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01-12-2013, 06:33 PM   #91
cak
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Hello people,
Although I have been a forum member for a while, I don't post very often, but because this has been such a fascinating discussion I thought I might chime in.
At first when discussing DOF from a given lens I wasn't grasping the concept that it could be different given different camera formats. After reading up a little regarding circles of confusion I decided to create some visual aids to help comprehend what's going on when a given lens is swapped between camera/sensor formats.
Utilizing my CAD background I came up with the following sketches:




I just a simple guy, so visualizing things with sketches helps. I hope my sketches are useful to others
other sketches to follow.
CAK


Last edited by cak; 01-12-2013 at 06:36 PM. Reason: to swap image for link
01-12-2013, 07:05 PM   #92
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My next step was to then go to the Depth of Field Calculator at: "cambridge in colour (dot) com" to look up specific DOF data regarding an average lens, lets say a 50mm @ f4.0 (I chose f4.0 because the difference between sensor formats was more dramatic than say f1.4 for the purpose of the diagrams I was drawing.

My first sketch shows Depth of Field for a 50mm lens @f4.0 focused at 20ft. if on a full frame sensor (36x24mm)


My second sketch shows the the same 50mm lens @f4.0 focused at 20ft. on an APS-C (24x16mm) sensor:


The following sketch shows that in order to obtain a similar image, with both similar field of view as well as depth of field, without changing vantage point that you will have to change both the lens and aperture.


Again, the data depicted in my sketches was obtained from the DOF calculator found at: A Flexible Depth of Field Calculator
I hope some of you find my sketches helpful
Sincerely,
Christopher A Kirk
01-12-2013, 07:11 PM   #93
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QuoteOriginally posted by Fogel70 Quote
Yep, but it is possible to create a "inverted TC" to make a FF lens into a APS-C lens, so that a APS-C sensor will capture all the light that is used for FF. And this double the light per unit area on the APS-C sensor.
One of the very fast leica lenses work a bit like that i believe...
01-12-2013, 07:13 PM   #94
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QuoteOriginally posted by cak Quote

Again, the data depicted in my sketches was obtained from the DOF calculator found at: A Flexible Depth of Field Calculator
Be careful though this one like most others don't take pupil magnification into account so they arent always correct.

01-12-2013, 07:44 PM   #95
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Depth of Field is a rather subjective topic to begin with as a person's visual acuity will vary so much between individuals. I was just using the Cambridge charts as a baseline for observing the differences in DOF from format to format.
01-12-2013, 08:08 PM   #96
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QuoteOriginally posted by Anvh Quote
Be careful though this one like most others don't take pupil magnification into account so they arent always correct.
I don't use these charts when I shoot anyway, they are far too anal for me. I usually just stop the lens down and if I like what I see I push the button.

Last edited by cak; 01-12-2013 at 08:10 PM. Reason: spelling error
01-13-2013, 12:23 AM   #97
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QuoteOriginally posted by Anvh Quote
Be careful though this one like most others don't take pupil magnification into account so they arent always correct.
It does if you click on "show advanced" - you can set print size, viewing distance and eyesight.
01-13-2013, 03:26 AM   #98
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QuoteOriginally posted by cak Quote
I don't use these charts when I shoot anyway, they are far too anal for me. I usually just stop the lens down and if I like what I see I push the button.
If we admit it, most of us shoot in this way. Where's the time and patience to sit down and calculate DoF for each shot being taken? Experience and scene visualisation gives us a good estimation to getting the intended DoF for each shot. Essentially, recognising that DoF diminishes from the point of focus by both decreasing subject-to-camera distance as well as enlarging aperture helps us to get results not far from our intended DoF.

01-13-2013, 05:42 AM   #99
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QuoteOriginally posted by m42man Quote
It does if you click on "show advanced" - you can set print size, viewing distance and eyesight.
uh yeah... but that isn't pupil magnification....
01-13-2013, 05:43 AM   #100
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QuoteOriginally posted by cak Quote
I don't use these charts when I shoot anyway, they are far too anal for me. I usually just stop the lens down and if I like what I see I push the button.
Sure, in the field i do the same but if we are discussing it however then the more correct you're the better right?
01-13-2013, 12:27 PM   #101
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According to Wikipedia:
The pupil magnification of an optical system is the ratio of the diameter of the exit pupil to the diameter of the entrance pupil. The pupil magnification is used in calculations of the effective f-number, which affects a number of important elements related to optics, such as exposure, diffraction, and depth of field. For all symmetric lenses, and for many conventional photographic lenses, the pupils appear the same size and so the pupil magnification is approximately 1.
01-13-2013, 12:46 PM   #102
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I have prepared yet another comparison sketch for everyone's amusement. This one shows The DOF region with a 50mm lens @f4.0 on a full frame camera, and when that lens is swapped onto an APS-C camera that stepping back by approximately 50% will yield a similar sized object on your plane of focus. However your background will be narrower and the foreground will be broader. Also the DOF will be increased so in order to achieve a "similar" DOF you will need to open the aperture to f2.8.



Again I am only using the data obtained from the DOF calculator as a baseline for comparing the effective DOF regions created by a given lens on various formats of sensors. The actual specific depth of field distances is not really relevant, it is the relationships of the regions that I am trying to depict.

Sincerely,
C. A. Kirk
01-13-2013, 02:31 PM   #103
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QuoteOriginally posted by cak Quote
According to Wikipedia:
The pupil magnification of an optical system is the ratio of the diameter of the exit pupil to the diameter of the entrance pupil. The pupil magnification is used in calculations of the effective f-number, which affects a number of important elements related to optics, such as exposure, diffraction, and depth of field. For all symmetric lenses, and for many conventional photographic lenses, the pupils appear the same size and so the pupil magnification is approximately 1.
Yeah but that is wikipedia....
Indeed for symmetrical lenses the pupil magnification is 1:1 but... not for conventional photographic lenses.
With 28mm lens the pupil magnification is very different then with a 200mm lens.
It's true that the differnce the pupil magnification gets less the further away you focus, so it's not that intersting for most type of photography but still if you want to be 100% correct it's a thing to include though.
01-13-2013, 04:11 PM   #104
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Can you show us a practical application for pupil magnification?
01-13-2013, 06:41 PM   #105
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It's like you're asking for a practical application for focal length...

pupil magnification is simply result of the magnification that happens to the image inside the lens, by this they can make long lenses shorter so that you get a Telephoto lens,
The reverse is used with lenses wider then register distance, they are called Angénieux retrofocus lens.

That's not really practical application of pupil magnification but more a by effect.

You can see it in macro images actually the effect of the pupil magnification.
The Pentax 35mm macro has a wider DOF as for example the 100mm or the 50mm macro at 1:1 magnification
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