Just like in film photography, the size of the recording medium directly impacts the quality of the enlarged image.   In digital systems size is viewed in two ways.  The number of pixels (receptors) the sensor contains and the size of the container which holds those pixels. 

     We know that digital noise is always there.  We know what creates it.  Then why does the sensor's size make a difference.   Remember what I asked you to remember.  No, okay, click here.

     So, magnifying the image makes grain and noise more visible.  Yes, it makes noise larger, too.  Why?  The camera sensor records images which could be considered less than dynamic compared to film.   The image is flat.   In that flat image, the noise is present, just like the grain in film.   That flat image has a limited amount of information recorded in it.   So, let's say the image has 5% noise. When the image is processed, either by the camera itself or the photographer, that flat image is tweaked to make it more vibrant.  When the contrast is increased, the noise is also increased.  When the colors are made stronger the noise is made stronger.   When the image or file size is made larger the noise is made larger.    All of this happens to the image file as it is adjusted.

     In digital photography, the more information that is recorded is directly proportional to the maximum size that it can be viewed at without loss of information or detail.    A 2mb file will make a good 5x7, but if you make an 11x14 from it, it will look like something is missing.  Something is missing;  Information.   So, more pixels let's us make bigger prints.  A 23mb file let's us make a 16x20 poster without loss of detail.   This is the reason why camera sensors have gotten more crammed with receptors over the last 15 years.  We, as consumers aren't satisfied with tiny prints or images that can't be zoomed into.  This sets us up for the problem of sensor size and it's relationship to noise.

     Large sensors produce cleaner, sharper images than small sensors.  This is a physical fact.  The more receptors there are crammed into a small space, the more they interfere with each other and produce noise.  In a large sensor, the receptors are larger, more energy efficient and spaced apart better.  They interfere less with each other.  The sensor itself is exposed to more light and light makes the image.  Clutter in any medium means noise.  Large sensors have a cleaner signal to noise ratio.   So, a 20mp full frame sensor has less noise than a 20mp sub frame sensor.  They hold the same amount of information but the full frame sensor has more actual photo, more useful information and less noise.   The sub frame sensor with smaller receptors crammed into a smaller space creates more noise. 

     The severity of noise grows as ISO settings rise.  The performance difference at ISO 100 can be barely noticeable but the difference in noise at ISO 800 is impossible to ignore and will be seen on monitors and in prints, especially enlargements.  The file sizes may be the same, but the image is much cleaner from a larger sensor.  

     Imagine a photo taken with a smaller sensor.  In order to get 20 megapixels into that sensor, they must be crammed in there so tightly that they interfere with each other to great extents.  Feedback from one receptor affects 20 other receptors causing errors all across the sensor.  It is like filling a room with mouse traps.   They all react to each other when one is tripped.  You don't want the trap next to the one that catches the mouse to respond but it is triggered to do so and before long, half of the room has tripped traps.  When that happens in a digital photo, the noise becomes overwhelming.   It is like looking at a static filled tv screen when the signal is weak. 

     Let's look at the sensor sizes of the most popular digital systems.   35mm/Full Frame is our reference size.