Posted by Peter Conway on July 4, 2008 at 12:58 AM
Shorter lenses give greater DOF at the same aperture. My question is this; at the same image size, which has the greater DOF.
In other words if I am closer with a 400mm so that the object of interest occupies the same amount of the frame as the same object does with a 600mm from further away, which gives the greater DOF at the same aperture setting?
DoF will be identical. Same composition (same reproduction ratio), same aperture = same DoF. Subject isolation will be better with the longer lens, though.
"Shorter lenses give greater DOF at the same aperture." If you mean the same aperture with a longer lens, no, shorter lenses don't have any greater depth of field than any other lens at a given opening. A shorter/wider lens only has an "apparent" greater dof because the objects are smaller in the frame.
So, you're closer to what's happening in your example. Assuming what you have in your field of view is the same regardless of what lens you're using, you will have the same dof at f/8 whether or not you're using 20mm or 200mm.
If you shoot something with the 600mm at f/8, and then move in closer to get the same exact field of view and shooting at f/8, you will have the same dof. Perspective may be different, but dof will be the same.
Another example... Say you're viewing a mountain scape. There's some tall trees a couple hundred yards out and the mountain peaks are a couple miles away (infinity.) You can take a broad scenic, say 20mm at f/16 focusing on the trees. You should get good "apparent" dof field and the mountain peaks should appear in focus (at the 20mm viewing.) Now, lets say you zoom in and shoot just the tree tops and mountains - at f/16. If you make a crop of the 20mm scene to match that of the 200mm view, you will be able to see the SAME dof.
Again, the wider lenses only seem to have a better dof - but it's just that everything is farther away and smaller.
So,
"if I am closer with a 400mm so that the object of interest occupies the same amount of the frame as the same object does with a 600mm from further away, which gives the greater DOF at the same aperture setting?"
Posted by Peter Conway on July 4, 2008 at 10:19 AM
Thank you very much. I now understand it.
One more question. Why does the longer lens give better "subject isolation"? What is the principle at work given that DOF is the same for the same field of view?
It has something to do with the "angle of view" of the lens. I don't know the physics, but maybe this crude diagram will help.
The longer lens has a narrow field or angle of view (blue lines.) You can see that both lenses here have the same image size in the frame, but there is less included in the background. The wider lens also has the same size image in the frame, but more is included in the background.
You also get a sense of "compression" when using the longer lens. ie. things appear closer together.
Lon (and Peter) - It's not really physics, but human vision that makes shorter focal lengths appear to have a greater DOF. If an object in the BG has the same amount of blur at both focal lengths (which it will) then it will more obviously be blurry in the telephoto case, simply because the object will be in a less cluttered BG and will be larger in the frame. The amount of blurriness is the same in the wider angle case, but there is more included in the BG, and the object is smaller. The way we see, smaller objects will not show the blur as obviously.
Sorry I don't have a terrific drawing to include...
For background isolation, one thing to keep in mind is magnification. Suppose an OOF object in the background is 1 mm across in the image with a 20 mm lens. Now change lenses to a 200 mm, with that 1 mm object still back there. With the longer lens, it is now 10 mm across, and still as OOF. The wash of color that it generates also blends in with the 1.5 mm object that was on the left, and the 2 mm object that was on the right, which are now 15 mm and 20 mm across. With the 20 mm lens, they were separated and OOF but with the 200 they are still OOF but their OOF regions on the recording medium have blended together in one bigger mush of color. That gives you the nice clean backgrounds that so often see with longer lenses.
(note: I don't think 'mush' qualifies as a technical term )
Another example... Say you're viewing a mountain scape. There's some tall trees a couple hundred yards out and the mountain peaks are a couple miles away (infinity.) You can take a broad scenic, say 20mm at f/16 focusing on the trees. You should get good "apparent" dof field and the mountain peaks should appear in focus (at the 20mm viewing.) Now, lets say you zoom in and shoot just the tree tops and mountains - at f/16. If you make a crop of the 20mm scene to match that of the 200mm view, you will be able to see the SAME dof.
I don't know if I understand correctly what you're saying, but if I do, then no, I don't think so. If you zoom in from 20 to 200 mm without changing your own position (and hence the focus distance) to match the composition, you do not get the same DoF.
BG
Posted by Peter Conway on July 6, 2008 at 12:55 AM
Thanks all, the discussion has been very helpful.
