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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 07 Feb 2014 (Friday) 09:17
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Aspect Ratio and Focal Length Questions

 
lsquare
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Feb 07, 2014 09:17 |  #1

So let's use the Canon 70D as an example. It doesn't have a multi-aspect ratio sensor. I'm assuming the 70D just crops the sensor base on which aspect ratio I select in the menu. So with a 18-135mm lens, the lens at 18mm is calculated base on a 3:2 aspect ratio. What happens if I keep the lens at 18mm (29mm equivalent in 35mm terms) and select 1:1 aspect ratio? Does the focal length changes now? If so, how does one calculate the true effective focal length now?




  
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gonzogolf
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Feb 07, 2014 09:20 |  #2

The focal length doesnt change. Only the shape of the frame changes.




  
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StillCrazy
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Feb 07, 2014 09:28 |  #3

On the T3i (same sensor?) choosing a different aspect ratio, only put up guide lines on Live View. Nothing changes, it just give you an idea of what's in the frame for a specific aspect ratio.


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archer1960
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Feb 07, 2014 09:37 |  #4

lsquare wrote in post #16670654 (external link)
So let's use the Canon 70D as an example. It doesn't have a multi-aspect ratio sensor. I'm assuming the 70D just crops the sensor base on which aspect ratio I select in the menu. So with a 18-135mm lens, the lens at 18mm is calculated base on a 3:2 aspect ratio. What happens if I keep the lens at 18mm (29mm equivalent in 35mm terms) and select 1:1 aspect ratio? Does the focal length changes now? If so, how does one calculate the true effective focal length now?

Remember that the focal length is an an inherent design characteristic of a lens, and has nothing to do with the body it's mounted on. An 18mm lens is 18mm whether it's on a micro 4/3 camera, or a 6x4 MF camera. Only the apparent field of view generated by the lens varies with the body.

Changing the aspect ratio in the camera body just crops off parts of the sensor data that don't fit the chosen image shape. The native aspect ratio of all Canon EOS bodies is 3:2, so narrower than that, such as 1:1 crop off the sides, and wider ratios such as 16:9 crop the top and bottom.


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nathancarter
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Feb 07, 2014 09:48 |  #5

The focal length never changes, regardless of the sensor size or the crop. You're just choosing to view less of the image projected by the lens. By choosing 1:1 you're choosing to see even less of the image projected by the lens, compared to an un-cropped crop sensor.

Focal length is the same regardless of sensor size or camera body. The crop sensor just sees less of the image than the full-frame sensor. The 1.6 multiplier is a silly thing - only useful if you've shot with full-frame sensors for so long that the focal lengths have become second nature, and you can see the image in your head and calculate the appropriate focal length before ever raising the viewfinder to your eye.


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Tony-S
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Feb 07, 2014 17:54 |  #6

nathancarter wrote in post #16670724 (external link)
The focal length never changes

It does with zoom lenses at each FL in their range, depending on the focus point.


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lsquare
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Feb 07, 2014 18:45 |  #7

archer1960 wrote in post #16670697 (external link)
Remember that the focal length is an an inherent design characteristic of a lens, and has nothing to do with the body it's mounted on. An 18mm lens is 18mm whether it's on a micro 4/3 camera, or a 6x4 MF camera. Only the apparent field of view generated by the lens varies with the body.

Changing the aspect ratio in the camera body just crops off parts of the sensor data that don't fit the chosen image shape. The native aspect ratio of all Canon EOS bodies is 3:2, so narrower than that, such as 1:1 crop off the sides, and wider ratios such as 16:9 crop the top and bottom.

Isn't this only true for non-multi-aspect-ratio sensors? For example, the Nokia Lumia 1020 have a 41MP sensor. Yet, it only takes a 38MP picture with a 4:3 ratio and a 34MP picture with a 16:9 ratio. The focal length also changes. At 16:9 it's the equivalent to 25mm in 35mm terms. So that's why this kind of piqued my curiosity.




  
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archer1960
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Feb 07, 2014 18:53 |  #8

lsquare wrote in post #16672247 (external link)
Isn't this only true for non-multi-aspect-ratio sensors? For example, the Nokia Lumia 1020 have a 41MP sensor. Yet, it only takes a 38MP picture with a 4:3 ratio and a 34MP picture with a 16:9 ratio. The focal length also changes. At 16:9 it's the equivalent to 25mm in 35mm terms. So that's why this kind of piqued my curiosity.

