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11-22-2009, 06:40 AM   #1
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DOF affected by focal length ?

This is a seperate issue to the interesting discussion going on about DOF in the Pentax DSLR Discussion Forum, and so I am creating a new thread here.

Many of us have seen links such as this one: DOF2 which argue that a longer focal length does not inherrently have a narrower DOF for any given aperture than wider lenses.

The pictures in the article seem to prove it, and the explanation that longer FL squeeze perspective and so give the appearance (and only appearance) of greater DOF seems a logical one.

However, I have been giving this some thought and two things strike me.

1. On my lenses which have a DOF scale I can clearly see that the Pentax technicians considered the wider FL lenses to have a greater DOF than longer ones.

2. The test in the above link is possibly fundamentally flawed. To conduct the test, the camera had to be continually moved further away from the "gremlin" as a longer focal length was used. This was necessary to keep the gremlin the same size.
Now my own knowlege tells me that the further away from a subject I am, the greater the DOF for a given aperture. So, when the test reveals the result of identical DOF for all the shots, can it not be interpreted that there is an inherrent narrower DOF in a longer lens: ie that each greater camera to subject distance (increased DOF) was negated by the use of a longer FL (decreased DOF) ?

Is it relevant anyway, we all know that a longer lens throws the background out of focus in a more pronounced way, does it matter why ?

11-22-2009, 07:22 AM   #2
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QuoteOriginally posted by pschlute Quote
This is a seperate issue to the interesting discussion going on about DOF in the Pentax DSLR Discussion Forum, and so I am creating a new thread here.

Many of us have seen links such as this one: DOF2 which argue that a longer focal length does not inherrently have a narrower DOF for any given aperture than wider lenses.

The pictures in the article seem to prove it, and the explanation that longer FL squeeze perspective and so give the appearance (and only appearance) of greater DOF seems a logical one.

However, I have been giving this some thought and two things strike me.

1. On my lenses which have a DOF scale I can clearly see that the Pentax technicians considered the wider FL lenses to have a greater DOF than longer ones.

2. The test in the above link is possibly fundamentally flawed. To conduct the test, the camera had to be continually moved further away from the "gremlin" as a longer focal length was used. This was necessary to keep the gremlin the same size.
Now my own knowlege tells me that the further away from a subject I am, the greater the DOF for a given aperture. So, when the test reveals the result of identical DOF for all the shots, can it not be interpreted that there is an inherrent narrower DOF in a longer lens: ie that each greater camera to subject distance (increased DOF) was negated by the use of a longer FL (decreased DOF) ?

Is it relevant anyway, we all know that a longer lens throws the background out of focus in a more pronounced way, does it matter why ?
There is a difference between "Subject Isolation" and DOF. Perspective effects are a useful way to isolate a subject from its background (or foreground), but they differ in a fundamental way from what's formally thought of as DOF.

Here's an edited clip from the link you provided emphasizing an aspect of the difference.


If one goes through the detailed maths on the subject, one finds that the diameter of an out-of-focus disk behind the in-focus plane increases in proportion to the distance behind *times* the apparent size reduction due to perspective. An out-of-focus point of light a certain distance behind a subject appears larger with a long lens hence it appears "more-out-of-focus".

A secondary effect of the longer lens is that the field behind the subject is greater so probably has more distractions in view.


Dave in Iowa

PS note that foreground items appear more out-of-focus with shorter lenses at constant subject size in the frame (ie. the situation is reversed.... look at the bunny on the right of the subject.)

Last edited by newarts; 11-22-2009 at 07:37 AM.
11-22-2009, 07:35 AM   #3
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QuoteOriginally posted by pschlute Quote
....
1. On my lenses which have a DOF scale I can clearly see that the Pentax technicians considered the wider FL lenses to have a greater DOF than longer ones.....
QuoteOriginally posted by from my earlier post about the math:
..Here's a useful form of standard theory for non-macro small DOF for a display viewed at a constant distance:

DOF=2*Display_Pixel_width*(Field_width/Display_width)*F-number*(Field_width/Sensor_width)
The last term is the important one here.

(1) If you stand at a fixed distance from the subject and use a shorter lens, the field (and DOF) will be wider.

(2) If you use a longer lens and move back so the width of the subject in the viewfinder is the same, there is no change in field width so DOF doesn't change.

The marks on your lenses take this into account. To have the same width of field in focus with the shorter lens, you must stand closer to the subject...this is the origin of the differences in the marks.

Dave in Iowa
11-22-2009, 08:54 AM   #4
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Depth of field is determined by reproduction ratio and aperture.
Focal length is not really a determinant.

11-22-2009, 09:10 AM   #5
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Which element would give you more out of focus
1. longer lense
or
2. distance
assuming the subject, background, foreground appears the same in both frame
11-22-2009, 09:28 AM   #6
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QuoteOriginally posted by rustynail925 Quote
Which element would give you more out of focus
1. longer lense
or
2. distance
assuming the subject, background, foreground appears the same in both frame

These two alternatives are coupled. You can't keep the subject at the same size in the frame for a different focal length unless you also change distance.

A longer lens/greater distance often allows you to compose so the background is less distracting but doesn't change the DOF.

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11-22-2009, 09:46 AM   #7
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QuoteOriginally posted by rustynail925 Quote
Which element would give you more out of focus
1. longer lense
or
2. distance
assuming the subject, background, foreground appears the same in both frame
You don't get to have this in the real world, so it's pointless to imagine it.
11-22-2009, 11:11 AM   #8
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You dont get what i mean. What i mean is
1 longer lense at a longer distance
2 short lense at shorter distance

11-22-2009, 11:23 AM   #9
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QuoteOriginally posted by rustynail925 Quote
You dont get what i mean. What i mean is
1 longer lense at a longer distance
2 short lense at shorter distance
OK.

