Question on DOF


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tjana

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dear CS-ers, recently i learned that in cropped sensor DSLR you don't get same kind of shallow DOF (with the nice bokeh we all love) like FF while using same aperture size. one guy explains that DOF is a function of sensor to object distance, and because cropped sensor camera has to be further from the object compared to FF, the DOF will not be as shallow.

can someone explain this further please? like in actual terms what's the comparison. i read somewhere f2.8 on cropped sensor will be the equivalent of f4 on FF. is that accurate?
 

ZerocoolAstra

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#2
you're like .... partially correct, but then you've been misled in a sense.

The main difference between crop sensor and FF is the field of view with the same lens.
With the crop sensor, you see less. So to get the same field of view, you step back.
So what do you change? The camera------ subject ------------------- background
distance. This affects the depth of field.
 

Jed

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#3
Approximately, yes. The sensor size is 1.5x smaller while each stop, such as from f2.8 to f4, is a factor of approximately 1.4x.

You can have a play around with an online DoF calculator such as the one at http://www.dofmaster.com/dofjs.html, although you will have to manually compensate the distance difference.
 

Fullerene

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#4
Whoa..
Actually I not aware of this, it's only after reading your post then I do a search and find this info.
http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

Depth of field requirement.

As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).

Think i really have lots of fundamental to read up on.
 

ZerocoolAstra

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#5
tjana said:
dear CS-ers, recently i learned that in cropped sensor DSLR you don't get same kind of shallow DOF (with the nice bokeh we all love) like FF while using same aperture size. one guy explains that DOF is a function of sensor to object distance, and because cropped sensor camera has to be further from the object compared to FF, the DOF will not be as shallow.

can someone explain this further please? like in actual terms what's the comparison. i read somewhere f2.8 on cropped sensor will be the equivalent of f4 on FF. is that accurate?
where is the justification for the point in bold? the aperture determines the size of the aperture opening in the lens. It's a characteristic of the lens, not the camera body.

If you're saying dof with an f/2.8 lens on crop sensor is same as f/4 lens on FF, that's a bit tough to prove. Do give it a try! :)
 

flipfreak

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#6
where is the justification for the point in bold? the aperture determines the size of the aperture opening in the lens. It's a characteristic of the lens, not the camera body.

If you're saying dof with an f/2.8 lens on crop sensor is same as f/4 lens on FF, that's a bit tough to prove. Do give it a try! :)
http://www.dofmaster.com/dofjs.html

give it a try.
 

flipfreak

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#8
a bit tough leh... coz with a FF, I won't know how far I have to step forward to get the same FOV, unless I experiment physically.
u can just input 1 crop with 1 ff and the same fov for both cams.

for eg, if u input d80 with 50mm at f/2 at 15 feet, u get almost the same dof as d3x at f/2.8 at 10 feet, the difference being 0.1 feet. of course the fov method i use is abit crude. but i reckon its the easiest way to get the same fov for the same lens.
 

tjana

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May 24, 2004
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#9
Zerocoolastra: yes, i got your point, that's what i meant by
camera has to be further from the object compared to FF
, must do the leg work to move the camera la. didn't express it very well, i'm afraid. re the justification of f2.8 similar to f4, i was repeating what i read elsewhere, want to find out if that's true or not. will check out the DOF calculator.
 

tjana

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May 24, 2004
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#10
Fullerene: yes, there is quite a bit to read up... thought i understood the basics of aperture/shutter/dof thing, until i switched to DSLR then a whole lot of new problems like white balance and cropped sensor need attention...
 

tjana

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May 24, 2004
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#11
thanks for the link Flipfreak!

eek, calculation seems to go all wrong... and how come 5Dmark2, 1Dmark3 and 1Ds all got different values... thought they're all full sensors... argh circle of confusion indeed.
 

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flipfreak

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#12
thanks for the link Flipfreak!

eek, calculation seems to go all wrong... and how come 5Dmark2, 1Dmark3 and 1Ds all got different values... thought they're all full sensors... argh circle of confusion indeed.
the sensor sizes are not the same. there are some minor size difference even between nikon fx and canon ff.
 

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Limsgp

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#13
If the resolution is different, the pixel size will be different for the same sensor size.

The DOF depends on the CoC which depends on the pixel size.


eek, calculation seems to go all wrong... and how come 5Dmark2, 1Dmark3 and 1Ds all got different values... thought they're all full sensors... argh circle of confusion indeed.
 

tjana

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May 24, 2004
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#15
ok, this is what i got so far, taking information from sites below and making my own conclusions using leftover secondary school physics knowledge...

http://www.sphoto.com/techinfo/dslrsensors/dslrsensors.htm
http://www.dofmaster.com/dofjs.html

1D camera is full frame, 1Ds is 1.3x, 40D is 1.6x
The angle of view depends on the ratio between the focal length and the film size.

the full frame one has to be closer to the object to take in the same FOV, and the distances are inversely proportional to the sensor size. so to get the same FOV using the same focal length, the distance from object got to be 0.625x the distance the cropped sensor camera is from object. someone please advise of this is accurate... :sweat:

now on with the calculator

cropped sensor (1.6x)

50mm f2.8
* subject at 100cm
DOF 4.09 (98-102.1cm)


full sensor

50mm f2.8
subject at 100cm
DOF 6.46cm (96.9-103.3cm)

subject at 80cm
DOF 4.08 (78-82.1cm)

* subject at 62.5cm
DOF 2.44cm (61.3-63.7cm)


compare the two equivalent FOVs, marked with (*). conclusion: the full frame sensor indeed has shallower DOF, but i don't know how much it translates in term of equivalent aperture sizes.
 

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tjana

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May 24, 2004
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#16
If the resolution is different, the pixel size will be different for the same sensor size.

The DOF depends on the CoC which depends on the pixel size.
i was using the CoC term anyhow, don't actually know what it means... ;p
 

calebk

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Jul 25, 2006
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#17
i was using the CoC term anyhow, don't actually know what it means... ;p
CoC is the largest diameter of a dot that is considered sharp. Any larger than that CoC reading and the image is considered blur.
 

Limsgp

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#18
When a subject is in focus, each point of the subject will form a point(dot) on the sensor. Move the subject nearer and the "point" will move to the back of the sensor. On the sensor itself, the "dot" will became bigger. Move the subject further and the dot will move to the front of the sensor. On the sensor itself, the dot will became also bigger. As you move the subject between the near distance and the far distance, and the size of the dot is still within the CoC size, then the difference between the near distance and the far distance is the DOF.


i was using the CoC term anyhow, don't actually know what it means... ;p
 

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