What is ISO?

[Poseidon]

Okay here is one that can be VERY HELPFUL, or RUIN your day... ISO is simply the "sensitivity to light" of the sensor. Meaning at lower ISO your sensor must be exposed to light LONGER because it is less sensitive. At HIGH ISO it is VERY sensitive to light, meaning you will get a picture in LESS time! This is great for fast action, or when taking pictures under low lighting conditions. All the photos that follow were taken with my 20D, and a 24-70 f2.8L lens, on a TRIPOD! The tank lights were OFF, with the only light coming from a widow to the right of the tank, and some light from my ceiling fan... NOt much light at all!

Also all of these pictures are 100% Crops, meaning what you are seeing is not ENLARGED or REDUCED, it is "full size" hence it is a very small part of the picture. I have also posted a full image so that you can see just how much of the actual picture you are actually looking at.

Each image is also Identified in the corner as to what ISO it was shot at. Making it easy to identify the differences. As the ISO gets higher, look especiually at the shadow areas for the increase in noise. You will notice a couple of the pictures have a "model" my hawkfish. He kept jumping in the picture... I think he wants to be famous!

Anyway, here are the pictures:















By the way, I mentioned earlier that ISO could RUIN your day... Imagine taking a full days pictures OUTSIDE under bright sun, and shooting at 3200 for NO REASON! Yup I DID that! I had lots of editing to do!

[zhenya]

Good read,Mike.
I think you forgot to add that some cameras handle noise at higher ISO numbers better than others, like for example your Canon 20D. I think it's rated like number one from all of the other comparable cameras. also know that my Rebel is not even close and I prefer not to go over 200 unless I shoot almost in the dark.
Any other suggestions you have as to how properly adjust for the loss of light.I mean I understand about the use of flash but is there other options? Like larger apertures, different shutter speeds? The reason I ask is that I'm trying to switch to exclusevely manual mode and it;s quite a bit of learning curve for me at the moment. For example, I've read once that optically speaking all lenses do their best at f8, so I try to stick to that quide number but I need to adjust for light differences sometimes and I'm having hard time setting correct shutter speeds.

[Poseidon]

That is a good point Gene, I know my camera does do well at higher ISO's. However I think the poitn is the same, as anyone increases ISO color noise will be more evident.

Your Rebel should actually do very well up to 400 or more. I shot several games at 1600 with my 10D, that has the same properties as your Rebel.

As far as the f8 rule, it is not so much a rule as a guide for balancing depth of field with sharpness. As you increase the f stop you increase the DoF, as you already know. Some photographers view this increase in DoF as an increase in overall quality as well. Canon and Leica both claim optimal sharpness from their lenses @ 1 stop slower then maximum apeture, and 1 stop faster then minimum apeture. So using my lens as an example, f4 and f16 are the "sharpest". Does that mean that other apetures are not "sharp" Not a chance. It just means Canon's measurements with highly sophisticated equipment that has much more accuity then our eyes will ever has, says that those are the "best".

Especially @ web resolution, we are not even close to the resolution limits of these lenses. In print, with your 28-135, I doubt you could tell a difference from f8 f11 or f16 while @ the tele end. f4 f5.6 or f8 should all appear equally sharp at the wide end.

As far as other ways to get around low light, shoot RAW, and under expose by 1 to 1.5 stops, then use Post to bring them up. That is a PITA though, and can introduce more noise, especially in the shadow areas. Other then that, you have the same options of shutter speed and apeture to get a proper exposure. I use ISO more and more lately, it is like having a 3rd option where there NEVER was in film days. Heck we even shot a good part of a wedding @ 1600 not long ago. The prints look GORGEOUS!

Just keep shooting, and don't get caught up in "pixel peeping"!