June 29, 2017, 08:50:20 AM

Author Topic: DOF and Magnification  (Read 1474 times)

chrysoberyl

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DOF and Magnification
« on: June 19, 2017, 10:42:38 AM »
I am considering buying the Sigma 135mm Art and want to compare DOF to my Milvus 100mm Makro.  I am aware of various DOF calculators, but I have yet to find one that considers magnification.  Is there such a thing?

Thanks!
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DOF and Magnification
« on: June 19, 2017, 10:42:38 AM »

neuroanatomist

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Re: DOF and Magnification
« Reply #1 on: June 19, 2017, 10:52:35 AM »
What do you mean by 'considers magnification'?  This one allow you to specify the pupil magnification, if that's what you mean. 
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Mikehit

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Re: DOF and Magnification
« Reply #2 on: June 19, 2017, 11:10:18 AM »
Also you need to bear in mind the minimal focal distance of the lens in question. If the Sigma can only focus at twice the distance of the Milvus (I have no idea if that is the case) then the extra focal length of the lens counts for little.
To add to the mix, lens focal lengths are based on a subject at infinity and at minimum focal distance many lenses are effectively shorter than this label suggests.

neuroanatomist

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Re: DOF and Magnification
« Reply #3 on: June 19, 2017, 11:12:03 AM »
To add to the mix, lens focal lengths are based on a subject at infinity and at minimum focal distance many lenses are effectively shorter than this label suggests.

+1 – for example, Canon's 100mm f/2.8L Macro IS is actually ~68mm when focused at 1:1.
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privatebydesign

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Re: DOF and Magnification
« Reply #4 on: June 19, 2017, 11:43:42 AM »
I am considering buying the Sigma 135mm Art and want to compare DOF to my Milvus 100mm Makro.  I am aware of various DOF calculators, but I have yet to find one that considers magnification.  Is there such a thing?

Thanks!

All dof calculators consider magnification, that is the very basis of dof! When they ask for sensor size, focal length and subject distance they are converting that to a magnification figure.

DOF is the sum of magnification (as viewed at output) and the size of the aperture (not the f stop), the definition of dof is subjective and is what you, personally, consider "acceptably sharp".
Too often we lose sight of the fact that photography is about capturing light, if we have the ability to take control of that light then we grow exponentially as photographers. More often than not the image is not about lens speed, sensor size, DR, MP's or AF, it is about the light.

chrysoberyl

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Re: DOF and Magnification
« Reply #5 on: June 19, 2017, 12:01:29 PM »
What do you mean by 'considers magnification'?  This one allow you to specify the pupil magnification, if that's what you mean.

By magnification, I mean the 0.5X for the Zeiss and 0.2X for the Sigma.  So, next question, where does one find pupil magnification?  And the approximate distance I have in mind is 2 meters, so where do I find actual focal lengths for that distance?

Or I can just buy the lens and see if I like it...but no, I want to learn, too.

I appreciate the information!

John
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Mikehit

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Re: DOF and Magnification
« Reply #6 on: June 19, 2017, 12:44:48 PM »
The 0.5 and 0.2 magnifications you quote arise from two things: the focal length (magnification proportional to focal length) and minimum focal distance (magnification is inversely proportional to the distance to the subject).

The 135mm Sigma will give you 1.35x bigger magnification than the Milvus at the same distance, but the Milvus is designed to let you get as close as 25cm opposed to the Sigma's 85cm. The closer distance of the Milvus more than offsets the   longer focal length of the Sigma.
The magnification as I understand it is the ratio of the distance from the pupil to the subject compared to the distance from the pupil to the sensor. They have already done the maths for you in those two numbers (0.2 and 0.5).

At 2m, the 0.2 and 0.5 are irrelevant and it will come down to a ratio of the focal lengths. The slightly different pupil positions will be so small you won't notice it.

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Re: DOF and Magnification
« Reply #6 on: June 19, 2017, 12:44:48 PM »

chrysoberyl

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Re: DOF and Magnification
« Reply #7 on: June 19, 2017, 01:05:50 PM »
I am considering buying the Sigma 135mm Art and want to compare DOF to my Milvus 100mm Makro.  I am aware of various DOF calculators, but I have yet to find one that considers magnification.  Is there such a thing?

Thanks!

All dof calculators consider magnification, that is the very basis of dof! When they ask for sensor size, focal length and subject distance they are converting that to a magnification figure.

DOF is the sum of magnification (as viewed at output) and the size of the aperture (not the f stop), the definition of dof is subjective and is what you, personally, consider "acceptably sharp".

Thanks, that means I should not buy the Sigma because it would have greater DOF at ~2 meters - it won't.  With the same framing, it will be equal.

John
6D, 80D, Zeiss Milvus 100mm Makro, Sigma 180mm/2.8 macro, Canon 100mm macro, Canon 24mm/1.4, Sigma 35/1.4, Tokina 16-28, Canon 1.4X III TC, Canon 2X III TC, Rokinon 14mm 2.4.

chrysoberyl

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Re: DOF and Magnification
« Reply #8 on: June 19, 2017, 01:06:37 PM »
The 0.5 and 0.2 magnifications you quote arise from two things: the focal length (magnification proportional to focal length) and minimum focal distance (magnification is inversely proportional to the distance to the subject).

