Pushing past the rated ISO

Elliot · March 5, 2009

I've been trying out low-light photography using the old fasioned push-pull technique. My Canon 450D (Rebel XSi) has a maximum ISO of 1600, however you can push this to 3200 and 6400 by using exposure compensation to under expose by 1 stop and 2 stops respectively, then bringing up the exposure by the same amount in Photoshop. It seems the 450D can cope at high ISO without too much noise. This one of my daughter going to bed is mostly red anyway so noise is even less of an issue. An acceptable bed-time shot though I think considering I'm using the kit lens at F3.5.

jeffreaux2 · March 5, 2009

If you dial back exposure compensation....and under expose by two stops.....then push it two stops in post......aren't you back to where you would have been with zero...exposure compensation?

Elliot · March 5, 2009

That's exactly it! The camera only goes to 1600, but I want 3200 or 6400 so that I can use the metering to get a usable shutter speed with the 3.5 kit lens. Pulling back by -2 gave me a usable shutter speed, 1/25 @ f3.5. Of course I could have gone manual at 1600 but I don't trust my eye as a light meter.

jeffreaux2 wrote:

If you dial back exposure compensation....and under expose by two stops.....then push it two stops in post......aren't you back to where you would have been with zero...exposure compensation?

Richard · March 5, 2009

jeffreaux2 wrote:

If you dial back exposure compensation....and under expose by two stops.....then push it two stops in post......aren't you back to where you would have been with zero...exposure compensation?

Not exactly, because you get a fourth the shutter speed. Of course, the bill comes due when you see the noise that is introduced by upping the exposure in post. Still, it's a good trick to have in your bag at times.

jeffreaux2 · March 5, 2009

Richard wrote:

Not exactly, because you get a fourth the shutter speed. Of course, the bill comes due when you see the noise that is introduced by upping the exposure in post. Still, it's a good trick to have in your bag at times.

Ill bet the noise is something else for sure.

ISO 12800 is in my current bag of tricks...though I can't imagine ever having to use it.

By chance, I did experiment with ISO3200...on the 50D

Elliot · March 5, 2009

It would be a bit extreme up there wouldn't it. Still, the 3200 you've done there looks great. I guess the high ISO use is ok on small prints.

jeffreaux2 · March 5, 2009

Elliot wrote:

It would be a bit extreme up there wouldn't it. Still, the 3200 you've done there looks great. I guess the high ISO use is ok on small prints.

Better than small really.... I currently use that same photo for the background on my desktop at work. Its a 20inch monitor. I suspect it would be fine even up to 8x10...and maybe a tad larger.

divamum · March 5, 2009

This is quite brilliant.

With the shoot I have coming up, gaining ANY extra shutter speed is going to be important, and it never occurred to me to do this. Yeah, the noise will be icky, but it would be icky anyway and I'd rather have a theoretically in focus and non-shaking shot with noise than one that's noisy AND has motion blur!!

Richard · March 6, 2009

divamum wrote:

This is quite brilliant.

With the shoot I have coming up, gaining ANY extra shutter speed is going to be important, and it never occurred to me to do this. Yeah, the noise will be icky, but it would be icky anyway and I'd rather have a theoretically in focus and non-shaking shot with noise than one that's noisy AND has motion blur!!

Yep--that's the idea.

anvilimage · March 6, 2009

So, I'm shooting with a 40D and it goes up to 1600 as well. But, I've seen mention of setting it to go to 3200 with an option in the camera. Is this only a pseudo-3200 ISO or is it a real ISO?

-joe

Elliot · March 6, 2009

(Richard, you might be better set to answer this). As far as I know there's not really such as thing as real ISO in a digital camera. It more about gain I think. So the pull-push technique is as real ISO as you're going to get in digi. Would love to hear a proper techinical answer from someone if they have it.

...and what I forgot to mention is that I was doing this in RAW. This makes a difference apparently...

http://en.wikipedia.org/wiki/Film_speed#Digital_camera_ISO_speed_and_exposure_index

Richard · March 6, 2009

Elliot wrote:

(Richard, you might be better set to answer this). As far as I know there's not really such as thing as real ISO in a digital camera. It more about gain I think. So the pull-push technique is as real ISO as you're going to get in digi. Would love to hear a proper techinical answer from someone if they have it.

...and what I forgot to mention is that I was doing this in RAW. This makes a difference apparently...

http://en.wikipedia.org/wiki/Film_speed#Digital_camera_ISO_speed_and_exposure_index

Elliot,

If you understand everything in that Wiki article, you know a lot more than I do about this stuff.

I have seen claims that 3200 ISO in Canon 20-40D cameras is not "real" (it shows as H for high when you select it in my 20D) but I suspect this is a distinction without a difference. What I see when shooting at 3200 is greater sensitivity to light and more noise than at 1600. The same is true when comparing 400 to 200.

ziggy53 · March 6, 2009

In Canon cameras the company will offer "calibrated" ISOs, and then sometimes additional "uncalibrated" ISOs.

The calibrated ISOs are a combination of signal levels, gain levels and additional processing of the image. The important thing to know about the calibrated ISOs is not that they are accurate relating to standard ISO rating, often they are off by a considerable margin. The calibrated ISOs are simply the best settings to use if you want the best combination of sensitivity, dynamic range, random noise levels and color accuracy.

The unclaibrated ISOs will have some additional compromise which is usually undefined.

As far as I know there is no industry standard for what a "calibrated ISO" means so suffice it to say that one manufacturer's method and definition is not transferable to another manufacturer.

To say that all ISO ratings are "real" or not is a bit of a misconception in that ISO is really a conditional sensitivity. It might be clearer to call them "equivalences" and then describe the conditions under which they are eqivalent.

One limiting factor is that in order to maintain color accuracy as the multiple gains are adjusted, it is necessary to adjust some color channels' gain more than others.

I must say that I am fascinated that Canon has chosen to make the ISO 6400 on the Canon 5D MKII a "calibarted" ISO.

Elliot · March 6, 2009

Thanks for that. It's an interesting subject, if not a bit heavyweight the more you get in to it. When I was a kid, ISO200 had a picture of a cloud next to it, and ISO100 had a picture of a sun next to it. Simpler times, with rubbish photos (from me anyway).

Toshido · March 6, 2009

So essentially this is just an underexposed photo. You are obviously not in Manual mode so I guess you are using Aperture priority mode for this.

I assume if you were in Manual mode you could simply use the light meter in the viewfinder to achieve the same results.

Either way neat trick to get increase shutter speeds if you are not ready to make the jump to full manual mode yet.

BTW, manual mode is not really that scary, I am trying to use it for most of my shots recently and have only really missed a few when I did not double check the odd setting, like ISO speeds.

Pushing past the rated ISO

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