# DOF and Magnification



## chrysoberyl (Jun 19, 2017)

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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## neuroanatomist (Jun 19, 2017)

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 (Jun 19, 2017)

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.


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## neuroanatomist (Jun 19, 2017)

Mikehit said:


> 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 (Jun 19, 2017)

chrysoberyl said:


> 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".


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## chrysoberyl (Jun 19, 2017)

neuroanatomist said:


> 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 (Jun 19, 2017)

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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## chrysoberyl (Jun 19, 2017)

privatebydesign said:


> chrysoberyl said:
> 
> 
> > 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, 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


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## chrysoberyl (Jun 19, 2017)

Mikehit said:


> 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).
> ...



Also very good!


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## chrysoberyl (Jun 19, 2017)

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.


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## Mikehit (Jun 19, 2017)

5% will dome nicely thank you ;D


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## Zeidora (Jun 20, 2017)

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 (Jun 20, 2017)

privatebydesign said:


> ...



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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## neuroanatomist (Jun 20, 2017)

@ 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 (Jun 20, 2017)

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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## hne (Jun 20, 2017)

tolusina said:


> 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.



What PBD did was actually to be concise and comprehensive. For a slightly more elaborate description of why PBD is right, I can recommend you to look at the diagrams and formulae in http://www.trenholm.org/hmmerk/index.html#TIAOOF (although that method is slightly simplified for the thin-lens case and tends to break down at macro distances, which is however properly noted on page 15)


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## dak723 (Jun 20, 2017)

tolusina said:


> 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),_".



This is not what PBD was saying or implying, as far as I know. The aperture diameter is used in these types of calculations rather than the f-stop because equal f-stop numbers on FF, crop, m4/3rds cameras, etc. do not have the same aperture diameter. It is the actual physical size of the aperture diameter that is important in calculating DOF.


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## neuroanatomist (Jun 20, 2017)

tolusina said:


> Please now explain how "magnification “_is the very basis of dof_”" I am unable to grasp the concept.



Perhaps you should read PBD's entire post, where he clearly states, "DOF is the sum of magnification (as viewed at output) and the size of the aperture (not the f stop)..." His initial statement that 'magnification is very the basis of DoF' was obviously a direct response to the OP's apparent inability to find an online DoF calculator that considers magnification. PBD's response was, quite correctly, that they all consider magnification (although in this case, the OP was talking about a different use of the term, which was why I asked the initial clarifying question that I posted). 

PBD's post was correct and concise. If you still believe it has 'multiple errors' and 'falsehoods', then it's clear you are failing to understand the relevant concepts. Frankly, your initial response exemplified a complete failure of understanding, combined with examples and statements that failed to refute anything PBD stated (e.g. "_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 fact that DoF isn't particularly relevant for a flat subject does not in any way mean that the image doesn't have DoF or that said DoF isn't determined by magnification and aperture diameter. It merely means that the flat subject is entirely within the DoF, so there's no point in discussing DoF.).



tolusina said:


> 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),_".



He implied nothing of the sort. He stated that the relevant factors underlying DoF calculations are [magnification and aperture size], not [magnification and f-stop]. The size of the aperture is a physical measurement (or if you prefer, an estimate of an optical measurement, but regardless, it's a value with a length-based unit). F-stop is a ratio of two measurements (focal length and aperture diameter), and sicne focal length is already incorporated into the magnification term, it should not also be incorporated into the aperture term. Of course, you can factor focal length out of magnification and use f-stop instead – algebra is good that way.


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