Crop factor and lenses

[G.T.]

Hi,

I just wanted to get someone to confirm this. I have a micro 4/3rds camera and when people talk about the camera/lens they sometime say about 35mm equivalents. Since the sensor is a quarter of full-frame it has a 2x crop factor. (I understand that if a m43 lens is 20mm then it is 20mm but if you wanted to compare to full-frame it would be 40mm equivalent). I have an adapter and use an an old film camera lens which is 50mm. This would however give a 100mm field of view because the sensor will not be receiving all the light coming through the lens, since it is smaller than what that lens was designed for, and therefore it is cropped.
So my question relates to APS-C lenses. Like I say most people always compare to full-frame, but it would be wrong to use those equivalents for an APS-C only lens? If my understanding is correct then I am right, but would like someone else to confirm or reject this.
I have read somewhere that the difference of m43 and APS-C sensors is about 1.25 so using that, if I got a lens only for an APS-C sensor that was 50mm then the field of view I would get is 62.5mm?

 

[Mrphildog]

Crop factor and lenses

The focal length of a lens is independent of the camera it is mounted on.

APS-C lenses are the focal length they say. So a lens designed for a full frame camera of say 50mm and a lens designed for an APS-C only camera of 50mm will have the same focal length.

This situation is unfortunately confused due to manufacturers of compact cameras quoting the focal lengths of their lenses in 35mm equivalents. This does not happen with interchangeable lens systems.

I expect someone will come along and tell you the opposite as people do get confused by this. It is easily tested however. Take a 50mm Canon EF lens and a 50mm EF-S lens and you will see that they have the same focal length (well very close taking into account that the focal length could be 49.x mm on one). The difference between the lenses is that the EF-S lens will only cast an image circle big enough to cover an APS-C sensor.

In short if you mount a 50mm APS-C lens to m4/3 system it will show the equivalent to 100mm lens field of view.

 

[G.T.]

In short if you mount a 50mm APS-C lens to m4/3 system it will show the equivalent to 100mm lens field of view.

Are you sure? Just because you say an "EF-S lens will only cast an image circle big enough to cover an APS-C sensor" which would mean if the sensor became micro 43 the difference between the two is not a 2x crop factor. Less image is lost than if the 50mm was one that is designed to cover a full-frame sensor.

Again I may still be misunderstanding. I've not got an APS-C lens to test this. But my other reasoning it when you see people putting lens that don't cover the whole sensor onto a camera and you therefore get black borders.

 

[Mrphildog]

Crop factor and lenses

Are you sure? Just because you say an "EF-S lens will only cast an image circle big enough to cover an APS-C sensor" which would mean if the sensor became micro 43 the difference between the two is not a 2x crop factor. Less image is lost than if the 50mm was one that is designed to cover a full-frame sensor.

Again I may still be misunderstanding. I've not got an APS-C lens to test this. But my other reasoning it when you see people putting lens that don't cover the whole sensor onto a camera and you therefore get black borders.

Yes. I am 100% sure. I do have full frame and APS-C lenses of the same focal length to compare.

The image circle is not the same as the field of view. When a full frame lens is attached to an APS-C camera then some of the light cast by the lens is off sensor. When a lens designed for APS-C only lens is attached to an APS-C camera then less of the image circle is off sensor. This is not the same thing as field of view.

The focal length of a lens is not a relative measure. It is an absolute measure.

Here is another forum with the same question and same answer.
http://www.dpreview.com/forums/thread/2624824

 

[G.T.]

Yes. I am 100% sure. I do have full frame and APS-C lenses of the same focal length to compare.

The image circle is not the same as the field of view. When a full frame lens is attached to an APS-C camera then some of the light cast by the lens is off sensor. When a lens designed for APS-C only lens is attached to an APS-C camera then less of the image circle is off sensor. This is not the same thing as field of view.

The focal length of a lens is not a relative measure. It is an absolute measure.

Ah, okay. Thanks.

 

[Mrphildog]

No problem. If it helps there's a page here that illustrates the concept.

http://www.creativephotobook.co.uk/pg04009.html

Cheers.

 

[G.T.]

