A beginner's guide to "is equal to" vs "is equivalent to".

[981S]

https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care

 

[Donald B]

Nice write up.

There is another dimension to this equivalence thing if we are also concerned with image quality: If the lens needs to project the same image on the smaller APS-C surface it requires a better lens to produce equal image quality.

the oly lens must be near perfect

em1mk2 iso 200 left d5 right iso 800







--
Olympus EM5, EM5mk2 my toys. EM1mk2

http://www.dpreview.com/galleries/9412035244

past toys. k100d, k10d,k7,fz5,fz150,500uz,canon G9, Olympus xz1

 

[stevo23]

... Only on this forum have I seen people trying to make the case of equivalent and equivalency.

Consider an 8x12 print taken with a full frame camera, using a 100mm lens at f/8, a shutter speed of 1/100, and a subject distance of 10 feet.

Would it have been possible to take a photo that looks the same with a crop body?

If the answer is "yes", then it makes sense to ask the question "how would I determine what focal length and settings would be needed?". The next question is "How do I determine which images can be visually matched?"

You're wasting your effort. Mr. Llama doesn't believe in a spherical earth...eh...equivalency.

 

[stevo23]

Nice write up.

There is another dimension to this equivalence thing if we are also concerned with image quality: If the lens needs to project the same image on the smaller APS-C surface it requires a better lens to produce equal image quality.

That issue manifests mainly at the widest entrance pupils of the fastest lenses; it is more difficult to find f/1.4 lenses, for example, that are sharp wide open.

It manifests itself at all apertures where diffraction isn't completely dominant.

When one is doing something like macro, though, or anything with specific deep DOF, diffraction is the dominant blur, and equivalence of analog image resolution is achieved.

There is another exception to equivalence here too, for smaller pixels on front-side-illuminated (FSI) sensors; they lose light at low f-numbers, generally somewhere around f/2.8 or so, where larger pixels might not lose until below f/2, but this is about pixel size; not sensor size per se.

Basically, the effect is like as if there were a circular neutral density filter in the entrance pupil that was clear over much of the inner circle but darkened near the opening edges, losing some light, and getting slightly deeper DOF than expected.

 

[Michael Fryd]

Concise definitions of the terms "equal" and "equivalent" would be a good start rather that trying to show the difference by example. Most good technical writing starts with a clear definition of terms before diving first into examples.

Sure. In the context of my post, "equal" means the same nominal value on the lens and/or camera whereas the term "equivalent" means the same effect in the photo.

Weak definitions, if even that. That is why your writings draw so much controversy.

Agreed, very weak. "Equal settings" and "Equal effect" might be simpler and clearer.

Even "equal settings" is unclear. The question is what aspects do you want "equal"?

Consider a full frame body set to a 24° angle of view with a 25mm aperture diameter.

On a 2X crop body, which of the following would be "the same settings"?

1. 24° angle of view and 25mm aperture diameter
2. 24° angle of view 12.5mm aperture diameter
3. 12° angle of view 25mm aperture diameter

====

Another way of describing these settings are :

a full frame with a 100mm lens at f/4 compared to a 2X crop body with

1. 50mm lens at f/2
2. 50mm lens at f/4
3. 100mm lens at f/4

====

We also get into issues of whether the resulting prints should be of "equal size" or "equal magnification" (ratio of print size to sensor size).

If we assume that the goal is to view the image from the entire sensor at the same size, and it is about getting the same composition, perspective, DOF, brightness and noise.

In that case the answer would be 50mm at f2. But you need to bring ISO in the picture.

The question is one of terminology. what does it mean to say "equal settings"? (in the context of "equal settings" vs. "equal effect") Is it reasonable to say that two cameras, both at a 24° angle of view and a 25mm aperture diameter, are at "equal settings".

Or would "equal settings" require the same focal length on both cameras?

 

[Mackiesback]

You are not a beginner and that post isn't a question.

I am curious. I have no interest in weighing in of this oft discussed topic, but how can a thread starter on this forum start a thread related to photography and have it be called "off topic"?

It's his topic, he started it and it is related to photography.

