Can somebody explaining rendering to me?

Drogo Nazhur

Some renders will be done in a matter of minutes with less than a hundred iterations while other renders will run for hours with a thousand or more interations.

Additionally, some renders, which I think are awful, might be 100% completed in several dozen iterations. Other renders, which I think are already perfect, have run for over a thousand iterations and are less than 50% complete.

Why?

Quasar

There is a lot that goes into render times. Things like the complexity of the scene, the type of lighting being used and the settings of the materials being used will all contribute to the render calculations. The good news is, if a render looks great at 50%, it's not likely to get a lot better by waiting until it's done, so there's no harm stopping it early.

nemesis10

Drogo Nazhur said:

Some renders will be done in a matter of minutes with less than a hundred iterations while other renders will run for hours with a thousand or more interations.

Additionally, some renders, which I think are awful, might be 100% completed in several dozen iterations. Other renders, which I think are already perfect, have run for over a thousand iterations and are less than 50% complete.

Why?

Wow, that is a big question!  However, some of the factors that determine how many iterations are 1) how bright the scene is, 2) how big the texture maps are (transparency and complex effxts are slower), 3) how complex the geometry is (hair notoriously slows things down or having loads of props), how the light bounces (if you lit a scene inside a box/room, light has to bounce around a lot so the calculations take longer), 4) the lighting (I find a good HDRI is the fastest, geometry heavy emessive surfaces the slowest), and on and on.  

kenshaw011267

lots of things make renders faster or slower.

Outdoor scenes generally render faster since there are many fewer bounces for rays to be pathed.

Scenes with more light generally render faster as it is more likely that a light ray will hit each pixel earlier in the process.

More objects in a scene tends to make renders take longer because there are so many more objects to interact with the bouncing light rays.

 

kyoto kid

...in Iray the more emissive sources used, the slower the render speed is as well.  

Also scene size, or better, memory load can be a factor, as if you have a low to mid range GPU card (4 - 6 GB) the scene might be handed off to the CPU (which is much slower) should it's size exceed the card's VRAM.