[Eric Auer]

If folk are not willing to learn (by taking pictures) how their particular format camera and lens perform with regard to image size/field of view/depth of field....... using a crop-formula is not going to help them.

There it is.

No angst involved at all.

Know your gear.

[stevebrot]

That confused me a little, since I understand somewhat that the "zoom equivalent" is due to the difference in size of the film, but I'm not sure I understand how the aperture changes?

It doesn't, f/8 is still f/8. That said, there are some who feel that the words "equivalent" and "equivalence" should always be pegged to the 24x36mm format and should include considerations such DOF and out-of-focus blur, sort of as a type of standard of behavior.

As a user of cameras from 35mmm up to 4x5 format, I find those views mildly humorous. That said, comments on this thread will likely drift that direction.


Steve

(...pass the popcorn...)

[Ash]

It's not a difficult concept, but it is very hard to compare with different formats because it is not a simple calculation.
Easiest way to conceptualise it is to consider each format as canvas sizes.
iPhone - tiny
4/3 - small
APS-C - modest
FF - large
645 - even larger
6x7 - huge
etc

The larger the canvas, the more you can get onto each photo,

Then if you compare results between formats, you are going to have to shrink one or enlarge another to make them equivalent sizes, but then you affect everything else about the photograph's properties, which then throws the equivalence argument out the window.

So just go by canvas size, and whatever canvas size tickles your fancy, go with that and you'll find the lenses that do the job for you after that.

[Eric Auer]

sixfourfive is the bestedest

[rdenney]

The digital world did not introduce the issue. Photographers have shot in multiple formats since the beginning.

Forget translating one format to another, which imposes mental gymnastics (easy for some, not so for others) every time reach into the bag for a different lens.

The way multi-format photographers have done it in the past starts with the diameter of the film. Yes, I said diameter--what's the distance across a circle that just touches the corners of the frame. Those diagonal measurements are (nominally, which means no intention or need for exactitude):

APS-C: 29mm
24x36: 43mm
33x44: 55mm
645: 75mm
6x6: 80mm
6x7: 90mm
6x9: 105mm
4x5 (inches): 160mm
8x10: 320mm

Lenses with these focal lengths are considered "normal" lenses, which simply means that they are the same focal length as the diameter of the format (and that's all it means--it says nothing about the design of the lens, which is and should be a whole separate taxonomy).

Once we establish the normal lens, everything else is a ratio of that.

(Sidebar for geeks--OP should skip: It's a mistake of terminology to refer to shorter-than-normal lenses as "wide angle" and longer-than-normal lenses as "telephoto". Those terms apply to the design of the lens. A wide-angle lens's physical glass is centered ahead of its optical center, which is necessary for cameras with reflex mirrors to get the glass in front of the mirror. Another term for these is "reversed telephoto" or "retrofocus" and the design was first commercialized in the late 40's by Angenieux It made possible short-focal-length lenses on the new SLR design that had just emerged--the Exakta. Telephoto lenses sit further back than their optical center, making it possible to have a physically shorter lens for a given focal length. They did by adding a magnification corrector behind a conventional lens, which forced a few compromises. These terms aren't as clear for modern lenses on smaller cameras because of the complexity of the designs. But they don't mean "short" and "long". It's quite possible to have a wide-angle lens with a longer focal length--the 55mm lens for the Pentax 67 is a wide-angle lens, even when it's adapted to an APS-C camera, on which it would be nearly double the diameter of the format. And a 45mm wide-angle lens for the 67 makes a perfectly usable normal lens when adapted to a 35mm camera, but it's still a wide-angle design, despite that the smaller format can't make use of that fact. Fun fact: Most lenses used on large-format cameras are neither wide-angle nor telephoto, being approximately centered physically around their optical center. Some are wide-field designs, confusingly called wide-angle, because their coverage angle is greater than about 45 degrees. That means simply: greater than is needed by a lens of normal focal length to cover the format. This allows shorter lenses to cover the format and provide for movements. I routinely use a 300mm lens on my 4x5 camera when I want a lens about twice normal, even though that particular lens was really marketed for use on 8x10 cameras. And so on. The point of the fun fact is that large-format photographers are as interested in lens design as in focal length, because it dictates what they can do with it on a view camera with movements. End of sidebar.)

