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Ronster
12-22-2009, 10:33 AM
Hey so this is a real tech question

basically I have found a bunch of depth of field calculators....but I want to make sure that I am plugging in the right lens for depth of field.

Here is an example:

if I am shooting with a 20mm lens on my 7d I know for mm lens I have to do a 1.6 convert on the lens to make sure it is the right lens.

so a 20mm lens multiplied by 1.6 for the crop factor is really a 32mm lens on the 7d.

Now my question is when calculating depth of field are we going off the 20mm lens that the lens actually is or off the 32mm readjustment for the crop sensor?

please help me out here

Ronster

awelgraven
12-22-2009, 11:02 AM
You'd calculate for a 20mm lens I believe. The crop factor doesn't change the optics of the lens. It's still a 20mm lens DOF-wise, you're just seeing a 32mm Field of View.

Michael Olsen
12-22-2009, 12:23 PM
Hi Ronster,

awelgraven has this correct.

A 20mm is always a 20mm lens. It will always converge light in the same way and will always display the same image circle regardless of what you mount it on...or if it is even mounted at all.

All that happens when you put it on the 7D is that your field of view is decreased in comparison to larger sensors (1Ds, 5D, medium format, and so on).

A 20mm lens on the 7D should give you about 9.3ft of DOF when the subject is at 8 feet.

Hyperfocal distance begins at roughly 17.

smelni
12-22-2009, 12:37 PM
i have to correct this info here.

the format of the camera does in fact have to be taken into account - while a 20mm lens is always a 20mm lens, the format will determine what Circle of confusion you choose - and that number is directly in your DOF formulas. Since the 7d is a smaller format than a 35mm - the circle of confusion is smaller because the image needs to be blown up more to get to a viewable format - hence the deeper DOF with the same lens at the same distance

convergence of light is not what is important - the COC is whats important and that depends on the format you shoot on and the final image size for viewing.

Ronster
12-22-2009, 12:46 PM
i have to correct this info here.

the format of the camera does in fact have to be taken into account - while a 20mm lens is always a 20mm lens, the format will determine what Circle of confusion you choose - and that number is directly in your DOF formulas. Since the 7d is a smaller format than a 35mm - the circle of confusion is smaller because the image needs to be blown up more to get to a viewable format - hence the deeper DOF with the same lens at the same distance

convergence of light is not what is important - the COC is whats important and that depends on the format you shoot on and the final image size for viewing.

so which is it?

I have been hearing from everyone pretty much that a 20mm lens regardless of the crop factor is still a 20mm lens in determining depth of field.....I want to know what to plug in to the different charts to determine my depth of field...I realize due to the crop factor that my 20mm lens is a 32mm lens in terms of the view because of the 1.6 crop on the 7d but for determining my depth of field....do I go off the 20mm lens, do I go off a 32mm lens, or something else???????

smelni
12-22-2009, 12:55 PM
You need a dof chart for the format size. That takes into account the coc and then a 20mm is correct. If you use crop factor on a 35mm chart you might be close but not exact. To approximate with that multiple the lens by crop but again that is not very accurate. You need a correct chart

Barry_Green
12-22-2009, 02:55 PM
Ignore ALL that crapola, and do as Seth says: just use a DOF calculator for the right format size. The 7D is basically identical in sensor size to 35mm movie film, so use a 35mm movie film DOF calculator. You can find one at panavision.co.nz

Michael Olsen
12-22-2009, 02:58 PM
You need a dof chart for the format size. That takes into account the coc and then a 20mm is correct. If you use crop factor on a 35mm chart you might be close but not exact. To approximate with that multiple the lens by crop but again that is not very accurate. You need a correct chart

Smelni, you are correct in that sensor size impacts the circle of confusion and, hence, DOF. We all know this as the reason why a 5D will have shallower DOF than a 7D, which will have shallower DOF than an 1/3" chip.

