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Thread started 23 Jan 2018 (Tuesday) 02:52
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Crop factor nonsense

 
Dalantech
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Jan 23, 2018 02:52 |  #1

Ever since the crop factor sensors came out it seems like peeps have been trying to find magic where none exists. Here's a short list of the nonsense that I've seen around the web. Please tell me if I'm out to lunch on any of it...

1) Crop factor sensors give more depth of field: This one is usually the result of trying to make the subject look the same size on both a crop factor sensor and full frame, so the full frame image is shot at a higher magnification. Magnification goes up, depth goes down when the aperture doesn't change. Or the full frame camera is moved closer to the subject to get the same apparent subject size and as the camera gets closer the magnification increase. Again mag goes up, depth goes down. Here's a simple test between a full frame sensor and a crop factor one. Notice how the depth doesn't change.




2) Crop factor sensors give you more magnification: This one should be common sense, but people think that because the subject looks larger on screen or in print with a crop sensor they assume that the image is magnified. But it's not, it's simply an enlargement and no different than cropping an image in post. Terms like "screen magnification" and "print magnification" don't help.

3) Crop factor sensors change the ISO because there is less surface area on a crop factor sensor: You can't think of sensor in terms of total surface area, because a digital sensor isn't a single light sensing device (like a solar cell), they are made of millions of light sensitive photo receptors (pixels). All the pixels on a full frame and a crop factor sensor get the same amount of light from the lens, and the intensity of the light that's projected by the lens onto the sensor doesn't change.

Here's my .02:

Canon's 5DS has a 50MP sensor, and if you were to take a shot with it and crop it down to a 1.6x APS-C crop factor you'd end up with roughly an 18MP image. So what would be the difference in the depth of field, ISO, resolution, or any other basic characteristic if you:

A. Cropped in post to that 1.6x?

B. Taped off the outside of the sensor so that it's now APS-C sized?

C. Canon making an APS-C sized sensor with the same physical characteristics as the taped off full frame sensor in B above?

The answer is nothing. The depth of field, ISO, aperture, etc. wouldn't change. The amount of light striking the sensor at a given aperture doesn't change. Would the subject look larger in the frame? Sure. But that's just an enlargement.

If there are any differences between a full frame and a crop factor camera then those differences are due to the way the chips are physically designed (pixel size, pixel density, etc.) and not the fact that one is physically smaller than the other. So cropping the image circle with the sensor and cropping in post are the same.

So where am I wrong?


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Perfectly ­ Frank
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Jan 23, 2018 05:28 |  #2

Dalantech wrote in post #18547067 (external link)
2) Crop factor sensors give you more magnification: This one should be common sense, but people think that because the subject looks larger on screen or in print with a crop sensor they assume that the image is magnified. But it's not, it's simply an enlargement and no different than cropping an image in post.

What if you have a 6D and 7DII, both with 20mp sensors. If you crop an image from the 6D until you have the fov that the 7DII provides, the resulting image will have less than 20mp. Doesn't sensor pixel density have to be considered?


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Post edited 4 months ago by Dalantech.
     
Jan 23, 2018 06:41 |  #3

Perfectly Frank wrote in post #18547104 (external link)
What if you have a 6D and 7DII, both with 20mp sensors. If you crop an image from the 6D until you have the fov that the 7DII provides, the resulting image will have less than 20mp. Doesn't sensor pixel density have to be considered?

True, you'd end up with fewer pixels to send to a printer so the 7DII image would probably make a better print. But that's because the 6D image was cropped though. I used the 5DS in my example because all of the physical characteristics of the full frame and cropped image (post or physical) are the same except for pixel count -makes it easier to visualize.

There might also be a slight difference in depth due to a difference in the Circle of Confusion (external link) (CoC) for both sensors. In a nutshell the sensor with the bigger pixels would have more apparent depth (even the gaps between pixels can influence the CoC). But CoC from my experience has very little impact on depth -not enough to choose one sensor over another. Also note that CoC has more to do with how a sensor is built than with the physical surface area of a sensor. You could have a full frame and a crop factor sensor and both of them could have the same CoC.


