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Author Topic: The mysteries of Depth of Field  (Read 6054 times)
nordattack
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« on: August 02, 2011, 07:23:28 PM »
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Can someone please explain the mysteries of Depth of Field and how this relates to lens focal length and sensor size?

For example:

Micro 4/3rds Sensor
Panasonic 20mm F1.7 lens
The actual focal length on 4/3 is 40mm correct?
But do we double the F stop as well?
Does this lens really have a DOF of F3.4?

Voigtlander 25mm F.95
The actual focal length on 4/3 is 50mm correct?
But do we double the F stop as well?
Does this lens really have a DOF of F1.9?

To get a true DOF of F.95 on 4/3 would the lens have to be a F.475?

What about Medium format 120 film?
A 50mm F1.7 lens
What is the real focal length on 120 film?
Less than 50mm?
What about the DOF?
Is it less than 1.7?
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feppe
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« Reply #1 on: August 02, 2011, 07:41:49 PM »
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This should get you started.
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Slobodan Blagojevic
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« Reply #2 on: August 02, 2011, 08:06:50 PM »
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And this might get you... brain dead (in other words, your head might explode) Smiley
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« Reply #3 on: August 02, 2011, 08:26:06 PM »
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"Micro 4/3rds Sensor
Panasonic 20mm F1.7 lens
The actual focal length on 4/3 is 40mm correct?"

"Voigtlander 25mm F.95
The actual focal length on 4/3 is 50mm correct?"

The focal length does not change , it is stil la 20mm lens . The focal length of a lens and so the depth of field at a given aperture  and camera/lens to subject distance is independent of the format size. But because, in this example the format,  is that much smaller the angle of view recorded by the sensor is comparable to a 40mm lens mounted on a (I assume) a 24x36mm format camera. This may cause you to back up to increase the angle of view taken in by the smaller format, and the increase in camera to subject distance will affect depth of field at a given aperture.

"What about Medium format 120 film?
A 50mm F1.7 lens
What is the real focal length on 120 film?"

Again the focal length is independent of the format size, the focal length remains 50mm. Assuming the lens covers the medium format frame you are workign with ( the most common ones are 6x4.5cm, 6x6cm, 6x7cm, 6x8cm, 6x9cm, 6x12cm, and 6x17cm ) the angle of view recorded within  the aspect ratio you are working with will range from wide to very wide.
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Ellis Vener
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nordattack
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« Reply #4 on: October 22, 2011, 03:12:13 PM »
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Ok, I understand now. Focal length stays the same.
But depth of field is not the same as 35mm.
In the case of the 20mm f1.7 Panasonic lens:
On micro 4/3 sensor it has an angle of view equivalent to a 40mm lens but not depth of field like a 40mm lens.
Its depth of field is not like a 40mm f1.7 on 35mm camera, it is really like a 40mm f3.4 on 35mm correct?
In other words when using the micro 4/3 format with lenses that are giving an angle of view equivalent to a 40mm or 50mm lens the user should not expect to get the same bokeh and narrow depth of field as a 40mm or 50mm lens of the same f number on 35mm.
Is this true?

So a 20mm F1.7 on micro 4/3 gives us:
35mm equivalent angle of view of 40mm focal length.
35mm equivalent light gathering power of F1.7
35mm equivalent Depth of field of 40mm at F3.4.
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PeterAit
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« Reply #5 on: October 22, 2011, 07:13:50 PM »
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A given lens/f stop/focus distance combo has a given depth of field, period, end of story. The sensor has nothing to do with it.

Here's an intuitive way to look at it. Put a (say) 100mm lens on a full frame SLR and set it to f/8. Focus at 20 feet and take a pic. The image has a certain DOF. Now magically replace the full frame sensor with a 4/3 sensor, changing nothing else. Take a pic. Do you think the DOF would change? Of course not - same image projected on a smaller sensor. Same experiment could be done by taking the full frame image and cropping it in PS to 4/3 size. So, the sensor has no DIRECT effect on DOF.

