# Regarding field of view and sensor size



## dsiglin (May 19, 2014)

This much I know: When you take a 50mm full frame lens and put it on say a Sony APS-C mirrorless you have the field of view equivalent of 75mm lens on full frame.
Here's my question: Those full frame lenses are designed to cast a wider circle of light than the APS-C sensor. This is why you get a narrower field of view. But what about a lens designed *for *APS-C? The image circle it produces is more focused/smaller than the full frame lens. Would that not mean a 50mm APS-C lens has an actual 50mm field of view? In my mind that makes sense but when I see APS-C lenses advertised they often state "equivalent to 75mm on full frame). What am I missing?


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## Overread (May 19, 2014)

Focal length is a property solely of the lens itself and is not measured based upon the angle of view that the lens is capable of rendering or is designed for. As a result 50mm is 50mm - no matter if its a lens made for a crop sensor camera or a large format camera. As a result the angle of view you get based upon the sensor/film size will change even though the focal length is the same.


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## dsiglin (May 19, 2014)

Yes, I understand focal length is focal length, hence my use of "equivalent". I'm not actually asking about focal length but about _field of view_. There are speed boosters which reduce the image circle of a full frame lens so that an APS-C sensor has almost the same field of view (to be technical the degree of view) as if it were a full frame sensor. What I'm asking is what about lenses designed specifically for APS-C sensors. They make a smaller more focused circle of light. It seems like it would be doing the same thing as a speed booster. Or is it that they are _designed_ to just show a smaller field of view?


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## astroNikon (May 19, 2014)

what Overread stated plus

on a FF lens the image circle is larger than that of a Crop specific lens ( such as if a Nikon lens has "DX" on it).
It's not a smaller more "focused" ... but basically cut off in it's total size. 

So if you focus on a house.  The house is the same size in both a FF and crop lens.  But the crop image is just less of the surrounding overall image.


this was a link in a previous thread - has good images of the "circle" and the crops of that circle
http://www.scantips.com/lights/cropfactor.html


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## dsiglin (May 19, 2014)

> It's not a smaller more "focused" ... but basically cut off in it's total size.



That's what I was asking. I started pondering this after reading about focal reducers. I guess it *is* possible for a lens designer to give a true 50mm field of view with an APS-C lens but it would require larger glass than normal and some kind of reducer.


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## astroNikon (May 19, 2014)

dsiglin said:


> > It's not a smaller more "focused" ... but basically cut off in it's total size.
> 
> 
> That's what I was asking. I started pondering this after reading about focal reducers. I guess it *is* possible for a lens designer to give a true 50mm field of view with an APS-C lens but it would require larger glass than normal and some kind of reducer.



yeah, but are you going to do that to all your lenses?

I got to a point where I was looking at a UltraWide Nikon lens .. it was close to $1300 ...but a refurb'd d600 FF was also $1300 ....
I just went FF and it resolved other problems too.  lol   I certainly love the decision now.

Basically, be careful how much money you spend trying to resolve a problem.  Another solution could be a lot quicker and better.


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## runnah (May 19, 2014)

I feel like everyone is going in different directions. 

Focal length of a lens doesn't change depending on the body, the field of view changes. So that is why people say "Oh a 50mm on a crop looks like an 85mm on a full frame. Focal length is constant.


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## Overread (May 19, 2014)

dsiglin said:


> Here's my question: Those full frame lenses are designed to cast a wider circle of light than the APS-C sensor. This is why you get a narrower field of view. But what about a lens designed *for *APS-C? The image circle it produces is more focused/smaller than the full frame lens. Would that not mean a 50mm APS-C lens has an actual 50mm field of view? In my mind that makes sense but when I see APS-C lenses advertised they often state "equivalent to 75mm on full frame). What am I missing?[/FONT][/COLOR]



The crop sensor designed lens still has the same 50mm focal length. Whilst the image circle it generates might be smaller, its still giving you the same field of view. Thus when you compare a fullframe 50mm lens and a crop sensor 50mm lens on the same camera body they'd both give the same fields of view (and if you compared them on a fullframe camera body the corners of the frame from the crop sensor lens might be significantly softer or even darker because its image circle is not necessarily as big or as well defined for those edge areas).