Basil, I think you get the same DOF if you crop the 20mm photo to match the 200mm photo.
If you remain in the same position and zoom in from 20 to 200mm, OR if you crop a 200mm image from a 20mm image, you WILL have the same depth of field. so,
"If you zoom in from 20 to 200 mm without changing your own position (and hence the focus distance) to match the composition, you do not get the same DoF."
AND you keep the same aperture setting, you WILL have the same amount of DOF at 20mm and at 200mm - assuming you are stationary and the aperture remains the same. "You do not get the same DoF" is incorrect - you DO get the same DOF.
DOF - or the distance of acceptable focus from near point to far point, is precisely the same whether or not you use a 20mm lens or if you use a 200mm lens - at the same aperture.
Lon - That's NOT true. Focus distance also matters. If you keep the subject the same size in the frame while switching between 20 and 200 mm, and you use the same aperture, THEN you will have the same DOF. Basil was correct - if you don't move your feet and just zoom on the same subject, DOF does change.
The equations for DOF involve distance from subject, focal length, and aperture. If you change the first two in order to keep the subject the same size in the frame, the DOF is the same. If you only change the focal length, so that the subject is larger in the frame, the DOF is not the same.
Here's a couple of references if anyone is interested :
Oh God, I knew someone would eventually link to a tutorial that involved the "circle of confusion." When I see that, I'm always reminded of the "pit of despair" in The Princess Bride. I get the principles DOF just fine until I run into the circle of confusion. I find the circle of confusion simply inconceivable, and feel like a farm boy when I try to understand it.
I got everything that was written here, and knew Lon was wrong on the last part, and am able to explain the ins and outs of DOF to clients accurately without overwhelming them. I guess (I hope) it's enough to know how to achieve the DOF effect I want. I'm of the philosophy that I don't need to know all the names of the parts of the bike, or explain the scientific principles of balance and defiance of gravity, in order to ride it.
Thanks. I think I stand corrected, but let me see if I get this right. Which, if any of the following are correct?
- You're stationary. You take an image at 20mm and another zoomed in to 200mm. the aperture is the same. The DOF will not be the same.
- If you crop the 20mm image down to the same "image size" as the 200mm frame and the aperture is still the same, The DOF will not be the same?
- If you move in with the 20mm to get the same "image size" as with the 200mm taken in the orinal position, you will have the same DOF.
David: "If you only change the focal length, so that the subject is larger in the frame, the DOF is not the same. " I agree and I think I wasn't clear on my statement. My intention was assuming the same image size, only cropping the 20mm image instead of zooming.
Heck, I'm getting confused now. I'm with Steve - I need to know enough to take a nice picture.... too much technical detail only clouds the mind... :-)
Agreed on the last sentiment Lon - and the other stuff you said sounds correct (although it's a little counterintuitive to me to talk about cropping - but that's half the problem - we all think differently). The problem I have is that the "circle of confusion" definitions don't seem like the simplest way to describe DOF to me.
And most people don't need to be that exact anyway, since you usually only need to know what makes DOF increase or decrease with you particular setup. Sometimes the mathematical, or theoretical, gets in the way of the practical.
Interesting discussion. I read the tutorials and they were helpful too.
From a practical viewpoint I don't think Ron's error really matters. If one were to blow up the 20mm to achieve the same magnification as was provided by the 200mm the loss of clarity from lack of pixels on the sensor screen would be more of an issue than DOF.
It also helped to think about the perspective issue and why the longer lens thus provides better separation of the subject from the BG.
Discussion
16 repliesIn other words if I am closer with a 400mm so that the object of interest occupies the same amount of the frame as the same object does with a 600mm from further away, which gives the greater DOF at the same aperture setting?
thanks,
Peter
BG
Let's see if I can get this right.
"Shorter lenses give greater DOF at the same aperture." If you mean the same aperture with a longer lens, no, shorter lenses don't have any greater depth of field than any other lens at a given opening. A shorter/wider lens only has an "apparent" greater dof because the objects are smaller in the frame.
So, you're closer to what's happening in your example. Assuming what you have in your field of view is the same regardless of what lens you're using, you will have the same dof at f/8 whether or not you're using 20mm or 200mm.
If you shoot something with the 600mm at f/8, and then move in closer to get the same exact field of view and shooting at f/8, you will have the same dof. Perspective may be different, but dof will be the same.