The reason it's fewer pixels is that some of them are cropped off the sides or top. The focal length isn't actually changing either, just the apparent field of view, because your sensor is effectively smaller (because you're using less of it). If they say it is, it's only because they're adjusting the number to correspond to the FOV.


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archer1960
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Feb 07, 2014 18:54 |  #9

Tony-S wrote in post #16672142 (external link)
It does with zoom lenses at each FL in their range, depending on the focus point.

Obviously, when you change the f/l on a zoom, it's different. But if you move the lens to a different camera without moving the zoom, its f/l doesn't change even if you see more (or less) due to the different sensor size.


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Tony-S
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Feb 07, 2014 21:50 |  #10

archer1960 wrote in post #16672262 (external link)
Obviously, when you change the f/l on a zoom, it's different. But if you move the lens to a different camera without moving the zoom, its f/l doesn't change even if you see more (or less) due to the different sensor size.

The focal lengths of zooms are based upon infinity focus. If you focus to infinity then comparison of to a fixed FL lens will be the same FL. However, if you focus each lens at 10 feet, the fixed FL lens will have a narrower angle of view because the zoom will have a shorter FL. For example, if you shoot the 70-200 at 200mm and the 200mm lenses at infinity, they will have the same focal length - 200mm. But at 10 feet the zoom will be closer to 175mm while the 200mm will be 200mm.


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20droger
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Feb 07, 2014 22:12 as a reply to  @ Tony-S's post |  #11

Boy oh boy! A lot of people really need to take Optics 101.

The focal length of a lens is a physical property of the lens, irrespective of the body with which it is used, the aspect ratio of the sensor or film in use, or even if the lens is attached to a camera at all. A 50mm lens is a 50mm lens if it is coupled to a full-frame camera, a crop camera, or a 120-film medium-format camera, if the aspect ratio is 3:2, 4:3, 5:4, 16:9, or 1:1, or if the lens is sitting by itself on a shelf gathering dust.

By definition, the lens' indicated focal length is equivalent to its physical focal length when the lens is focused at infinity. If a lens is focused closer than infinity, its physical focal length will always be less than its indicated focal length. This is especially apparent with macro lenses, where the difference can be enormous. There are no exceptions to this; it's a law of optics. Camera lenses are converging lenses, and that's the way converging lenses operate.

With multi-element lenses, the focal length has very little to do with the physical length of the lens. Normally focused lenses (those greater than 50mm) are almost always shorter than their focal length. Retrofocus lenses (those less than 40mm) are almost always longer than their focal length. Note—if you're going to start measuring your lenses, bear in mind that focal length is measured from the point of focus on the film/sensor plane, not the back of the lens. For Canon lenses, this is 44mm past the lens' mounting ring.

A prime lens is a lens with a single, fixed focal length, measured with the lens focused at infinity. A zoom lens is a lens with a focal length that varies continuously between a minimum focal length and a maximum focal length, also measured with the lens focused at infinity. All the rules of optics apply identically to prime lenses and zoom lenses. For example, a 70-200mm zoom lens set to 85mm behaves exactly like an 85mm prime lens. That is, if either were focused at 2m instead of infinity, its physical focal length would be less than 85mm. Note than the zoom lens would still be set to 85mm; its zoom setting would not have been changed.

Why are prime lens usually better than equivalent-quality zoom lenses encompassing the same focal length? Two reasons.

First, prime lenses typically have fewer elements than equivalent zoom lenses. (The don't have all those zoom elements.) The less glass the light has to pass through, the less distortion it acquires.

Second, the elements in prime lenses typically bend the light less than those in zoom lenses. Those zoom elements can really stress the light, introducing distortion. And the greater the zoom, the greater the stress. This is why superzoom lenses (10× or more) not only have lower I.Q. than equivalent prime lenses, they have lower I.Q. than equivalent non-super zooms. A 28-300mm zoom lens set to 135mm will have lower I.Q. than an equivalent 70-200mm zoom lens set to 135mm, which itself will have lower I.Q. than an equivalent 135mm prime lens.

You can't beat prime lenses for I.Q.




  
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melcat
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Feb 08, 2014 00:19 |  #12

lsquare wrote in post #16672247 (external link)
Isn't this only true for non-multi-aspect-ratio sensors? For example, the Nokia Lumia 1020 have a 41MP sensor. Yet, it only takes a 38MP picture with a 4:3 ratio and a 34MP picture with a 16:9 ratio. The focal length also changes. At 16:9 it's the equivalent to 25mm in 35mm terms. So that's why this kind of piqued my curiosity.

