Resolution and Sensitivity

[jtnorth]

Just had a thought that I thought I'd throw up for immediate shoot-down or acceptance.

When shooting at higher resolutions, utilizing more of the sensor, would it make sense that overall sensitivity would increase and depth of field decrease?

Would this be a good consideration when deciding to shoot 4 or 5K over 3K?

I understand that this may have more to do with pixel size than sensor size, though specifics I would be fuzzy on.

Any thoughts?

 

[David Mullen ASC]

The sensitivity is partially a function of the size of each individual photosite, the theory being, like grains in film, that a larger surface area can collect more light. So a sensor with a high pixel count tends to be less sensitive because the size of the pixels were limited by how many you could physically cram into the total area of the sensor.

After that, whether you use all of the sensor or crop to a smaller area of the sensor isn't going to change sensitivity. But enlarging a smaller image may make noise artifacts more visible. In other words, whether you shoot 6K on the Dragon or crop it to 1K, the sensitivity is the same.

Depth of field characteristics are more shallow when a larger sensor area is used and you switch to longer focal lengths to maintain the same field of view as a shorter focal length on a smaller sensor. So again, putting a 50mm lens on and shooting at 6K or cropping the image to 1K doesn't change the depth of field, but switching from a 50mm at 6K to a 25mm at 3K to maintain the same field of view at the same distance and f-stop means that the 6K image with the 50mm would feel shallower in focus than the 25mm lens image on the smaller 3K area.

 

[Justin McAleece]

A great explanation you've no doubt given a hundred times. No one can argue with that, David!

 

[jtnorth]

Thanks so much David. Clarifies the day-dream.

One more question if you have the time:
Thought I'd heard recently that the Dragon sensor will be sporting smaller photosites than the MX chip, but is nevertheless be more sensitive than the previous generation. Is this owing to a different material or just magic?

 

[David Mullen ASC]

It's not necessarily more sensitive -- in fact, because you can rate the Dragon sensor at lower ISO's without losing highlight detail tends to suggest the opposite -- but that the Dragon sensor is much less noisy than the MX sensor so that it can be rated faster with less noise than the MX sensor can even if the MX sensor might have a base sensitivity that is slightly higher. The sensors in DSLR's like Canons are like that -- they actually have the widest dynamic range nearer to 100 ISO but can be rated faster than 3200 ISO, so it's not so much that the sensor is more sensitive but that the noise is lower. So the "magic" is that despite using smaller photosites, they have reduced the noise in the system so that you can underexpose the sensor even more than you can with the MX sensor.