# Question on relationship between "focal length" and "FOV"...



## Matt English (Feb 16, 2020)

With my M50 having a crop factor of 1.6, I have been told different things as to how it relates to lens focal length and FOV...It was my understanding that, though not synonymous, they were certainly closely related. Several graphs even show "crop factor equivalents" for lenses on M's.
My main question was whether the 1.6x math should be applied even when using "native" M lenses on my m50.
Mt second question is whether

the "crop factor equivalent" should be utilized when considering focal length, or does it apply only to FOV???

TY!


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## Viggo (Feb 16, 2020)

Yes, a lens focal length doesn’t change no matter what sensor you put it in front of. So a 20mm for M series or 20mm for a FF is the same 20mm. But, the fov is 20x1,6 when used on a crop sensor body.


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## AlanF (Feb 16, 2020)

A simple way of looking at it is that a crop sensor is 1.6x smaller in height and width than a full frame so you capture a field of view that is 1.6x narrower. And a 1.6x narrower field of view is what you would get with a lens that has 1.6x greater focal length.


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## Matt English (Feb 16, 2020)

Viggo said:


> Yes, a lens focal length doesn’t change no matter what sensor you put it in front of. So a 20mm for M series or 20mm for a FF is the same 20mm. But, the fov is 20x1,6 when used on a crop sensor body.


I see, so the M series lenses do not make a compensation for "equivalent focal length" in their listing of focal length. This info was what I sought. But we must remember that crop sensors do indeed give a practical change to the image, hence the universally accepted "equivalent..."


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## Mt Spokane Photography (Feb 16, 2020)

Matt English said:


> I see, so the M series lenses do not make a compensation for "equivalent focal length" in their listing of focal length. This info was what I sought. But we must remember that crop sensors do indeed give a practical change to the image, hence the universally accepted "equivalent..."


The focal length of a lens is a property of the lens, as others have said, it does not change due to the sensor.

It does not matter if its a camera lens, a telescope, or whatever, its the same.

However, its common for a camera with a fixed lens permanently mounted to specify the equivalent full frame focal length, you may see that in a point shoot or a camera phone. It can be confusing.






Focal length - Wikipedia







en.wikipedia.org


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## SteveC (Feb 16, 2020)

This makes sense.

You wouldn't want to (for example) call the 32mm a 50mm just because it's designed for a crop sensor camera. If you do that then it messes up any sort of comparison with adaped EF lenses designed for a full frame, such as (for instance) the EF 50mm F/1.4. If the 32mm EF-M were called a 50 mm you'd be misled into thinking it would be similar to the EF50 mm with adapter.

(As it happens I own both of these lenses, an M6 MkII, and no full frame camera so this example isn't contrived by any means.)


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## Matt English (Feb 16, 2020)

SteveC said:


> This makes sense.
> 
> You wouldn't want to (for example) call the 32mm a 50mm just because it's designed for a crop sensor camera. If you do that then it messes up any sort of comparison with adaped EF lenses designed for a full frame, such as (for instance) the EF 50mm F/1.4. If the 32mm EF-M were called a 50 mm you'd be misled into thinking it would be similar to the EF50 mm with adapter.
> 
> (As it happens I own both of these lenses, an M6 MkII, and no full frame camera so this example isn't contrived by any means.)


INDEED!!!! That was my question...I was merely confirming that the M-series lenses did not label their focal length the "equivalent focal length", assuming they would always be used on M bodies and therefore give one an apples to apples comparison to what the FOV is on a FF camera. Being new, I never take anything for granted, and just wanted to make sure the M lens was the ACTUAL focal length. Now I know, and know it will apply universally..TY


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## SteveC (Feb 16, 2020)

Matt English said:


> INDEED!!!! That was my question...I was merely confirming that the M-series lenses did not label their focal length the "equivalent focal length", assuming they would always be used on M bodies and therefore give one an apples to apples comparison to what the FOV is on a FF camera. Being new, I never take anything for granted, and just wanted to make sure the M lens was the ACTUAL focal length. Now I know, and know it will apply universally..TY



If you can get to a camera store, try this experiment, it should make you more confident: borrow an M mount camera, an EF-M to EF adapter, then borrow an EF 50mm lens. Find a picture on the wall or something like that, and stand so the picture fills your viewfinder. Remember exactly where you stood. Then borrow an EF-M lens, and stand on that same spot. You SHOULD see the picture sized appropriately for whatever lens you borrowed (if the 32 mm, the picture should be considerably smaller in the viewfinder than it was with the 50mm; if they don't have that but do have a zoom, set the zoom to 50mm and see that the picture looks the same).

My 50mm EF is not ready to hand, but I put the 85mm on my M-50, powered it up, found the spot where the picture fills the frame, then swapped for the 32mm EF-M. Standing on the same spot, the picture was only a bit more than a third as wide as the viewfinder. This makes sense, because 85/32=2.65. If the 32 had really been a shorter focal length only *labelled* as 32 MM because it's "equivalent", then the picture would have been tiny in the viewfinder.

Nope, the lenses are "interchangeable" without the spec being misleading. This would have been a problem years ago with the introduction of "EF-S" (lenses designed for APS-C sensors on the EF mount) if they hadn't done it exactly this way.

Now you WOULD see interesting differences swapping out a full frame for an APSC, but you knew that. Your M camera with a 32mm should show you about the same picture as a full frame with a 50mm. In fact, that's why Canon chose that particular focal length, because that particular field of view (50mm on a full frame) is regarded as "standard" by photographers, and a 32mm very nearly duplicates it on an APS-C sensor.


