# Advice for nube badly needed! CMOS sensor pixel size question.



## has981 (Sep 15, 2010)

Hi all,

This is the first time I post in this forum so apologies if I'm posting in the wrong place.

Could anybody please shed some light on how the sensor pixel size affect the image quality, depth of field, selection of lenses, or any other aspect of the image?

I'm asking because I'm trying to make a selection between different CMOS image sensors (mostly from micron) which differ in their sizes (from 2.2µm up to 12.0µm) and I'm not sure which I should favor. 
See:
Image Sensors - Aptina Imaging

I do appreciate any kind of help or advice on this.

Regards,
Hasan.


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## Big Mike (Sep 15, 2010)

Welcome to the forum,

Are you building your own digital camera? Or just doing a lot of research before buying a camera?  :scratch: 

I'm no expert, but AFAIK, larger photosites on the sensor, can usually make for better image quality.  Especially when it comes to higher ISO because they produce a cleaner image (less digital noise).
But as the demand for more pixels has risen, and they cram more pixels onto the sensor, the photosites get smaller, or at least closer together...which tends to produce more noise, especially at higher ISO.  That's why most 'point & shoot' digital cameras don't perform as well as DSLR camera...their sensors are tiny in comparison and packed with pixels.  

As for how this affects DOF, lenses etc....I think that has more to do with the overall size of the sensor and the design of the camera etc.


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## KmH (Sep 15, 2010)

Are you asking about individual pixel size? 

Image pixel dimensions, or image sensor sizes like Micro 4/3, APS-C, APS-H and full frame?


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## Garbz (Sep 16, 2010)

Large pixels will capture more light and thus produce a greater signal to noise ratio. That's the only factor individual pixel sizes will have.

Of course the signal to noise ratio in the sensor at that point will absolutely pale in comparison to the signal to noise ratio of your amplifier on the other end when you read out the sensor data, so really the question comes down to what are you trying to do.


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## has981 (Sep 19, 2010)

Thanks all for the informative replies..

KhM: yes it is the individual pixel size I'm asking about.

I've been reading about the topic.. obviously plenty of articles discuss this issue...
most notables were:
Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks
Digital Camera Sensor Sizes: How it Influences Your Photography

which -according to my understanding- confirms the views of replies.

It's become obvious to me after the research it depends on what I'm trying to do.

I've been able to come up with the following comparison between both:
(Please do comment and/or correct)

Large Pixel
* Better ISO
* Less diffraction aberration => higher f/n possible => better DoF.
* Cheaper (Low Mp) lenses possible to use.
* Higher signal to noise ratio => better performance in low light conditions.


Smaller Pixel:
*Cheaper sensor.
* Better DoF due smaller Circle of Confusion (Not quite sure about this point though)


Other useful articles:
Clarkvision: Does Pixel Size Matter
Understanding Sensor Design
Sensor Pixel Size as a Determinant of Digital Camera Image Quality


Thanks again for all the replies.

Hasan.


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## Garbz (Sep 20, 2010)

If you analyse pixels out of context then your conclusions would be correct. But on the contrary it's actually the larger pixel that is cheaper due to manufacturing difficulties in high resolution sensors. However then if you look at physical area of the entire sensor a 35mm sensor will be more expensive than an APS sensor regardless of the size of the individual pixels simply due to the amount of realestate it takes up on the silicon wafer. 

With depth of field you also need to look at it in context of the sensor. A 2.5mpx 35mm sensor will produce the same depth of field as a 10mpx 35mm sensor despite the pixels being a quarter of the surface area. However a 10mpx 35mm sensor will produce a much smaller depth of field compared to a 2.5mpx 1/3" sensor due to the smaller projection area requiring a lens with a wider angled design to achieve the same physical field of view. 

Diffraction also takes into account a common final outlet for the image. If you compare two sensors of the same size with different pixel densities, the one with the smaller pixels will be more challenging on the lens, and thus will show diffraction related sharpness problems more apparently (more pixels worth of bluryness). Larger pixels are defeating diffraction by reducing resolution, hardly an optimal solution. In the end if your sensor outperforms you lens, it doesn't matter what physical size the pixels have, the diffraction limit of the lens remains unchanged. Now contrast this to physically increasing the sensor size. Now the airy discs don't just have a changing resolution to deal with, but rather a larger total surface area to play with. They become smaller in the whole image so now the diffraction limit is pushed further back. 

This is explained poorly, but in essence: D40 and D300 both have identical limits. Diffraction limiting sharpness sets in at beyond f/11. D700 and D3 on the other hand with redesigned lenses to suit casting an image over the larger sensor have diffraction limits starting at around f/16 purposes. In all 4 cases we assume the sensor outperforms the lens.


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## has981 (Sep 22, 2010)

Thanks again Garbz,

Truly appreciate it. please bear with me as I'm just beginning to see the picture.

So if I understand you correctly, fixing the resolution (say at 320x240), larger pixels would produce larger sensor area (obviously) which implies shallower DoF. Whereas smaller pixels would produce smaller sensor area increasing the DoF. Also, fixing the resolution, I would benefit from the larger pixels with (and I'm assuming here): 1- less diffraction (without compromising the resolution). 2- better ISO or smaller aperture at the same ISO.

Should that be true, my next question should be: Does the benefit on DoF obtained by the smaller aperture (in the case of the large pixels) exceed the benefit on DoF produced by the smaller sensor size (of the smaller pixels)?

Hasan.


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