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Depth of field (DOF) describes the amount of distance from the camera sensor) that the object appears sharp
A shallow/small DOF means only objects very close to the actual point of focus will appear sharp in the picture.
A very large DOF means almost every object int he picture is sharp

DOF is one of the most important topic for photography. Instead of using hard to understand mathematical formulas to explain how different factors can affect DOF, I'll try to show you some example pictures instead.

The DOF is determined by three factors

1. the subject distance
2. the lens f-number (aperture size).
3. the lens focal length



Subject Distance
Pic A: Object very close


Pic B: Object further away


Close Object Distance = small DOF
Far Object Distance = large DOF


Aperture Size

Pic C: f/1.4


Pic D: f/5.6


Pic E: f/16


Large Aperture (small F number) = small DOF
Small Aperture (large F number) = large DOF


Focal Length

These two pictures were took from the same distance, with the same aperture and both focused on

Pic F: 50mm


Pic G: 85mm


To make it easier to see the difference, here are two center crops:

Pic H: Center Crop of pic F


Pic I: Center Crop of pic G


Longer Focal Lengh = small DOF
Shorter Focal Lengh = large DOF


What affects the DOF?
From the example pictures above, you can see that you will have a larger DOF if you either:
1. use a smaller aperture
2. use a shorter focal length
3. the object is further away.




So, does Cropped Sensor (DX) camera give you more DOF compare to a ull frame (FX)camera ?

The answer is no and yes

Answer is NO because a smaller sensor by itself won't suddenly change the DOF.

Answer is YES because if you are using a camera with smaller cropped sensor and you want to take a photo that is similar to one from a full frame sensor, you'll need to either stand further away or use a wider lens (shorter focal length), both of these will give you a bigger DOF as we've already seen above


Why do we need to stand further away with a small cropped sensor?

This is a picture took with a full frame camera, the whole object is just within the picture:
Pic J:


Now, if we switch to a cropped sensor camera, and take another picture again at the same position with the same lens:
Pic K:


Because of the cropped sensor, which cuts off the edges, now part of the object goes outside the picture. Please notice that if you compare pic J with Pic K, the depth of field from these two pics are exactly the same

To get the whole object inside the picture again with the same lens and focal length, you'll have to move a bit further back or use a wider lens.


So here are two comparison photo, both took with a 50mm lens at f/1.4.

The first photo was taken with a full frame camera: (distance to object approx 75cm)


And the 2nd photo was taken with a cropped sensor camera with the same 50mm lens at f/1.4: (distance to object approx 135cm)


Both photos were took with the same lens at same aperture, and you can see the photo took with a DX lens has a larger DOF.

Isn't it great that the D700 can be a FX and DX at the same time?

Nice write up. Its easier to get a larger DOF than it is to get a smaller DOF. Thanks!!!

Thx Yellow~~~~~~~


GREAT article~~~

much more easy to understand now....

Thx alot Thx alot~~

Is there any way to activate all 11 AF points on my camera at the same time?

go to menu 02 "AF-Area Mode", set to Auto Area AF and the camera will pick one of the 11 AF point for you.

(09-30-2009 01:35 PM)yellow15 Wrote: [ -> ]go to menu 02 "AF-Area Mode", set to Auto Area AF and the camera will pick one of the 11 AF point for you.

Thanks, yellow15, I did know that.
What I actually meant - is it possible to focus on all 11 points at the same time.

how do you focus on all 11 points? unless the objects at the 11 AF points are all at the same distance from the camera
you can only focus on 1 distance so in theory you can only focus on 1 AF point.

I see...
Once I read a lens review on a Canon body and one of the images claimed it was foucused on all points, I think.

(09-30-2009 05:37 PM)ArcspacE Wrote: [ -> ]I see...
Once I read a lens review on a Canon body and one of the images claimed it was foucused on all points, I think.

Which means that the subject cold have been large enough to cover all 11 a/f points. The subject would also need to be flat or on the same focal plane.

(09-30-2009 09:18 PM)CTM Wrote: [ -> ]

(09-30-2009 05:37 PM)ArcspacE Wrote: [ -> ]I see...
Once I read a lens review on a Canon body and one of the images claimed it was foucused on all points, I think.

Which means that the subject cold have been large enough to cover all 11 a/f points. The subject would also need to be flat or on the same focal plane.

Yeah, that's what I thought as well although the image was at an angle.
Anyways, I could have also read it wrong.
Sorry, I'm still learning every day.