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Wilt
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Post edited 4 months ago by Wilt. (13 edits in all)
     
Jan 23, 2018 14:05 |  #4

Dalantech wrote in post #18547067 (external link)
1) Crop factor sensors give more depth of field

2) Crop factor sensors give you more magnification

3) Crop factor sensors change the ISO

So where am I wrong?



Yes, #2 and #3 are fallacies.
Because of the smaller sensor, the APS-C sensor captures a smaller section of the image circle than FF, yielding the impressions of 'more magnification'...but effectively no different than chopping off 9mm x 14mm off a 135 negative and then making a print from it (if we ignore pixel counts and pixel-to-pixel distances)!
And putting a smaller sensor into the image path of identical lens does not alter the amount of light passed by the lens, and when the manufacturer designs the software for the sensors, the larger and the smaller sensor have similar responses to the same amount of light ...'same ISO'.

About #1, though...

If I mount a 50mm on APS-C and an 80mm on FF, mount both cameras at the same tripod position, both cameras frame the same area at a distance of 10' with a field of view of 3' x 4.5' in both cases and a 36" yardstick is as tall as the frame height for both images

  • The APS-C image enlarged to 8" print height will be perceived by a person with 20/20 visual acuity to have a DOF zone of 0.64' at f/4
  • The FF image enlarged to 8" print height will be perceived by a person with 20/20 visual acuity to have a DOF zone of 0.40' at f/4


The CofC size on sensor is indeed different for APS-C vs. FF, but that is due to the fact that the APS-C image is enlarged by about 13.5X to make the 8" tall print, while the FF image is enlarged by 8.5X to make an 8" tall print.
On the print, the DETECTABLE CofC size is the same size for both images to the eye
, but because of the use of the longer FL for FF, the blur circle for not-perfect-focus grows more dramatically and the eye sees the larger blur circles and concludes 'less DOF in FF' as a result.

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Dalantech
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Jan 23, 2018 14:50 |  #5

Wilt wrote in post #18547401 (external link)
Yes, #2 and #3 are fallacies.
About #1, though...

If I mount a 50mm on APS-C and an 80mm on FF, mount both cameras at the same tripod position, both cameras frame the same area at a distance of 10' with a field of view of 3' x 4.5' in both cases and a 36" yardstick is as tall as the frame height for both images
  • The APS-C image enlarged to 8" print height will be perceived by a person with 20/20 visual acuity to have a DOF zone of 0.64' at f/4
  • The FF image enlarged to 8" print height will be perceived by a person with 20/20 visual acuity to have a DOF zone of 0.40' at f/4


The CofC size on sensor is indeed different for APS-C vs. FF, but that is due to the fact that the APS-C image is enlarged by about 13.5X to make the 8" tall print, while the FF image is enlarged by 8.5X to make an 8" tall print.
On the print, the DETECTABLE CofC size is the same size for both images to the eye
, but because of the use of the longer FL for FF, the blur circle for not-perfect-focus grows more dramatically and the eye sees the larger blur circles and concludes 'less DOF in FF' as a result.

Understood Wilt, and thanks for bringing your expertise to the discussion!

The only "knit picking" I have is...

1) The magnification of those two lenses is going to be different, and that can effect depth of field.

2) The CoC is effected by characteristics of the sensor construction, and not really a result of the crop itself. It's possible for a crop factor sensor and a full frame sensor to have the same CoC -although the crop sensor would be pretty pixel poor ;)


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Post edited 4 months ago by Wilt. (2 edits in all)
     
Jan 23, 2018 14:59 |  #6

Dalantech wrote in post #18547436 (external link)
Understood Wilt, and thanks for bringing your expertise to the discussion!

The only "knit picking" I have is...

1) The magnification of those two lenses is going to be different, and that can effect depth of field.

