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Thread started 21 Jan 2016 (Thursday) 14:23
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Crop factor explained

 
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Jan 21, 2016 19:40 as a reply to  @ post 17867639 |  #16

Keep in mind if I had taken this same shot with the 5D3, and cropped it down to the 1.6 factor, then cropped it even further down to what I had there, the image might be pretty good still, but you wouldn't have enough resolution to do anything with it. You might be able to print it as a 4x6, maybe.

The juggling act you are faced with is how much reach you get + how much you can crop what you want compared to what resolution result do you require for what you want to do with it. If you just want web images, that should be fine, but do larger prints after you are done, and the results won't be all that good if you had a FF (other than the 5DS/R) and cropped it down this far.


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Jan 21, 2016 20:30 |  #17

Wilt wrote in post #17867543 (external link)
The reminder that needs to be refreshed in everyone's minds...

The LENS can only deliver a finite amount of detail to the focal plane. So if you 'crop' (digital, or what had been done with film in the darkroom for almost 6 generations of photographers) you throw away lens resolution, and it does not matter how many pixels on sensor per square millimeter!

This is true, the lens can only deliver so much resolution to the sensor. Lens resolution is usually measured in Line Pairs per unit length, although since the introduction of digital, it has tended to be normalised as per image height, which is a useless measure for this conversation. In order to convert an analogue signal (that comes from the lens) to a digital one you need to record two digital samples for each line pair. If you record less than two samples per line pair you end up recording a distorted signal at a much lower resolution. This is set by the Nyquist-Shannon Sampling Theorem (it's not really an unproved theory, it is what a lot of people might consider a law). So you have to have a minimum of two samples per line pair, but more are fine, but could be wasted if the signal never reaches a high enough frequency. From what I have been able to deduce, because most tests normalise to per picture height, based on testing with a digital sensor, the very best lenses for DSLRs seem to be capable of providing around 120 LP/mm. To record that signal correctly you then need to have 240 image sensels per mm, so you would need to have a 22.5×15mm sensor record 5400×3600 or 19.44 MP from a crop camera or 49.77 MP from a 35mm frame. If the sensor has less resolution than this you have to add a filter to reduce the maximum frequency that the lens can transmit, these are known as Optical Low Pass Filters (OLPF) or Anti Aliasing (AA) filters. The interference caused by aliasing is impossible to filter out AFTER the image has been digitized. It is impossible to tell the low frequency signal caused by aliasing, from a wanted signal at the same frequency.

Back in the days of analogue film what you wanted to have happen was that the lens could deliver more resolution than the film could record, because the film was the limiting factor, and having too much resolution had no drawbacks, except in the possible costs of the lenses. With digital though we need to have the sensor record just a bit more resolution than the lens can actually deliver, as now the sensor is the limiting factor, and exceeding the limit of the sensor has serious problems that cannot be undone.

It seems as if the current sensors are just reaching the the optical limits of the current best of breed lenses, but they still need to improve in resolution quite a bit, as the above figures are for a monochromatic sensor. Most digital cameras use a Bayer Colour Filter Array with alternating Red/Green and Green/Blue rows of filters over the sensels, this effectively reduces the colour resolution, as to get full colour we have to use a square group of 4 pixels in an RGGB layout. Full colour information is really recorded at only half of the sensors grayscale sensitivity. Tricks can be done to improve things, but colour resolution is still reduced by about 25%. To record the full resolution from the best lenses in colour would require a doubling of the monochromatic resolution, or about 480 sensels/mm, requiring a 100MP 35mm sensor.

It is only when you get to a sensor resolution above 4× (for a Bayer CFA) the maximum resolution of the lens that adding more resolution to the sensor will not result in a better quality final digital image. With the exception of the 5DS/5DSR the current Canon FF sensors are only capable of recording around 76 LP/mm from a lens, compared to the approximately 120 LP/mm from an APS-C sensor. Is it surprising that most lenses seem to max out at about 75 LP/mm when mounted on a FF camera? You have simply hit the sensor resolution. Since we can only see about 6 LP/mm at about a 12" viewing distance current best lens designs, that provide resolution that just about matches the Nyquist limit of our best sensors at 120 LP/mm allow for images at are 18"×12" without adding any digital interpolation, or reducing the maximum image resolution below that of the average human observer. We can actually reduce the output resolution by about one third, to about 2LP/mm (100PPI) and the image still looks generally very good, if it didn't we would all be clamouring for 15" 4K displays for editing. Given that, it is possible to take a 5×3.3mm crop from a current APS-C sensor and get a useable 12×8 image. That is a 60× enlargement which would be approx 85"×57" for a full size same resolution image from the 5DS series cameras, or 53×35 from a 7DII.

Adding extra resolution to our highest resolution sensors, even without an improvement in lens resolution will give a much better colour resolution in images that have been enlarged to this size, so is actually still very worth doing. Wanting a lens to outresolve a digital sensor is the biggest misconception in digital photography after the crop factor and DoF.

