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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 06 Jul 2012 (Friday) 15:26
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Confused about crop lenses

 
SkipD
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Jul 08, 2012 14:22 |  #31

mrkgoo wrote in post #14688069 (external link)
Exactly.

On wayne's post:
I do understand the point of non interchangeable lenses, such that FOV just becomes a practicality when using a P&S (like just look on the LCD if your FOV is what you want).

I was referring to when you purchase a new camera, it,s nice to have SOME measure to compare. For example, it would be nice if companies tried to educate field of view in terms of AOV in degrees as opposed to pushing the Focal Length as a measure of field of view.

On top of just purchasing, i do check the exifs of P&S cameras. You don't always remember how you shot something, and the focal length is a meaningless value for comparisons sake without knowing the sensor size.

Most (if not all) point-n-shoot cameras I've seen in the last ten years or so have their "35mm equivalent focal length" published. That would provide enough information for you to create your own "crop factor" for any other known camera format which you cared to compare the point-n-shoot to.


Skip Douglas
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mrkgoo
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Jul 08, 2012 14:29 |  #32

SkipD wrote in post #14688089 (external link)
Most (if not all) point-n-shoot cameras I've seen in the last ten years or so have their "35mm equivalent focal length" published. That would provide enough information for you to create you own "crop factor" for any other known camera format which you cared to compare the point-n-shoot to.

Yup. A little legwork goes a long way.




  
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xarqi
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Jul 08, 2012 18:40 as a reply to  @ mrkgoo's post |  #33

Wilt wrote in post #14687837 (external link)
But they do!
  • FF with 50mm normal lens sees 46.79 degrees diagonal AOV, with DOF zone of 25' when focused at 30' at f/4
  • APS-C with 31.6mm normal lens sees 46.33 degrees diagonal AOV, with DOF zone of 50.8' when focused at 30' at f/4

No, they don't. The effect is due to the difference in focal length, not the difference in format.




  
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runninmann
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Jul 08, 2012 19:35 |  #34

If I understand Wilt's point, he's showing that in order to take the "same" picture, that is the same framing, you'd have to use ~30mm fl on the APS-C camera to achieve the same framing as a 50mm on "full frame" camera, and given that, the crop would yield greater DOF.


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SkipD
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Jul 08, 2012 19:49 |  #35

runninmann wrote in post #14689236 (external link)
If I understand Wilt's point, he's showing that in order to take the "same" picture, that is the same framing, you'd have to use ~30mm fl on the APS-C camera to achieve the same framing as a 50mm on "full frame" camera, and given that, the crop would yield greater DOF.

That's very true.

There is a little additional effect on depth of field which is due strictly to the size of the film frame or sensor in the camera. The "format size" effects the "Circle of Confusion" factor in the formula. This is because there is a difference in the magnification (commonly called "enlargement") from the in-camera image size (the size of the film frame of sensor in the camera) and the size of the standard print used to analyze depth of field. The image from the smaller format camera will need more magnification than the image from the larger format camera. This makes any blurs in the in-camera image look larger in the standard print made with the smaller format camera's image.


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Tim ­ S
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Jul 08, 2012 21:27 |  #36

Geckotek wrote in post #14680199 (external link)
Ok, I understand that when I buy an EF lens and place it on a crop body that I'm looking at a 1.6x crop factor (for example.)

However, if I buy a crop lens (let's use my Sigma 17-50mm EX) and place it on a crop body (T2i in this situation), is there still a 1.6x crop factor to consider or are those focal lengths already adjusted for the crop?

Part of what I'm wondering is, exactly how much width I lost moving from a 17mm on a T2i to a 24mm on a 5D II.

(I'll be going home and taking comparison shots to see for myself, but I'm in a debate with someone and the office and wanting to clear up our confusion.) :confused:

Well, you won't find out with that lens as it won't mount on a full frame body....
And, as several have already pointed out- 17mm is 17mm. You won't have magically gained any lens length. You have a narrower field of view as compared to a full frame camera, roughly the field of view of a 28mm lens(27.2mm).


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pyrojim
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Jul 09, 2012 00:06 |  #37

xarqi wrote in post #14689015 (external link)
No, they don't. The effect is due to the difference in focal length, not the difference in format.

