Depth of the field.

[paulherrin]

everyone is talking about something different... sorry i got roped into it

 

[Joe D'Arcy]

Here is a good example. I shot a multicam show last week. I have taken a mid and an M/C/U and gave them the same field of view by cropping the mid to match the close.

Same F stop, only difference is the close is a 300mm and the mid is 85mm. The 85mm is zoomed up to match so it's a bit gritty, but the DOF is clear.

Difference in DOF is pretty clear to me.

Here are the originals:

Hi Jim,
What I find remarkable about these shots is that you manage to keep such a steady hand filming Carl Barron?

 

[jimhare]

Heh, you know it Joe, that's why I only hire deaf operators so they don't shake from laughter!

 

[jimhare]

everyone is talking about something different... sorry i got roped into it

I'm with you Paul, starting to think I'm talking about something completely different as well! I'll leave it to the mathematicians cause I certainly can't get those results on my 11-16mm! :cursing:

 

[Peter Dmitriyev]

Nick Gardner is correct. Him and David Mullen amongst others have responded in my thread a while back which started off on a slightly different topic but if you read the whole thread you will see this very same discussion noted with a few better explanations

http://reduser.net/forum/showthread.php?82937-Why-Do-Anamorphics-Have-Shallower-DoF


point is if you frame a subject where they take up the same exact space in a given frame, for instance frame a person's face with a 20mm lens, and then frame their face so it takes up the same exact amount of the frame with a 120mm lens, the DOF will be identical because the lenses have the same DOF.

 

[Mitch Gross]

The difference y'all are talking about here is APPARENT Depth of Field v. ACTUAL Depth of Field. Most people think in terms of Apparent DoF, where wider lenses and greater depth than more telephoto ones. But as accurately noted, Actual DoF functions independent of focal length.

Ths is more a semantic argument than anything else, because it is about how one uses and perceives the image.

 

[Peter Dmitriyev]

Here is a good example. I shot a multicam show last week. I have taken a mid and an M/C/U and gave them the same field of view by cropping the mid to match the close.

Same F stop, only difference is the close is a 300mm and the mid is 85mm. The 85mm is zoomed up to match so it's a bit gritty, but the DOF is clear.

Difference in DOF is pretty clear to me.

Here are the originals:

You can't just crop shots in post to make the image magnification match and then derive depth of field differences from that, it doesn't work that way. The image magnification would have had to have taken place during the actual shoot. I.e. if you actually shot your 300mm such that the comedian's body took up the same exact space/size on the screen/viewfinder as it did on the 85mm then you would have seen that the Depth of field is identical and the curtains behind him would have had the same exact level of blurred/out of focus.

However what you apparently did was shot the 300mm as a much closer image where the DoF looked much more shallow and then the 85 you shot much further away thus the difference.

You simply aren't understanding the mechanics properly as per this test http://www.luminous-landscape.com/tutorials/dof2.shtml

as you can see, the camera is moved so that the gremlin subject takes up the same exact space in the image in each shot NOT via a post crop but in the actual shot. You did not do that with your shots however if you moved the 85mm camera much closer to the comedian such that his body takes up the same exact space as in the 300mm frame then you would have seen that they have identical DoF and the curtains behind him would have the same exact level of blurriness. Or vice versa if you would have backed your 300mm camera up so much that the comedian took up the same amount of the frame as in the 85mm shot, you would see that the 300mm shot's DF would look just as deep as in the 85mm frame.

 

[Jay Kim]

Okay. I can see the problem here.
You cannot combine the two variables to compare DOF and say they have the same DOF when the image size is same.
To make the image size same you have to move the camera closer when you using a wide lens. At the same time, if you are using a telephoto lens, you have to move your camera away from the subject to make the image size same.
Wider lens:greater DOF + closer focus distance:shallower DOF = Telephoto: shallower DOF + Further Focus Distance: greater DOF
It is almost same as you run the camera fast with lower iris and saying Iris doesn't affect exposure since you get same exposure as slow shutter with higher iris. However, when you look at Iris alone, there shouldn't be any doubt that iris affects Exposure.
Same theory applys here. You cannot simply combine two variables together and saying one variable doesn't affect the result since you forgot to take the other variable from the table.

You have to look at each variable without changing the others to see if it affects DOF.
If I keep the same F/Stop and Focus distance, then there is no way all focal length lenses make the same DOF.

I can see some of you want to argue that Focal Length doesn't affect DOF, but the Image size, please look at the image size as Focal Length + Focus Distance.

 

[Björn Benckert]

Here is a good example. I shot a multicam show last week. I have taken a mid and an M/C/U and gave them the same field of view by cropping the mid to match the close.

Same F stop, only difference is the close is a 300mm and the mid is 85mm. The 85mm is zoomed up to match so it's a bit gritty, but the DOF is clear.

Difference in DOF is pretty clear to me.

Here are the originals:

Ok, this is a bit odd... Are you sure you had the cameras on the exact same distance to the subject when you shot the two shots? IF that was the case then the DOF should match up better. Still it's not to far off as if you take the 300 mm picture and scale it down and put it ontop of the 85mm then it will look quite close. As if you zoom into the 85mm you enlarge the little differences that are there so going the other way around you will see that they match up fairley good as you take sensor resolution and lens resolution out of the equation.

 

[Jay Kim]

I think you don't see much difference here due to the sensor size (seems 1/3" or 1/2" to me) or the studio was very bright with higher f/stop. However, it is still clear that 85mm and 300mm had different DOF.

 

[Stephen Williams]

I think you don't see much difference here due to the sensor size (seems 1/3" or 1/2" to me) or the studio was very bright with higher f/stop. However, it is still clear that 85mm and 300mm had different DOF.

DOP is what actually in focus, not to do with how sharp the out of focus parts look. If you use a box of cornflakes at 45 degrees to the camera & see how much lettering on the pack is in focus you would see that the DOF is the same regardless of lens. The camera will be 5 x closer to the pack with a 20mm lens than with a 100m lens.