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Thread started 10 Feb 2013 (Sunday) 16:55
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Why does FF yield shallower DOF?

 
JohnB57
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Feb 11, 2013 07:45 |  #16

For the same framing, perspective and final image size, 1.6x crop sensors give 1.6x more depth of field. This equates to one and a third stops - more than the difference between f/2.8 and f/4 - so hardly minimal or insignificant.




  
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davidc502
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Feb 11, 2013 07:47 |  #17

I recommend the OP read the following -- http://www.cambridgein​colour.com …al-camera-sensor-size.htm (external link)


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marsaz
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Feb 11, 2013 07:49 as a reply to  @ post 15597429 |  #18

Sensor size does not change DOF. If you speak numbers.

Taking the same picture from the same distance with same lense on crop and then on FF will yield same DOF. Except that FF will capture more of that same scene so it might look less flat but if you measure how much space is in focus. It'll be the same.

You can get more bokeh by getting closer to the subject with the same lense when sensor size increases without changing the framing but that is not the same thing.

DOF is determined by the lense and not the sensor. Bigger sensor helps to feel the depth more though. It helps to isolate subject from background and foreground more since it captures more of that same scene.


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Feb 11, 2013 07:49 |  #19

TSchrief wrote in post #15597318 (external link)
The fact is that sensor size has (almost) NO impact on DOF. Several things have a much larger influence.

1.) Focal length of the lens in use.
2.) The aperture setting.
3.) Distance from sensor to subject.

If you mount lens A on a full frame camera and take a photo, then mount that same lens on an APS-c camera and take a photo framed exactly like the first one you took, you will notice that DOF is EXACTLY the same, assuming the same aperture. In photographic terms, sensor size has no effect on DOF. Period.

Sensor size does play a very minimal role in the calculation of the 'circle of confusion'. If you want to put some numbers on it, you'll have to do the research. Look into "Optics" and/or "Physics of Electromagnetic Radiation". As far as photography goes, sensor size has no impact on DOF. DOF is governed completely by items 1 through 3 above.

I find it curious that you "KNOW" something that isn't true. Do you also know that tomato juice will remove skunk smell? Or that aspirin is a blood thinner? There is an interesting book out, the title of which is, I believe, 101 Things Everyone "KNOWS" That Aren't True. Lots of people believe something called 'centrifugal force' exits. No such animal in the world of Physics.

The bolded line is where you went wrong. In order to achieve the same framing with the same focal length on a different sensor size, it will be necessary to change the distance--which is a DoF factor you had just yourself identified.




  
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gjl711
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Feb 11, 2013 07:53 |  #20

smorter wrote in post #15597454 (external link)
It needs to be noted though that for a given output size, when you can't change your FL or position, the APS-C camera gives LESS DOF. By definition it would be a more magnified view = less DOF at any given output size

I know that applies when you look at the full frame because of the magnification needed to make the same size print. Any DOF calculator will show that. However, in this example where you are shooting with a full frame camera but cropping out just the center 1.6 or 1.3 crop sensor area, the magnification will be the same so the DOF will be the same as well. In this case even though you are using a FF camera, you are not using the entire sensor area so in essence you have turned your FF sensor into a crop sensor.


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msowsun
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Feb 11, 2013 07:53 |  #21

smorter wrote in post #15597462 (external link)
But Jeffrey the issue is being able to equalise framing is a theoretical fantasy. The reality is, we are always focal length limited, or always position limited. Who here can foot zoom to whereever they want unless they are Superman? Who here can use any focal length from 8mm to 1200mm? Who here actually will equalise their framing when they change lenses or camera? The reality is they can't or don't.

Therefore Zivnuska's example, whilst only showing one side of the story (and yours showing the other), is no more or less valid than your scenario

The OP asks a very valid question. In the context of the OP's question, we are not "always focal length limited, or always position limited".

It is very easy to choose either 50mm or 85mm lens, or change your position slightly to equalize framing between Crop and Full Frame.

So yes, Full Frame cameras have less Depth of Field than Crop cameras. If you still have trouble grasping this simple truth, just try comparing images from a point and shoot camera to those made with with a Full frame (or larger) camera.


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JohnB57
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Feb 11, 2013 08:00 |  #22

msowsun wrote in post #15597510 (external link)
The OP asks a very valid question. In the context of the OP's question, we are not "always focal length limited, or always position limited".

It is very easy to choose either 50mm or 85mm lens, or change your position slightly to equalize framing between Crop and Full Frame.

