Depth of field and circle of CONFUSION!


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Jan 14, 2005
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#1
Let me try to explain this here...

Theoretically, when a lens focuses on a scene, there will only be a single plane where the image is in focused. Moving away from the plane of focus, the image will get increasingly blur.

Our eyes have limited resolution. In a photo, although the image is blurred immediately away from the plane of focus, our eyes do not detect it and still perceive it to be still in focused. This total distance in front and behind the plane of focus where the eyes still perceive as sharp is known as the depth of field. The circle of confusion is defined as how much a point needs to be blurred before the eye perceives it to be out of focus.
 

Jan 14, 2005
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#2
So what will influence DOF? Let’s start with the viewing of the final print.

Viewing distance
The nearer you view a photo, the more your eyes are able to detect the out of focus.

Use the photo below as an example.


If you are sitting right in front of the monitor, you will notice the “-2” is out of focus. If you move back far enough, you will start perceiving “-2” to be in focus.

The DOF is shallower when you are viewing at a nearer distance
 

Jan 14, 2005
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#3
Print size/magnification
When an image is magnified into a large print size, the more your eyes are able to detect the out-of-focus.

The following photos are all from the same image magnified into different sizes.
Compare this


with this (2x)


and this (4x) cropped...


and if blown up extreme big (12.1x)... and seeing just a crop of it


In the smallest photo, you will probably see that 3 to -2 are in focused. However, in the largest magnification, only 1 and 0 appear to be in focus.

The larger the magnification, the shallower the DOF.


Now, this complicates things quite a bit when we try to define DOF with print size and viewing distance. Therefore, when nothing is stated, it is usually assumed that the final enlargement size is 10” x 8” and the viewing distance is 1ft.

So if you are making really large prints bigger than 10" x 8", take the print size into consideration for DOF. If you have making a poster to viewed at a distance, consider that too.
 

Jan 14, 2005
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#4
Let us now consider the image making.
The factors we more commonly know that affect the DOF are: focal length, aperture and distance to subject.

Longer the focal length, the shallower is the DOF and vice-versa.

Larger the aperture, the shallower is the DOF and vice-versa.

Shorter the distance to subject, the shallower the DOF and vice-versa.
 

Jan 14, 2005
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#5
How about sensor size?

Remember that DOF is defined in the final viewing image. The image created by the lens (focal length, aperture & subject distance) is captured by the sensor/film and then enlarged into the final viewing image (usually assumed to be 10” x 8” viewed @ 1ft). Therefore, if the sensor is small, the more enlargement is needed to project the image into the final image size. This will enlarge the “blurness” too and make the out-of-focus easier to detect. Therefore, keep all other factors (focal length, aperture, subject distance, etc.) constant, smaller sensor size will give shallower DOF.

Now, why we always say that full-frame/35mm/FX format has shallower DOF compared to cropped sensors or small sensors on PNS cameras? Often, we are also referring to same FOV. Therefore, if you compare a same image with same FOV, we need to compare a longer focal length using the full-frame to a shorter focal length of the cropped sensor and extremely short focal lengths for PNS cameras. In this case, it is true that full-frame has shallower DOF than cropped sensors, mainly due to the focal length.
 

lsisaxon

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Nov 29, 2004
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#6
How about sensor size?

Remember that DOF is defined in the final viewing image. The image created by the lens (focal length, aperture & subject distance) is captured by the sensor/film and then enlarged into the final viewing image (usually assumed to be 10” x 8” viewed @ 1ft). Therefore, if the sensor is small, the more enlargement is needed to project the image into the final image size. This will enlarge the “blurness” too and make the out-of-focus easier to detect. Therefore, keep all other factors (focal length, aperture, subject distance, etc.) constant, smaller sensor size will give shallower DOF.

