MP's & Crop Factor vs Lens

A lot of "hype" is made over high megapixel cameras and "crop factor" giving you additional "focal length" from your lens. I even use a high MP camera in DX mode sometimes. BUT,  crop factor or high MP "cropability" is NOT the same as using a longer lens. All it does is give you the recorded field of view of a longer lens.

 

The difference here is important. 

 

A longer lens has greater magnification and it provides more detail to the sensor to be recorded in the first place. More detail available to be recorded is always going to be better. With a longer lens you are using larger pixels to capture larger deatils.


More pixels lacking detail is still an image without detail. Imagine taking a picture of a flat colored wall.... it will look the same regardless of how many pixels are used to record it. 

 

More pixels in the resulting image *IS* more detail *IF* the detail was there in the first place. But this means the lens has to be "better." It is much harder to for a lens to resolve a detail at 1 pixel if that pixel is much smaller. That's why (currently in 2013) a 41MP cell phone camera can't match a DSLR...the lenses used just aren't up to the task.


Keep in mind, "crop factor" is only of any benefit *if* the resulting image will have at least the same number of pixels remaining. For example, there is no advantage of using a 16MP D7000 as opposed to using my 36MP D800 in DX mode. And there is no advantage (as far as IQ is concerned) in using the D800 in DX mode as opposed to cropping a FF image in post.

 

More pixels in a smaller image FOV means the pixels are smaller. With "crop factor," or "cropability," you are using smaller pixels to record smaller details. Smaller pixels (sensor photo sites) are less effective at gathering light...they *require* more light to perform well. Almost all of the penalties of smaller pixels (and higher ISO's) are due to less ability to fully saturate the sensor photosites (pixels). These penalties include lower contrast, lower dynamic range, lower color sensitivity, and greater image noise.

 

Additionally, smaller pixels require more light in order to generate higher shutter speeds in order to prevent motion blur due to subject or camera movement. Motion blur is caused by light (a "detail") moving across more than one pixel. As long as the movement is smaller than the size of the pixel no motion blur will be recorded. Obviously, the smaller a pixel is, the harder it is to keep movement within it. So higher shutter speeds are required in order to record a smaller portion of the movement in order to keep it within the size of one pixel.

This is why you will often hear that you have to increase the minimum shutter speed for a lens due to crop factor. This isn't exactly true. I can handhold a 12MP DX (crop) sensor at the same speed that I can a 16MP full frame sensor. I cannot hand hold my 36MP FF D800 at as low of a shutter speed as either of the others because it has smaller pixels.

 

Don't get me wrong, sometimes MP's and crop factor are the "practical solution" to the issue of focal length/magnification. You can buy three D800's for the price of one 400mm f/2.8 VRII lens, plus they're smaller and they weigh a whole lot less. 

 

Tele converters fall somewhere in the middle. They increase the magnification making details larger, but they also reduce the amount of light that gets to the sensor and degrade the image sharpness to some extent. It becomes a question of how much detail there really is, how much degradation will occur, and how much light do you have available. Sometimes a TC is a better answer and sometimes crop factor/MP's is a better answer.


And here's an odd "circle." Lets say you have an f/2.8 lens a 1.5x TC +16MP FF and a 16MP (1.5 factor)DX body...which combination to choose? It could make almost no difference....
The TC makes the lens an f/4 and degrades the image ~ 1stop so we wind up at f/5.6.

The DX body makes greater demands of the lens and needs ~1stop so we wind up at f/4. It also has ~ 1stop worse ISO performance than the FF body but we haven't lost a stop by changing the lens's actual focal length (adding a TC)... So we wind up with slightly different settings, but very comparable image IQ. The DX body would have ~1 stop less DOF (same effective focal length, from the same distance, wider aperture).

 

The "best answer" is always a longer lens with high IQ and a fast/sharp enough aperture. But, as with all things in photography, you usually can't have everything you would want and it becomes a balancing act.


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