Lenses focal length

[simsurace]

Added for clarity:
...

I agree with everything you write in your clarification, since it is logically equivalent to what I have written earlier.
The important thing is that something changes between formats, and how that thing scales with the size of the sensor. Whether you call it 'the secondary magnification that is required to produce the same output' or something else does not matter. What matters is the set of predictions derived from the argument, and whether those predictions turn out to be correct. It is only because a digital file loses its physical dimension whereas a negative does not that some people now prefer not to talk of secondary magnification. So they use different words to describe the same fundamental relationships.

Of course many people misunderstand this stuff. But they would likewise misunderstand any other terminology, even more if they never worked with film formats. The misunderstandings won't go away by abolishing a terminology, but hopefully they will diminish by being explained.

Please don't call this debate 'creationist'. First, you are putting people who understand the stuff and those who don't in the same bin just because they are using the same terms, which creates more confusion among those that don't understand it. Secondly, we are engaging in a rational/scientific discussion on a technical topic, which is quite unlike 'creationism'.

I'm also aware that this discussion has very little to do with the artistic aspects of photography, but this is not the point here. My aim is to create clarity about the technical stuff. This discussion relates to photography in the same way that a discussion of chemical reactions relates to culinary arts. It is up to the individual photographer/cook to decide how important it is or whether they want to use these insights in their daily work or to guide their purchasing decision, but the truth or falsehood of it is independent of this individual decision.

[Frank Fremerey]

Thank you Simone.

Now I will extract the original files of the "practical (non)equivalens group shot" from one of my backup disks and make them available to download

[Frank Fremerey]

May I note that the size of the aperture (not the aperture ratio) is larger for the 35/1.4 lens than for the 24/1.4 lens. It's my understanding that when the background is well outside the DoF zone the lens with the larger aperture (not the aperture ratio) will give more background blurring.
How Much Blur: 105/2.8 FX v. 70/2.8 DX v. 70/2.0 DX
How Much Blur: 35/1.4 FX v. 24/1.4 DX
Bokeh and Background Blur - Bob Atkins
Dave

Nono, this is not what I meant. For the group shot I shoot FX at f=11 and DX at f=8 as stated in the theoretical formula given by physics.

My question aims at the:

1) drawing (lens geometry and other (non)corrections)
2) background compression (more tele in 35 than in 24)

[simsurace]

May I note that the size of the aperture (not the aperture ratio) is larger for the 35/1.4 lens than for the 24/1.4 lens. It's my understanding that when the background is well outside the DoF zone the lens with the larger aperture (not the aperture ratio) will give more background blurring.

How Much Blur: 105/2.8 FX v. 70/2.8 DX v. 70/2.0 DX

How Much Blur: 35/1.4 FX v. 24/1.4 DX

Bokeh and Background Blur - Bob Atkins

Dave

What I'm seeing in Frank's comparison are geometric differences, which I would attribute to the lens distortion characteristics and the potentially different position of the entrance pupil between the two shots, which creates a different perspective. Also, the angle of view is only approximately equivalent.

[Frank Fremerey]

Full size.
Full EXIF.
Sorry, both developed in Photo Ninja.
Results would be much better with NX-D.
Instead of f=5.6 (DX) and f=8.0 (FX) I accidentially dialed in 5.6 & 11.

http://fotokontext.de/ZENTRALKRAFT/email_ANP_9908_v1.JPG
http://fotokontext.de/ZENTRALKRAFT/email_DSC_1107_v1.JPG

[Anthony]

Thanks, Simone, I am not so much seeking equivalence, but trying to get a view of what changes with changes in equipment.  Your comments help me.

[Frank Fremerey]

What I'm seeing in Frank's comparison are geometric differences, which I would attribute to the lens distortion characteristics and the potentially different position of the entrance pupil between the two shots, which creates a different perspective. Also, the angle of view is only approximately equivalent.

real world lenses neither have exactly the focal length stated (not even at infinity **ggg**) nor are they perfectly corrected for geometry and all other drawing characteristics.

That is why I write "equivalens" not "equivalence"

The theory is good, but is has not much practical meaning.


For day-to-day work the competent use of perspective is much more important.

One of the first things (just after: tell me where the light sources are) I teach to people coming to me to learn photography basics from me is, that perspective distortion is independent of camera and lens!

So I suggest to keep the same distance (perspective) in portraiture, roughly 2.5 to 3.5 meters and choose focal length just to add or leave out context.

I can easily show the "big nose" and "bended head" effect of a perspective too near to the subjects.

[Frank Fremerey]

difference between the pictures

the magnification (21MP vs. 24 MP) and (35mm and 24mm) and (FX vs. DX) is normalized to the kids in the first row

verdict:
1) I will do the test again under more controlled conditions.
2) From what I see the group is more or less rendered the same modulo subject movement, the BG looks much different though

[Les Olson]

May I note that the size of the aperture (not the aperture ratio) is larger for the 35/1.4 lens than for the 24/1.4 lens. It's my understanding that when the background is well outside the DoF zone the lens with the larger aperture (not the aperture ratio) will give more background blurring.

It is not the aperture, it is the magnification.  The key is to distinguish absolute and relative blur.  Absolute blur is the size of the blur circle of a point on the print or screen, and that, of course, is greater for lenses of longer focal length.  If you normalise for image magnification you have relative blur and that depends on F number.  There is a good example at http://toothwalker.org/optics/dof.html - scroll down to images 3 and 4.