35mm equivalents are mostly stated because the majority of photographers (esp in the film days) use or at least have experience of 35mm so its a base line comparison point people can work with.


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## dsiglin (May 19, 2014)

Ok so my confusion has been cleared because I discovered focal reducers actually _*change the focal length of a lens*. _I did not know this but it perfectly explains why you get a wider angle of view with them.



> Basically, be careful how much money you spend trying to resolve a problem. Another solution could be a lot quicker and better.



I'm not looking to buy a focal reducer. That previous post was me pondering how a lens designer could give a 50mm field of view on an APS-C lens.


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## Derrel (May 19, 2014)

Focal length and the size of the image circle are two UTTERLY SEPARATE issues. Not. Even. Related. Two, totally different issues, entirely.

There are 300mm lenses designed to cover a 24x36mm sensor or piece of film, and there are 300mm view camera lenses that are designed to cover 4x5 or 8x10 inch sheet film. And yet, the 300mm lenses designed for 35mm film (aka 24x36 format or "135 format") have image circles that cannot even cover a 6x6 centimeter piece of 120 rollfilm--and yet, they are genuine 300mm focal length lenses.


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## dsiglin (May 19, 2014)

The speed booster / focal reducer works as the inverse of a tele-convertor adapter. Decreasing the focal length and increasing the field of view.


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## robbins.photo (May 19, 2014)

Derrel said:


> Focal length and the size of the image circle are two UTTERLY SEPARATE issues. Not. Even. Related. Two, totally different issues, entirely.



Hmm.  could you go over that middle part again?  I'm afraid you lost me.. lol


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## KmH (May 19, 2014)

dsiglin said:


> Would that not mean a 50mm APS-C lens has an actual 50mm field of view?


No.
FWIW - all of Nikon's 50 mm prime lenses are FX lenses.

Here are the specs for the AF-S 50 mm f/1.8G


*Maximum Angle of View (DX-format)* - 31°30'
*Maximum Angle of View (FX-format)* - 47°


A DX (APS-C) 50 mm prime or a zoom set to 50 mm also delivers a FoV very close to or the same as the 31° 30' DX-format FoV.


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## Derrel (May 19, 2014)

robbins.photo said:


> Derrel said:
> 
> 
> > Focal length and the size of the image circle are two UTTERLY SEPARATE issues. Not. Even. Related. Two, totally different issues, entirely.
> ...



There is the angle of view that a lens exhibits. A wide-angle has a wide angle of view. A telephoto lens has a narrow angle of view.

There is also the image circle's coverage; large-format lenses can literally "cover" a LARGE piece of film with light rays. Lenses designed for APS-C cameras "cover" only a small circular area.

This entire area, of lenses, focal lengths, angles of view, and image circle sizes, is fraught with misunderstanding and misconceptions.

Check this thread for more information, and some numbers. Image Circle Coverage and Angle of View - how to calculate?

I only mentioned this because the OP posed a question and it had some erros in understanding in this part: " Those full frame lenses are designed to cast a wider circle of light than the APS-C sensor. This is why you get a narrower field of view. But what about a lens designed *for APS-C? The image circle it produces is more focused/smaller than the full frame lens."
*
I don't even want to get into this whole issue, but the idea that the image circle affects the angle of view is NOT correct, at all. Not correct at all.


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## robbins.photo (May 19, 2014)

Derrel said:


> robbins.photo said:
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> > Derrel said:
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Lol.. Derrel, I avoid these discussions of focal length/field of view deals like the plague myself, but had to make an exception just to yank your chain..


