Full-frame lens on a crop sensor camera

[Bonesh]

Hi everyone!

I'm struggling a bit with understanding the crop sensor/full-frame in a specific case and would appreciate your help.

I do understand that a full-frame lens on a full-frame camera is not the same as on a crop sensor camera. But are the numbers on the APS-C lens already 'calibrated' or does the same rule apply also for these lenses?
So let's say I have a 50 mm APS-C lens and a 50 mm full-frame lens on a crop sensor camera - will the focal length be ~80mm on both lenses or is it 80 mm on full-frame and 50 on APS-C?

I hope that I explained my question clear enough and I thank you in advance.

 

[MarshallG]

The focal length does not change.
If you put a 50mm full frame lens on a crop sensor body, it is still a 50mm lens.

The crop factor occurs because the smaller sensor in the camera simply crops the image: Only the central portion of the image is exposed to the sensor, and the rest is ignored, or “cropped out.”

You can achieve the same result with a full frame camera by simply cropping the image in Photoshop. if you crop an image and then enlarge the image, you will have magnified the image. That’s why the magnification of a 50mm lens on a crop sensor body is equivalent to an 80mm lens on a full frame body.

 

[Dem Bell]

So let's say I have a 50 mm APS-C lens and a 50 mm full-frame lens on a crop sensor camera - will the focal length be ~80mm on both lenses or is it 80 mm on full-frame and 50 on APS-C?

Both lenses are 50 mm and will have the number "50" engraved on their barrels to prove it. That is their focal length: 50 mm.

If you attach either lens to your Canon APS-C camera, they will give you the same angle of view. This angle of view is the same as any hypothetical 80 mm lens would give on a full frame camera.

I hope that I explained my question clear enough and I thank you in advance.

 

[FrancoD]

it is really very simple to understand.







With the same lens, on FF you see the Green area , with DX you see the red area.

 

[HappyCampers]

manufacturers alway (I think) refer to a full frame focal length when describing lenses, even if that lens cannot be used on FF

for example I have an APSC only 35mm Macro, the focal length is 35mm but the field of view (al la pic above) means it behaves like a 50mm on APSC

Focal length is defined by the lens design, field of view is dicated by the sensor size - so if you could put my 35mm macro on a 4/3 sensor camera it would behave like a 70mm

 

[mclewis]

manufacturers alway (I think) refer to a full frame focal length when describing lenses, even if that lens cannot be used on FF

Manufacturers always put the actual focal length of the lens on their lenses. This has absolutely nothing to do with the sensor/film size. It is a physical property of the lens itself.

 

[FingerPainter]

it is really very simple to understand.



With the same lens, on FF you see the Green area , with DX you see the red area.

And that area in red is very similar to what you would see over the whole frame if you put a 75mm lens on a FF camera.

 

[FingerPainter]

manufacturers alway (I think) refer to a full frame focal length when describing lenses, even if that lens cannot be used on FF

While that may be true of manufacturers of APS-C and larger format cameras, it is not always true of makers of smaller format systems. Many of these will refer to their lenses in FF equivalent focal length (but never mention FF equivalent apertures). Even these manufacturers do engrave actual focal lengths on their lenses though.

for example I have an APSC only 35mm Macro, the focal length is 35mm but the field of view (al la pic above) means it behaves like a 50mm on APSC

Focal length is defined by the lens design,

field of view is dicated by the sensor size

and its interaction with the focal length. The interaction is predictable The field of view on a smaller format than FF is always the same as that of a lens on FF whose focal length is equal to the focal length of the lens used on the smaller format multiplied by the diagonal dimension of a FF sensor and divided by the diagonal dimension of the smaller sensor. This ratio of sensor diagonals is called the "crop factor".

- so if you could put my 35mm macro on a 4/3 sensor camera it would behave like a 70mm

 

[Barry Twycross]

The crop factor applies to the camera, it has nothing to do with the lens. The lens does not change, the focal lengths is a fundamental property of the lens. it is always the focal length written on it.

