Lens and what mm

Hadley

Hadley

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James
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From what I can work out the magnification on lens is measure in mm, and that they stand for millimeter.

However what I don't understand is where this unit comes from and why a 28mm lens for a 35mm-film camera looks alot wider than 28mm lens on a DSLR.
 
Hadley said:
From what I can work out the magnification on lens is measure in mm, and that they stand for millimeter.

However what I don't understand is where this unit comes from and why a 28mm lens for a 35mm-film camera looks alot wider than 28mm lens on a DSLR.
Click to expand...

The measurement is the focal length of the lens which refers to the distance between the front of your lens and the imaging plane (the sensor or film).

It's essentially a indication of the angle of view of the lens rather than it's magnification - so a long lens, eg 300mm, will have a much narrower angle of view compared to a wide lens, eg 10mm.
This isn't always the physical length of the lens as many will use optics and sometimes mirrors to achieve this length inside the lens.

By convention it's quoted as though it was mounted on a 35mm camera although due to the smaller size of a digital sensor the same lens will appear to have a longer focal length. This is typically referred to as the crop factor; for example many Canon dSLRs have a crop factor of 1.6x which means for a given length of lens the 35mm-equivalent focal length is 1.6x the quoted length.
ie a 28mm lens on a Canon 40D gives me the same angle of view as if I had used a 45mm lens on a full-frame or 35mm camera.

- there's been lots of discussion on here before over what is actually happening in the camera, it's enough to say that your lens is not magically longer but that the sensor captures a smaller area which is then blown up more to fit the equivalent print size.


Does any of that help?!? :bonk:
 
The best way to look at it is this...

Get a kitchen roll holder and look through it with the roll against your eye. So what you can see..... The roll is maybe 300mm long, just imagine that what you can see will fill the screen.

Now cut it in half and do the same. Ignore the actual size but see what you can see, it will be more. Again just imagine that what you can see will fill the screen.

Do the same again cutting it in half to make it 75

Then chop off a third to get 50

Then chop it in half again to get 25

Then again to get 12 ish

This is HOW it works.....
 
On the same note, if your eye is touching the kitchen roll holder, that's like full frame, if you have the toilet roll holder about 5/10cm away, thats like 1.5/1.6x (the cheaper digital cameras)
 
Another way of explaing the crop factor - if a full frame sensor was a sheet of A4 paper (representing a 35mm film) then a crop sensor would be a sheet of A5 in the middle of it (this isn't to ratio before anyone says something!).
Because the smaller sensor is only looking at the middle of the image you are effectively gaining on the length of the lens.

I hope I've not confused you further!
 
I think aligibbs FF/crop example is best :thumbs: and it is how it actually is!
 
Hadley said:
Thanks to all who replied, it now makes sense.
Click to expand...

:thinking:

The focal length defines the magnification, nothing else.

The field of view, also known as the angle of view (or more simply what you get in the picture) is defined by two things, a) the magnification - how large the objects in the projected image appear, ie focal length, and b) the size of the sensor, which determines how much of that projected image it is able to capture.

A small sensor only captures a small area of the projected image, over a narrow angle. A larger sensor captures more, over a broader angle, recording a wider field of view.

To take an extreme example, a 300mm lens is normally regarded as a long telephoto lens because on a camera with a small sensor you don't get very much in the picture. But on a large format camera that takes 10 x 8inch sheet film, 300mm is a 'standard' lens and will capture such a wide field of view it is equivalent to a 30mm lens on a crop format DSLR. Of course the two lenses are very different animals but if you were to fit the 300mm lens intended for a 10 x 8in camera onto a DSLR what you would see in the viewfinder would be exactly the same as with any other 300mm lens.

For clarity, the 35mm format (also known as full frame) is named after the width of the film, including the sprocket holes. The actual image area is 24 x 36mm. In this context, '35mm' has nothing to do with lenses.

The crop factor was originaly intended to help film users wanting to use their old lenses on new digital cameras to get an idea of what they new framing would look like. Canon use a 1.6x (linear) crop factor because they use a sensor that is 22.3 x 14.9mm which has an area less than half the size of full frame. Nikon/Sony are fractionally larger with a 1.5x crop factor and the 4/3rds format (Olympus/Panasonic) is quite a bit smaller, being one quarter the area of full frame and so it has a 2x crop factor.

To establish how much you will actually get in the picture from a given lens when used on a camera with a different sized sensor to full frame, you multiply/divide the focal length by the crop factor. The focal doesn't change, but the area of the captured image does, so that gives you the new equivalent focal length.

For example, if you have a 50mm lens on a full frame camera, and then fit it to a 1.6x crop format DSLR, the smaller sensor will capture a smaller area of the scene, equivalent to using an 80mm on full frame (50 x 1.6 = 80).
 
HoppyUK said:
:thinking:

The focal length defines the magnification, nothing else.

The field of view, also known as the angle of view (or more simply what you get in the picture) is defined by two things, a) the magnification - how large the objects in the projected image appear, ie focal length, and b) the size of the sensor, which determines how much of that projected image it is able to capture.

A small sensor only captures a small area of the projected image, over a narrow angle. A larger sensor captures more, over a broader angle, recording a wider field of view.

To take an extreme example, a 300mm lens is normally regarded as a long telephoto lens because on a camera with a small sensor you don't get very much in the picture. But on a large format camera that takes 10 x 8inch sheet film, 300mm is a 'standard' lens and will capture such a wide field of view it is equivalent to a 30mm lens on a crop format DSLR. Of course the two lenses are very different animals but if you were to fit the 300mm lens intended for a 10 x 8in camera onto a DSLR what you would see in the viewfinder would be exactly the same as with any other 300mm lens.

For clarity, the 35mm format (also known as full frame) is named after the width of the film, including the sprocket holes. The actual image area is 24 x 36mm. In this context, '35mm' has nothing to do with lenses.

The crop factor was originaly intended to help film users wanting to use their old lenses on new digital cameras to get an idea of what they new framing would look like. Canon use a 1.6x (linear) crop factor because they use a sensor that is 22.3 x 14.9mm which has an area less than half the size of full frame. Nikon/Sony are fractionally larger with a 1.5x crop factor and the 4/3rds format (Olympus/Panasonic) is quite a bit smaller, being one quarter the area of full frame and so it has a 2x crop factor.

To establish how much you will actually get in the picture from a given lens when used on a camera with a different sized sensor to full frame, you multiply/divide the focal length by the crop factor. The focal doesn't change, but the area of the captured image does, so that gives you the new equivalent focal length.

For example, if you have a 50mm lens on a full frame camera, and then fit it to a 1.6x crop format DSLR, the smaller sensor will capture a smaller area of the scene, equivalent to using an 80mm on full frame (50 x 1.6 = 80).
Click to expand...

Richard,

He DID understand it :bonk: ;)
 
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