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## TCampbell (May 19, 2014)

robbins.photo said:


> Derrel said:
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> > robbins.photo said:
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He said "utterly separate".  I think it has something to do with a dairy cow.


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## dsiglin (May 20, 2014)

Derrel said:
			
		

> _I only mentioned this because the OP posed a question and it had some erros in understanding in this part: " Those full frame lenses are designed to cast a wider circle of light than the APS-C sensor. This is why you get a narrower field of view. But what about a lens designed _*for APS-C? The image circle it produces is more focused/smaller than the full frame lens."
> *



That questions stems from my not understanding how focal reducers / speed boosters work. I was under the impression they "bent" the incoming light and focused it into a smaller area but giving the FX field of view when in fact they (as the name implies) reduce the focal length.

Another common misconception is that when you get a 75mm FX field of view by using a 50mm DX lens you are also getting extra zoom as if you were using a 75mm FX lens. I do know enough to know that is not true.


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## WayneF (May 20, 2014)

dsiglin said:


> Another common misconception is that when you get a 75mm FX field of view by using a 50mm DX lens you are also getting extra zoom as if you were using a 75mm FX lens. I do know enough to know that is not true.



Be careful.       Understanding is important.   Assuming both camera bodies are using the same lens at the same subject distance, the FX / DX  difference is just a crop difference.  There is no magic, the DX sensor is simply smaller, and can not show the full field of view that the larger FX sensor sees  (with same lens at same subject distance).

This  DX crop is the same effect as if you simply cropped the FX image smaller in your  photo editor later. DX is simply cropped smaller.

* If you crop any image smaller (anytime)*, and then enlarge it to  show it same size again, it does appear as a telephoto effect.  Cropping does  magnify the smaller view, much like zooming in.     It will hard to tell it from actually using a  telephoto lens. Yes, there can be small differences with a different lens, in Depth of Field or  in perspective (when all things are not equal), but when cropping, all things are equal (except magnification). So regarding the field  of view seen, *any cropping anytime anywhere shows as a telephoto effect* (as if zoomed, shows a more narrow view, which we subsequently enlarge again).   This is simply true of the cropped DX  sensor size too.

Not saying the lens becomes telephoto (it remains the same lens), but the CROPPED DX  FIELD OF VIEW has a telephoto effect, as if zoomed.

To see the same uncropped field of view as FX, DX has to stand back 1.5x farther with same lens, which is a telephoto effect  (or for the same DX view, FX actually has to use a 1.5x telephoto lens if standing at same place, which is the meaning of Effective Focal Length, DX as compared to FX).    Or if standing at same place, to see same FX view, DX must use a focal length 2/3 as long (1/1.5).


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## Norten (Apr 21, 2017)

Derrel said:


> robbins.photo said:
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> > Derrel said:
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I was just reading this thread because of my interest in the the focal reducer (Speed booster), however, there has to be a simpler way of describing the difference effected to the "perceived" focal length of lenses when used on FF or APS-C.

So I'll take a stab at this.

Let's disregard the purpose built APS-C lens for a minute.
Let's say you are using a FF lens on a FF, AND an APS-C sensored  camera. 

First of all. The image sensor in your camera is essentially fitted upside down (a very simplistic view I know but bear with me). That means that the light must at some point cross-over or invert itself due to lens properties. This cross-over point is what determines the FOCAL LENTH of a lens and roughly means that the cross-over point is "x"mm ahead of the sensor plane, be it film or chip.

Now in the case of the FF x FF setup, if you were to scribe a simple line-diagram of the light path from the edges of the sensor to a cross-over point (somewhere in the lens) and on-ward to whatever subject, you'd effectively determine a certain angle (or AOV).
Now, if you change only the size of the sensor in the diagram leaving all other points as is and then re-scribe the the lines from this smaller sensor's edges, you'd find that the focal length is effectively the same but the AOV has changed.

