Regarding field of view and sensor size

[TCampbell]

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

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.

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..

He said "utterly separate". I think it has something to do with a dairy cow.

 

[dsiglin]

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.

 

[WayneF]

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).

 

[Norten]

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

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.

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.

 

[table1349]

 

[Bebulamar]

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.

 

[table1349]

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.

 

[Bebulamar]

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.
View attachment 139040

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.

 

[petrochemist]

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.

 

[table1349]

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

the focal length of the lens itself doesn't change

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?????