How does sensor size produce a shallower DOF?...

[EOSMan]

I understand aperture size, subject to camera distance and focal length and I understand that all of these change compared to APC size sensor in order to get the same FOV. But how does the size of the sensor contribute?

Thanks

--
EOSMan (got print button?)
http://www.pbase.com/eosman

 

[john cornicello]

Sensor size doesn't directly affect depth of field. But with a larger sensor you have to be closer to your subject or use a longer focal length lens to get the same framing. Being closer or using a longer lens has the effect of decreasing DOF.

If you stood at the same place with the two sensors and didn't change lenses the DOF would be the same, but there would be more space around the subject in the full frame image.

John

I understand aperture size, subject to camera distance and focal
length and I understand that all of these change compared to APC
size sensor in order to get the same FOV. But how does the size of
the sensor contribute?

Thanks

--
EOSMan (got print button?)
http://www.pbase.com/eosman

 

[EOSMan]

I only bring this up because of thread like this one (see link).

http://forums.dpreview.com/forums/read.asp?forum=1032&message=20141323

Thanks

I understand aperture size, subject to camera distance and focal
length and I understand that all of these change compared to APC
size sensor in order to get the same FOV. But how does the size of
the sensor contribute?

Thanks

--
EOSMan (got print button?)
http://www.pbase.com/eosman

--
EOSMan (got print button?)
http://www.pbase.com/eosman

 

[dwalby]

In addition to the issues you mentioned, sensor size also affects the DOF because it determines what enlargement ratio you will use to get the final print. The Circle of Confusion value is directly related to how much you blow up the image to get the final print size. Since FF sensors require 1.6x less enlargement to get the same size print, the CoC value used is different from that of a 1.6x sensor.

I don't have the link handy, but do a search on Bob Atkins and DOF on Google and you'll find an excellent article he wrote a long time ago that explains this very well.

If you stood at the same place with the two sensors and didn't
change lenses the DOF would be the same, but there would be more
space around the subject in the full frame image.

John

I understand aperture size, subject to camera distance and focal
length and I understand that all of these change compared to APC
size sensor in order to get the same FOV. But how does the size of
the sensor contribute?

Thanks

--
EOSMan (got print button?)
http://www.pbase.com/eosman

 

[Lee Jay]

More enlargement ratio means smaller CoC for the same blur circle in the final print and that means less DOF.

You heard right. You get less DOF with smaller sensors. It's a linear relationship.

The reason the prevailing wisdom is the opposite of that is the basic assumption used - constant framing. For constant framing, you need to use a shorter focal length on the smaller sensors.

Shorter focal lengths have more DOF and the relationship is (kind of) quadratic. That means it cancels out the linear of the sensor-size term and reverses the effect so that, with constant framing, you get more DOF with smaller sensors, and the effect is (roughly) linear.

The "kind of" and "roughly" above are because the effect is only as described for subject distance

--
Lee Jay
(see profile for equipment)

 

[Yohan Pamudji]

This debate comes up much too often, cluttering up the forums. It's times like these I wish Phil would implement "sticky" threads to put a FAQ at the top of the forums to stop repetitive questions like this.

-Yohan

 

[Roger Krueger]

More enlargement ratio means smaller CoC for the same blur circle
in the final print and that means less DOF.

You heard right. You get less DOF with smaller sensors. It's a
linear relationship.

The smaller CoC and larger degree of enlargement exactly cancel each other out. DOF at the sensor/film plane is a dumb, confusing concept anyway--DOF on identical 8x12/8x10 prints is a lot more useful.

 

[Lee Jay]

The smaller CoC and larger degree of enlargement exactly cancel
each other out.

They smaller CoC is caused by the increased enlargement. And that's why smaller sensors have less DOF at the same focal length - smaller CoC.

5D, 100mm, f2.8, 10 feet, DOF = 0.5 feet
20D, 100mm, f2.8, 10 feet, DOF = 0.32 feet
Pro 1, 100mm, f2.8, 10 feet, DOF = 0.13 feet
S3, 100mm, f2.8, 10 feet, DOF = 0.08 feet

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

--
Lee Jay
(see profile for equipment)

 

[Roger Krueger]

Of course, I don't know what I was thinking.

No more posting right after a big lunch for me.

 

[Lee Jay]

;-)

--
Lee Jay
(see profile for equipment)

 

[Olga Johnson]

...even if repeated, is a lot more useful than a lot of the other threads that are thrown around in these forums.

Olga

 

[thw]

but it's too complicated to remember when shooting.

Guess I will just point and shoot.

-------------------------------------------
See the colors of my world in:
thw.smugmug.com

 

[PhotoTom]

More enlargement ratio means smaller CoC for the same blur circle
in the final print and that means less DOF.

You heard right. You get less DOF with smaller sensors. It's a
linear relationship.

The reason the prevailing wisdom is the opposite of that is the
basic assumption used - constant framing. For constant framing,
you need to use a shorter focal length on the smaller sensors.

Indeed, constant framing is the key.

Shorter focal lengths have more DOF and the relationship is (kind
of) quadratic. That means it cancels out the linear of the
sensor-size term and reverses the effect so that, with constant
framing, you get more DOF with smaller sensors, and the effect is
(roughly) linear.