"Standard" lenses are not exactly "normal" lenses, though the terms are used interchangeably. The "standard" lens for a 35mm camera is traditionally 50mm, while the normal lens is 43mm. The standard lens for 8x10 is traditionally 12" (305mm), while the normal lens is a bit longer. But here's what you should remember: The normal lens will provide a similar field of view on its respective format as any other format's normal lens. So, a lens that is one half the focal length of the normal lens will provide the same wide field of view on its respective format. The 28mm end of the 28-45 zoom on the 645z is right at one-half of the normal lens. A 21mm lens on a 35mm camera is very close to one-half of the normal lens on that format. A 75mm large-format lens on a 4x5 camera is not far from one half of the normal lens for that format. And they will all provide similar fields of view, as long as one compensates for the shape of the format.

I know what lenses that are one-half of the normal lens do on some of my cameras, and so I can know about what they will do on all of my cameras. By "know what they do" I can tell from looking at the scene that it's the lens I want to use.

Going longer than normal: A popular focal length for portraits is not quite twice the normal focal length, say, 80mm on a 35mm camera (when twice the normal focal length would be closer to 90). On a 645z, with a normal focal length of 55mm, not quite double that is--shazam!--105mm, which is why a certain fast "standard" lens for the P67 is particularly popular for adaptation. And it's why the preferred portrait lens for the P67 is the 165/2.8 (and why Pentax designed a lens of that focal length with such a large aperture). And it's why the 50/1.4 "standard" lenses for 35mm make such good portrait lenses for APS-C. I know what a lens a little less than twice the normal focal length does on any camera--and it does about the same thing on all cameras.

Greater precision than "half" and "about twice" is a waste of arithmetic--it will get you no closer to a worthwhile conclusion. But it will help you decide which lens to grab. Your feet can make further adjustments.

Now, about the f/stop and aperture--a rabbit hole if there ever was one. The aperture is the size of the hole as projected to the rear of the lens, but nobody ever measures that diameter. It doesn't matter. What matters is the ratio of that diameter to the focal length of the lens, because that's what describes how much light is getting through no matter what that focal length happens to be. So, f/4 on one lens moves the same light as f/4 on any other lens no matter the focal length (roughly speaking--there are differences in transmission efficiency that work at the margins). Format size has no role in defining the f/stop. Format size is just how much of the scene projected by the lens we happen to be recording.

BUT...(and there's always a "but"), some people are interested in matching the rendering of the out-of-focus areas between lenses sized for different formats. Depth of field is dictated by focal ratio (f/stop) and magnification (focal length). To get the same degree of blur in that distant background, you can't change one without changing the other. A lens of low magnification (as in, shorter than normal) needs a lower f/stop than a lens of high magnification. This is why a lens that twice-normal portrait lens needs a wider aperture (i.e., lower ratio, or f/number) to achieve the same degree of out-of-focus blur. One can consult depth of field tables to determine the similarities, and I agree with the point upthread that similarities are the best you'll get. Experience with specific lenses will take you the rest of the way--there are too many confounding factors at the subtlety level.

Rick "working in multiple formats for 45 years" Denney

[UncleVanya]

The way multi-format photographers have done it in the past starts with the diameter of the film. Yes, I said diameter--what's the distance across a circle that just touches the corners of the frame. Those diagonal measurements are (nominally, which means no intention or need for exactitude):
….
Now, about the f/stop and aperture--a rabbit hole if there ever was one. The aperture is the size of the hole as projected to the rear of the lens, but nobody ever measures that diameter. It doesn't matter. What matters is the ratio of that diameter to the focal length of the lens, because that's what describes how much light is getting through no matter what that focal length happens to be. So, f/4 on one lens moves the same light as f/4 on any other lens no matter the focal length (roughly speaking--there are differences in transmission efficiency that work at the margins). Format size has no role in defining the f/stop. Format size is just how much of the scene projected by the lens we happen to be recording.

BUT...(and there's always a "but"), some people are interested in matching the rendering of the out-of-focus areas between lenses sized for different formats. Depth of field is dictated by focal ratio (f/stop) and magnification (focal length). To get the same degree of blur in that distant background, you can't change one without changing the other. A lens of low magnification (as in, shorter than normal) needs a lower f/stop than a lens of high magnification. This is why a lens that twice-normal portrait lens needs a wider aperture (i.e., lower ratio, or f/number) to achieve the same degree of out-of-focus blur. One can consult depth of field tables to determine the similarities, and I agree with the point upthread that similarities are the best you'll get. Experience with specific lenses will take you the rest of the way--there are too many confounding factors at the subtlety level.

