where I expected it to be.

 

[Petraio Prime]

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.

As far as I can tell it's just the 35 1.8. My 18-55 and 70-300 behave exactly as I would expect them to, focus point being the middle of the DoF.

I don't have anybody around that I know that has a nikon system to check it out on, unfortunately.

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

I know that I don't have DoF preview. Do they even make cameras that have a DoF preview anymore? I also know, given a distance and aperture and lens length, approximately how much DoF I should see and where I should see once the image is made.

I am not talking about that, I'm talking about the DoF not being where I expected it to be.

I still don't follow. The DoF lies in front of and behind the focus plane. I think you're saying the focus isn't where you expect it to be.

 

[benlonghair]

I still don't follow. The DoF lies in front of and behind the focus plane. I think you're saying the focus isn't where you expect it to be.

Fine, yes. The focus isn't where I put it. Maybe I'm looking at it backward.

 

[Petraio Prime]

Do they even make cameras that have a DoF preview anymore?

Yes... my camera has it and it just came out in March.

This is a feature that not all SLRs now have. It is often left off for reasons of economy.

 

[Petraio Prime]

I still don't follow. The DoF lies in front of and behind the focus plane. I think you're saying the focus isn't where you expect it to be.

Fine, yes. The focus isn't where I put it. Maybe I'm looking at it backward.

Think of DoF as an accordion. You squeeze the ends it in and out, but the middle stays stationary. The middle is the plane of focus.

 

[KmH]

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?

With a 35 mm at f/1.8 the focal point to image sensor distance will be critical.

With the focal point 5 feet from the sensor the total DOF is only 0.43 ft or about 3 inches, and it gets smaller as you get closer to the focal point. With the focal point 10 feet away the total DOF will expand to about 1.75 feet. DOF is not always 50% in front/50% behind. As the aperture opening gets smaller there is more DOF behind and can get to 20% in front/80% behind which is one of the reasons it's difficult to get a nice blurred background when you stop a lens down.

Focus will only be sharp at the focus point. Sharpness in the photo will start to diminish both in front of and behiind the focal point. How quickly it falls off is a function of the total DOF depth.

When the DOF is only 3 inches deep and you have a persons eyes as the focal point anything 1.5 inches in front of and behind the eyes will be OOF.

Now that assumes the plane of the image sensor is parallel to and perpendicular with the plane of both eyes. I you have the camera angled up or down the image sensor plane is no longer perpendicular.

Check out this DOF calculator. Their graphic depicts the DOF as if the camera was to the left of the graphic and does a good job displaying how focus sharpness drops off relative to total DOF:

http://www.tawbaware.com/maxlyons/calc.htm

 

[benlonghair]

I still don't follow. The DoF lies in front of and behind the focus plane. I think you're saying the focus isn't where you expect it to be.

Fine, yes. The focus isn't where I put it. Maybe I'm looking at it backward.

Think of DoF as an accordion. You squeeze the ends it in and out, but the middle stays stationary. The middle is the plane of focus.

Dude, no matter what you think of me, I'm not a moron. I know how DoF works. What I don't understand is why my focus in the viewfinder and the center of my DoF in the image isn't the same.

 

[Dao]

Do they even make cameras that have a DoF preview anymore?

Yes... my camera has it and it just came out in March.

Most of the Canon EOS has it. Even the 1st Rebel or the D30 that came out 10 years ago.

 

[benlonghair]

Thanks, KmH, but it still doesn't address the problem of what I see in the viewfinder being different from what I see in the captured image.

 

[Petraio Prime]

Fine, yes. The focus isn't where I put it. Maybe I'm looking at it backward.

Think of DoF as an accordion. You squeeze the ends it in and out, but the middle stays stationary. The middle is the plane of focus.

Dude, no matter what you think of me, I'm not a moron. I know how DoF works. What I don't understand is why my focus in the viewfinder and the center of my DoF in the image isn't the same.

I'm just trying to make sure we are both talking about the same thing. The way you worded your question indicates I need to establish what you are talking about. It isn't clear what the problem is.

 

[KmH]

We need to see a photo and a description of your intended focal point.

Unfortunately, because people have come to rely on auto this and auto that, many handy and very useful lens features are being discarded by the lens makers.

The AF-S 35 mm f/1.8 doesn't have a focus distance scale on it, one of the reasons is only a $200 lens. But the feature would make it fairly easy for you to check the accuracy of the focus.

So, you're likely going to wind up sending it to Nikon USA service to be checked out. be sure it's registered with Nikon so the warranty gets extended to 5 years.

 

[benlonghair]

The AF-S 35 mm f/1.8 doesn't have a focus distance scale on it, one of the reasons is only a $200 lens. But the feature would make it fairly easy for you to check the accuracy of the focus.

Believe me, I'm really not happy about having no distance scale at all. It's bad enough that my 70-300 doesn't have aperture markings, but having no distance scale at all is just cheap as hell.

 

[DerekSalem]

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.

I'm guessing you mean 1" not 1' (1 inch not 1 foot). If you're 10ft away from your focal point (10') then the depth of field would *definitely* not be 1ft (1') in either direction. There would be 8" (8 inches) in front and 10.8" (10.8 inches) behind. There are other factors though. If you're closer than that, it can drop *drastically*. If you think it's 10' (10ft) but it's actually only around 6' (6ft)...the values are half of that or less. That's a very thin plane of focus.

 

[benlonghair]

I'm guessing you mean 1" not 1' (1 inch not 1 foot). If you're 10ft away from your focal point (10') then the depth of field would *definitely* not be 1ft (1') in either direction. There would be 8" (8 inches) in front and 10.8" (10.8 inches) behind. There are other factors though. If you're closer than that, it can drop *drastically*. If you think it's 10' (10ft) but it's actually only around 6' (6ft)...the values are half of that or less. That's a very thin plane of focus.

If you look at DoFMaster, it claims the DoF of that lens on my camera at 10' (which is a guess, but doesn't really matter) is like 10" on either side of the focal point.

But that's not the point. I know where I focused it, and the image isn't focused there.

I'll try to run some tests tonight if I have time.

 

[Gaerek]

:addpics:

I don't think anyone completely understands your problem. If I'm understanding you correctly (and I'm not sure I am), I think your lens is messed up, and you need to send it in. Get some pics of what you're talking about and you might get some advice that will actually help.