 

[Tom Axford]

You are not a beginner and that post isn't a question.

I am curious. I have no interest in weighing in of this oft discussed topic, but how can a thread starter on this forum start a thread related to photography and have it be called "off topic"?

It's his topic, he started it and it is related to photography.

I'm sorry for the confusion. This thread was originally posted in "Beginners' Questions".

When it was moved to "Open Forum", I tried to delete my post but, of course, it was not possible to do so!

 

[stevo23]

You are not a beginner and that post isn't a question.

I am curious. I have no interest in weighing in of this oft discussed topic, but how can a thread starter on this forum start a thread related to photography and have it be called "off topic"?

It's his topic, he started it and it is related to photography.

I'm sorry for the confusion. This thread was originally posted in "Beginners' Questions".

When it was moved to "Open Forum", I tried to delete my post but, of course, it was not possible to do so!

Ah, that helps. I was trying to place the comments.

 

[Zed Man]

A 16:9 image and a 1:1 can be "equivalent", but never equal.

A OOC image taken with a FF 3:2 camera can be equivalent to an OOC M43 image, but never equal.

Because they have different aspect ratios.

 

[tbcass]

equivalence noun equiv·a·lence | \ i-ˈkwiv-lən(t)s, -ˈkwi-və-\

Definition of equivalence

1a: the state or property of being equivalent

b: the relation holding between two statements if they are either both true or both false so that to affirm one and to deny the other would result in a contradiction

2: a presentation of terms as equivalent

3: equality in metrical value of a regular foot and one in which there are substitutions.

equal adjective\ ˈē-kwəl \

Definition of equal

(Entry 1 of 3)

1a(1): of the same measure, quantity, amount, or number as another

(2): identical in mathematical value or logical denotation : EQUIVALENTb: like in quality, nature, or statusc: like for each member of a group, class, or societyprovide equal employment opportunities2: regarding or affecting all objects in the same way : IMPARTIAL

3: free from extremes: such asa: tranquil in mind or moodb: not showing variation in appearance, structure, or proportion

4a: capable of meeting the requirements of a situation or a task

b: SUITABLEbored with work not equal to his abilities

equal

noun

Definition of equal (Entry 2 of 3)

1: one that is equal; insists that women can be absolute equals with men— Anne Bernays

2: an equal quantity

equal

verb

equaled or equalled; equaling or equalling

Definition of equal (Entry 3 of 3)

transitive verb

1: to be equal toespecially : to be identical in value to

2archaic : EQUALIZE

3: to make or produce something equal to

Equivalence as it's related to photography seems to me to be a new use of the term as it relates to cameras with different sized sensors and how it relates to lens focal length.

--
Tom

 

[tbcass]

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www.dpreview.com

 

[tbcass]

A 16:9 image and a 1:1 can be "equivalent", but never equal.

A OOC image taken with a FF 3:2 camera can be equivalent to an OOC M43 image, but never equal.

Because they have different aspect ratios.

Used in that way you must be specify how they are equivalent. Surface area? Field of view? something else?

--
Tom

 

[Michael Fryd]

A 16:9 image and a 1:1 can be "equivalent", but never equal.

A OOC image taken with a FF 3:2 camera can be equivalent to an OOC M43 image, but never equal.

Because they have different aspect ratios.

My full frame DSLR (Canon 5D mark IV) allows me to choose from a handful of aspect ratios, 3:2, 4:3, 16:9 and 1:1.

Is it possible for an OOC (Out Of Camera) JPEG from my DSLR to be "equal" to an OOC JPEG from a M43, even though my DSLR is using a larger sensor area than the M43?

 

[riveredger]

A recent thread has spawned some confusion on the use of the word "equals" with respect to focal length and relative aperture.

There was no confusion, just people like you who did not take the effort to read until the third post in the thread...

https://www.dpreview.com/forums/post/62175139

= means I see the same «window»

Could be better expressed but his logic was correct.

This attitude is not very nice in the beginner forum.

I think you are a good member on dpr, honestly, but my feelings were that this was not very nice.