2) The CoC is effected by characteristics of the sensor construction, and not really a result of the crop itself. It's possible for a crop factor sensor and a full frame sensor to have the same CoC -although the crop sensor would be pretty pixel poor ;)


I stated that both cameras mounted at the identical location would frame the identical 3' x 4.5' area...with identical scene from identical position, it was the lens FL that determined how rapidly the blur circles would grow (for out-of-focus points). Magnification had nothing to do with nothing, since the final images in both 8" tall prints was identical for the two prints.

CofC is NOT affected per se by sensor construction, no more than CofC is altered by changing film from Tri-X to Velvia or Plus-X.
The ON-SENSOR CofC size is a reverse calculation of what the eye can SEE in an 8" tall print. Period.

  • Divide that blur circle size (at 8" print) by 13.5X = APS-C CofC size;
  • divide that same blur circle size (at 8" print) by 8.5X = FF CofC size;
  • divide that blur circle size (at 8" print) by 4X = 4x5 sheetfilm CofC size


To illustrate the concept of CofC blur circles and what fools the eye to be judged 'in focus' vs. 'out of focus', and how different format sizes relate

IMAGE: http://i69.photobucket.com/albums/i63/wiltonw/Principles/CofC%20blur_zpstliq8kqb.jpg

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Post edited 4 months ago by Dalantech.
     
Jan 23, 2018 15:16 |  #7

Wilt wrote in post #18547440 (external link)
I stated that both cameras mounted at the identical location would frame the identical 3' x 4.5' area...with identical scene from identical position, it was the lens FL that determined how rapidly the blur circles would grow (for out-of-focus points). Magnification had nothing to do with nothing, since the final images in both 8" tall prints was identical for the two prints.

Understood your initial post Wilt. Magnification does come into play though -just like any other aspect of a lens. You can't change something and then start selectively throwing things out...

Wilt wrote in post #18547440 (external link)
CofC is NOT affected per se by sensor construction, no more than CofC is altered by changing film from Tri-X to Velvia or Plus-X.
The ON-SENSOR CofC size is a reverse calculation of what the eye can SEE in an 8" tall print. Period.
[/LIST]

Then why does the CoC change (external link) depending on what digital camera you use if sensor construction does not alter it?...


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Jan 23, 2018 16:06 |  #8

Dalantech wrote in post #18547452 (external link)
Then why does the CoC change (external link) depending on what digital camera you use if sensor construction does not alter it?...

DOF characteristics only change when the sensor size changes, since the mag factor to enlarge to the same 8" print size will change; that and the use of different FL in order to achive the same field of view from the same camera position. Sometimes the 'normal' for the FL is not truly in scale with the frame dimension.

  • 4/3 has 13mm tall sensor, 25mm 'normal' is 1.92 * frame height
  • APS-C has 15mm (14.9mm) tall sensor, 32mm 'normal' is 2.13 * frame height
  • FF has 24mm sensor, 50mm 'normal' is 2.08 * frame height


so the true FOV varies because identical AOV is not achieved with 'normal'

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Jan 23, 2018 23:41 |  #9

Not that it matters much but shouldn't you be using the diagonals in in that comparison?




  
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Jan 24, 2018 00:44 as a reply to  @ soeren's post |  #10

Folks compare diagonals when the use of that fails to address when different formats have different aspect ratio frames! It is a very poor way to compare, for example, when trying to come up with 'equivalent FL' for 6x6 vs. 135 format! If you instead compare the SHORT DIMENSION of the frame, then 135 format has 24mm vs. Hasselblad has 56mm.
If I put a 24mm lens on 135, and I put a 56mm lens on Hassy, I can put both bodies on the same tripod and I can capture the EXACT SAME amount of subject area at the same subject distance. That does NOT work if I tried to use the diagonal measures of the two frames (43mm vs. 79mm lenses).


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Post edited 4 months ago by Dalantech.
     