The source of your confusion is the widespread misuse of "35mm equivalent focal length" as a surrogate for angle of view. Although it is well defined enough for some cameras to write it into EXIF, it is a computed quantity and not actually the focal length. If it were, you wouldn't be able to fit that phone in your pocket!

I haven't checked, but I imagine the formula they use probably uses the diagonal of the image size.




  
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Tony-S
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Feb 08, 2014 01:02 |  #13

20droger wrote in post #16672638 (external link)
All the rules of optics apply identically to prime lenses and zoom lenses. For example, a 70-200mm zoom lens set to 85mm behaves exactly like an 85mm prime lens. That is, if either were focused at 2m instead of infinity, its physical focal length would be less than 85mm. Note than the zoom lens would still be set to 85mm; its zoom setting would not have been changed.

Canon 200mm f/2.8L at infinity.

IMAGE: http://farm4.staticflickr.com/3758/12378920644_e8dac75aa2_o.jpg
IMAGE LINK: http://www.flickr.com/​photos/digi-film/12378920644/  (external link)
canon200 (external link)

Sigma 18-200mm at 200mm and infinity.
IMAGE: http://farm8.staticflickr.com/7366/12378472035_85b39fba40_o.jpg
IMAGE LINK: http://www.flickr.com/​photos/digi-film/12378472035/  (external link)
sig200 (external link)

Canon 200mm f/2.8 at close focus.
IMAGE: http://farm4.staticflickr.com/3750/12378472135_569af278e2_o.jpg
IMAGE LINK: http://www.flickr.com/​photos/digi-film/12378472135/  (external link)
ef200Lclose (external link)

Sigma 18-200 at 200mm and same close focus as photo above.
IMAGE: http://farm8.staticflickr.com/7366/12378920424_b3b117aac0_o.jpg
IMAGE LINK: http://www.flickr.com/​photos/digi-film/12378920424/  (external link)
sig18-200osclose (external link)

"Raw" is not an acronym, abbreviation, nor a proper noun; thus, it should not be in capital letters.

  
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archer1960
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Feb 08, 2014 06:36 |  #14

Tony-S wrote in post #16672607 (external link)
The focal lengths of zooms are based upon infinity focus. If you focus to infinity then comparison of to a fixed FL lens will be the same FL. However, if you focus each lens at 10 feet, the fixed FL lens will have a narrower angle of view because the zoom will have a shorter FL. For example, if you shoot the 70-200 at 200mm and the 200mm lenses at infinity, they will have the same focal length - 200mm. But at 10 feet the zoom will be closer to 175mm while the 200mm will be 200mm.

The f/l of primes is also based on infinity focus (it's part of the standard definition of focal length), and will also vary slightly with focus distance.


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BigAl007
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Feb 08, 2014 07:24 |  #15

Tony-S wrote in post #16672853 (external link)
Canon 200mm f/2.8L at infinity.
QUOTED IMAGE
IMAGE LINK: http://www.flickr.com/​photos/digi-film/12378920644/  (external link)
canon200 (external link)

Sigma 18-200mm at 200mm and infinity.
QUOTED IMAGE
IMAGE LINK: http://www.flickr.com/​photos/digi-film/12378472035/  (external link)
sig200 (external link)

Canon 200mm f/2.8 at close focus.
QUOTED IMAGE
IMAGE LINK: http://www.flickr.com/​photos/digi-film/12378472135/  (external link)
ef200Lclose (external link)

Sigma 18-200 at 200mm and same close focus as photo above.
QUOTED IMAGE
IMAGE LINK: http://www.flickr.com/​photos/digi-film/12378920424/  (external link)
sig18-200osclose (external link)

Yes but the focal length change with focus distance, while different for different lenses, even of or at the same focal length, may be different. The focal length/focal distance combination for THAT lens will be constant, regardless of the camera format it is mounted on, or even if it is not mounted on a camera. So if we keep the lens set at the same focal distance, and if a zoom lens focal length, and change the image format behind the lens nothing happens to the physical properties of the lens. The angle of view may change, but in this situation that is strictly speaking a property of the image size format, not the focal length/focal distance combination.

Alan


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Aspect Ratio and Focal Length Questions
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