Alan


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Jan 22, 2016 05:15 |  #18

nqjudo wrote in post #17867587 (external link)
FPS is frames per second and that's exactly what I was referring to. It has a direct relationship to this conversation in the context that I used it; there are always tradeoffs. A 50 MP image comes at monetary and technical cost that may not be ideal for some. Cropping 80% of an image and still get something useable may be tempting but that ability, in the case of the 5DS comes at a cost of being limited to 5 FPS. This may be enough of a consideration for someone shooting sports or wildlife to go with a 7D2 that shoots at 10 FPS. The OP mentioned photographic pursuits where long focal length are required so my comments are not out of context. Simply put one needs the best tool for the job so all factors have to be considered. To focus on a sensor and ability to crop an image without considering anything else is myopic. I hope that clarifies things for you.

The FPS issue, applied in this thread, does not help the OP understand the issues that are being discussed. He's still trying to understand about image sizes on various sensor sizes with the same focal length, etc.


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Jan 22, 2016 06:28 |  #19

Fps as per the first mention of it in this thread fit at the time. Cause it was how many fos at full resolution.


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Jan 22, 2016 08:37 |  #20

SkipD wrote in post #17868069 (external link)
The FPS issue, applied in this thread, does not help the OP understand the issues that are being discussed. He's still trying to understand about image sizes on various sensor sizes with the same focal length, etc.

Look, these threads are fluid. They move from a central point in all directions all the time. If someone asks what 2+2 is there is nothing wrong with someone also volunteering what 3+3 is as long as it is relevant. Again, may comments were to bring to light the fact that focusing solely on the pixel count of a sensor is of little use and putting the cart before the horse. That's what a conversation is; an exchange of ideas. I understand that you may not find the FPS information relevant but others, including the OP above might not share your opinion. You have stated not once but twice that you find my information of no use. I accept that. In that sprit I'm sure you'll agree that information offered by a member that is of little use is far less offensive and damaging than superfluous comments designed to do nothing more than police a thread and derail a conversation. Time to move on.


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Jan 22, 2016 08:48 |  #21

In earlier days, the FF sensor were also said to be better because the pixel density was not as compact. Understanding the crop factor, my question is actually about improvements in current sensor manufacturing technology - specifically for crop sensors. That is, is the quality of crop sensors now equal to that of FF sensors?

An explanation:

compare two theoretical cameras:
- one a 1.6 crop, the other a full frame
- both 20 megapixels
- using the same lens, shooting the same scene
- the 1.6 shot is foot-zoomed so the uncropped framing of the scene is the same as the FF shot
- both are printed, uncropped, at 24" x 36"

Is the quality of the crop shot now equal to the quality of the FF shot?

I do not care about pixel-peeping in photoshop zoomed to 9000%.

I care about a 24" x 36" print, handing on a wall, viewed from 6 feet. Will the human eye be able to tell the difference?


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Jan 22, 2016 09:01 |  #22

I just know that mediocre lenses that resolved detail enough to wow people back in the day with less dense sensors no longer produce that same kind of detail rendering at nearly 100% on newer bodies. Whatever the reason that newer lenses produce better detail, I don't care, I do know that vs II of the 24-70 and 70-200 is much better than version 1, ditto with the 100-400L, and also even with the kit lenses.

So indeed we do need newer better lenses as sensor densities increase. We just don't know what the limit is now with the latest vs II lenses, it could be 100Mpx, or larger before we need vs III lenses.

The expectation with the newer bodies should also include newer lenses, if somebody pixel peeps.


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Jan 22, 2016 09:29 |  #23

Picture North Carolina wrote in post #17868218 (external link)
- the 1.6 shot is foot-zoomed so the uncropped framing of the scene is the same as the FF shot
- both are printed, uncropped, at 24" x 36"

Is the quality of the crop shot now equal to the quality of the FF shot?
...............

Will the human eye be able to tell the difference?

In this particular case, yes there will be a difference in the two images. It won't have anything to do with resolution, though. The perspective will have changed because of the "foot zooming". By perspective, I mean relative sizes of elements of the scene that are different distances from the camera. The scenes will be different even if the primary subject is framed the same.

The only way to make two images that are, for all practical purposes, the same with two different format cameras is to use different focal lengths on each camera and make the two images with the cameras in the same position.

The resolution and technical quality of both images should be pretty much the same. It's the content of the images that will be different in this case.


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Jan 22, 2016 10:00 |  #24

SkipD wrote in post #17868265 (external link)
The only way to make two images that are, for all practical purposes, the same with two different format cameras is to use different focal lengths on each camera and make the two images with the cameras in the same position.

Your are correct. Forgot about the obvious - the subject to camera distance created by the foot zoom.

Basically, I'm just trying to understand the difference in manufacturing technology (print quality) of newer crop sensors compared to my older (5D MK2) sensors and processor.


