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DPChallenge Forums >> Tips, Tricks, and Q&A >> Sigma compatibility with Canon 20D & ISO...
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 06/16/2005 12:26:46 PM · #1
(This posting has two separate questions on two different issues)

First question: Does anyone else have problems with Sigma lenses and Canon 20D bodies (or any Canon bodies for that matter). My camera will not autofocus at night. Any suggestions?

Second question: I'm relatively new to photography. I've been reading a lot of material about aperature sizes (f2.8, f11, etc.) and shutter speeds...I'm starting to understand how that works. But all of the online tutorials typically just talk about those two things. They rarely mention anything about how ISO fits into the equation. What role DOES the ISO setting play? What exactly does a lower or higher ISO do? It's not talked about that often. I'm confused. Thanks a lot.
 06/16/2005 12:33:58 PM · #2
ISO refers to the speed of the film...

ie: 100 ISO is equivalent to Kodak 100, 400 ISO to 400 speed film. And so on...

ISO is sensitivity. How sensitive to light the film/sensor is. A 3200 ISO film or sensor is extremely sensitive to light. Even a brief moment of light will alter it's state and thus produce an image. Hope that helps!
06/16/2005 01:12:54 PM · #3
Russel, I have a Sigma 105mm macro lens that I love and adore. Sometimes the AF hunts for a while, but the lens is so nice and sharp that I'm willing to just put up with the hunting.

re ISO - what theSaj said, plus add this:
slow ISO (e.g. 50 or 100) gives the better quality, but needs more light. Sometimes that is just not suitable, e.g. indoor photography, or when you really need short shutter speeds.
Higher ISO gives you more speed, needs less light, but becomes grainy or noisy, so you always have that trade off between the two.

Usually you want to use as low an ISO as you can get away with, however, the 20D is rather impressive in that regard- some ISO 1600 pics are still quite usable - my last P&S camera started getting really ugly at only 200!!!!
 06/16/2005 02:09:28 PM · #4
The make of the lens has nothing to do with your autofocus, it is the camera that autofocuses. Of course how fast and precise the motor in the lens is makes a difference, but even the crummiest lens will eventually focus. If it's failing to lock focus, there's probably nothing to focus on - as you say, you have difficulty autofocusing at night. Of course it'll have more difficulty focusing on things it can't see!

As for iso nobody seems to have answered your question so i'll have a go:
iso is not the same sort of thing as shutter speed and aperture. Those two modify how much light reaches the film or sensor. ISO on the other hand is a measure that determines a sensor's sensitivity to the light that hits it. ISO 200 is twice as sensitive as 100 etc, and so takes half as much light to produce the same exposure - one f-stop lower, so twice the shutter speed or one stop narrower aperture.

In an ideal world, ISO would have nothing to do with image quality, but higher iso's generally produce more grain in both film and digital, for different reasons. On film coarser grain is required so what light hits the film darkens a larger area of the surface, and on digital more electronic noise is picked up by the more sensitive sensor.
 06/16/2005 02:26:22 PM · #5
riot,

thanks for the explanation...that helped a lot. i'll try to take night shots that have some sort of light to focus on!

russell
 06/16/2005 02:45:48 PM · #6
Originally posted by chafer:
riot,
thanks for the explanation...that helped a lot. russell


Russell, I apologize for wasting your time with a post that was obviously totally useless to you.
Or did you perhaps save time by not even reading it (as well as theSaj's) - that would make me feel much better.
 06/16/2005 02:49:05 PM · #7
To focus at night - sometimes a camera has an focus assit light as part of the flash or an IR one. You can use a pen light flashlight, get focus and shut it off.

The camera needs a given amount of light to focus, and needs to see some contrast in the subject. Point it at a blank wall and it will hunt and seek like all heck - nothing with contrast to focus on. Also, Canon's camera will not AF well if it all if the widest aperature is less than 5.6 (for exapmple, you use a 70-300mm 4-5.6 lens, at 300mm the max ap is 5.6. Fine. Addin a 1.4 or 2.0x teleconverter and you now do not have enough light to AF.

ISO - International Standards Organisation is what is stands for. Before ISO, we in america use ASA (american standards assoc.) - same numbers. smae theory.

As you double the ISO (100 to 200, 200 to 400, etc) you double the amount of light - so that 1/100 at f2.8 at ISO 200 is the same as 1/200 at f2.8 at ISO 400 or 1/50 at f2.8 at iso 100.