Another example... Say you're viewing a mountain scape. There's some tall trees a couple hundred yards out and the mountain peaks are a couple miles away (infinity.) You can take a broad scenic, say 20mm at f/16 focusing on the trees. You should get good "apparent" dof field and the mountain peaks should appear in focus (at the 20mm viewing.) Now, lets say you zoom in and shoot just the tree tops and mountains - at f/16. If you make a crop of the 20mm scene to match that of the 200mm view, you will be able to see the SAME dof.
Again, the wider lenses only seem to have a better dof - but it's just that everything is farther away and smaller.
So,
"if I am closer with a 400mm so that the object of interest occupies the same amount of the frame as the same object does with a 600mm from further away, which gives the greater DOF at the same aperture setting?"
They will both have the same dof.
Hope this makes sense.
Lon
One more question. Why does the longer lens give better "subject isolation"? What is the principle at work given that DOF is the same for the same field of view?
Thanks in advance.
Peter
It has something to do with the "angle of view" of the lens. I don't know the physics, but maybe this crude diagram will help.
The longer lens has a narrow field or angle of view (blue lines.) You can see that both lenses here have the same image size in the frame, but there is less included in the background. The wider lens also has the same size image in the frame, but more is included in the background.
You also get a sense of "compression" when using the longer lens. ie. things appear closer together.
Lon
Sorry I don't have a terrific drawing to include...
(note: I don't think 'mush' qualifies as a technical term
I don't know if I understand correctly what you're saying, but if I do, then no, I don't think so. If you zoom in from 20 to 200 mm without changing your own position (and hence the focus distance) to match the composition, you do not get the same DoF.
BG
Basil, I think you get the same DOF if you crop the 20mm photo to match the 200mm photo.
Peter
If you remain in the same position and zoom in from 20 to 200mm, OR if you crop a 200mm image from a 20mm image, you WILL have the same depth of field. so,
"If you zoom in from 20 to 200 mm without changing your own position (and hence the focus distance) to match the composition, you do not get the same DoF."
AND you keep the same aperture setting, you WILL have the same amount of DOF at 20mm and at 200mm - assuming you are stationary and the aperture remains the same. "You do not get the same DoF" is incorrect - you DO get the same DOF.
DOF - or the distance of acceptable focus from near point to far point, is precisely the same whether or not you use a 20mm lens or if you use a 200mm lens - at the same aperture.
Hope this makes sense.
Lon
The equations for DOF involve distance from subject, focal length, and aperture. If you change the first two in order to keep the subject the same size in the frame, the DOF is the same. If you only change the focal length, so that the subject is larger in the frame, the DOF is not the same.
Here's a couple of references if anyone is interested :
CIC DOF Tutorial
Luminous Landscape DOF Tutorial 2
I got everything that was written here, and knew Lon was wrong on the last part, and am able to explain the ins and outs of DOF to clients accurately without overwhelming them. I guess (I hope) it's enough to know how to achieve the DOF effect I want. I'm of the philosophy that I don't need to know all the names of the parts of the bike, or explain the scientific principles of balance and defiance of gravity, in order to ride it.
Thanks. I think I stand corrected, but let me see if I get this right. Which, if any of the following are correct?
- You're stationary. You take an image at 20mm and another zoomed in to 200mm. the aperture is the same. The DOF will not be the same.
- If you crop the 20mm image down to the same "image size" as the 200mm frame and the aperture is still the same, The DOF will not be the same?
- If you move in with the 20mm to get the same "image size" as with the 200mm taken in the orinal position, you will have the same DOF.
David: "If you only change the focal length, so that the subject is larger in the frame, the DOF is not the same. " I agree and I think I wasn't clear on my statement. My intention was assuming the same image size, only cropping the 20mm image instead of zooming.
Heck, I'm getting confused now. I'm with Steve - I need to know enough to take a nice picture.... too much technical detail only clouds the mind... :-)
thanks guys,
Lon
And most people don't need to be that exact anyway, since you usually only need to know what makes DOF increase or decrease with you particular setup. Sometimes the mathematical, or theoretical, gets in the way of the practical.
From a practical viewpoint I don't think Ron's error really matters. If one were to blow up the 20mm to achieve the same magnification as was provided by the 200mm the loss of clarity from lack of pixels on the sensor screen would be more of an issue than DOF.
It also helped to think about the perspective issue and why the longer lens thus provides better separation of the subject from the BG.