The OP was asking about using a DOF calculator in which you input

the size of the sensor (either the dimensions, the crop factor, or the model of camera)
the focal length of your lens
the f-stop of your lens
the focus distance


All of this outputs data: Near limit, far limit, and total DOF, as well as the relative location of the DOF to the subject, the circle of confusion, as well as the hyperfocal distance.

My statement was in regard to field of view and magnification - letting the OP know that a 20mm does not magically become any other focal length simply by applying a different camera to it. Judging by his use of the 1.6x crop number, it sounds as though he knew he needed to include the size of the sensor in his work.


I have been hearing from everyone pretty much that a 20mm lens regardless of the crop factor is still a 20mm lens in determining depth of field.....I want to know what to plug in to the different charts to determine my depth of field...I realize due to the crop factor that my 20mm lens is a 32mm lens in terms of the view because of the 1.6 crop on the 7d but for determining my depth of field....do I go off the 20mm lens, do I go off a 32mm lens, or something else???????

Whenever you are using a chart or calculator to determine DOF, you definitely need to take into account the size of the sensor. I assume you know that the Canon EOS 7D uses a single CMOS 1.6x crop APS-C sensor sized 22.3 14.9 mm. If you don't include that information, or use some other standard such as FF 35mm, as Smelni says, your calculations will be inaccurate.

You also need to include the focal length of your lens. Look on the lens. Somewhere it will say "XXmm". In your case, I believe it says 20mm. If it says 20mm, it's 20mm no matter what you try to do with it. It will always be a 20mm lens.

So what it boils down to, in your case.

You are correct if you enter 22.3 14.9 mm as your sensor size and 20mm as your lens focal length.

You are incorrect if you enter 35mm as your sensor size and 20mm as your lens focal length. This is because the 7D is not a 35mm camera!

You are incorrect if you enter 22.3 14.9 mm as your sensor size and 36mm as your lens focal length. This is because your lens is a 20mm lens, not a 36mm lens. It simply has the field of view of a 36mm lens when mounted on a 1.6x crop sensor (as the 7D).

Michael Olsen
12-22-2009, 03:10 PM
Ignore ALL that crapola, and do as Seth says: just use a DOF calculator for the right format size. The 7D is basically identical in sensor size to 35mm movie film, so use a 35mm movie film DOF calculator. You can find one at panavision.co.nz

I might just be doing this wrong, but I get rather different data using the Panasonic DOF calculator for 35mm film (COC = .001mm) and when using DOF calculators for the 7D (COC = .019mm).

I'm using 10 feet as the subject distance, f-stop of 4, and focal length of 20mm. The difference between the near limits is 0.7 feet, and the hyperfocal distance varies by almost 5 feet.

smelni
12-23-2009, 09:23 AM
sensor size is NOT in the formula for DOF - COC is. when a chart has an option to choose a sensor size it is really choose a COC that is generally accepted for that format - normally that is sufficient but if you want to be more precise you should use a chart that lets you input a COC -

remember a COC takes into account your sensor size AND your distribution screen size - to be more precise its really about aperture and distro size

Ronster
12-23-2009, 12:28 PM
so I found this website:

http://www.dofmaster.com/dofjs.html

and you can plug in "canon 7d" under:

Camera, film format, or circle of confusion

I guess under circle of confusion for the 7d it is 0.019 and you can plug that in too.

This seems like the right calculator...if i plug in 50d, 40d, etc or 7d or COC .019 the numbers come out the same....

here is something interesting

when I switch to "Canon 5d" it gives me a greater depth of field.

why is the "Canon 7d" shallower than the "Canon 5d" full frame sensor? It appears that the 7d has the shallowest depth of field of all the canon line up?????

I'm learning but this stuff is confusing to start...I notice this website also has an iphone app for 1.99

if this seems legit I might get this one....can some of you chime in if this seems right

smelni
12-23-2009, 12:43 PM
to compare dof for 2 formats (5d and 7d) you need to compare at the same image size - NOT at the same lens -

so a 20mm lens on a 5d has a WIDER frame of view then the 20mm on the 7d - a wider frame of view generally had a deeper DOF - but if you take the lens for the 5d that had a comparitive same field of view (like about a 33mm ) then it will have a shallower DOF.