Just to give a bit of background to my question. I was thinking in terms of wide-angle lens. Since to get a wide field of view on a small sensor you have to buy quite a wide-angled lens. But there are so many older cameras out there, that I could get second hand (in theory cheaper, though I've not really researched prices/quality of image). Like I mentioned I have my 50mm but the pictures don't let me fit so much in the image (obviously). I just thought if I was to get something that was say 14mm for APS-C sensors only compared to a 14mm that was made to cover 35mm frames then I would get a wider field of view.

I'm probably going to change systems anyway soon. Choosing M43 was just to see how I liked photography and the smaller form of the cameras was appealing.
I just wondered about the maths last night as I was thinking my options and thought I'd ask someone else who probably knew.

Thanks again

 

[G.T.]

No problem. If it helps there's a page here that illustrates the concept.

http://www.creativephotobook.co.uk/pg04009.html

Cheers.

That link helped.
So I should think of focal length as a magnification value, therefore, 50mm will make objects a certain size on the sensor and the size of the sensor just determines how much of the scene you will actually see? 20mm changes this magnification (squeezes more in) but again the sensor size doesn't change this it just crops how much of the scene is shown?

Which mean I should really be looking at changing sensor size and getting one that is bigger. (I could get a really wide m43 lens but the sensor is still limited in a few ways)

 

[Mrphildog]

That link helped.
So I should think of focal length as a magnification value, therefore, 50mm will make objects a certain size on the sensor and the size of the sensor just determines how much of the scene you will actually see? 20mm changes this magnification (squeezes more in) but again the sensor size doesn't change this it just crops how much of the scene is shown?

Which mean I should really be looking at changing sensor size and getting one that is bigger. (I could get a really wide m43 lens but the sensor is still limited in a few ways)

Yes that's a good way to look at it. Unfortunately with m4/3 you have to get a lens with a very short focal length to get wide angle. This is why they have lenses like 7-14mm and they are unfortunately quite expensive.

To get the same angle of view as a 7-14mm m4/3 lens you'd need a 7-14mm lens and whether it was designed for full frame or APS-C wouldn't matter. There's no easy shortcut unfortunately.

Personally I have a m4/3 camera with a 20mm lens and a 14mm lens as a carry everywhere camera and an APS-C camera which I use with a 10-20 lens for wide angle stuff.

 

[G.T.]

Yes that's a good way to look at it. Unfortunately with m4/3 you have to get a lens with a very short focal length to get wide angle. This is why they have lenses like 7-14mm and they are unfortunately quite expensive.

To get the same angle of view as a 7-14mm m4/3 lens you'd need a 7-14mm lens and whether it was designed for full frame or APS-C wouldn't matter. There's no easy shortcut unfortunately.

Personally I have a m4/3 camera with a 20mm lens and a 14mm lens as a carry everywhere camera and an APS-C camera which I use with a 10-20 lens for wide angle stuff.

Yeah I have thought about the 7-14mm but like you say it costs lots. I imagine with time m43 may get cheaper to make but then the engineering must be a challenge with the small form factor or the system. I have the 20mm and 14mm too, it's a nice carry everywhere set. But I think I won't put anymore money into it and just choose a new system now and go from there. Like I said I like night photography/low light and while the sensors have come a long way for noise it is still limited because the pixels are so close together.

 

[ElectronGuru]

Having used a few formats, I find FOV to be confusing. The two key 'view' concepts are the size of the image circle and the angle of view. The point of a wide lens is getting a wide angle. Lenses aren't labeled in degrees (100, 120, 140), but that's actually what you are buying.

Putting a FF 24mm lens onto a crop body doesn't change the angle of view, it just crops the image circle - so you are confined to the center if a wide view. The real advantage of a crop camera is that lenses can be smaller and lighter for the same angle. In theory this should also make them cheaper, but newness often overrides that. Especially when the whole format is new.

In practice, the easiest way to do wide is with a larger sensor. That's also how you get higher ISOs, but new FF glass is not the way to go to save $$.

