Kippy's posts with tag: aperture
 | ISO | Apr 17, '08 1:11 PM
for everyone |
 No, no, no. Not the Infrared Space Observatory (photo courtesy of NASA, via Wikipedia)! The International Organization for Standardization. Um, yes. An organization which "standardizes" everything .. except acronyms, obviously. Otherwise shouldn't it be IOS? Ok, well, whatever. I digress.
ISO is basically "film speed." Also sometimes referred to (in them thar olden days) as ASA.
Yesterday a friend of mine sent me an email asking my thoughts on defining how ISO applies to digital photography. So last night Jeremy and I were discussing it while driving up to Reno. Well, that and the physiological basis of fear response in human beings as it applies to post-traumatic-stress disorder and evolution of society as we know it. *whew* Yep, it was one of those nights. :)
So I decided to blog about "What ISO means to me."
I'll warn you right now this is not going to be technical. I'm not going to use the word "halide" in my entire post (well, except for just now). For those of you who are just joooonesing for something technical, here, again from Wiki:
 Film speed is found from a plot of optical density vs. log of exposure for the film, known as the D–log H curve or Hurter–Driffield curve. There typically are five regions in the curve: the base + fog, the toe, the linear region, the shoulder, and the overexposed region. For black and white negative film, the “speed point” m is the point on the curve where density exceeds the base + fog density by 0.1 when the negative is developed so that a point n where the log of exposure is 1.3 units greater than the exposure at point m has a density 0.8 greater than the density at point m. The exposure Hm, in lux-s, is that for point m when the specified contrast condition is satisfied. The ISO arithmetic speed then is
S = \frac {0.8} {H_\mathrm{m}} .
Determining speed for color negative film is similar in concept but more complex because it involves separate curves for blue, green, and red. The film is processed according to the film manufacturer’s recommendations rather than to a specified contrast. ISO speed for color reversal film is determined from the middle rather than the threshold of the curve; it again involves separate curves for blue, green, and red, and the film is processed according to the film manufacturer’s recommendations.
Riiiight. Jeremy started launching into a definition of film grain referring to silver um, "that word I'm not going to use" and emulsion and I flashed over to something I read from Susan and my brain checked out. As I told Jeremy, he and Susan are gonna have a blast talking about all that technical crap. But it's soooo not me.
I can tell you how I shoot. I can show you, it is something that has just become "intuitive" to me. But I can't tell you all the technical workings behind it. Jeremy could probably explain every little function within the camera, what it does, why it works the way it works. I ADMIRE it, I just can't force my brain to work that way. So Susan, when you come up, you and Jeremy can talk all the technical jargon to your little hearts' content. He'll probably love it, because I don't really have those conversations with him.
Anyway .. ISO. I think that the general opinion is:
Low ISO = clear, sharp images
High ISO = noisy, not-so-sharp images
We-ellll .. maybe. What I have found is that moreso than ISO (and this is all as regards shooting digitally .. I can't speak to film usage), amount of light greatly affects noise in images. As I understand it, images that are clipped (underexposed) actually contain LESS color data than images that are blown (overexposed), and the dark areas hold more of the noise than the light. So, when you open an underexposed image in Photoshop and try to lighten it, you're also lightening the noisiness and therefore making it more obvious.
My mentor (who has shot for Audi and Ferrarri and some diamond manufacturers, etc.) told me that he shoots EVERYTHING at ISO 200. Why? Because he claimed that it gives better tonal range. But doesn't it add more noise, I asked? Nope, not if you allow enough light in. Hm. Interesting. So I tried it. You know what? I think there "might" be a little more noise at 200, but the images are soooo much nicer! They just "feel" better. The contrasts are not so harsh, and the images are warm. And he was right! When you allow enough light, images at ISO 200 just feel better.
Do I still shoot at ISO 100? Yep, sadly I do. Why? Because most stock sites are noise nazis. They are not interested in the "feel" of an image and rarely take into account how it will print. They just want no noise, period. When I'm shooting artsy stuff for myself, I often bump up to 200. When I'm shooting for stock I try very hard to keep at 100.
So what's all this about light, then? You're supposed to what, carry a big flashlight?
Nope, there are a few different ways to get "more light" into the picture (so to speak). You can increase your shutter speed (shutter open for a longer time, allows more light in) or widen your aperture (set a lower f-stop number). Widening your aperture basically means when the camera opens up to take the picture, it's going to increase the opening through which light is admitted a little more. The reason a smaller number means a larger aperture is because you're dealing with a ratio. For instance, when you see something like:
f/8. What that means is f (focal length) divided by 8. If you set your camera to f/2.8 then you get focal length divided by 2.8. Now, it's not that you need to know all the math behind it, I'm just explaining why a lower number means more light, because this was something that confused me to the point of near tears when I was trying to wrap my mind around it. Say your focal length is 10 meters (in reality it doesn't matter WHAT it is .. I'm just illustrating there).
f/8 = 10 divided by 8 = 1.25 (for argument let's just call this "units of light" being allowed into the camera)
f/2.8 = 10 divided by 2.8 = 3.57 "units of light"
So, see why a lower number = more light being let into the camera? It's a ratio, so you get more units of light the lower you set your f-stop. This also affects your depth of field, but I won't go into it now except to say that a lower (wider) f-stop will give you a more shallow depth-of-field. That means less of the image will be in focus. So if you're trying to shoot an object and make it stand out from the background using focal plane (ie - you want the background really fuzzy), use a lower (wider) f-stop. If you want a lot of the image, front to back, in focus, you need to increase (tighten) your aperture, so use a higher (smaller) f-stop.
*sigh* once more this is getting away from me and ending up longer than I wanted it to be. Not too much longer now, I promise! I'll curtail my tangents.
So, the thing is .. when people bump up their ISO (to like 800, 1600, even 3200 on some digital cameras) they are usually doing so because they are shooting in low-light situations. A restaurant, a concert, whatever. They increase the ISO (effective "light sensitivity") because they want to be able to shoot a faster shutter speed. Say you're trying to get pics of your kid's birthday party and the restaurant is kind of dark, if you have a low ISO and your camera operates on "auto", it may well be taking images at something like 1/10 of a second .. or even longer! Maybe a full second - it's trying to get enough light let into the shot to make it something recognizable. Think how quickly your kids move! You need a way faster shutter speed to catch something that is not a complete blur. So you bump up the ISO, to make the camera "more sensitive" to the light that is available. Awesome .. so now the camera is shooting a faster shutter speed, but the pictures are going to be noisy. The reasons for that are, when the camera is "more sensitive" to light, it is more sensitive to ALL ambient factors, including dust in the air, fluctuating light patterns based on reflections and refractions, etc. It is picking up all kinds of particles that our naked eye isn't even seeing. Think a camera can't pick up on something invisible to the naked eye? Ask Andi about shooting with my macro lens last week. :)
So, you're shooting in a dark restaurant, with a camera set for "increased sensitivity" and you are likely still shooting underexposed images, to boot, because the shadows are darker and more vast than the camera will likely be able to judge. That's a triple threat. If you were shooting ISO 1600 in a studio, with plenty of lighting and ample shutter speed, you'd still have noise, but it wouldn't be NEAR as much.
I guess what I'm trying to get at here is, yes. A good "general rule of thumb" is that lower ISO will give you less noise and clearer images and higher ISO will give you more noise in your images. But there are a LOT of exceptions to that generality, and the more you learn about the relationship between ISO, shutter speed, aperture and available light, the better off you'll be.
If you actually made it through this entire post, you deserve a cookie. Go treat yourself! :)
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