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Answer to question on sensor size vs crop factor
Old 01-02-2006, 10:02 AM   #1 (permalink)
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There was an excellent question about crop factor from John Paul which has made me want to share crop factor and image size information with all of you as I recently posted a question about Canon vs Nikon images due to sensor size / pixel size differences. The original post asked about mathematics of crop factors which has a less than simple answer but may be helpful for those also in question.

You may know much of the information below but I would like to share it with the masses as some are not aware of some of the technical reasoning behind the answer.

As I am an engineer and have a degree in mathematics, I believe I can be overly helpful. As I mentioned, this same sensor size issue is one that effects me greatly as a Nikon D2X guy wanting the same shallow Depth of Field as a full frame piece of film. One interesting point to those that don't know is to note that a 35mm piece of film does not measure 35mm in either direction. It really measures 24 x 36mm in film as it is double frame (height) of the old 18 x 24 movie film.

I think it is also important to note that magnification is done through the lens and the cropping factor (something invented by communists like Nikon / Sony to keep costs down and customer bases up) simply means the same size image comes through the same lens but some of that image spills past the edges of the sensor and into "nowhere" land off the sensor. An advantage of smaller sensors, aside from cost, is the edges of the lens (full frame sensors will see this) will vignette / darken as they can not disburse the light as efficiently as the center glass can. Simply understanding the concept of why fiber optic lines require a sheathing layer helps explain this refraction issue.

Besides sensor cost, please note that smaller sensors require smaller lens, also keeping lens cost and weight down. The issue of economics drives is what drives digital cameras anyway. Let's face it, if we really cared about Depth of Field or image quality, we would be shooting medium format Hassleblads!
In cameras like my D2X, it makes sense that the image quality is high, not because of any cropping factor, but simply that the receptors on the CMOS sensor are very small and close together. Nikon offers the highest image definition of any digital camera on the market simply because the sensors provide a finer image. Nikon leaves Canon in the dust when measuring resolving power simply because of this reason. The D2X has the highest resolving power of any digital camera. One note to remember is the Glamor business doesn't want resolving power as we don't want to see flaws in skin anyway. Black scarf filter anyone? NASA on the other hand uses D2X's to shoot the bottom of the shuttle as they do want resolving power. Something about things hanging off the bottom of their vehicle and 17,000 mph. The problem with the smaller sensors is when an image is "stretched" to be the same image size as a full frame camera it looks as if the resolving detail is not the same quality as the full frame camera. This effect was demonstrated in a previous post where two images (one D2X and one Mark II DS) where shown next to one another. The flaw in that remonstration is the Nikon camera would have to be further away or the image cropped to give the same starting point image.

To answer your question about sensor size cropping factor, the issue is slightly more complex that just that of sensor size as, for example, the Nikon has a sensor height of apx. 24mm and a Mark II DS has a sensor size of apx. 36mm. Simply dividing 36 / 24 gives you 1.5, or an apparent 1.5 crop factor. Just like horsepower of a car, you can measure power at the tire or the engine. You get different numbers depending on the point of measure. With cameras, you can measure the image at the sensor or on the final digital file. The only issue to keep in mind is that the image size has little to do with sensor size and more to do with pixel size and pixel count. Just as an example, there is an interesting article explaining this resolving power at:

http://www.lonestardigital.com/D2X.htm

To underscore the issue, please note that neither Nikon nor Canon define their image size by sensor size but rather by pixel count. To calculate a true comparison, you would need to know the pixel count and the pitch size of each photosite on the sensor.

All that being said, if the pitch size is somewhat similar in your question, simply 24 / 5.32 for an answer of around 4 1/2 or 5. I would dare to add that a sensor so small would undoubtedly have as fine a pitch count as the full frame camera as I suspect price / cost was a driving force to manufacture such a small sensor in the first place.

I still want a Canon D5 because the pixel count and photosite pitch size allow me to fit the image I want onto the sensor at a closer range than my Nikon therefore, blurring the background (similar to 1 1/3 greater stops) at a wide open F Stop.


Too much information?

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Re: Answer to question on sensor size vs crop factor
Old 01-02-2006, 11:56 AM   #2 (permalink)
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Ok, so, in laymans terms....

I can blur the background on my photos with my 10d with no problem. I can get some pretty nice close-ups of flowers for example with my 50mm f1.8 (took some over the weekend, yet to download them though). So, I'm not sure what you're getting at. Do you mean to say a full frame sensor would allow me to get the same image from closer up? Wouldn't I, in this instance, still be limited by the physical distance the lens needs to be from the subject in order to focus?
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Re: Answer to question on sensor size vs crop factor
Old 01-02-2006, 12:07 PM   #3 (permalink)
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David,

I would be happy to answer your question. Since you 20D has a smaller sensor (images still computed by pixel count and size to be exact) than, let's say, a 35mm film camera, you are forced to stand back further to get the whole flower in the picture than you would if you were shooting with the film camera. The further you back up from the flower, the greater the depth of field is going to be. Your 20D may blur the background all you want, but the D5 with blur the background of the same flower even greater as you could stand closer to the flower to get the same fished image.

The below shallow DOF shot was taken with my full frame F5 film camera. I could never duplicate this shot on my non-full frame digital.

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Re: Answer to question on sensor size vs crop factor
Old 01-02-2006, 12:36 PM   #4 (permalink)
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[ QUOTE ]
The below shallow DOF shot was taken with my full frame F5 film camera. I could never duplicate this shot on my non-full frame digital.


[/ QUOTE ]

only if you were shooting each image wide-open with the same lens at the same shutter speed. proximity to the subject ain't the only thing effecting DOF.
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Re: Answer to question on sensor size vs crop factor
Old 01-02-2006, 12:57 PM   #5 (permalink)
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Jimmy,

Very true. Holding all other variables the same (F stop and focal length), distance would be the factor determining DOF.
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Re: Answer to question on sensor size vs crop factor
Old 01-02-2006, 01:44 PM   #6 (permalink)
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Sorry that my first post is about this, but I couldnt resist.

http://www.dofmaster.com says that its the other way around ?

I am a Point and shooter, so dont get angry that I disagree.



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Re: Answer to question on sensor size vs crop factor
Old 01-02-2006, 01:47 PM   #7 (permalink)
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What exactly are you saying is the other way around?
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Re: Answer to question on sensor size vs crop factor
Old 01-02-2006, 01:50 PM   #8 (permalink)
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I´m not good at posting, sorry for that as well.

What I mean is that when you put the values of a Canon 5D in the calculator and put the same values for a D2X in the calculator, the D2X DOF is smaller. (not larger)


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Re: Answer to question on sensor size vs crop factor
Old 01-02-2006, 01:59 PM   #9 (permalink)
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The DOF is not a factor of the brand of camera but rather the focal length to the subject (along with F stop and aperture). To keep the subject size equal in the viewfinder, you need to MOVE BACK from the subject when using the D2X as opposed to the D5. When you increase the distance from the subject, you increase DOF.

What kind of camera are you using?
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Re: Answer to question on sensor size vs crop factor
Old 01-02-2006, 02:08 PM   #10 (permalink)
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aargh, your right.

To get the equivalent focal length, I need to devide the focal length by 1,5. Then you do get a bigger DOF in a cropped sensor camera.

DOF is tied into the Camera brand because the circle of confusion is camera dependant. (different sensor sizes).

You do not need to step back to get the same frame, you do need to use a different focal length. i.e. 50mm full frame equals 33,3 mm 1,5 cropped sensor.

Well, great made a fool of myself.

Regards,
P

EDIT: let me check this again cause it doesnt add up in my mind.

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