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Jan 22, 2016 10:45 |  #25

nqjudo wrote in post #17867587 (external link)
FPS is frames per second and that's exactly what I was referring to. It has a direct relationship to this conversation in the context that I used it; there are always tradeoffs. A 50 MP image comes at monetary and technical cost that may not be ideal for some. Cropping 80% of an image and still get something useable may be tempting but that ability, in the case of the 5DS comes at a cost of being limited to 5 FPS. This may be enough of a consideration for someone shooting sports or wildlife to go with a 7D2 that shoots at 10 FPS. The OP mentioned photographic pursuits where long focal length are required so my comments are not out of context. Simply put one needs the best tool for the job so all factors have to be considered. To focus on a sensor and ability to crop an image without considering anything else is myopic. I hope that clarifies things for you.

Ah, you were referring to multishot BURST speed, and not to video frame rates! What confused folks is that 'FPS' was used for video/cinema long before it was used for multishot stills.


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Jan 22, 2016 10:49 as a reply to  @ Wilt's post |  #26

Yes indeed. The OP was referring to still images so I was responding in kind.


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Jan 22, 2016 11:12 |  #27

BigAl007 wrote in post #17867727 (external link)
From what I have been able to deduce, because most tests normalise to per picture height, based on testing with a digital sensor, the very best lenses for DSLRs seem to be capable of providing around 120 LP/mm.... With the exception of the 5DS/5DSR the current Canon FF sensors are only capable of recording around 76 LP/mm from a lens, compared to the approximately 120 LP/mm from an APS-C sensor. Is it surprising that most lenses seem to max out at about 75 LP/mm when mounted on a FF camera?

An interesting historical perspective to add to the above observations...even back in the 1970s an 'excellent' lens achieved about 80 lp/mm, a 'very good' lens achieved about 64 lp/mm, and the very rare 'exceptional' lens achieved around 120 lp/mm in the subjective ratings.

BigAl007 wrote in post #17867727 (external link)
You have simply hit the sensor resolution. Since we can only see about 6 LP/mm at about a 12" viewing distance current best lens designs, that provide resolution that just about matches the Nyquist limit of our best sensors at 120 LP/mm allow for images at are 18"×12" without adding any digital interpolation, or reducing the maximum image resolution below that of the average human observer. We can actually reduce the output resolution by about one third, to about 2LP/mm (100PPI) and the image still looks generally very good, if it didn't we would all be clamouring for 15" 4K displays for editing. Given that, it is possible to take a 5×3.3mm crop from a current APS-C sensor and get a useable 12×8 image. That is a 60× enlargement which would be approx 85"×57" for a full size same resolution image from the 5DS series cameras, or 53×35 from a 7DII.

Adding extra resolution to our highest resolution sensors, even without an improvement in lens resolution will give a much better colour resolution in images that have been enlarged to this size, so is actually still very worth doing. Wanting a lens to outresolve a digital sensor is the biggest misconception in digital photography after the crop factor and DoF.

Alan

...and in print evaluation, the threshhold was a minimum of 5 lp/mm for the print to be perceived as 'sharp', and that threshhold applied as we enlarged to increasingly larger print magnifications and determined the effective max print size from the 135 format negative...about 16X was typically a threshhold determined by the grain size of the emulsion becoming objectionable. And if you started with a 'very good' lens' delivery of 60 lp/mm (not uncommon), at 16X it was providing 3.75 lp/mm at the point at which the Tri-X grain was becoming noticeable yet still acceptable. So grain and lens resolution set 16X magnfication as the typical acceptable limit of both grain and the 'sharp' assessment, mandating larger formats for larger actual print sizes.


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Jan 22, 2016 11:25 |  #28

Picture North Carolina wrote in post #17868218 (external link)
Is the quality of the crop shot now equal to the quality of the FF shot?

I do not care about pixel-peeping in photoshop zoomed to 9000%.

I care about a 24" x 36" print, handing on a wall, viewed from 6 feet. Will the human eye be able to tell the difference?

Assuming that the semiconductor manufacturing process is identical for 15x22 mm sensor as it is for 24x36 mm sensor, the fundamental no-signal circult noise would be the same.
If the area of the single pixel is the same for both FF and APS-C, the number of photons striking a single pixel per unit time is also the same. So both should have the same high ISO low light gathering; S/N would be the same.

Assuming that the lens can provide 120 lp/mm to focal plane, both FF and APS-C captures the same number of line pairs since the pixels/mm are the same.
So when we enlarge both to the same final print size, the APS-C frame is magnified by 1.6X greater factor, reducing the on-print resolution seen by the eye, and the FF image will seem to be sharper and more detailed, about 25X mag to make 24x36" print from FF, 40X mag to make same print from APS-C, or 4.8 lp/mm from FF and 3 lp/mm from APS-C
...if we used lens FL appropriate to the format size, for the shots with both bodies...'short telephoto' 50mm on APS-C vs. 80mm on FF, or 'long telephoto' 500mm on APS-C vs. 800mm on FF, for example.


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Jan 22, 2016 13:33 |  #29

Thanks, Wilt. Appreciate the help.


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Crop factor explained
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