I have a CMOS camera in which I can change the digital gain (ISO) and the exposure time. I should use a fixed frame rate (at least 100 fps) and a fixed f-number (i.e. I can't change the aperture size), so I have limitation on the upper limits of the exposure time. The only parameters I can change are exposure time and ISO.
I was wondering if anyone knows which strategy is better to have less noisy images: reducing the gain (ISO) and increasing the exposure time or the other way around?
Thank you.
Hey Amir,
as you talk about fps, are we talking about an image or a video? When taking Images, there should be nothing about fps, right? May be I just did not really understand your questions !?
Another issue is the fact around your aperture (f-number). You say you can't change the shutter speed but on the other side ask whether you might increase the exposure time. I am not sure whether you are mixing things up a little. The exposure time is like the same as the shutter speed, just differently expressed.
So once talking about images, you only have three variables:
* Shutter speed (exposure time)
* Aperture (f-number)
* ISO
What are your constraints here?
* Do the objects in the image move? (related to exposure time)
* What is the greatest aperture your lens supports (smallest f-number)?
* What do you consider the highest acceptable noise (at which ISO)?
Hi Mathias,
Thanks for your reply. In my setup, I want to analyse images in the video. So they are not single shots, they are frames of a video. That's why I discussed about frame rate.
Thank you for mentioning this. You are right about the relation of the shutter speed and exposure time (I fixed that on my question). However, because of the depth of field I need for my images I can't use wide aperture (i.e. small f-number). The smallest f-number that I can use is 12.
So, I have control over these two parameters:
1. Exposure time (shutter speed)
2. ISO
My constraints are:
1. Yes, they move. Minimum frame rate to capture the movements is 100 fps.
2. The smallest I can use is 12. However, the smallest that camera supports is 2.5.
3. In any case the noise is in an acceptable range for me. I am looking for a way to minimise that.
Thank you very much.
The only thing I can tell you so far is, that most probably you will have some opportunity to decrease the noise by lowering the ISO. In order to do so, I propose to lower the aperture, even knowing you need the depth of field. From my experiences as a photographer I would propose to go down to lets say f/8. However, this depends on two things:
Are there any objects close to the camera (changing the depth of field issue)?
What are the light conditions? Do you have direct sun light (danger of overcast) or do you film during dusk / artificial light? The less direct light you have (not talking about too few) the lower you can go with the f-number while still keeping the depth of field.
Hello,
the best steps would be first to use manual settings, lower the ISO e.g. to minimum, then adapt the light conditions (e.g. ring light, soft box) to reach the desired exposure time (< 1/100) by considering the desired depth of field. Aperture was fixed as you said.
You already have a set of parameters to tune to reduce noise as suggested. One other possibility (not sure if your setup is allowing you to do it though) to cool your camera using fan, peletier system or any similar trick so you can reduce thermal noise. That is how astrophotographers usually do.