So yes, Full Frame cameras have less Depth of Field than Crop cameras. If you still have trouble grasping this simple truth, just try comparing images from a point and shoot camera to those made with with a Full frame (or larger) camera.

Your argument breaks down when you move Mike. At the same FL, equal framing gives identical DoF. It's only through adjusting focal length that it changes between formats.




  
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smorter
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Feb 11, 2013 08:05 |  #23

msowsun wrote in post #15597510 (external link)
It is very easy to choose either 50mm or 85mm lens, or change your position slightly to equalize framing between Crop and Full Frame.

I guess everyone has a 50mm or 85mm lens then?

msowsun wrote in post #15597510 (external link)
So yes, Full Frame cameras have less Depth of Field than Crop cameras. If you still have trouble grasping this simple truth, just try comparing images from a point and shoot camera to those made with with a Full frame (or larger) camera.

Apples to oranges. You're comparing a 2mm lens against a 28mm lens. Of course there will be differences when you equalise the framing - but is that the sensor size? Indirectly, yes, but from an optical mechanics perspective, no.


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msowsun
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Feb 11, 2013 08:11 |  #24

It is not apples to oranges. A Point and Shoot camera is a Crop camera. Full Frame cameras have less depth of field than Crop cameras.

Why are you making it so difficult to understand?


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gjl711
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Feb 11, 2013 08:17 |  #25

n1as wrote in post #15595460 (external link)
If you shoot with the same lens on both cameras (same scene, same shooting position, same f/stop) and then crop the FF image to equal the FoV of the 1.6 crop-factor sensor will you have the same DOF?

You guys are totally missing the question. Same lens, same position, same f/stop, same crop = same DOF.


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JohnB57
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Feb 11, 2013 08:20 |  #26

msowsun wrote in post #15597566 (external link)
It is not apples to oranges. A Point and Shoot camera is a Crop camera. Full Frame cameras have less depth of field than Crop cameras.

Why are you making it so difficult to understand?

Because for the purposes of his argument, he is ignoring the last part of the DoF calculation - final enlargement.

smorter wrote in post #15597550 (external link)
I guess everyone has a 50mm or 85mm lens then?

Your gear list includes the following: -

85mm f/1.2L II
70-200mm f/4L IS
Canon 24-105 f/4L IS
Canion 70-200 f/2.8L IS II
50mm f/1.8 II - Sharp even at f/1.8
18-55 f/3.5-5.6 IS
Tamron 17-50 f/2.8

So you have quite a selection that include these FLs.




  
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JohnB57
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Feb 11, 2013 08:21 |  #27

gjl711 wrote in post #15597579 (external link)
You guys are totally missing the question. Same lens, same position, same f/stop, same crop = same DOF.

Also = different picture.




  
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msowsun
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Feb 11, 2013 08:21 |  #28

gjl711 wrote in post #15597579 (external link)
You guys are totally missing the question. Same lens, same position, same f/stop, same crop = same DOF.

Yes, the OP's question asked about cropping a FF to match the crop camera with the same position, same f/stop, same crop .

But then the FF becomes a crop camera doesn't it? And the same basic truth still applies.

A FF camera has less DOF than a Crop camera.

.


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francis_a
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Feb 11, 2013 08:28 |  #29

If the crop is done in post, wouldn't the DOF be exactly the same?

However, if the framing of the FF is done to match the crop, then the FF has to zoom in which increases the FL, which then decreases DOF.


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JeffreyG
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Feb 11, 2013 08:29 |  #30

gjl711 wrote in post #15597579 (external link)
You guys are totally missing the question. Same lens, same position, same f/stop, same crop = same DOF.

But then is isn't the same picture. Who uses cameras this way? Would you really try to shoot the same picture at 50mm on a compact digital (where it is a long telephoto) and then also at 50mm on a 5D where it is a normal lens?

The fact is, people typically select and use lenses that cover the same kinds of FOV ranges no matter what sensor size they have. So a compact digital camera often will come with a zoom lens that covers something like 6mm - 72mm or so. A 5D owner might use a pair of zooms to cover 24mm - 300mm for the same FOV range.

So in short, to take the same photo with a larger format, you use a longer focal length. This gives less DOF.

And since some people cannot grasp this, if you are focal length limited on the larger format and you crop it down to the same physical size on the sensor, then the DOF is the same. But this is because by cropping you are no longer using two different formats. You crop to one size, then you are using only one size.


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Why does FF yield shallower DOF?
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