It happens all the time on these forums. Watch what happens in this thread...

 

[onlyfreeman]

Bigger sensor is always better, so no, it's impossible.



Am I doing it right?

 

[Mackiesback]

You are not a beginner and that post isn't a question.

I am curious. I have no interest in weighing in of this oft discussed topic, but how can a thread starter on this forum start a thread related to photography and have it be called "off topic"?

It's his topic, he started it and it is related to photography.

I'm sorry for the confusion. This thread was originally posted in "Beginners' Questions".

When it was moved to "Open Forum", I tried to delete my post but, of course, it was not possible to do so!

Ah, that helps. I was trying to place the comments.

Yes it does. Moderator ninja edit strikes again!

 

[Michael Fryd]

equivalence nounequiv·a·lence | \ i-ˈkwiv-lən(t)s, -ˈkwi-və-\

Definition of equivalence

...

3: to make or produce something equal to

Equivalence as it's related to photography seems to me to be a new use of the term as it relates to cameras with different sized sensors and how it relates to lens focal length.

In terms of photography, "Equivalence" is used to refer to settings that yield the pictures of the same subject that "look" the same. The common criteria are:

• same field of view
• same perspective
• same depth of field
• same motion blur
• same image noise

In other words the settings that give you equal results.

This is most often used when comparing cameras with different sensor sizes. For instance:

• Full frame, 100mm, f/4, 1/60, ISO 400, 10 foot subject distance

and

• 2X Crop, 50mm, f/2, i/60, ISO 100, 10 foot subject distance

are generally considered "equivalent" as they produce images that look the same according to the above criteria.

 

[tbcass]

In terms of photography, "Equivalence" is used to refer to settings that yield the pictures of the same subject that "look" the same. The common criteria are:

• same field of view
• same perspective
• same depth of field
• same motion blur
• same image noise

In other words the settings that give you equal results.

This is most often used when comparing cameras with different sensor sizes. For instance:

• Full frame, 100mm, f/4, 1/60, ISO 400, 10 foot subject distance

and

• 2X Crop, 50mm, f/2, i/60, ISO 100, 10 foot subject distance

are generally considered "equivalent" as they produce images that look the same according to the above criteria.

Excellent summary except, when referring to noise, you must specify sensors of the same or equal technology. Should we add the same sensor resolution?

 

[Michael Fryd]

In terms of photography, "Equivalence" is used to refer to settings that yield the pictures of the same subject that "look" the same. The common criteria are:

• same field of view
• same perspective
• same depth of field
• same motion blur
• same image noise

In other words the settings that give you equal results.

This is most often used when comparing cameras with different sensor sizes. For instance:

• Full frame, 100mm, f/4, 1/60, ISO 400, 10 foot subject distance

and

• 2X Crop, 50mm, f/2, i/60, ISO 100, 10 foot subject distance

are generally considered "equivalent" as they produce images that look the same according to the above criteria.

Excellent summary except, when referring to noise, you must specify sensors of the same or equal technology. Should we add the same sensor resolution?

I agree, I should add a note about equivalent sensor technology, or simply leave the noise issue out.

I left off sensor resolution as I was using the resulting print as my metric.

I view resolution as an implementation detail. Modern cameras have more than enough resolution for most situations. At that point the differences in resolution are likely not visible to the unaided eye in the final print.

I have both a 20 megapixel and a 50 megapixel full frame camera. At ISO 100, f/4, 100mm and the same subject, 8x12 prints from both are going to look the same to the naked eye.

 

[Zed Man]

Anything you select on your DSLR that is not 3:2 is really just cropping.

If you select 4:3, then the resulting image will have fewer pixels and a narrower angle of view.

To get a 4:3 image that is equivalent to what you get with 3:2, you would need to add some to the top and bottom as well as crop on the sides. You camera only crops on the sides.

Maybe someone can explain better. I feel I am not doing this well.

Pictures would help.

Oh, and in rare cases there are cameras with multi aspect ratio sensors. GH1 for example. They CAN take equivalent images with different aspect ratios. Those sensors are "oversized".