ISO vs. EC

[slejhamer]

Is there any difference (technically, theoretically, aesthetically) between bumping up ISO to gain additional light sensitivity and increasing exposure comp?

Example: Shooting in natural light on a 10D, Av mode with a 100mm lens wide open at ISO 800, the camera gives me a shutter speed of 1/60s.

For one reason or another, I want the shutter speed to be 1/125s. I can increase ISO sensitivity to 1600 to get there, or I can switch to Tv, dial in the shutter speed (the aperture should set to wide open) and set EC to +1.

Will EC result in less noise than ISO 1600? Are there any other differences between the two? Or is ISO 800 with EC at +1 really the same thing as ISO 1600 with EC at zero? I note that the user's guide mentions noise with respect to the higher ISOs but is mute on the subject when discussing EC. One advantage I see is that EC can be adjusted in 1/3 or 1/2 stop increments while ISO can not (as far as I can tell. :) )

Thanks in advance,

[CyberDyneSystems]

YES!

They are totally different and you would use the two methods to acheive completely different results! :)

Icreasing ISO allows the film or CCD/CMOS to be more sensitive to light,. thus allowing it to take the same picture with a shorter(faster) shutter time. (and or smaller aperture)

If you overexpose,. the amount of ight needed increases and thus you need a longer(slower) shutter time. (and or larger aperture)

Increasing ISO with a camera set to to any form of auto exposure will not change the overall exposure as the camera will take the ISO into account in its metering. For manual exposure you will need to do the same.

[daveh]

slejhamer wrote:
Is there any difference (technically, theoretically, aesthetically) between bumping up ISO to gain additional light sensitivity and increasing exposure comp?

I think perhaps you're thinking of film cameras. When you change the film speed on a film camera you're just lying to it which achieves the same effect as EC. Not so on a digital camera. Now when you change the speed it's like loading different film.

[slejhamer]

Thanks for responding CDS and daveh.

Yes, daveh, in a sense I am trying to "lie" to the camera, like rating film differently than its nominal speed, but in this case I'm using EC to do it.

Let me make sure you both understand the context of my question: What I'm really getting at is reciprocity - I want to achieve the same exposure but with different camera settings that better suit my needs.

I am holding aperture (already wide open) and ISO as constants but I set a faster shutter - I know I will underexpose if nothing else is done. However, I can bump up EC to offset this, getting me back to a correct exposure. (Correct me if I'm wrong.)

So I guess my question is: in terms of image quality (noise) and general camera functionality, is changing EC (in the context in which I am using it) better than or worse than or the exact same as simply bumping up ISO to achieve the faster shutter?

[daveh]

If I understand you, playing with the EC will do nothing more than make the meter read "right". You can get the same result in manual by just increasing the shutter speed. (From your description it sounds like you're already in manual where the camera won't let you do EC anyway because it's basically redundant.)

So perhaps your question is whether you can underexpose, and then use post-processing to make it look the same as or better than a shot properly exposed at a higher speed? I think not. Simple (extreme) experiment: shoot something 5 stops under at ISO 100 and see if you can get something that looks like an ISO 3200 shot. (I haven't tried this but there doesn't seem to be enough range captured in 12 bits to pull this off.)

[DNHayashida]

If I am understanding your question right, you are already at full wide open aperture and you want to set a faster shutter speed. Instead of bumping up ISO you set exposure compensation to +1.

The camera cannot do something magical to increase the light getting to the sensor. Setting the exposure comp to +1 most probably will take the picture at your settings without doing anything at all and be under exposed. The only things the camera can do to achieve a +1 exposure is either slow down the shutter speed or bump up the ISO (already at full open aperture). As far as I can tell EC manipulates the aperture only, but here is a way you can tell. Take your picture then take the same picture with EC at +1. Look at the EXIF data and see what changed.
Darryl Hayashida

[slejhamer]

Argh! Thanks Darryl. I've realized my mistake. What I didn't understand is that EC actually overrides the camera's exposure settings. So, if my lens is already wide open and I have a fixed shutter and ISO, then EC has nowhere to go to increase exposure. I had mistakenly thought that EC had an effect on sensor sensitivity in the same way that ISO does ...

So it seems that in my situtation where I want a certain shutter speed and already have my lens open to the max, then I have no choice but to boost ISO.

Helps to read the manual, and reread it, and reread it again... :O

Thanks for the helpful responses.

[CyberDyneSystems]

Slejhammer,.. Yes,. exposure is an equation involving shutter speed, aperute and ISO (or film speed) You can't change it without adjusting one of them :)


FYI,. do not be afraid to boost the ISO on the 10D up as far as 800!

The noise reduction in the 10D is fantastic.

1600 is very usable,. 400 to me looks just as good as 100! and 800 is a godsend with very clear detailed images and if the grain is at all noticeble,. it is in fact a quite pleasing look.

Use 1600 and 3200 sparingly :)

I was going to load in an image example of 800 ISO,. but fotopic is down. Check my galleries under the fashion show. All of those images were shot at ISOs 800 and 1600.

[dleewo]

What would be interesting is say, shooting at ISO 200, but UNDEREXPOSE by 2 stops and then adjust the RAW file.

Would that produce less or more noise than shooting the correct exposure at ISO 800?

I will try some tests this weekend, but maybe someone here has already tried that and can share their experience.

[daveh]

dleewo wrote:
I will try some tests this weekend, but maybe someone here has already tried that and can share their experience.

I haven't tied but given that there's a finite (and not very large) dynamic range, you're going to throw away a lot of it in order to do this. Your first stab might be to just punch up the brightness. That is likely to give you an image that doesn't look particularly noisy but does look extremely flat and washed out. If you try to fix that problem (curves, contrast etc.), that's when I would expect you to start seeing a lot of noise. Bottom line, is that I don't see how fewer bits can be better.

As I suggested above though, try a 5 f-stop correction to make it really dramatic. I think an exaggerated test like that will show you what's going on quicker and you won't waste as much time thinking you can tweak it back to something useful if you can just find the right Photoshop filter.

[BrettD]

dleewo wrote:
What would be interesting is say, shooting at ISO 200, but UNDEREXPOSE by 2 stops and then adjust the RAW file.

Would that produce less or more noise than shooting the correct exposure at ISO 800?


From my experience results are similar, but slightly better using the camera at the higher ISO. This makes sense as it allows the noise reduction methods on the sensor to functin as designed by the Canon engineers.

To explain why high ISO images get noisy think of it this way: The CMOS/CCD sensor has a more or less fixed amount of noise, but when more signal (light) is allowed throught the lense, the signal to noise ratio increases.
Higher ISO's are literally trying to make the same exposure with less signal, so lower signal to noise ratio = more visable noise.

Brett D

[slejhamer]

CyberDyneSystems wrote:
FYI,. do not be afraid to boost the ISO on the 10D up as far as 800!


Thanks CDS - I'm okay with ISO 800, as it's better than my G1 at ISO 100! But 1600 is pushing it, and that's where I needed to go...