If it's a 50mm lens, it's a 50mm lens on FF, and it's a 50mm lens when on APS-C.

Thinking the lens changes because it's put on a different body causes all sorts of confusion like you have.

What changes when you put the lens on a different format camera is the angle of view, as illustrated by a previous reply. The crop factor is telling you is you use a different format camera, how much you'd need to change the lens by to keep the angle of view the same.

So if you have an APS-C camera with a 1.6x crop, and a certain lens (50mm in your case). If you wanted to have something which looked the same on full frame, you'd need a lens which was 1.6x longer (had a 1.6x focal length), that is you'd need an 80mm lens to do the same job. That's telling you about the camera, not the lens.

 

[Michael Fryd]

Hi everyone!

I'm struggling a bit with understanding the crop sensor/full-frame in a specific case and would appreciate your help.

I do understand that a full-frame lens on a full-frame camera is not the same as on a crop sensor camera. But are the numbers on the APS-C lens already 'calibrated' or does the same rule apply also for these lenses?
So let's say I have a 50 mm APS-C lens and a 50 mm full-frame lens on a crop sensor camera - will the focal length be ~80mm on both lenses or is it 80 mm on full-frame and 50 on APS-C?

I hope that I explained my question clear enough and I thank you in advance.

The fundamental problem is that "focal length" is not a good way of describing "angle of view".

Focal length is a physical property of the lens, and is independent of the sensor size. A 50mm lens is always 50mm, no matter which body it is mounted on (and even if it is not on any body).

Angle of view is determined by both focal length and sensor size.

A 50mm lens on a full frame body will yield a 47° angle of view. That same lens on a 2X crop body will yield a 24° angle of view. In both cases the focal length remains 50mm.

The difference between a 50mm full frame and 50mm crop body lens is not related to the angle of view. It has more to do with "vignetting". The crop body lens will likely vignette more, but the difference won't be noticeable on a crop body.

For historical reasons, Angle of View is frequently expressed in terms of the corresponding focal length for a 35mm film SLR.

For instance a 100mm lens on a 35mm film SLR yields a 24° angle of view. Therefore any combination of lens/body that results in a 24° angle of view, may be called "100mm equivalent"

A 50mm lens on a 2X crop body is one combination that yields that 24° angle of view. Therefore a 50mm lens on a 2X crop body is sometimes called "100mm equivalent", even though the lens remains a 50mm focal length lens

 

[Great Bustard]

Hi everyone!

I'm struggling a bit with understanding the crop sensor/full-frame in a specific case and would appreciate your help.

I do understand that a full-frame lens on a full-frame camera is not the same as on a crop sensor camera. But are the numbers on the APS-C lens already 'calibrated' or does the same rule apply also for these lenses?
So let's say I have a 50 mm APS-C lens and a 50 mm full-frame lens on a crop sensor camera - will the focal length be ~80mm on both lenses or is it 80 mm on full-frame and 50 on APS-C?

I hope that I explained my question clear enough and I thank you in advance.

A 50mm FF lens mounted on an APS-C camera will give the same angle of view as a 50mm APS-C lens mounted on an APS-C camera (plus or minus slight differences between the actual focal length and marked focal length -- e.g. one 50mm lens might actually be 48mm and another might actually be 52mm).

In addition, the f-numbers will have the same effect. For example, a 50mm FF lens at f/1.4 on the APS-C camera will result in the same exposure as a 50mm APS-C lens at f/1.4 on an APS-C camera (barring in differences in t-stops and vignetting -- e.g. one lens might be t/1.8 at f/1.4 and another t/1.6 at f/1.4; one might have 2 stops of vignetting at f/1.4 and another might have 3 stops of vignetting at f/1.4).

If, however, we mounted a 50mm APS-C lens on a FF camera, the image circle may not be large enough to cover the entire sensor. However, if the image circle were large enough, all the above applies.

