Basic Question.

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richardjjj

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Name
Richard
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Firstly, Hello to all.
I have recently rekindled my interest in photography and have been reading this excellent and other sites to get up to speed with the current technology available.
One element (among many) that confuses me is the expression 'that on a cropped sensor body a lens has 'extra reach' as opposed to when used on a full frame body'
I understand that the field of view will be smaller on a cropped sensor but cannot understand the 'extra reach'
Any clarification much appreciated.
Thanks.
Richard
 
canon cropped sensors are a factor of 1.6 compared with canon full frame. Therefore a 100mm lens on a cropped sensor will in effect be equivalent to 160mm on a full frame. ie 1.6 times closer.

Hope that makes sense
 
There are many mis-understandings about how cropping factor works in DSLRs. From my understanding (and I am likely to be contradicted) - the focal length of the lens is an absolute. However, because the image is cropped, you will get the image from the central section of the frame and hence be the equivalent of cropping and magnifying during printing - as such it appears to behave as a longer lens in terms of perspective and image size.

However, as the focal length of the lens remains the same, the depth of field is greater than the longer "equivalent" lens on a full frame body.
 
Thanks both for the replies.
My understanding is as Graham's however I can understand that the view will be larger on full frame but wouldn't the depth of field also be the same if the same apeture was used? or am I now confused by the terminology?
Regards
 
no because depth of field is to do with magnification not absolute focal length

think of it as 35mm (crop) versus medium format (full frame) for film
 
If you use the Depth of Field Calculator you will see that at 10ft range and a 100mm lens at f/5.6 on a crop sensor (40d) the depth of field is 0.63ft (6.3%). For an equivalent view on full frame (1D) using 160mm lens at the same f/5.6, the depth of field reduces to 0.38ft (3.8%) for exactly the same perspective.

Equally, the 1D with the 100mm lens at f/5.6 at a range of 6ft (to give the same image size but different perspective) has a depth of field of 0.35ft (5.8%)

So the depth of field is to do with magnification and absolute focal length of the lens, distance from subject and the size of sensor (degree of magnification to get teh same final image). Assuming that the final images are all viewed from the same distance.
 
Both.
Thanks again for your time and the handy link. Understanding clearer now.
Regards
 
I think others have pretty much hit the nail on the head so I won't go over the same explanation again. What I will add, however, is to be aware that with crop sensors, the focal length of a lens won't be what its rated to be - ie. a 50mm lens on a body with a cropped sensor will not *actually* give you a 50mm image. The so-called 'crop factor' is particularly easy to calculate on a Nikon body as it's 1.5x the lens focal length - ie. stick a 50mm lens onto a Nikon body with a 1.5x and you're effectively getting a 75mm image (ie. 50mm x 1.5). Even if the lens is specifically designed for a crop sensor body (a so-called 'DX' lens on Nikons, for example), this rule will still apply.

Whilst this generally doesn't make much difference to most people, it does make a difference when you're buying a lens for a particular purpose. Take the good old nifty fifty, for example - it's a perfect portrait lens on a full frame body but a bit on the tight side on a crop body. For a 50mm equivelent on a crop body, you'd actually want a 35mm lens (ie. 35 x 1.5 = 52mm).

Of course when you're talking about zoom lenses, this crop factor is actually a god send as your lowly 200mm zoom is suddenly a 300mm! Also, if you're using a full frame lens on a cropped body, you benefit from the 'sweet spot' and avoid the nasty blurred glass towards the edge of the lens on cheaper lenses.
 
grahamsewelluk said:
There are many mis-understandings about how cropping factor works in DSLRs. From my understanding (and I am likely to be contradicted) - the focal length of the lens is an absolute. However, because the image is cropped, you will get the image from the central section of the frame and hence be the equivalent of cropping and magnifying during printing - as such it appears to behave as a longer lens in terms of perspective and image size.

However, as the focal length of the lens remains the same, the depth of field is greater than the longer "equivalent" lens on a full frame body.
Click to expand...

The perspective will not change as this is all about distance from the subject and not focal length. same for DOF this is also about aperture and distance all focal length have same DOF for same distance
 
Chaz Photos said:
same for DOF this is also about aperture and distance all focal length have same DOF for same distance
Click to expand...

So a 10mm wide angle has the same DOF as a 200mm tele when viewing a subject 10 ft away? Nope, for the wide angle you'll be well within the hyperfocal distance and yet the DoF on the Tele can be measured in inches.

Yes, perspective is the relative sizes and positions within the scene of objects at different distances from the lens.

So if you keep your feet in the same place, the perspective will not change but the focal length of the lens needs to change to achieve the same image on a full frame, medium format or sub-frame camera.

My point about perspective change was that if you do move yor feet (reducing distance to subject from 10ft to 6ft as mentioned) that will change perspective while keeping the image size of the subject the same (relative to the frame size) for the change in format.

With this change of focal length will come changes to DoF.
 
Thanks again everyone.
Could you please review the following to make sure I have understood. Sorry its a bit longwinded.

Take 2 identical camera bodies, 1 with ff sensor, 1 cropped, sensor pixel count = same. Sensor focal plane from lens mount distance = same.

Fix ff body and mount lens. Focus on subject and take image.

Change body to cropped and and mount same lens with no settings changed and take photo.

The resulting images will have the same DOF and same magnification.
The ff image will have a larger field of view.
The subject will appear larger on the cropped image if both images are the same dimensions when processed
To achieve the same field of view as the cropped image, I would need to move the ff body closer to the subject and this would change the DOF.

Please correct as required.
Thanks.
Richard
 
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