Now, why we always say that full-frame/35mm/FX format has shallower DOF compared to cropped sensors? Often, we are also referring to same FOV. Therefore, if you compare a same image with same FOV, we need to compare a longer focal length using the full-frame to a shorter focal length of the cropped sensor. In this case, it is true that full-frame has shallower DOF than cropped sensors, mainly due to the focal length.
:thumbsup: This is worth a sticky. :)
 

knpan

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May 2, 2006
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#7
i think it is quite difficult to understand for starters lol.

But good explanation.
 

cairocks

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Jul 8, 2006
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#10
Wow,

Learn something so early in the morning. :thumbsup:
Thanks:)
 

V

vince123123

Guest
#11
That is because as explained so well by the TS, your compact cameras typically have lenses with very short focal lengths to maintain the same FOV as the SLR sized sensors. With short focal lengths come big DOFs.

What about compact cameras/PnS? Their sensors are so tiny and yet it is virtually impossile to achieve shallow DOF even when using f2.8 or when zoom to the max. The "only" time I can see shallow DOF with these compact PnS cameras are when using the macro mode. (in this case subject distance)

If smaller sensor size = shallower DOF, how come not in the case of compacts?

Hope you can shed some light into this. Btw, great explanation so far :thumbsup:
 

Redsun

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Nov 27, 2005
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www.flickr.com
#12
This is a very good explanation of circles of confusion!
I have never truly understood it
Now i have a clearer picture
Thanks!
 

SolBadGuy

Senior Member
Nov 5, 2006
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#13
Thanks for sharing! When my lecturer 1st mentioned it last semester, it was so confusing (though I did understand after his 2nd/3rd explanation)! Your explanation is quite apt and easy for people to understand. :)

Yeah, this should be made a sticky! ;)
 

blazer_workz

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May 8, 2006
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#14
Very good explaination by TS!!! :thumbsup:

What about compact cameras/PnS? Their sensors are so tiny and yet it is virtually impossile to achieve shallow DOF even when using f2.8 or when zoom to the max. The "only" time I can see shallow DOF with these compact PnS cameras are when using the macro mode. (in this case subject distance)

If smaller sensor size = shallower DOF, how come not in the case of compacts?

Hope you can shed some light into this. Btw, great explanation so far :thumbsup:
I too felt the same doubt as u earlier..but do read vince123123 post..he made it even clearer now..

That is because as explained so well by the TS, your compact cameras typically have lenses with very short focal lengths to maintain the same FOV as the SLR sized sensors. With short focal lengths come big DOFs.
 

Jan 14, 2005
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#16
Thank you, guys! I'm just sharing something that I learnt. I too found it confusing last time until I sat down to read and understand about it.

Hope it helps.

BC
 

Apr 12, 2005
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#17
Nice post.

I think last time you also posted those pictures before to explain CoC and why sensor size is one of the factor affecting DOF.
 

kelccm

Senior Member
Mar 2, 2004
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#18
It is great that you are willing to share your knowledge and spend the time and effort to present it. A good read.:thumbsup:
 

Apr 12, 2005
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#19
Illustration 1 : DOF effect of sensor size [same focal length and same shooting distance (i.e. same perspective)]



DOF is reduced by the smaller sensor size by a factor of 1.5 (which is the relative size of the sensor).

4.25/1.5 = 2.8 ft (very close to the the 2.76 ft above).
 

Apr 12, 2005
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#20
Illustration 2 : DOF effect of focal length [Same sensor size, same shooting distance and same aperture F number]



With a 1.5x longer focal length, the DOF is reduced by a factor of (1.5)^2 when the focal length is increased by a factor of 1.5x.

4.25ft/[(1.5)^2] = 1.9 ft (very close to 1.81 ft shown above).

Note that the diameter of the lens diaphragm is now :
50mm @F/5.6 = 50/5.6 = 8.93mm
75mm @F/5.6 = 75/5.6 = (50 x 1.5)/5.6 = 8.93mm x 1.5 (meaning that the opening is 1.5^2 = 2.25x bigger in terms of area).

The equivalent DOF is therefore :


5.6/(1.5)^2 = 2.5

DOF @F/5.6 for 75mm = DOF @F/2.5 for 50mm
 

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