 

[simsurace]

difference between the pictures

the magnification (21MP vs. 24 MP) and (35mm and 24mm) and (FX vs. DX) is normalized to the kids in the first row

verdict:
1) I will do the test again under more controlled conditions.
2) From what I see the group is more or less rendered the same modulo subject movement, the BG looks much different though

The point is that you managed to produce almost the same shot using two different formats. It is possible using the correct settings. There are minor differences but they would not matter to anyone unless they could choose between the two images. You could have done a similar experiment using a cell phone and a medium format system. At some point you will run into limitations, and this is where the right choice of format becomes important, i.e. it actually matters.

[Frank Fremerey]

And: Is "almost the same shot within given limitations" = "equivalence"

[simsurace]

And: Is "almost the same shot within given limitations" = "equivalence"

No, equivalence is simply the idea that you can in principle set the two cameras in a way that ensures that certain parameters are the same. In practice, there are many caveats of course, but they are exactly the things that constrain your choice of format/equipment/system (e.g. not being able to choose a certain lens).
Why don't you do your formal group shots with a cell phone? It is because you can't get an equivalent shot as with your DSLR, or the equivalent shot puts you in a setting where you are not comfortable and where you don't need to go if you use your DSLR.

[simsurace]

It is not the aperture, it is the magnification.  The key is to distinguish absolute and relative blur.  Absolute blur is the size of the blur circle of a point on the print or screen, and that, of course, is greater for lenses of longer focal length.  If you normalise for image magnification you have relative blur and that depends on F number.  There is a good example at http://toothwalker.org/optics/dof.html - scroll down to images 3 and 4.

 

If you are interested in how big the bokeh balls are relative to the imaged subject (as opposed to the scale of the blurred object), then it is indeed the absolute size of the entrance pupil that matters -- irrespective of magnification or focal length (assuming that the point light source is far away and that the angle of the parallel rays to the optical axis is small).

In the gromit example in your link, the bokeh balls of the 100mm/4 shot are bigger than in the 28mm/4 shot when measured relative to the gromit. The sizes of appear roughly as 25mm and 7mm measured in the plane of the gromit. These are the respective sizes of the entrance pupils.

[chambeshi]

Alongside probably many on this forum, I've been following this thread with interest, and learnt how confused I have been about the fundamentals of optics WRT different film formats/sensors. And I am not alone in it being decades since I gave due thought to the attributes of lenses (as demanded in physics in high school and 1st year university). Nothing in my personal photography library touches on this stuff, and even some academic ebooks I checked this weekend are either superficial and/or vague, or far too technical. So I'm sure I'm not alone in having already learnt new and important insights within the thread. So thanks for inputs, and honest questions

And I agree fully only science can sift fact from myth, yes, our aim "is to create clarity about the technical stuff" and it's up to the individual how they work with such insights.

It will be most valuable as resolution consolidates :-)  to summarize the axioms in point form

kind regards

Woody

I agree with everything you write in your clarification, since it is logically equivalent to what I have written earlier.
The important thing is that something changes between formats, and how that thing scales with the size of the sensor. Whether you call it 'the secondary magnification that is required to produce the same output' or something else does not matter. What matters is the set of predictions derived from the argument, and whether those predictions turn out to be correct. It is only because a digital file loses its physical dimension whereas a negative does not that some people now prefer not to talk of secondary magnification. So they use different words to describe the same fundamental relationships.

Of course many people misunderstand this stuff. But they would likewise misunderstand any other terminology, even more if they never worked with film formats. The misunderstandings won't go away by abolishing a terminology, but hopefully they will diminish by being explained.

Please don't call this debate 'creationist'. First, you are putting people who understand the stuff and those who don't in the same bin just because they are using the same terms, which creates more confusion among those that don't understand it. Secondly, we are engaging in a rational/scientific discussion on a technical topic, which is quite unlike 'creationism'.

I'm also aware that this discussion has very little to do with the artistic aspects of photography, but this is not the point here. My aim is to create clarity about the technical stuff. This discussion relates to photography in the same way that a discussion of chemical reactions relates to culinary arts. It is up to the individual photographer/cook to decide how important it is or whether they want to use these insights in their daily work or to guide their purchasing decision, but the truth or falsehood of it is independent of this individual decision.

[Almass]

Phewww, I start a very clear thread and come back few days later to find this:

"Round, Like a circle in a spiral
Like a wheel within a wheel,
Never ending or beginning,
On an ever-spinning reel"

My thread is very simple, if you mount the same focal lens on an Fx camera or Dx camera you still have the exact focal lenght.
Your camera or lens does not become a factorial of the cropped sensor. All what is being done is a CROP.

I also mentioned as a "fail safe" point that people who like to be anal about the statement, will find a "minute" difference in the DoF.

Par Bjorn and like minded people, It seems that the Anal's are taking over the asylum with ad nauseum half baked optical and physics refer3ences.

THE FACT OF THE MATTER IS THIS:

1- Fx provides a better picture than Dx, you are free to understand a (better picture) in any which way you like sunny side up.

2- In fact Dx existence is a product aberration in as so much to have an answer to Canon at the time. Which resulted in having a camera at a lower price point than FX.......that's it.......it is a money issue disguised as a marketing exercise.

3- I do not know how many on this board are versed in design or painting in the sense of having learned and practiced design and painting where Perspective is your Vista and Color Harmony is your palette.
Most importantly what you learn in Design and Painting and Photography is the following:

"IF IT IS GOOD TO YOUR EYE, IT IS GOOD TO GO"

and that is why and how I am what I am today in the Art and Photography industry.
My eternal thanks to my Design and Cinematography instructors.

I will leave you with this:

https://www.youtube.com/watch?v=7VydfUohuaM

.......or indulge in continuing the thread to your liking........