Paul Hudson
12-23-2009, 01:39 PM
If you have an IPhone the Pcam app is an essential tool. Check it out. It is kind of pricey but I use it all the time.

Ronster
12-23-2009, 04:52 PM
to compare dof for 2 formats (5d and 7d) you need to compare at the same image size - NOT at the same lens -

so a 20mm lens on a 5d has a WIDER frame of view then the 20mm on the 7d - a wider frame of view generally had a deeper DOF - but if you take the lens for the 5d that had a comparitive same field of view (like about a 33mm ) then it will have a shallower DOF.

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....

ROCKMORE
12-24-2009, 04:59 AM
i have to correct this info here.

the format of the camera does in fact have to be taken into account - while a 20mm lens is always a 20mm lens, the format will determine what Circle of confusion you choose -


Where did the term "Circle of Confusion" originate?

smelni
12-24-2009, 05:07 AM
http://en.wikipedia.org/wiki/Circle_of_confusion

Ronster
12-27-2009, 12:46 PM
maybe someone can chime in on this...because I feel like i get it with this website:

http://www.dofmaster.com/dofjs.html

I plug in type of camera as 7d, the lens, the distance, the f stop and it gives me my depth of field...when I switch the camera type to 5d it is a greater depth of field. Is the 5d shallower or have a greater depth of field than the 7d?

I also can pick circle of confusion instead of camera...7d I believe is .019 and 5d is .030

when you make those changes the 7d has a far shallower depth of field when all the numbers are the same....it just doesn't make much sense. If I were to make the change in lenses the 7d would have an even more shallow depth of field so I don't believe it is changing the lens mm that is throwing this off?

can someone please help!!!!!!!!!!!!

smelni
12-27-2009, 01:15 PM
You get a shallower dog for the same image size. Not the same lens

Ronster
12-28-2009, 01:57 PM
let me try this one last time:

i go to dopmaster.com and click on the onlline calculator

i plug in 7d in format/type

then i plug in:

10 feet for distance
17mm for lens
f2.8 for apeture

I then switch between 7d and 5d and it gives me 2 sets of near and far for focus and total focus is different when i switch back and forth.

on the 7d I can choose 50d, 40d, etc or .019 on circle of confusion and the numbers come out the same and I can pick 5d or .030 and the numbers come out the same (I believe the first is basically for a 1.6 crop sensor and the latter is for full frame).....

so everything is identical...just switching between 5d and 7d
again: focus is set to 10 feet away, 17mm lens, f2.8 for iris...only thing shifting is camera/film format between 7d and 5d.

7d goes from 6.4 feet to 22.9 in focus and 16.5 feet total focus when set to these settings

when I go to 5d ALL THE SETTINGS ARE THE SAME EXCEPT I GO TO 5D INSTEAD OF 7D

5D goes to 5.9 feet to 90.8 feet in focus...total focus is 85.6

its still set to 10 feet away, f2.8 for the iris, and 17mm lens

can you explain?

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

smelni
12-28-2009, 02:10 PM
no. You can only compare at the same image size. Some Calcs have that as a value
So a 17lens on the 7d is pro about equivalent on image size to 28 on the 5d. If you compare the two with those lenses you will get a shallowerer dof

Common misconception that larger format has shallower dog with same lens but that's not true. Its shallowrre for same image size

smelni
12-28-2009, 02:14 PM
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
...

smelni
12-28-2009, 02:15 PM
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
...

Michael Olsen
12-28-2009, 04:30 PM
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

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.

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.

Ronster
12-28-2009, 09:01 PM
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.


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

Michael Olsen
12-28-2009, 11:07 PM
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

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.

j1clark@ucsd.edu
12-29-2009, 06:56 PM
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....

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.

Ronster
12-30-2009, 01:57 PM
the 7d has a smaller sensor than the 5d