what causes grainy images?

[Charliedelta]

I was wondering, what is the biggest cause for grainy images? What would less grainy images, a high quality lens on a camera like a sony nex 5 or a more "regular" lens (something better than a kit lens though) on a better camera like a nikon d7100?

 

[snowbear118 49]

Film has grain, digital has noise. Using a high ISO can cause noise in the dark areas.

 

[Charliedelta]

Thanks. I forgot to say it, but I meant at low, identical, ISO.

 

[Ysarex87 81]

Thanks. I forgot to say it, but I meant at low, identical, ISO.

Under exposure.

Joe

 

[goodguy]

Thanks. I forgot to say it, but I meant at low, identical, ISO.

Getting a grainy image is a combination of variety of condition.
Sensors and in camera processors will make difference.
Lenses has less impact on grain, to really answer such a question you need to post 2 pictures and we might be able to tell from them what cause one to be grainier then the other.
Lens might offer better CA, less distortion, sharper images but I don't think grain is something that really is affected a lot by lens.

 

[KmH]

There are a lot of different causes of digital image noise, and every digital photo has some amount of image noise.

Image noise causes:
Electronic noise
Shot noise
Gaussian noise
Noise from sensor dark current
Analog to digital converter error noise
Quantization noise
Ansiotropic noise

If you want to minimize the amount of digital image noise you need to optimize exposure using Expose-To-The-Right - ETTR.
Gaining an understanding of how digital images work also helps a lot.
http://www.lumita.com/site_media/work/whitepapers/files/pscs3_rendering_image.pdf
ETTR
Optimizing Exposure - Luminous Landscape

 

[BrickHouse]

Excellent articles! Very interesting.