Rick "working in multiple formats for 45 years" Denney

Excellent explanation. I like the geeky parts a lot! I still use equivalence and find it helpful for my brain but your approach makes a lot of sense. Thank you!

A minor point about aperture:
Aperture is defined by the entrance pupil not the exit. This means those telephoto lenses and others can have larger apparent apertures than the actual physical opening in the iris.

[leekil]

The digital world did not introduce the issue. Photographers have shot in multiple formats since the beginning.

The digital world introduced the issue to most people, because it made different-sensor-format cameras (APS-C) available with a mount that would take the full-frame (35mm) lenses. I'm not sure who came up with the term "equivalence", though, maybe that was the real mistake.

[UncleVanya]

The digital world introduced the issue to most people, because it made different-sensor-format cameras (APS-C) available with a mount that would take the full-frame (35mm) lenses. I'm not sure who came up with the term "equivalence", though, maybe that was the real mistake.

Apsc predates digital. It was originally a film format.

[Ian Stuart Forsyth]

This means those telephoto lenses and others can have larger apparent apertures than the actual physical opening in the iris.

An interesting is that irises for different lenses can be the same size take the sigma 105 ƒ2.8 and the sigma 24 1.8 side by side have very similar size iris but one has a 38 mm entrance pupil while the other has a 13 mm entrance pupil

[TDvN57]

I have such a terrible time wrapping my head around any of this. I've previously only used aps-c sensors so I didn't care what 'equivalence' even meant, since I just used the one format.
but... I've been shooting 35mm and some 120 film and the topic is a bit more interesting now...
It seems to me that the quality of light leaving the rear element towards the sensor would be the exact same no matter what format it's mounted on.
All that changes is the amount of 2D surface area captured from a lenses potential image circle.
For instance, the quality of brightness does not change between formats.
So why is it that an image quality such as depth-of-field change between formats?
Wouldn't the depth of field have been 'decided' at the time the light left the lens?
Yet I've seen images where people display the bokeh and field differences and equivalences.
I just don't get it haha

This may help to add more color to the mosaic of info on this topic.... A video by Mat Marrash about depth of field in large format photography, which may be a helpful addition.

Youtu.be

Youtu.be

-

[Ian Stuart Forsyth]

A 75mm lens on 6x7 isn't really "equivalent" to a 35mm on full frame. Instead, it's more accurate to say that a 75mm lens on 6x7 makes images with a similar field of view as a 35mm on full frame.

Is not equivalent and similar similar or are they equivalent ?
And when did equivalent always mean equal



Last time went to the gym for an hour it sure didn't taste like a glass of wine but they do give equivalent heath benefits according to this study.

[biz-engineer]

Lens/format equivalence has a practical use for selecting the right lenses for a particular format. It's useful. Using a calculator makes it even easier: https://mmcalc.com/

---------- Post added 21-10-21 at 09:55 ----------

Like they referred to the 75mm f4.5 as corresponding to a 35mm f2.2, and the 135mm f4 lens as corresponding to a 68mm f2.

75mm f4.5 on a 6x7 is equivalent to 37.5mm f2.25 on full frame. 135mm f4 on 6x7 is equivalent to 67.5mm f2 on full frame.

It's interesting for shooting panorama / stitched image mosaic. Ansel Adams used lenses mostly between 200mm and 300mm on 4x5 and 8x10. I use a 70-210 zoom lens for 3x3 full frame stitches, which is roughly equivalent to shooting with a 48mmx72mm camera , still 70-210 f4 lens, which would be equivalent to shooting with 24x36 full frame and 35-105mm f2 zoom.

[Serkevan]

Jumping in Rdenney's excellent post, I'd argue that "wide angle" doesn't necessarily mean retrofocus (FF mirrorless lenses down to ~18mm need not be retrofocus).
However, for the long end we don't use "narrow angle"... which reminds me that the DA560 is *not* a super tele... because it's not a telephoto at all!

[texandrews]

The digital world did not introduce the issue. Photographers have shot in multiple formats since the beginning.

Forget translating one format to another, which imposes mental gymnastics (easy for some, not so for others) every time reach into the bag for a different lens.