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## Matt English (Feb 16, 2020)

SteveC said:


> If you can get to a camera store, try this experiment, it should make you more confident: borrow an M mount camera, an EF-M to EF adapter, then borrow an EF 50mm lens. Find a picture on the wall or something like that, and stand so the picture fills your viewfinder. Remember exactly where you stood. Then borrow an EF-M lens, and stand on that same spot. You SHOULD see the picture sized appropriately for whatever lens you borrowed (if the 32 mm, the picture should be considerably smaller in the viewfinder than it was with the 50mm; if they don't have that but do have a zoom, set the zoom to 50mm and see that the picture looks the same).
> 
> My 50mm EF is not ready to hand, but I put the 85mm on my M-50, powered it up, found the spot where the picture fills the frame, then swapped for the 32mm EF-M. Standing on the same spot, the picture was only a bit more than a third as wide as the viewfinder. This makes sense, because 85/32=2.65. If the 32 had really been a shorter focal length only *labelled* as 32 MM because it's "equivalent", then the picture would have been tiny in the viewfinder.
> 
> ...


Great idea...the experiment...but I chose to merely type a question into this forum and get my confirmation. 100% dead-on confident, thank you


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## tron (Feb 17, 2020)

Something partially on topic which came to mind when I read about the crop factor in this thread.

I was on an daily excursion to a wetland with a forest and we would split depending on preference. One team would walk through a small forest with a leader and the other would comprise of people who wanted to take pictures and would follow another path with another leader. A little before the split we found a little bird close to us and started taking pictures. I was shooting with 5DsR and 400mm DO II. The leader of the photography team saw my picture and he said that there is an issue with Full Frame cameras.

I wanted to get rid of him and *I purposefully answered wrongly - at least for the specific case -* and said yes I know the depth of field and he said exactly. But I said I was going to walk through the forest - that was true - so a crop wouldn't be so practical.

I didn't want to argue by saying that FF or crop I would stand at the same spot (=same distance from target) and shoot the bird with exactly the same Focal length lens and aperture so I could not see how the depth of field would be different especially since my crop camera would be a 7DII which has the same pixel density with 5DsR. 

P.S The only interesting bird photos proved to be the ones taken before the split: Cattle Egrets on sheep which for fun were momentarily named sheep egrets 

It wasn't a bird rich area so the forest proved a nice choice both as a walking experience and for taking photos .


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## Matt English (Feb 17, 2020)

tron said:


> Something partially on topic which came to mind when I read about the crop factor in this thread.
> 
> I was on an daily excursion to a wetland with a forest and we would split depending on preference. One team would walk through a small forest with a leader and the other would comprise of people who wanted to take pictures and would follow another path with another leader. A little before the split we found a little bird close to us and started taking pictures. I was shooting with 5DsR and 400mm DO II. The leader of the photography team saw my picture and he said that there is an issue with Full Frame cameras.
> 
> ...


LOL!!!!


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## Joules (Feb 17, 2020)

tron said:


> I didn't want to argue by saying that FF or crop I would stand at the same spot (=same distance from target) and shoot the bird with exactly the same Focal length lens and aperture so I could not see how the depth of field would be different especially since my crop camera would be a 7DII which has the same pixel density with 5DsR.


There is a lot of confusion around Depth of Field, because I think it gets used in two ways.

There is an intuitive meaning and an actual formula.

Intuitively to me, two images should have the same depth of field if the blur is the same size relative to the subject. In my Intuition, it shouldn't matter if I view the image at 1:1 or fully. That may be because when I started learning photography, it seemed many people used the term to describe the amount of blur in the background and I picked that up. 

The actual definition isn't about that though. It is about the physical distances from the subject where blur becomes noticeable, and that calculation involves the amount of magnification that's applied to an image when it is viewed. What this results in is that for the same distance to the subject, same Focal length and aperture, a FF sensor will have a *greater* Depth of Field than a crop sensor. The way to think about that is this: Once you zoom into an image, small amounts of blur become more noticeable. So the DoF decreases as you zoom in, because the blur becomes more pronounced. By the time you have zoomed into the FF sensor as much as into the crop sensor, there is no difference. 

Depending on what people talk about, there can be an endless amount of back and forth that helps nobody. It certainly doesn't make the discussion around equivalence any more fun.


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## tron (Feb 17, 2020)

Joules said:


> There is a lot of confusion around Depth of Field, because I think it gets used in two ways.
> 
> There is an intuitive meaning and an actual formula.
> 
> ...


Hence the same depth of field I was thinking about. In small bird cases I crop the same irrespective of camera used. And since the two mentioned cameras have the same pixel density I end with the same number of pixels per bird (What has been humorously defined in another forum as pixels per duck  )


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## AlanF (Feb 17, 2020)

I love how a simple question expands into a wider discussion - and that’s for real as I believe in it. Here is useful little article https://www.opticallimits.com/Reviews/986-equivalence from one of the really honest sites.


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## privatebydesign (Feb 17, 2020)

Yes DOF is greatly misunderstood. Try this thought experiment, take a single picture, print it at 8"x12" and also 24"x36", place those two prints on a wall and stand back and look at them. 

The DOF in the larger print is smaller, how can that be when it is the same picture? Well the magnification has changed so the DOF has changed.

To work out DOF you need to know two things, the physical size of the aperture used, not the f number the actual apparent size, and the reproduction ratio, that is, how large is the subject in your sight now, just those two. When you plug focal length, distance, etc into a DOF calculator it is just changing that data into the numbers it needs to work out aperture size and reproduction size.


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## Matt English (Feb 18, 2020)

AlanF said:


> I love how a simple question expands into a wider discussion - and that’s for real as I believe in it. Here is useful little article https://www.opticallimits.com/Reviews/986-equivalence from one of the really honest sites.


BINGO!!!! Thank you so much!


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