AND the difference in aperature. F4 for the two lenses mean very different hole sizes. But that information is encoded in the focal length!

Thank you!!!!! (see im not crazy!!!!)


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mwsilver
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Jul 09, 2012 00:35 |  #38

SkipD wrote in post #14688089 (external link)
Most (if not all) point-n-shoot cameras I've seen in the last ten years or so have their "35mm equivalent focal length" published. That would provide enough information for you to create your own "crop factor" for any other known camera format which you cared to compare the point-n-shoot to.

Exactly, but its very misleading. My fiend has a Nikon p500 which is a "36x" zoom and has a advertised "focal range" of 22.5 to 810mm. Only problem is the front of the lens indicates the zoom range is 4 to 144mm, which probably means around a 5.6 "crop factor". Most references to this camera, that I've seen, ignore or downplay the 35mm equivalent reference. As a result many or most owners of this camera, who are relative novices, are mislead. Since megapixels and zoom are everything in that market my friend has opted not to upgrade to a DSLR because he thinks his "810mm" camera has superior performance.


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tkbslc
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Jul 09, 2012 00:57 |  #39

Why can't anyone ever just answer the freaking question. We always devolve into random arguments that do nothing but confuse people who don't know this stuff, and it becomes a pissing match for those that do.

OP: Your 17-50 on APS-C works out to 27-80mm Full frame equivalent. Your 24-105 on FF is of course 24-105mm equivalent. So compare 27-80 and 24-105 and have a good night! :)


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Wilt
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Jul 09, 2012 00:59 |  #40

xarqi wrote in post #14689015 (external link)
No, they don't. The effect is due to the difference in focal length, not the difference in format.

We're now debating semantics. Find anyone who denies, "Large format has shallower DOF than 135 format". The DOF difference is associated with the format size in the minds of most. The FL difference in lenses used for same FOV at same camera position is not commonly the logical link.

I think the real answer to the debate is whether the equation which represents to DOF difference includes FL or includes some other format-size-dependent factor (such as mag factor to make final print). I don't claim to really know, but will attempt to look this up.


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Wilt
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Jul 09, 2012 01:14 |  #41

Wilt wrote in post #14690341 (external link)
We're now debating semantics. Find anyone who denies, "Large format has shallower DOF than 135 format". The DOF difference is associated with the format size in the minds of most. The FL difference in lenses used for same FOV at same camera position is not commonly the logical link.

I think the real answer to the debate is whether the equation which represents to DOF difference includes FL or includes some other format-size-dependent factor (such as mag factor to make final print). I don't claim to really know, but will attempt to look this up.

I found this information within Wikipedia http://en.wikipedia.or​g/wiki/Depth_of_field (external link) :

"“Same picture” for both formats
The results of the comparison depend on what is assumed. One approach is to assume that essentially the same picture is taken with each format and enlarged to produce the same size final image, so the subject distance remains the same, the focal length is adjusted to maintain the same angle of view, and to a first approximation, magnification is in direct proportion to some characteristic dimension of each format. If both pictures are enlarged to give the same size final images with the same sharpness criteria, the circle of confusion is also in direct proportion to the format size. Thus if

IMAGE: http://upload.wikimedia.org/wikipedia/en/math/2/d/b/2db95e8e1a9267b7a1188556b2013b33.png
is the characteristic dimension of the format,

IMAGE: http://upload.wikimedia.org/wikipedia/en/math/3/8/f/38f2722eab5603ca9a13dc722bc56f36.png
With the same f-number, the DOF ratio is then

IMAGE: http://upload.wikimedia.org/wikipedia/en/math/3/1/b/31bfceeaa855740e06c8d3858c3a1401.png
so the DOF ratio is in inverse proportion to the format size.

This ratio is approximate, and breaks down in the macro range of the larger format (the value of
IMAGE: http://upload.wikimedia.org/wikipedia/en/math/6/f/8/6f8f57715090da2632453988d9a1501b.png
in the numerator is no longer negligible) or as distance approaches the hyperfocal distance for the smaller format (the DOF of the smaller format approaches infinity).
If the formats have approximately the same aspect ratios, the characteristic dimensions can be the format diagonals; if the aspect ratios differ considerably (e.g., 4×5 vs. 6×17), the dimensions must be chosen more carefully, and the DOF comparison may not even be meaningful.
If the DOF is to be the same for both formats the required f-number is in direct proportion to the format size:

IMAGE: http://upload.wikimedia.org/wikipedia/en/math/c/6/4/c64392c8c4bc4e850a38ae5840b6707d.png
Adjusting the f-number in proportion to format size is equivalent to using the same absolute aperture diameter for both formats, discussed in detail below in Use of absolute aperture diameter (external link)."