The way multi-format photographers have done it in the past starts with the diameter of the film. Yes, I said diameter--what's the distance across a circle that just touches the corners of the frame. Those diagonal measurements are (nominally, which means no intention or need for exactitude):

APS-C: 29mm
24x36: 43mm
33x44: 55mm
645: 75mm
6x6: 80mm
6x7: 90mm
6x9: 105mm
4x5 (inches): 160mm
8x10: 320mm

Lenses with these focal lengths are considered "normal" lenses, which simply means that they are the same focal length as the diameter of the format (and that's all it means--it says nothing about the design of the lens, which is and should be a whole separate taxonomy).

Once we establish the normal lens, everything else is a ratio of that.

(Sidebar for geeks--OP should skip: It's a mistake of terminology to refer to shorter-than-normal lenses as "wide angle" and longer-than-normal lenses as "telephoto". Those terms apply to the design of the lens. A wide-angle lens's physical glass is centered ahead of its optical center, which is necessary for cameras with reflex mirrors to get the glass in front of the mirror. Another term for these is "reversed telephoto" or "retrofocus" and the design was first commercialized in the late 40's by Angenieux It made possible short-focal-length lenses on the new SLR design that had just emerged--the Exakta. Telephoto lenses sit further back than their optical center, making it possible to have a physically shorter lens for a given focal length. They did by adding a magnification corrector behind a conventional lens, which forced a few compromises. These terms aren't as clear for modern lenses on smaller cameras because of the complexity of the designs. But they don't mean "short" and "long". It's quite possible to have a wide-angle lens with a longer focal length--the 55mm lens for the Pentax 67 is a wide-angle lens, even when it's adapted to an APS-C camera, on which it would be nearly double the diameter of the format. And a 45mm wide-angle lens for the 67 makes a perfectly usable normal lens when adapted to a 35mm camera, but it's still a wide-angle design, despite that the smaller format can't make use of that fact. Fun fact: Most lenses used on large-format cameras are neither wide-angle nor telephoto, being approximately centered physically around their optical center. Some are wide-field designs, confusingly called wide-angle, because their coverage angle is greater than about 45 degrees. That means simply: greater than is needed by a lens of normal focal length to cover the format. This allows shorter lenses to cover the format and provide for movements. I routinely use a 300mm lens on my 4x5 camera when I want a lens about twice normal, even though that particular lens was really marketed for use on 8x10 cameras. And so on. The point of the fun fact is that large-format photographers are as interested in lens design as in focal length, because it dictates what they can do with it on a view camera with movements. End of sidebar.)

"Standard" lenses are not exactly "normal" lenses, though the terms are used interchangeably. The "standard" lens for a 35mm camera is traditionally 50mm, while the normal lens is 43mm. The standard lens for 8x10 is traditionally 12" (305mm), while the normal lens is a bit longer. But here's what you should remember: The normal lens will provide a similar field of view on its respective format as any other format's normal lens. So, a lens that is one half the focal length of the normal lens will provide the same wide field of view on its respective format. The 28mm end of the 28-45 zoom on the 645z is right at one-half of the normal lens. A 21mm lens on a 35mm camera is very close to one-half of the normal lens on that format. A 75mm large-format lens on a 4x5 camera is not far from one half of the normal lens for that format. And they will all provide similar fields of view, as long as one compensates for the shape of the format.

I know what lenses that are one-half of the normal lens do on some of my cameras, and so I can know about what they will do on all of my cameras. By "know what they do" I can tell from looking at the scene that it's the lens I want to use.

Going longer than normal: A popular focal length for portraits is not quite twice the normal focal length, say, 80mm on a 35mm camera (when twice the normal focal length would be closer to 90). On a 645z, with a normal focal length of 55mm, not quite double that is--shazam!--105mm, which is why a certain fast "standard" lens for the P67 is particularly popular for adaptation. And it's why the preferred portrait lens for the P67 is the 165/2.8 (and why Pentax designed a lens of that focal length with such a large aperture). And it's why the 50/1.4 "standard" lenses for 35mm make such good portrait lenses for APS-C. I know what a lens a little less than twice the normal focal length does on any camera--and it does about the same thing on all cameras.

Greater precision than "half" and "about twice" is a waste of arithmetic--it will get you no closer to a worthwhile conclusion. But it will help you decide which lens to grab. Your feet can make further adjustments.