IN film as you up the ISO you get more grain in the picture, digitally you get more noise. Like turning up a stereo you get more distortion in the sound, same thing.


06/16/2005 02:50:50 PM · #8
Originally posted by chafer:
riot,

thanks for the explanation...that helped a lot. i'll try to take night shots that have some sort of light to focus on!

russell


Try taking a spot light or laser pointer with you at night to shoot. You light up the subject, get an autofocus then switch the lens to manual focus. Then meter on the subject and shoot away!
 06/16/2005 03:03:03 PM · #9
Originally posted by Beetle:
Originally posted by chafer:
riot,
thanks for the explanation...that helped a lot. russell


Russell, I apologize for wasting your time with a post that was obviously totally useless to you.
Or did you perhaps save time by not even reading it (as well as theSaj's) - that would make me feel much better.


How stuck up are you! Neither your nor thesaj actually answered his original question, to be fair. Aren't you the one who was once picking on someone because of their average vote cast? Now you're attacking someone for not writing a special letter of thanks for your brief and vague comment?
 06/16/2005 03:27:48 PM · #10
Originally posted by riot:

How stuck up are you! Neither your nor thesaj actually answered his original question, to be fair. Aren't you the one who was once picking on someone because of their average vote cast? Now you're attacking someone for not writing a special letter of thanks for your brief and vague comment?


Are you always this vicious, especially when it isn't even YOUR business? There was NO need for you to get this rude about it.

Now let me explain this to you:

There have been times where I have asked a question and a number of people have answered me. Not every answer was equally useful to me at the time for whatever reason.
However, I think it is fantastic that people will take the time to help others out.

Let's say I had response A,B,C,D,and E. C was the one that answered it most clearly. The way I would deal with this (and other people do it like this too)is to say:

Thanks everyone for your help. Thank you, "C", that did the trick.

Do you get the difference? I still showed my appreciation for all the lovely people that tried, but managed to give extra credit where it was due. I would not want to totally ignore the other answers, after all, they DID take the time to try and help, and that one little thank you to them didn't even cost me a dime.

And all that without any name calling :-)
 06/16/2005 03:38:36 PM · #11
Originally posted by Beetle:
Originally posted by riot:

How stuck up are you! Neither your nor thesaj actually answered his original question, to be fair. Aren't you the one who was once picking on someone because of their average vote cast? Now you're attacking someone for not writing a special letter of thanks for your brief and vague comment?


Are you always this vicious, especially when it isn't even YOUR business? There was NO need for you to get this rude about it.

Now let me explain this to you:

There have been times where I have asked a question and a number of people have answered me. Not every answer was equally useful to me at the time for whatever reason.
However, I think it is fantastic that people will take the time to help others out.

Let's say I had response A,B,C,D,and E. C was the one that answered it most clearly. The way I would deal with this (and other people do it like this too)is to say:

Thanks everyone for your help. Thank you, "C", that did the trick.

Do you get the difference? I still showed my appreciation for all the lovely people that tried, but managed to give extra credit where it was due. I would not want to totally ignore the other answers, after all, they DID take the time to try and help, and that one little thank you to them didn't even cost me a dime.

And all that without any name calling :-)


Actually, it is my business since i found your comment directly offensive, and since it was me he was thanking i felt your attack on him was in some way my fault, so i had to speak out and defend this innocent person you are having a fit at because he didn't thank you in the way you thought proper. If the only reason you comment on this site is to collect accolade for your wonderful help, then good luck with that. I personally would never be so presumptious as to expect thanks, even if i took the time to research my comment.

I admit i am very opinionated at times and can often be very aggressive, but this time it was warranted - it is nothing short of extremely rude of you to attack someone in such a way because of whom he thanked. But i especially wonder if you'd have said anything at all if chafer hadn't thanked anyone.

When you comment here, you're trying to help someone. That's an act of voluntary kindness. But to think you have a right to demand something back, to the degree of being offensive when you don't get it, shows that you have other motives for commenting in the first place.
 06/16/2005 03:45:27 PM · #12
No riot, don't hold back, it's not good for you..... let it out!

I was born roman catholic. I am caucasian. I don't like jazz. I have killed cockroaches. I love oldies (the music). I hate it when people are late. I eat meat.

There you go, got enough fuel to go hammer me some more? I'm sure you can get a few choice attacks and names out of that lot, whether or not they are relevant or any of your business doesn't seem to matter to you anyway. So go ahead, give me your best shot.

No thanks expected from a sweetie like you. Glad to be of service :-)
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