 

[Designer Dale]

Please read the last paragraph of this link before the comments section. It explains lenses and crop/full frame cameras in simple terms. It doesn't address M4/3.

http://improvephotography.com/699/the-difference-between-full-frame-and-crop-sensor-dslr-cameras/

I'm pretty sure a lens for a micro 4/3 camera is physically smaller than a DSLR lens and won't fit. So the DSLR on my M4/3 is a mute point.

Focal length is not to be thought of as a magnification factor. I'll see if I can look it up and edit this post.
-----------------------------------------
Here's my edit:

This link explains focal length without all the math involved:

http://www.paragon-press.com/lens/lenchart.htm

Here's a screenshot:



If you have a short lens like a 12mm, the lens is close to the sensor plane (12mm away) and the field of view is wide (wide angle). It the lens is long, the distance between the lens and sensor when the lens is focused at infinity might be 400mm and the field of view would be narrow (telephoto). It's not about magnification, it's about the angle of view of the lens.

Take a few moments to imagine what happens to those lines from the edges of the sensor as the oval that represents the lens moves closer or further from the sensor plane and you will get "focal length" and how it relates to "field of view".


Dale

 

[acearchie]

If you have a short lens like a 12mm, the lens is close to the sensor plane (12mm away) and the field of view is wide (wide angle).

Be careful here. Regardless of what lens you put on a camera you have to take into account the flange distance.

You will see from that link that a Canon EF and EF-S mounts for example have a flange distance of 44mm

 

[Designer Dale]

Be careful here. Regardless of what lens you put on a camera you have to take into account the flange distance.

You will see from that link that a Canon EF and EF-S mounts for example have a flange distance of 44mm

I had a thing in the back of my head that you couldn't use some crop lenses on ff or ff on crop because of the distance relation with the mirror plane, but I couldn't straighten that out as to what didn't fit what so I skipped it...

Sometimes I say what I seem to recall in the back of my head and get lectured to for all eternity by Internet smarties. (You aren't classified as such)

Dale

 

[acearchie]

Sometimes I say what I seem to recall in the back of my head and get lectured to for all eternity by Internet smarties. (You aren't classified as such)

Dale

I often know what you mean.

Often I don't bother saying anything because I know someone will probably prove me wrong and then insult me along the way but fortunately the digital photography sub-forum appears to have some more mature members!

 

[ChrisA]

Here is an easy way to do the computations, not 100% accurate but very close for normal and long lenses for subjects that are not to close.

The method is graphical you need a ruler with mm scale and blank paper.

Draw a line that is as wide as your camera's sensor. For an APS-C that you be about one inch, for full from 35mm that would be 35mm long. This line is the film or sensor plane.

Now place a dot one focal length away from the first line and centered over it. Now make a triangle by conecting the dot to each end of the line.

You see now that if the line is wider with the same lens the field of view is wider.

What you have done here is drawn a full size model of a pin-hole camera. The focal length of a pin hole camera is the hole to film distance. It is easy to see that putting in bigger film will crop less of the image.

 

[The Mad Hatter]

Hi,

I just wanted to get someone to confirm this. I have a micro 4/3rds camera and when people talk about the camera/lens they sometime say about 35mm equivalents. Since the sensor is a quarter of full-frame it has a 2x crop factor. (I understand that if a m43 lens is 20mm then it is 20mm but if you wanted to compare to full-frame it would be 40mm equivalent). I have an adapter and use an an old film camera lens which is 50mm. This would however give a 100mm field of view because the sensor will not be receiving all the light coming through the lens, since it is smaller than what that lens was designed for, and therefore it is cropped.
So my question relates to APS-C lenses. Like I say most people always compare to full-frame, but it would be wrong to use those equivalents for an APS-C only lens? If my understanding is correct then I am right, but would like someone else to confirm or reject this.
I have read somewhere that the difference of m43 and APS-C sensors is about 1.25 so using that, if I got a lens only for an APS-C sensor that was 50mm then the field of view I would get is 62.5mm?

What's even more confusing is that now Nikon has a camera (D7100) that is already a 1.5 crop sensor. However, you can also shoot in 1.3x mode. So you have cropping in a cropped sensor.

Geez, what's the focal conversion on that? ;-)