If the subject is the same size in the viewfinder (reproduction ratio) and the aperture is the same, depth of field will also be the same.
What will change is the amount of background that is visible (and this might mask the fact that DOF hasn't changed).
11-22-2009, 11:27 AM   #10
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For example you are going to take 2 pictures both at the same aperture at different focal length but you have to have the same result.
1 first is the shorter lens like 35mm - so when you have a shorter lense in order for you to achieve that frame you have to come closer to the subject.
or
2 second is the longer lens like 70mm - then you have to move away from the subject to achieve that frame you want to capture

a 35mm f2.8 and a 70mm f2.8 can both get this kind of framing by moving away or coming closer to the subject.. Right? so my question is which one would achieve a more out of focus the ist or the 2nd.. ill insert a sample picture here

Last edited by rustynail925; 01-31-2010 at 07:43 AM.
11-22-2009, 12:13 PM   #11
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QuoteOriginally posted by rustynail925 Quote
For example you are going to take 2 pictures both at the same aperture at different focal length but you have to have the same result.....
You cannot get the same result. If you use the longer lens, you'll see less background width & what you do see will be larger.

Look at the Gremlin example above.


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11-22-2009, 12:25 PM   #12
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QuoteOriginally posted by rustynail925 Quote
For example you are going to take 2 pictures both at the same aperture at different focal length but you have to have the same result.
1 first is the shorter lens like 35mm - so when you have a shorter lense in order for you to achieve that frame you have to come closer to the subject.
or
2 second is the longer lens like 70mm - then you have to move away from the subject to achieve that frame you want to capture

a 35mm f2.8 and a 70mm f2.8 can both get this kind of framing by moving away or coming closer to the subject.. Right? so my question is which one would achieve a more out of focus the ist or the 2nd.. ill insert a sample picture here
You've imagined an impossible scenario. If you change focal length, you can keep the same reproduction ratio by moving the camera, and you can keep depth of field the same by maintaining reproduction ratio and aperture, but you cannot keep the results "the same" because you are altering the perspective by moving the camera.
11-22-2009, 01:19 PM   #13
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Newarts and Wheatfield are correct; you will not get the same result, and any "answer" given can not match your question as it is based on bad assumptions.

But what the heck, I can't finish the half bath remodel until my boy wakes up from his nap, so here's my attempt to further confuzzle things...

In order to achieve the same basic framing of your subject going from 35mm to 70mm lens you must change your distance to the subject. The longer lens has a narrower angle of view (roughly 23deg for 70mm vs 44deg for 35mm) so you must back up from the subject in order to get the same amount of that subject in frame.

Here's the important part: When you change your distance to subject you change the perspective. Perspective defined as the appearance of objects, relative to each other determined by their individual distances to the observer - in this case the camera's image frame.

But if we throw caution to the wind and ignore reality for minute ...

Assuming the lens specs are perfectly accurate, to get the "same framing" of your subject with a 70mm lens that you have with a 35mm lens you would need to double your distance to the subject.

The depth of field (DOF), mathematically speaking, is a different matter.

If we start with the 35mm lens at 10ft from subject and the 70mm lens at 20ft to get the "same frame", both lenses set to f/2.8, on a Pentax APS-C DSLR, the difference in DOF is fairly negligible: 2.30ft for 35mm vs 2.28ft for 70mm. However...
When you stop down to f/16 the difference in DOF becomes quite noticable: 22.2ft for 35mm lens and 14.5ft for the 70mm lens.
Similarly, if you increases the distances to 50ft for the 35mm and 100ft for the 70mm lens you, at f/2.8, you end up with DOF of: 84ft for 35mm and 62ft for 70mm.

The reason for this is as you stop down the lens and/or increase your distance to subject you decrease the differece between the hyperfocal distance and focus distance. And when those two numbers meet you reach nirvana, or "infinite" DOF whichever pleases you more.
11-22-2009, 01:39 PM   #14
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Excellent comments Dave & Wheatfield.
Your answers made understanding this relationship quite easy.

The apparent shallower DoF with longer focal lengths at the same aperture is simply a magnification of the OOF area within a frame taken by a wider FL. So aperture's really the only factor in DoF.

The change in perspective by shifting camera-to-subject distance may be important for the image captured, such as in portraits.
11-23-2009, 01:15 AM   #15
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QuoteOriginally posted by Venturi Quote
If we start with the 35mm lens at 10ft from subject and the 70mm lens at 20ft to get the "same frame", both lenses set to f/2.8, on a Pentax APS-C DSLR, the difference in DOF is fairly negligible: 2.30ft for 35mm vs 2.28ft for 70mm. However...
When you stop down to f/16 the difference in DOF becomes quite noticable: 22.2ft for 35mm lens and 14.5ft for the 70mm lens.
Similarly, if you increases the distances to 50ft for the 35mm and 100ft for the 70mm lens you, at f/2.8, you end up with DOF of: 84ft for 35mm and 62ft for 70mm.

The reason for this is as you stop down the lens and/or increase your distance to subject you decrease the differece between the hyperfocal distance and focus distance. And when those two numbers meet you reach nirvana, or "infinite" DOF whichever pleases you more.
This answers my question on which would give the narrow DOF.. and thx also for the correction
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