The 135mm Sigma will give you 1.35x bigger magnification than the Milvus at the same distance, but the Milvus is designed to let you get as close as 25cm opposed to the Sigma's 85cm. The closer distance of the Milvus more than offsets the   longer focal length of the Sigma.
The magnification as I understand it is the ratio of the distance from the pupil to the subject compared to the distance from the pupil to the sensor. They have already done the maths for you in those two numbers (0.2 and 0.5).

At 2m, the 0.2 and 0.5 are irrelevant and it will come down to a ratio of the focal lengths. The slightly different pupil positions will be so small you won't notice it.

Also very good!
6D, 80D, Zeiss Milvus 100mm Makro, Sigma 180mm/2.8 macro, Canon 100mm macro, Canon 24mm/1.4, Sigma 35/1.4, Tokina 16-28, Canon 1.4X III TC, Canon 2X III TC, Rokinon 14mm 2.4.

chrysoberyl

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Re: DOF and Magnification
« Reply #9 on: June 19, 2017, 01:09:11 PM »
Thanks, all.  You just saved me $1400!

But not really, I still want this lens for portraiture/AF.  Plus, it is sharper than the Zeiss.
6D, 80D, Zeiss Milvus 100mm Makro, Sigma 180mm/2.8 macro, Canon 100mm macro, Canon 24mm/1.4, Sigma 35/1.4, Tokina 16-28, Canon 1.4X III TC, Canon 2X III TC, Rokinon 14mm 2.4.

Mikehit

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Re: DOF and Magnification
« Reply #10 on: June 19, 2017, 01:59:26 PM »
5% will dome nicely thank you  ;D

Zeidora

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Re: DOF and Magnification
« Reply #11 on: June 20, 2017, 01:05:16 AM »
Pupil magnification or pupil factor (I assume that is what neuro was alluding to) comes into play in the context of effective f-stop. Has nothing to do with magnification of the subject.

For a more complete discussion (also on pupil position), see e.g. Savazzi's book
https://www.amazon.com/Digital-photography-science-paperback-Savazzi/dp/0557925371
Pretty good read.

Fortunately, most lenses (except for micronikkors and a few others) do not adjust f-stop as you focus closer, so effective f-stop is the usual f-stop-set x magnification+1. DOF is standard function of magnification and effective f-stop at given circle of confusion value. Notice that focal length is not part of DOF calculation. Pupil factor may enter the DOF equation as it affects effective f-stop.
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tolusina

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Re: DOF and Magnification
« Reply #12 on: June 20, 2017, 02:33:36 AM »
...

 Sample images hot linked from http://www.learn.usa.canon.com

 
Magnification;

f16, 100mm
Depth of field is not a significant point of discussion in this photo as the entire scene is only the thickness of the quarter deep.
The scene is magnified.

 
- - -
Deep, natural depth of field;

EXIF removed, 16-35mm is part of the file name.

 
- - -
Shallow, un-natural depth of field, “looky what my gear can do, look what I do with gear”. No EXIF

The shallow depth of field emphasizes the relatively more magnified dandelions in the foreground, de-emphasizes the (distracting?) aeroplane in the background.
- - -
 Certainly, as with almost every other technical parameter of almost every photograph, there is interaction between magnification and depth of field. The 'balance' of this particular interaction is controlled by the photographer's aperture selection.
But, to state that magnification “is the very basis of dof” is not only misleading in its incompleteness, it is WRONG.
Also, when did “size of the aperture” become “(not the f stop)”?
- - -
Apologies for the distraction of highlighting PBD's multiple errors, but allowing such falsehoods unchallenged leads directly to the currently too common 'fake news syndrome'.

 

 

 
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Re: DOF and Magnification
« Reply #12 on: June 20, 2017, 02:33:36 AM »

neuroanatomist

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Re: DOF and Magnification
« Reply #13 on: June 20, 2017, 06:26:14 AM »
@ tolusina – go out and take pretty pictures, use DoF how you want and don't worry about this technical stuff.

@ PBD – I wouldn't bother.  If you drop a bowling ball, it will fall to the ground.  But if you try to explain the concepts and mathematics that underlie gravity to that bowling ball, you'll only frustrate yourself.  The bowling ball won't ever understand.
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tolusina

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Re: DOF and Magnification
« Reply #14 on: June 20, 2017, 10:55:25 AM »
John,
Take any adjustable camera of your choice with stop down preview, any lens, compose any scene, focus on whatever object you desire at any distance.
Next, activate the stop down preview while adjusting the aperture or f-stop, (whichever of those apparently different parameters you use), depth of field increases with f-number, magnification does not change at all.
Please now explain how "magnification “is the very basis of dof" I am unable to grasp the concept.

Also, while I have a scientist's attention, please explain how it is that when f-stop is changed, the aperture does not change as implied in PBD's statement " size of the aperture (not the f stop),".

Please, be concise and comprehensive rather than ambiguous in incompleteness as PBD has done. Links to reputable and falsifiable sources are fine.

Thank you for your consideration.
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Re: DOF and Magnification
« Reply #14 on: June 20, 2017, 10:55:25 AM »