Jan 24, 2018 02:07 |  #11

Wilt wrote in post #18547500 (external link)
DOF characteristics only change when the sensor size changes, since the mag factor to enlarge to the same 8" print size will change; that and the use of different FL in order to achive the same field of view from the same camera position. Sometimes the 'normal' for the FL is not truly in scale with the frame dimension.

OK, thanks! Looking at the chart that makes sense. CoC is pretty minor for the most part, so I'm not really concerned with it. Would be a bigger issue comparing an APS-C to a medium or large format negative.


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Jan 24, 2018 06:08 |  #12

Wilt wrote in post #18547812 (external link)
Folks compare diagonals when the use of that fails to address when different formats have different aspect ratio frames! It is a very poor way to compare, for example, when trying to come up with 'equivalent FL' for 6x6 vs. 135 format! If you instead compare the SHORT DIMENSION of the frame, then 135 format has 24mm vs. Hasselblad has 56mm.
If I put a 24mm lens on 135, and I put a 56mm lens on Hassy, I can put both bodies on the same tripod and I can capture the EXACT SAME amount of subject area at the same subject distance. That does NOT work if I tried to use the diagonal measures of the two frames (43mm vs. 79mm lenses).

Hmmm we can agree that though 6x6, 6x7, 6x8 and even 6x9 shares og the same "short" side length only 6x9 will give you the "exact" same subject area at the same distance as 24x36?




  
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Jan 24, 2018 06:33 |  #13

Reading all of this excellent information is making me dizzy.
In laymans terms then. If I shoot a picture of, say a bird on a post with a FF camera and then a crop camera with the same lens then enlarged the FF image to the same as the crop would both images yield the same quality. (lets assume they have similar sensors). I ask this not to be argumentative but purely to know if it is worth hanging on to my 7D11 when I have a 1DXmk2.




  
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Jan 24, 2018 08:37 |  #14

Fordsabroad wrote in post #18547884 (external link)
Reading all of this excellent information is making me dizzy.
In laymans terms then. If I shoot a picture of, say a bird on a post with a FF camera and then a crop camera with the same lens then enlarged the FF image to the same as the crop would both images yield the same quality. (lets assume they have similar sensors). I ask this not to be argumentative but purely to know if it is worth hanging on to my 7D11 when I have a 1DXmk2.

Depends. How many pixels are you going to have to send to a printer after cropping the full frame image? A lot of people claim that a print has to have 300 dots per inch -that's the "holy grail" of print quality. But at what distance are you viewing the print? How picky are you? I've made poster size prints with 10 and 12 MP images and the people who viewed them raved about the detail in them. But they were standing a couple of meters from the print so they could comfortably view it edge to edge. That's just the sharpness though. What about the dynamic range difference between the two senors? I noticed a change in the color of my images going from a 70D to a 1D MK III because the dynamic range of the 1D MK III's larger sensor seemed to allow me to capture more graduation in color, and in some cases that translated into more detail.

Also depends on how you're going to display them. A lot of the images I see on the web look great, but they're so heavily cropped that even an 8x10 might look about as pixelated as Japaneses porn. I'm sure there is some "cutoff point" where taking a shot with your 7D II makes more sense than cropping the same scene taken with your 1DX mkII, and I'm sure someone will work out the math. But IMHO the numbers won't tell the whole story and the real answer is gonna be pretty subjective.


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Jan 24, 2018 09:16 as a reply to  @ Dalantech's post |  #15

Dalantech

Thanks for your swift reply. My pictures are purely for personal use. I print my own and a few hang on the walls of my friends and family. I use an Epson SC-P800 and until recently printed at 300DPI however it was recommended that i tried printing at 180DPI and I have to say I cannot notice the difference.
As you say, I am sure someone will do the math and tell me that a crop v FF is better/worse.
The 1DXmk 2 is a very capable camera with excellent quality images but i suppose the real test for me would be to get of my backside and shoot some images with both cameras and see for myself. you never know I may just be able to sell the 7Dii and buy a new lens!!




  
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