The "kind of" and "roughly" above are because the effect is only as
described for subject distance
not true, then the effect is non-linear.

--
Lee Jay
(see profile for equipment)

My crude comparison with "almost" constant framing between a 5D at 155 mm and a 30D at 100 mm from the same shooting location (constant framing and constant perspective) and same f/2.8 aperture:



--
Tom

 

[EOSMan]

and not directly related by the 5D FF sensor. Only indirectly related to the 5D FF sensor because of the increase focal length to get the same FOV. Right?

More enlargement ratio means smaller CoC for the same blur circle
in the final print and that means less DOF.

You heard right. You get less DOF with smaller sensors. It's a
linear relationship.

The reason the prevailing wisdom is the opposite of that is the
basic assumption used - constant framing. For constant framing,
you need to use a shorter focal length on the smaller sensors.

Indeed, constant framing is the key.

Shorter focal lengths have more DOF and the relationship is (kind
of) quadratic. That means it cancels out the linear of the
sensor-size term and reverses the effect so that, with constant
framing, you get more DOF with smaller sensors, and the effect is
(roughly) linear.

The "kind of" and "roughly" above are because the effect is only as
described for subject distance
not true, then the effect is non-linear.

--
Lee Jay
(see profile for equipment)

My crude comparison with "almost" constant framing between a 5D at
155 mm and a 30D at 100 mm from the same shooting location
(constant framing and constant perspective) and same f/2.8 aperture:



--
Tom

--
EOSMan (got print button?)
http://www.pbase.com/eosman

 

[David J. Littleboy]

and not directly related by the 5D FF sensor. Only indirectly
related to the 5D FF sensor because of the increase focal length to
get the same FOV. Right?

Not quite; the DOF for the same focal length is directly proportional to the format size. A 50mm lens on an FF camera will have twice the DOF (at the same f stop at the same subject distance at the same print size) as a 50mm lens on a 4/3 camera. (With a much smaller rendition of the subject.)

This is because the enlargement to get to the final print is smaller for the FF camera. (Remember, DOF is something that happens in the head of the viewer; it's not a physical phenomenon!)

Whereas the DOF for the same sensor size is inversely proportional to the square of the focal length. A 50mm lens has 1/4 the DOF as a 24mm lens on the same format.

Note that both of the above comparisons are a tad bogus, since the viewer will be comparing different images!

But if you multiply (directly proportional to) by (inversely proportional to the square of), you get (inversely proportional to). Which means:

A 50mm lens on FF has 1/2 the DOF of a 24mm lens on a 4/3 camera.

And this comparison is quite valid and sensible, since the viewer is looking at identically framed images (in prints of the same size, of course).

--
David J. Littleboy
Tokyo, Japan

 

[malcolm82]

A 50mm lens on FF has 1/2 the DOF of a 24mm lens on a 4/3 camera.

And this comparison is quite valid and sensible, since the viewer
is looking at identically framed images (in prints of the same
size, of course).

Identically framed but with different apertures. Why should you use a larger aperture just because youre using a larger sensor? It just doesnt make sense you should have to change your shot because of different camera specifications. Its not a logical comparison. DOF depends on field of view and aperture only.

 

[PhotoTom]

and not directly related by the 5D FF sensor. Only indirectly
related to the 5D FF sensor because of the increase focal length to
get the same FOV. Right?

--
EOSMan (got print button?)
http://www.pbase.com/eosman

Two things changed in the comparison - the sensor size (5D vs. 30D) and the focal length. I could have kept the same focal length, but then I would have had to change either the framing or the shooting location.

Changing the framing by keeping the same location and focal length would have made the primary subject (the 2X teleconverter) less significant in the image.

Changing the location to keep the primary subject the same size in the frame would have resulted in a different perspective with a differrence in how the components of the image appear in relation to one another - it would be a different picture.

One can argue from several points of view since perspective, aperture, and focal length all play a part in the DOF equation (as does circle-of-confusion). My choice was to maintain framing and perspective as that's the way I think is the most realistic for photography purposes.

--
Tom

 

[mgates]

Cut the center rectangle out of any print to see the effect of a smaller sensor on what is in focus. There is none.

True if you use lenses with different focal lengths you get different DOF effects. That is true without changing the sensor.

 

[Hellashot]

.. will make the lens look like it was shot on an f4 lens. Taking this to the extreme - this is why there is so very little DOF control on a P&S camera - because so little of the lens is being used that bascially everything is in focus (unless your focus distance is very close to the lens on a P&S)

P&S's are about a 7x crop factor.

 

[mgates]

.. will make the lens look like it was shot on an f4 lens. Taking
this to the extreme - this is why there is so very little DOF
control on a P&S camera - because so little of the lens is being
used that bascially everything is in focus (unless your focus
distance is very close to the lens on a P&S)

P&S's are about a 7x crop factor.

A 2.8 lens produces the same DOF regardless of sensor size. The properties of any lens projection is unchanged by film.

True you cut away some of the image. The discarded outer part of the frame may or may not contain something in focus. It is possible that the focal point might be located in the outer part of the frame. Then the full frame has more in focus then the cropper.