Now, about the f/stop and aperture--a rabbit hole if there ever was one. The aperture is the size of the hole as projected to the rear of the lens, but nobody ever measures that diameter. It doesn't matter. What matters is the ratio of that diameter to the focal length of the lens, because that's what describes how much light is getting through no matter what that focal length happens to be. So, f/4 on one lens moves the same light as f/4 on any other lens no matter the focal length (roughly speaking--there are differences in transmission efficiency that work at the margins). Format size has no role in defining the f/stop. Format size is just how much of the scene projected by the lens we happen to be recording.

BUT...(and there's always a "but"), some people are interested in matching the rendering of the out-of-focus areas between lenses sized for different formats. Depth of field is dictated by focal ratio (f/stop) and magnification (focal length). To get the same degree of blur in that distant background, you can't change one without changing the other. A lens of low magnification (as in, shorter than normal) needs a lower f/stop than a lens of high magnification. This is why a lens that twice-normal portrait lens needs a wider aperture (i.e., lower ratio, or f/number) to achieve the same degree of out-of-focus blur. One can consult depth of field tables to determine the similarities, and I agree with the point upthread that similarities are the best you'll get. Experience with specific lenses will take you the rest of the way--there are too many confounding factors at the subtlety level.

Rick "working in multiple formats for 45 years" Denney

Thank you, once again Mr.Denney, for that magisterial explanation. BTW, we still need to get together about that DA 25 business on 645...

[normhead]

Everything you need to know, you can see in the viewfinder.

There is no equivalence that resolves exposure value, field of view and depth of field.

AN example....


Lumix ZS100 at ƒ3.2, 1/125s
1/125s is the preferred speed for freezing people in informal snapshots.
If I want to take this with my K-1 I have to use ƒ/16.
The resultant shutter speed would be 1/6s.


In other words, if you try and include DoF in equivalence, you get shutter speeds that aren't functional. Practically you can't take that shot with the same results.

Better to just realize some sensor sizes are better than others and use the cameras to their strength. Don't take snapshots on dull days with large sensor cameras, don't take landscapes with large DR values with small sensor cameras. NeIther large format nor small format can be used interchangeably. In that sense equivalence including DoF is a complete lie.

There simply is no equivlance that can resolve both exposure values and DoF values. And if shutter speed, aperture and ISO and DoF are not all equivalent, (which they aren't) there is no equivalence.

For depth of field ƒ/2.8 small sensor might be 16 FF.
But there is no scenario where ƒ/2.8 small sensor is the same as ƒ/16 FF for exposure. One will be 1/8 the other. You can shoot more depth of field at faster shutter speeds with a small sensor.
Equivalence including DoF does not coincide with equal exposures. Equivalence including DoF is a myth when looking at a camera systems. And pretty much useless in any case.

What I do is get to know my lenses. Shoot a "base" ƒ/5.6 on APS-c, ƒ/8 on full frame and base ƒ3.2 or 2.8 on my small sensor cameras. Experience has taught me what that looks like at various distances. So when I approach a scene, my internal discussion is "do I need more than my base aperture or less? " This is based on my understanding of taking an image with different sensor sizes. There is no way to make this less complicated, using some formula. You have to learn your gear.

You need to understand what you'r base aperture is for each camera...after that, you'll never need to work with equivalence again.
Field of view equivalence you need when you buy a lens if you use multiple systems if you don't understand one of those systems.

The whole "will provide the same image on different systems" is bogus to start with. First of all, if you're taking the same picture with both systems you are by definition making a mistake. Different systems have different strengths. Selecting the right system is the way to go, not trying to take the same image with two different systems. If what you are trying to do is something nobody ever does, maybe you need to rethink the utility of the proposed information.

It's kind of like,"If I have to push my car a km to the nearest gas station a smaller car is better." Finding one isolated reason for buying a smaller car, doesn't make it a better choice overall. There are other factors to consider. It is so annoying to see DoF equivalent proponents excluding exposure time and claiming that a difference of 1/125s is somehow equivalent to 1/6s exposure time. It's not. The whole thing is a fallacy.

How people can with a straight face claim ƒ/2.8 small sensor is equivalent to ƒ/16 FF without mentioning the problem created by shooting 1/8 the shutter speed on the FF is beyond me. It's a clear misstatement of the issues at hand to the point of being intentional mis-information. IN the image above, it could not have been taken with my K-1. There is no equivalent FF image to be had. There is no equivalent image taken on an FF nor could there have been one, given the conditions that day. Equivalence applies only to field of view and focal length. And even then, with two different systems, both will have strengths and weaknesses. They are never truly equivalent.