Stupid question about exposure

dougietee

dougietee

Suspended / Banned
Messages
190
Name
Doug
Edit My Images
Yes
Well it might be a stupid question. I was wondering why digital cameras still need an ND filter for when the conditions are too bright, and whether they could achieve the same thing by simply reducing the gain on the sensor?

Can anyone explain this to me?
 
Depends entirely on whether the conditions allow you to take the shutter speed required for the desired effect. Modern sensors don't seem to do much below ISO100 and diffraction is an issue for high f/ numbers so, sometimes yes, sometimes no.
 
The ISO setting on a digital camera is a measure of how much the signal from the sensor is amplified. If the signal is too large to start with there's nothing to be done to improve it. Streaking off the sun or speculars spring to mind.

You might also want to open up the lens to get a shallow depth of field. Without ND, on a bright day this would cause severe overexposure.
 
dougietee said:
Well it might be a stupid question. I was wondering why digital cameras still need an ND filter for when the conditions are too bright, and whether they could achieve the same thing by simply reducing the gain on the sensor?

Can anyone explain this to me?
Click to expand...

Hi Doug,

If you have auto-ISO set then the camera will try to lower the ISO if the adjustments to shutter speed and aperture can't get the light level down far enough. This is achieved by reducing the gain on the sensor as that is how a digital camera fakes ISO.

There is a practical limit to how low you can reduce the gain on any given sensor, however, before running into other electronic induced problems and this is why ND filters still have a place in digital photography.
 
Interesting question actually, thinking aloud i know the iso 50 setting on the 5d2 is what you see be going too far here, while it is less than iso100 the dynamic range is dropped as well so if you'd taken the shot at iso100 and dropped it a stop in post then you'd have more data to play with.

But that doesn't really explain why, only that there must be drawbacks.

But then looking at MFDB, they go down to iso25 or so and have a much larger sensor so less gain to begin with and they look lush.

My guess is the sensors can only really be built to cope with a specific range and would you rather buy the camera that does iso 102400 or the camera that does iso 12? Then keep in mind the answer you or i give is probably different from the soccer mom deciding what to buy at walmart
 
nano said:
My guess is the sensors can only really be built to cope with a specific range and would you rather buy the camera that does iso 102400 or the camera that does iso 12? Then keep in mind the answer you or i give is probably different from the soccer mom deciding what to buy at walmart
Click to expand...

I think your guess is correct. And I'm with the soccer mom in my choice of ISO range. After all, it's easy to simulate ISO 0.1 with an ND filter whereas there's no sensible way to simulate ISO 102400.
 
Interesting responses, thanks. I suppose it does make sense that the sensor could only cope with a certain range of sensitivity. I was just thinking about it the other day and wondered whether or not the effects of lowering the light with a filter could be easily simulated electronically, and then if that were the case, why would ND filters be needed for those bright/long exposure situations.
 
dougietee said:
Interesting responses, thanks. I suppose it does make sense that the sensor could only cope with a certain range of sensitivity. I was just thinking about it the other day and wondered whether or not the effects of lowering the light with a filter could be easily simulated electronically, and then if that were the case, why would ND filters be needed for those bright/long exposure situations.
Click to expand...

Was just thinking this myself, I believe it's a conspiracy camera companies and filter companies have to keep selling ND filters..haha
 
Wierd, I was thinking something similar the other day!

I'm sure there is a technical reason as to why it cant be done. My thinking wasn't just for the sensor as a whole though. My thought was why can't different parts of the sensor have their 'sensitivity' adjusted - was out on the North Yorkshire Moors Railway the other day - dark steam trains and bright blue skies. When framing a shot with both in you have a choice, meter for the sky and underexposed the train or meter for the train and blow the sky. Now a graduated ND filter would help - but I wondered if the camera can detect and display shadows/highlights why can't the sensor adjust its sensitivity to best exposed different areas of the shot?

Maybe this could be the 'next big thing' in the world of digital photography!!

Neil
 
You must log in or register to reply here.
Back
Top