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Thread started 08 Mar 2007 (Thursday) 16:03
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Crop factor/sensor math

 
fuzzygraphics
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Mar 08, 2007 16:03 |  #1

I have Rebel XT and the kit lens (18-55mm). If the Rebel XT has a crop factor of 1.6 for the smaller sensor, does that mean that my kit lens is really 29mm-88mm? Or is the 18-55mm already taking the crop factor in to account?

I'm trying to compare the 17-40L to the 18-55 kit lens in terms of how wide it will shoot. As I understand the 17mm aspect of the 17-40 will actually be closer to 27mm because of the crop, but I'm wondering if that is still slightly wider than the kit lens or not.


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Jarrad
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Mar 08, 2007 16:11 |  #2

yes, 27.2mm is wider than 28.8mm :)


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chris ­ clements
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Mar 08, 2007 16:11 |  #3

Crop factor has no relevance.
What can't you compare between 17 and 18, or 40 and 55??

* I'm wondering if your question might have been asked here before - you DID 'search' before posting, didn't you ???




  
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crn3371
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Mar 08, 2007 16:12 |  #4

The 17-40 is 1mm wider than your kit lens.




  
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Andy_T
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Mar 08, 2007 16:21 |  #5

On a crop camera, 1.6 mm wider :wink:

If you compare the field-of-view on your new digital camera with the field-of-view you got on your old digital camera, then you are absolutely correct.

The 18 mm lens on the digital camera will give you the same field of view that using a 28 mm lens on a 35 mm film camera would give you.
So for the sake of convenience, if you are used to using a 28 mm lens for something on your film camera, use an 18 mm lens now.


Don't get confused by those that will come to this thread and


- point out that the actual focal length of the lens will not change (They' re right. It will not change. But it will give you a different image :p )

- or who will tell you that a picture taken with film with a 28 mm lens will be different to the image taken with the 18 mm lens on a digital camera
(they're right, too. Using the same aperture and a shorter lens, the digital camera will give you larger depth-of-field. If you want to have the same, just use f/1.8 with the digital camera to get a similar effect to using f/2.8 on the longer lens on film).

But don't let them confuse you or tell you you are completely wrong.

You are *basically* right with the effect of the focal length on the image you will get :wink:.

Best regards,
Andy

Best regards,
Andy


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AeroSmith
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Mar 08, 2007 16:27 as a reply to  @ Andy_T's post |  #6

Once Andy has spoken I think we can lock the thread. ;)


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Curtis ­ N
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Mar 08, 2007 16:29 |  #7

fuzzygraphics wrote in post #2838690 (external link)
If the Rebel XT has a crop factor of 1.6 for the smaller sensor, does that mean that my kit lens is really 29mm-88mm?

If it was really a 29-88, it would say 29-88.

But it says 18-55, because that's what it is.

And the 17-40 says 17-40 because it really is 17-40.

And my Tamron 17-50 says 17-50 because it really is 17-50.

I hope that clears things up.


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Wilt
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Mar 08, 2007 17:03 |  #8

The 'crop' effect is due to the difference in sensor size (36mm vs. 22.5mm horizontal, 24mm vs. 15mm vertical) So if you put a 28mm lens on the FF camera, you need to use a 17.5mm lens on the APS-C camera in order for the same FOV to be captured in the final image.

A 17mm lens is really a 17mm lens on any camera body, it is merely the FOV that can be captured by the film/sensor within the boundaries of the frame edges which changes!

Using 17mm on APS-C will result in the same FOV being captured in the frame as using a 27.2mm lens on FF camera.
Using 18mm on APS-C will result in the same FOV being captured in the frame as using a 28.8mm lens on FF camera.


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fuzzygraphics
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Mar 08, 2007 17:23 |  #9

Wilt wrote in post #2838990 (external link)
The 'crop' effect is due to the difference in sensor size (36mm vs. 22.5mm horizontal, 24mm vs. 15mm vertical) So if you put a 28mm lens on the FF camera, you need to use a 17.5mm lens on the APS-C camera in order for the same FOV to be captured in the final image.

A 17mm lens is really a 17mm lens on any camera body, it is merely the FOV that can be captured by the film/sensor within the boundaries of the frame edges which changes!

Using 17mm on APS-C will result in the same FOV being captured in the frame as using a 27.2mm lens on FF camera.
Using 18mm on APS-C will result in the same FOV being captured in the frame as using a 28.8mm lens on FF camera.

Thank for explaining this. This is where I was getting hung up. I'm looking at a lot of amazing photos taken with the 17-40L and just wondering if the wides are all with FF sensors. I guess I'm just trying to avoid buying a new lens and not being able to go as wide as I'd like.


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fuzzygraphics
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Mar 08, 2007 17:27 |  #10

Curtis N wrote in post #2838819 (external link)
If it was really a 29-88, it would say 29-88.

But it says 18-55, because that's what it is.

And the 17-40 says 17-40 because it really is 17-40.

And my Tamron 17-50 says 17-50 because it really is 17-50.

I hope that clears things up.

It does. Does anyone have the number for 411?


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Wilt
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Mar 08, 2007 17:30 |  #11

fuzzygraphics wrote in post #2839108 (external link)
I guess I'm just trying to avoid buying a new lens and not being able to go as wide as I'd like.

Yeah, that was an adjustment for me, too, after 40 years of shooting with 35mm film SLRs. OTOH, all of the other film format cameras I also have, the idea of buying lenses to suit was never a bother! Everyone is simply spoiled at being able to re-use 35mm SLR lenses on APS-C dSLR!

The only thing that makes it a bother now is the relative scarcity (two years ago) of truly wide lenses and fast apertures at more reasonable prices that would provide UWA on APS-C. (Can you readily find a 15mm f/2 lens for APS-C at the same price I can buy a 24mm f/2 lens for 35mm?)


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Crop factor/sensor math
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