There is, however, an INDIRECT effect. The 4/3 image takes in only about 1/2 the scene of the full frame image. This is why they say "200mm equivalent," but the fact is it is still a 100mm lens. So, to get the same scene with the 4/3 sensor you would have to back away from the subject to about 2x the distance. And, you would have to focus the lens at 40 ft, not 20 ft. And, focusing further from the camera results in greater DOF for any lens. So, smaller sensors give greater DOF INDIRECTLY by forcing the photographer to be further from the subject.
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Peter
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Sheldon N
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« Reply #6 on: October 22, 2011, 10:01:45 PM »
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A given lens/f stop/focus distance combo has a given depth of field, period, end of story.

This is not at the heart of what depth of field is. Depth of field is the perception of which parts of an image appear reasonably sharp in a given set of viewing conditions.  Lenses don't have depth of field, apertures don't have depth of field, etc.

There are so many variables that go into the equation, including underlying assumptions about size of print, viewing distance, estimations about the average visual acuity of the human eye, the degree of magnification from the sensor size, in addition to the standard focal length/distance/aperture that you can't say it's just one thing. You also can't say that a given lens/focal length/distance combo has a given depth of field, because there are many variables that remain flexible.

At it's heart, the "measurement" of depth of field is the attempt at quantifying perception, and that is a difficult thing to nail down in any concrete fashion.
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RFPhotography
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« Reply #7 on: October 23, 2011, 06:35:54 AM »
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That's not correct, Sheldon.  There are very well defined terms for dept of field.

Depth of field is defined as circles of confusion of 1/100" or smaller in an 8x10 print.  Circles of confusion are the individual 'dots' that make up an image caused by the round aperture opening.

Depth of field is also very specific as a product of aperture and magnification.  Magnification is a product of focal length and distance to subject.  There's really nothing subjective about it.

Here's the thing.  When most people talk about DOF, they talk about it at the image plane.  Based on the standard definition noted above, that's not correct.  If you have a given lens at a given aperture and a given magnification DOF will be the same at the image plane.  Doesn't matter if it's 4/3, 1.6 crop, full frame 35mm, 6x6, 4x5 or 8x10.  A 50mm lens on an 8x10 and a 50mm lens on a 35mm next to each other shooting the same subject will have the same DOF at the image plane.  But that's not the determining factor.  The standard is that 8x10 print.  When the print is produced, the 35mm image needs to be enlarged (effectively magnified) more than the large format image.  That effective increase in magnification reduces DOF in the standard print.  That's why smaller formats have shallower DOF than larger. 

Carry that through now to the idea of getting the same image in frame with the smaller format.  What do you need to do to achieve that?  You need to (a) use a shorter focal length lens, (b) move back from the subject, or (c) some combination of the two.  This results in a reduction of magnification and a resultant increase in DOF at the image plane.  When both images are printed to that standard 8x10 print size; however, the image from the smaller sensor will have to be 'enlarged' more.  This effective increase in magnification reduces DOF to the point where DOF in the two prints will be roughly equal. 
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PeterAit
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« Reply #8 on: October 23, 2011, 11:11:12 AM »
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This is not at the heart of what depth of field is. Depth of field is the perception of which parts of an image appear reasonably sharp in a given set of viewing conditions.  Lenses don't have depth of field, apertures don't have depth of field, etc.

There are so many variables that go into the equation, including underlying assumptions about size of print, viewing distance, estimations about the average visual acuity of the human eye, the degree of magnification from the sensor size, in addition to the standard focal length/distance/aperture that you can't say it's just one thing. You also can't say that a given lens/focal length/distance combo has a given depth of field, because there are many variables that remain flexible.

At it's heart, the "measurement" of depth of field is the attempt at quantifying perception, and that is a difficult thing to nail down in any concrete fashion.

Didn't read the question, didja?
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Peter
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Sheldon N
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« Reply #9 on: October 24, 2011, 01:31:56 AM »
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That's not correct, Sheldon.  There are very well defined terms for dept of field.

Depth of field is defined as circles of confusion of 1/100" or smaller in an 8x10 print.  Circles of confusion are the individual 'dots' that make up an image caused by the round aperture opening.