Notice that neither FL nor aperture diameter are parameters within any of the three equations, but
(DUPLICATE IMAGE)
, the characteristic dimension of the format format size is in all three!

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Jul 09, 2012 01:31 as a reply to  @ post 14687907 |  #42

SkipD wrote in post #14687444 (external link)
....... the 35mm film format (24mm by 36mm) is not, never has been, and never will be the format standard against which all other camera formats (film and digital) are measured.

SkipD wrote in post #14688089 (external link)
Most (if not all) point-n-shoot cameras I've seen in the last ten years or so have their "35mm equivalent focal length" published.

@Skip, Hi,

Even a cursory read through a few posts here and on other sites suggests differently to the assertion in your first post above. Your second post (above) directly contradicts your first statement. It seems to me that the 35mm (or Full Frame as it is commonly known) format is exactly that, what every other format is compared to. Even the term crop is a reference to the Full Frame format.

I hate to be contrary but these are my observations of how the Full Frame (35mm) format is referred to and how it is used widely.




  
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Wilt
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Jul 09, 2012 01:32 |  #43

And found in http://www.bobatkins.c​om …technical/digit​aldof.html (external link)

is the statement, "So the bottom line - and all you really need to know - is that DOF is inversely proportional to format size. Note that format size is inversely proportional to the "digital multiplier".

It's somewhat like debating the cause of children, was it 'sex' or 'marriage'...ultimatel​y it comes down to "fertilization of egg with spermatazoa" as the root cause that matters.


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xarqi
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Jul 09, 2012 02:11 |  #44

Wilt wrote in post #14690375 (external link)
I found this information within Wikipedia http://en.wikipedia.or​g/wiki/Depth_of_field (external link) :

"“Same picture” for both formats
... the focal length is adjusted to maintain the same angle of view, ...so the DOF ratio is in inverse proportion to the format size.

True - if you focus solely on format, and completely gloss over the fundamental fact that the focal length must be changed to fit the test parameters, then yes, you'll find that DoF is inversely proportional to sensor size. No surprise there. But, it is important not to confuse correlation with causation. The cause is the change in focal length. If that is because of a choice made to match fields of view for different formats, so what?

I wouldn't care so much if format size were actually irrelevant to DoF, but the fact is that it is not, and further, it operates in the opposite way from that which is typically stated. Just increase the format size, and the DoF will be increased, as a lesser enlargement will be needed to reach "standard" size. This is why I take exception to the direct attribution of any decrease in DoF to increased format size, when the ONLY thing responsible for that decrease is the change in focal length.

Wilt wrote in post #14690428 (external link)
And found in http://www.bobatkins.c​om …technical/digit​aldof.html (external link)

is the statement, "So the bottom line - and all you really need to know - is that DOF is inversely proportional to format size. Note that format size is inversely proportional to the "digital multiplier".

This may be true if the person has no desire to understand what is actually going on, and is happy that focal lengths are being changed willy-nilly in the background.

It's somewhat like debating the cause of children, was it 'sex' or 'marriage'...ultimatel​y it comes down to "fertilization of egg with spermatazoa" as the root cause that matters.

"Root cause" - I like that - those with a British English background will appreciate it too.
Humour aside, I don't think it's much like that at all.

Scenario A: smaller sensor + shorter lens = image
Scenario B: larger sensor + longer lens = similar image with shallower DoF.

What caused the shallower DoF? The change in focal length, not the change in format. It's really that simple.

If the photographer elected to use that longer focal length to achieve optimal framing on the larger format, that in no way means that that larger format caused the change in DoF.

To argue this way would be to give credence to the notion that winter was actually warmer than summer because we are forced to turn on the heater.




  
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runninmann
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Jul 09, 2012 05:43 |  #45

If the same focal length were used with both formats and picture from the larger format were cropped to match the AOV of the smaller format, how would the DOF compare?


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