Full-frame / 1.6x explained please!

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Ian T

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Ok, I'm confused. From what I gather, a full frame DSLR is one that has a sensor equivalent in size to a 35mm frame. Is this right?

But with lenses that fit both film and digital SLRs, how are the focal lengths affected for example? If I was to fit a Nikon 18-70 DX lens from a D70 onto a Nikon film body with the correct mount, would it work? How would the focal length of the lens be affected?

Also, when looking at compact cameras I see the lenses shown as '28mm equivalent', etc. Does this mean I'd have the same field of view as if I looked through the viewfinder of an SLR with a 28mm lens attached?
 
Basically focal length is NOT affected at all, you get the same amount of magnification from a 300mm lens on a 1.6 sensor as with a full frame, what you don't get is the same coverage of area. A 1.6 size sensor will see less (the centre portion) of what the lens projects than a full sensor would.
 
Ian T said:
Also, when looking at compact cameras I see the lenses shown as '28mm equivalent', etc. Does this mean I'd have the same field of view as if I looked through the viewfinder of an SLR with a 28mm lens attached?
Click to expand...

Basically.... Yes.

You have to take the true focal length of the lens (note this is Steep'ies bug bear :p ) and multiply it by the crop factor of the sensor to get the "effective focal length".
You get more effective zoom for "free", which is a pain if you want to do wide angle shots, but great if you are shooting things far away.. wildlife etc..
 
oldgit said:
You get more effective zoom for "free",
Click to expand...

No no no no no, images taken with the same lens will be exactly the same size on both sensor types. it's only the field of view that changes.


/edit honest guv!
 
Here's an example of what I mean. Note the baby is the same size in each ;)

Full sensor
fullsensor.jpg

1.6 crop factor sensor
onepointsixsensor.jpg
 
Here's an explanation that I typed up a while ago on a different forum:


Most modern film cameras have a film size of 35mm (diagonal).

Most digital cameras (aside from those such as the Canon 1DS MkII, which are marked as 'full frame' cameras) do not have a digital sensor this size. The sensor is smaller. This produces the following effect (image courtesy of Canon):
angle-main04.jpg


The full frame sensor captures more of the scene, but when the image files of a APS-C camera are printed/displayed with the same physical dimensions, the APS-C camera shot appears to have further reach. (Of course, the same effect could be achieved by cropping the full-frame camera's image).

Thus you can multiply the focal length of a lens (eg 300mm) by the crop factor (eg, 1.6 for Canon SLRs) to get the efl, 480mm.


EDIT - dammit, I went to the trouble of typing all that and then I find this link
http://en.wikipedia.org/wiki/Crop_factor
 
Steep said:
No no no no no, images taken with the same lens will be exactly the same size on both sensor types. it's only the field of view that changes.
/edit honest guv!
Click to expand...

:geek: mode on...

Nope.... not quite...

What we are missing from this argument is the number of pixels on the captured image.

If the 1.5x sensor had correspondingly less pixels then your statment would be true.
(my maths is probably wrong here, but here goes)

ie: if the full frame sensor has 100 pixels (10x10) then for the 1.5x sensor to have no magnification then it would have to have 66.6.. pixels (approx 8x8).
If you printed both images out using the same print resolution (say 1mm per pixel) then
the 1.5x image would indeed lie perfectly on the 1x image.
1x image = 10mm x 10mm
1.5x image = 8mm x 8mm

However if both the 1x and 1.5x sensor have exactly the same number of pixels (10*10) then when printed out using the same 1mm pixel size then
1x image = 10x10mm
1.5x image = 10x10mm
so 1.5x image will be 1.5X larger and have captured 1.5X more detail. = 1.5x effective zoom.

This is the sort of maths that makes sense to me... rather than measuring over the diagonals of a 35mm film (could go into grain sizes of the emulsion and relativistic effects of photon sizes, but that would be plain silly).

So what is quoted by manufacturors etc. is the assumption that if you had a 1X sensor and a 1.5x sensor with the same number of pixels you do get an "effective" zoom. ("effective" being the key word)

:geek: mode off

:)
 
LOL. Aren't we making hard work of this 'magnification gain' when using tele lenses?

It comes about surely from comparisons with similar focal lengths used on 35mm cameras?

It's absolutely true that whatever the size of the sensor you record your image on, the image size will be the same on the sensor with any given focal length lens - the focal length of the lens doesn't change.

So if we stick a 100mm lens on a 1.6X crop camera we have to multiply by that crop factor, so 100mm x 1.6 = 160mm.

What that means is that you get the same field of view with your 100mm lens as you would have got with a 160mm lens on a 35mm film camera which has the equivalent size to a full frame sensor (24X36mm).
 
The sensor does not magnifiy the image oldgit, the lens does. Obviously the more pixels on the sensor the larger you can display the image but the components of the image (ie the baby or Adwrights parasols) are still physically the same size on the sensor. The images from my 30D are bigger than the images from my 300d because it has more pixels but the sensor is the same 1.6 crop size in both. If pixels (size or number) had any bearing on it, you would not be able to say effective zoom of 300mm = 480mm for all 1,6 crop factor cameras because the pixel count would be different and so the 'effective' zoom would also be different.

Cheers CT :)
 
The zoom never changes, irrespective of the size of the sensor.

On a Canon APS-C sensor a 70-200mm lens will give the same field of view as a 112-320mm lens would on a full frame camera.

The amount of zoom from 70 to 200 is x2.86.
The amount of zoom from 112 to 320 is x2.86.

You never get any extra zoom for free, nor would you want to. It does not imply anything about the sensor size of the magnification of an object.

Sigma's 18-200mm lens has an x11.1 zoom, while their 50-500mm Bigma is only a ten times zoom. The Bigma will magnify an object far more than the former despite having less zoom.

It seems zoom is being confused with magnification and resolution.

Zoom is the factor of magnification between two focal lengths.
Magnification is the ability to enlarge something.
Resolution is the density of pixels (and detail) in a captured image.

Michael.
 
Ian T said:
But with lenses that fit both film and digital SLRs, how are the focal lengths affected for example?
Click to expand...

Unless you have used, or are planning to use, a full 35mm frame camera (digital or film) ignore the issue completely.

Focal lengths are a property of the lens regardless of the size of the film or sensor size and so they are all measured the same. All that matters is understanding the relationship between the image your camera captures and how it magnifies the scene.

The only reason focal lengths are usually given in those terms is so that people familiar with 35mm film cameras can relate to the different field of view. Unfortunately this has also lead to a lot of confusion.

For a digital only shooter (and one not planning on getting a full frame DSLR) there is as much point as worrying about the focal length in 35mm terms as there is in converting everything to medium format. The only reason to relate everything to 35mm frames is if you are converting from a system you already know, or want to share lenses between cameras with different frame sizes.

If you want to read all that said in more words, lol, then I posted it on this thread a few months ago:
http://www.talkphotography.co.uk/forums/showthread.php?t=15233

Michael.
 
mij said:
Resolution is the density of pixels (and detail) in a captured image.

Michael.
Click to expand...

And that's basicaly the bottom line and the key to the end product, with certain caveats such as the interaction between the photosites on a sensor.
 
I love it when this emotive thread resurfaces every quarter or so :lol: :lol: :lol:

its the field of view not the focal length that is cropped. A 100mm lens retains a 100mm focal length on a cropped sensor, but the field of view is reduced. Clear as mud.

Adam - i believe a Canon / Darkside collaboration may be required to illustrate this graphically ;)
 
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