The reason therefore, for so-called APS-C lenses becomes more obvious when one considers the cost of producing a FF lens that can project light onto a large sensor,  and then end up being used on a smaller APS-C sensor. There is a LOT of light that is going to "waste" on an APS-C sensor, hence it is unnecessary to manufacture such an expensive lens when you only need a smaller project area of light.

Using the APS-C crop factor is simply to give an indication of what lens would give a similar AOV performance should you switch to FF overnight. Or a marketing ploy to get you to aim for the bigger bucks of the FF body and lenses.

So, Derrel, I agree with you but I don't really agree. You are right, the image circle doesn't affect the angle of view but the size of the sensor does, and THAT is what makes APS-C lenses "longer". 
And the APS-C lenses DOES NOT produce a more "focused" light circle. It produces exactly what a FF lens does, it's just looking through a smaller (and more importantly - CHEAPER) hole.


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## table1349 (Apr 21, 2017)




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## Bebulamar (Apr 30, 2017)

The field reducer like Metabones changes the focal length of the lens. So when you put it on the the 50mm f/2 FF lens it becomes a 32mm f/1.4 APS-C lens. Not quite because the metabone isn't very good.


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## table1349 (Apr 30, 2017)

Bebulamar said:


> The field reducer like Metabones changes the focal length of the lens. So when you put it on the the 50mm f/2 FF lens it becomes a 32mm f/1.4 APS-C lens. Not quite because the metabone isn't very good.



1.  NO  you are wrong.  Focal Length is Focal Length, PERIOD.  It is a physical property of the lens.  No adapter changes it's focal length.  A 50mm lens is 50mm lens no matter if it is on a $50 P&S or on the Hubble Telescope.  You might want to peruse this. Digital Photography Tutorials

2.  This thread is THREE YEARS OLD.  The OP hasn't been around for the last three years.  

3.


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## Bebulamar (Apr 30, 2017)

gryphonslair99 said:


> Bebulamar said:
> 
> 
> > The field reducer like Metabones changes the focal length of the lens. So when you put it on the the 50mm f/2 FF lens it becomes a 32mm f/1.4 APS-C lens. Not quite because the metabone isn't very good.
> ...


You are not familiar with the field reducer but I am sure you're very familiar with the tele converter. So does the tele converter changes the focal length of the lens? If you add optical elements to a lens you can change its focal length. They used to make the triple convertible which is 2 pieces and each has its own focal length and if you put them together you get a lens of another focal length.


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## petrochemist (Apr 30, 2017)

gryphonslair99 said:


> 1.  NO  you are wrong.  Focal Length is Focal Length, PERIOD.  It is a physical property of the lens.  No adapter changes it's focal length.  A 50mm lens is 50mm lens no matter if it is on a $50 P&S or on the Hubble Telescope.  You might want to peruse this. Digital Photography Tutorials



Any adapter with optical elements (lenses) will change the focal length of the lens combination. This applies to teleconverters, close up diopters, focal reducers, wide angle adapters or adapters that have an element added to allow infinity focus on lenses designed for shorter registration. the focal length of the lens itself doesn't change but it is the combination that is being used with the adapter fitted, so that is effectively the lens.


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## table1349 (Apr 30, 2017)

petrochemist said:


> gryphonslair99 said:
> 
> 
> > 1.  NO  you are wrong.  Focal Length is Focal Length, PERIOD.  It is a physical property of the lens.  No adapter changes it's focal length.  A 50mm lens is 50mm lens no matter if it is on a $50 P&S or on the Hubble Telescope.  You might want to peruse this. Digital Photography Tutorials
> ...



*Exactly. * 

Just as a APS-c sensor does not change the focal length of the lens.  Just as a magnifying glass or a pair of glasses does not change the size of the print on the page.  

The point of convergence from the lens DOES NOT CHANGE, there for the focal length DOES NOT CHANGE.  It can be manipulated via external forces but the focal length of the lens DOES NOT CHANGE.

Now can we let this zombie thread die?????


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