While I agree with all the later portion of your post, the main point I was getting at is this: the above specification you cited not only requires that print size be a fixed absolute, it also is based upon assumptions about visual acuity and viewing distance. In reality people often print larger or smaller than 8x10, view in different ways (nose to print versus angle of view that allows them to see the whole print at once), etc. The math and formulas may be very concrete, but the reality is not. My reply was more of a contradiction to PeterAlt's incorrect pronouncement.

As a side note, I've always thought of it in pretty simple terms. Larger formats require you to use a longer focal length lens to take a given picture, and the net effect is a reduction of depth of field in practical use.

Didn't read the question, didja?

No, I was only replying to your statement. And you didn't read Bob's reply did you?

He indicated (and I agree) that for a given print size (viewing conditions/etc), given focal length, given aperture, and given focus distance, a smaller format will have LESS depth of field than a larger format. So sensor size DOES play into the equation. This is specifically because you have to enlarge the smaller format more to reach the same print size. It's only when you consider the longer focal lengths required by larger formats that you see expected effect of reduced depth of field from larger formats.

Your statement that "A given lens/f stop/focus distance combo has a given depth of field, period, end of story. The sensor has nothing to do with it." is fundamentally wrong.
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stamper
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« Reply #10 on: October 24, 2011, 03:08:12 AM »
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Quote

And, focusing further from the camera results in greater DOF for any lens. So, smaller sensors give greater DOF INDIRECTLY by forcing the photographer to be further from the subject.

Unquote

I don't think so? If you focus at infinity in a landscape scene the the area nearest you wouldn't be sharp? That is why you have to focus about 1/3 of the way into a scene. 2/3 of DOF is behind the subject focused on so at infinity some of the DOF is lost? I assume that DOF refers to the area that is sharp?
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pegelli
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« Reply #11 on: October 24, 2011, 03:17:18 AM »
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He indicated (and I agree) that for a given print size (viewing conditions/etc), given focal length, given aperture, and given focus distance, a smaller format will have LESS depth of field than a larger format. So sensor size DOES play into the equation. This is specifically because you have to enlarge the smaller format more to reach the same print size. It's only when you consider the longer focal lengths required by larger formats that you see expected effect of reduced depth of field from larger formats.


While this is absolutely correct for me I always think of a given scene I want to photograph from the same spot.

In that case for a given print size (viewing conditions/etc), given field of view (so a shorter lens for a smaller sensor/film strip), given aperture, and given focus distance, a smaller format will have MORE depth of field than a larger format.

I think this fact (given lens vs. given field of view) is leading to the confusion on the topic and why some people say you get more dof with a larger format while others say you get less. Both are right, it just depends what your basic assumptions are  Wink
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pieter, aka pegelli
Sheldon N
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« Reply #12 on: October 24, 2011, 11:07:33 AM »
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While this is absolutely correct for me I always think of a given scene I want to photograph from the same spot.

In that case for a given print size (viewing conditions/etc), given field of view (so a shorter lens for a smaller sensor/film strip), given aperture, and given focus distance, a smaller format will have MORE depth of field than a larger format.

I think this fact (given lens vs. given field of view) is leading to the confusion on the topic and why some people say you get more dof with a larger format while others say you get less. Both are right, it just depends what your basic assumptions are  Wink

Exactly. The practical reality is larger formats require longer lenses to take any given picture (fixed FOV). Longer lenses result in shallower depth of field, all else being held constant. So larger formats have shallower depth of field in real world usage.

The only reason I brought up the other example where the smaller sensor had less depth of field was to show that if you hold all other variables constant and only changed sensor size, that depth of field does change. So it's not accurate for someone to say that sensor size has nothing to do with depth of field. 
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pegelli
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« Reply #13 on: October 24, 2011, 12:53:50 PM »
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So it's not accurate for someone to say that sensor size has nothing to do with depth of field. 

Again fully agree, only the circle of confusion (pun intended  Cheesy) on the sensor/film remains constant irrespective of sensor size (everything else being the same, i.e focal length, focus distance, aperture)
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pieter, aka pegelli
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