 

[Michael Fryd]

Hi everyone!

I'm struggling a bit with understanding the crop sensor/full-frame in a specific case and would appreciate your help.

I do understand that a full-frame lens on a full-frame camera is not the same as on a crop sensor camera. But are the numbers on the APS-C lens already 'calibrated' or does the same rule apply also for these lenses?
So let's say I have a 50 mm APS-C lens and a 50 mm full-frame lens on a crop sensor camera - will the focal length be ~80mm on both lenses or is it 80 mm on full-frame and 50 on APS-C?

I hope that I explained my question clear enough and I thank you in advance.

A 50mm FF lens mounted on an APS-C camera will give the same angle of view as a 50mm APS-C lens mounted on an APS-C camera (plus or minus slight differences between the actual focal length and marked focal length -- e.g. one 50mm lens might actually be 48mm and another might actually be 52mm).

In addition, the f-numbers will have the same effect. For example, a 50mm FF lens at f/1.4 on the APS-C camera will result in the same exposure as a 50mm APS-C lens at f/1.4 on an APS-C camera (barring in differences in t-stops and vignetting -- e.g. one lens might be t/1.8 at f/1.4 and another t/1.6 at f/1.4; one might have 2 stops of vignetting at f/1.4 and another might have 3 stops of vignetting at f/1.4).

f/1.4 will provide the same exposure (light per unit area), but I would not say it has the same effect.

At the same exposure, overall image noise tends to go down as sensor size increases. Thus f/1.4 on a smaller sensor tends to yield a noisier image than f/1.4 on a larger sensor.

Focal length does not change with sensor size, but the effect on angle of view does.

Aperture does not change with sensor size, but the effect on overall image noise does.

If, however, we mounted a 50mm APS-C lens on a FF camera, the image circle may not be large enough to cover the entire sensor. However, if the image circle were large enough, all the above applies.

 

[Great Bustard]

Hi everyone!

I'm struggling a bit with understanding the crop sensor/full-frame in a specific case and would appreciate your help.

I do understand that a full-frame lens on a full-frame camera is not the same as on a crop sensor camera. But are the numbers on the APS-C lens already 'calibrated' or does the same rule apply also for these lenses?
So let's say I have a 50 mm APS-C lens and a 50 mm full-frame lens on a crop sensor camera - will the focal length be ~80mm on both lenses or is it 80 mm on full-frame and 50 on APS-C?

I hope that I explained my question clear enough and I thank you in advance.

A 50mm FF lens mounted on an APS-C camera will give the same angle of view as a 50mm APS-C lens mounted on an APS-C camera (plus or minus slight differences between the actual focal length and marked focal length -- e.g. one 50mm lens might actually be 48mm and another might actually be 52mm).

In addition, the f-numbers will have the same effect. For example, a 50mm FF lens at f/1.4 on the APS-C camera will result in the same exposure as a 50mm APS-C lens at f/1.4 on an APS-C camera (barring in differences in t-stops and vignetting -- e.g. one lens might be t/1.8 at f/1.4 and another t/1.6 at f/1.4; one might have 2 stops of vignetting at f/1.4 and another might have 3 stops of vignetting at f/1.4).

f/1.4 will provide the same exposure (light per unit area), but I would not say it has the same effect.

At the same exposure, overall image noise tends to go down as sensor size increases. Thus f/1.4 on a smaller sensor tends to yield a noisier image than f/1.4 on a larger sensor.

Focal length does not change with sensor size, but the effect on angle of view does.

Aperture does not change with sensor size, but the effect on overall image noise does.

I was talking about two 50mm lenses, one designed for FF, the other designed for APS-C, both mounted on an APS-C camera (but not at the same time, obviously!) as opposed to talking about a 50 / 1.4 on a FF camera vs a 50 / 1.4 on an APS-C camera.

 

[Bonesh]

Thank you everyone for clearing up my confusion!

I'll have to dive deeper into how cameras are made, but for now I learned what I wanted to know.