7d calculating depth of field

For help understanding just try to get a shallow dof shot on Amy can with a 17 lens. You can't

When you have a larger sensor all your shots are wider on the same lens giving deeper dof

But at same image size
...
 
For help understanding just try to get a shallow dof shot on Amy can with a 17 lens. You can't

When you have a larger sensor all your shots are wider on the same lens giving deeper dof

But at same image size
...
 
Ronster said:
I realize the two images won't be exact....the 7d will be a tighter frame but that is regardless...shouldn't the depth of field be greater on the 7d?


Ronster
Click to expand...

Here's the best explanation I've found so far.

Since the way the circle of confusion is derived includes the evaluation of a printed image, in order to be consistent, we'd need to use the same size printed image regardless of whether our test image came from film or digital. But since the sensor in almost all digital cameras is smaller than a frame of 35mm film, we'll need to enlarge the resulting digital capture more to achieve that print size than we would from a film capture. Because of this, any potential focus errors will be more visible on the print from digital as they will be enlarged during printing too. To compensate, we'll need to use a lower circle of confusion value. How much lower depends on how much smaller the sensor is than film. Using the same standards of judgment, we'll need to divide 0.03 (the circle of confusion for 35mm film) by the crop factor for the DX digital format we are using to get the CoC value for that format. So, for my Nikon D2x which has a 1.5x crop factor, I'd need to use a circle of confusion value of 0.02 (that is, 0.03 divided by 1.5). Circle of confusion values for other digital cameras can be calculated similarly, or simply looked up on this helpful page.
Click to expand...

from this site: http://www.earthboundlight.com/phototips/crop-factor-depth-of-field.html

Essentially, since a crop sensor is smaller than a full-frame sensor but still needs to record the same amount of information in similar detail to produce a quality print rendition of the same size, the COC needs to be smaller as well. If it wasn't everything shot on a crop lens would look a bit fuzzy next to something shot on full frame. So focus is more critical on crop sensors, which is really a fun way of saying there is less depth of field.

Ceteris paribus, the DOF of a 7D at 10' with a 17mm set to f/2.8 will have less field of view and less depth of field than a 5D MKII at 10' with a 17mm set to f/2.8.

However, as has been pointed out, in order to get approximately the same image in terms of framing and composition with the crop camera, one has to either 1) move away from the subject or 2) use a wider lens. Either of these will lengthen the depth of field on the crop camera so much that the full frame camera will have the shallower DOF.
 
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Michael Olsen said:
Here's the best explanation I've found so far.



from this site: http://www.earthboundlight.com/phototips/crop-factor-depth-of-field.html

Essentially, since a crop sensor is smaller than a full-frame sensor but still needs to record the same amount of information in similar detail to produce a quality print rendition of the same size, the COC needs to be smaller as well. If it wasn't everything shot on a crop lens would look a bit fuzzy next to something shot on full frame. So focus is more critical on crop sensors, which is really a fun way of saying there is less depth of field.

Ceteris paribus, the DOF of a 7D at 10' with a 17mm set to f/2.8 will have less field of view and less depth of field than a 5D MKII at 10' with a 17mm set to f/2.8.

However, as has been pointed out, in order to get approximately the same image in terms of framing and composition with the crop camera, one has to either 1) move away from the subject or 2) use a wider lens. Either of these will lengthen the depth of field on the crop camera so much that the full frame camera will have the shallower DOF.
Click to expand...


The second half of what you wrote here is exactly what a friend of mine told me...which makes total sense now....basically if you are getting an image that is on a 17mm lens, at 10 feet, f2.8

if its an image of a person it will be there whole body on the 5d....but on a 7d it will be just their upper body...so to get the same image you have to pull back the 7d say to 16 feet and then the two frames will be the same but by pulling back the 7d now it has a depth of field greater than the 5d.

I think it makes sense now....

still trying to fully digest...I realize the whole blowing up and circle of confusion comes in...but i think i have a bit better of a grasp

greatly appreciated

Ronster
 
Ronster said:
The second half of what you wrote here is exactly what a friend of mine told me...which makes total sense now....basically if you are getting an image that is on a 17mm lens, at 10 feet, f2.8

if its an image of a person it will be there whole body on the 5d....but on a 7d it will be just their upper body...so to get the same image you have to pull back the 7d say to 16 feet and then the two frames will be the same but by pulling back the 7d now it has a depth of field greater than the 5d.

I think it makes sense now....

still trying to fully digest...I realize the whole blowing up and circle of confusion comes in...but i think i have a bit better of a grasp

greatly appreciated

Ronster
Click to expand...

I'm glad it helped. I'm learning a lot as I read through all of this about DOF - there is a lot to it and I've never really looking into the details of COC. Very interesting stuff, that.
 
Ronster said:
everyone has said you go by the lens you don't go by the converted lens....so this seems wrong by what everyone has said so far

I just don't get why the 5d when I switch up the selection has a broader depth of field than the 7d....in a way I could see how you are right but it just doesn't make sense....
Click to expand...

Check the sensor size, which relates to the 'circle of confusion', and hence, relates to DoF.

I don't know the specs on either, but apparently the 7D may have a larger sensor.


Here's a reference URL:

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

That reference seems to cover a number of topics that are useful.
 
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