Pages: [1]   Go Down

Author Topic: Best ISO to use?  (Read 12050 times)

jule

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 738
    • http://www.juliestephenson.net
Best ISO to use?
« on: March 20, 2006, 03:19:07 pm »

Coming from limited ASA options in film now to an extensive range of ISO choices, I am floundering a bit working out how to determine the best ISO to use. I get the general gist of a ball park figure, but how does one determine whether to choose something like 160??? or 640??? I would appreciate some guidance please. ...hmmm, ...and I don't use a light meter (perhaps I should?)
Julie
Logged

DarkPenguin

  • Guest
Best ISO to use?
« Reply #1 on: March 20, 2006, 03:26:26 pm »

I'm not sure I understand the question.

Your shutter speed should determine this.  You want as low as possible to achieve the shutter speed you want.

(I'd love to have the ability to set a min shutter speed and let the camera adjust the ISO so it is always at or above that speed.  Does any DSLR offer such a feature?)
Logged

Andrew Larkin

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 85
Best ISO to use?
« Reply #2 on: March 20, 2006, 04:09:27 pm »

There are now three parameter available to the photographer to control the exposure of a scene:

1. Shutter
2. Aperture
3. ISO

These three values work against each other in combination to create your exposure's "firing solution".  You could imaging these as being the axes of a cube.  The particular combination you use for an image depend on your creative intent.

The shutter axis gives the photographer options between stop motion (high shutter speed) and blurred motion (low shutter speed).

The aperture axis gives the photographer options over depth of field (amongst other things).

The ISO axis gives the photographer a compromise between sensitivity and image noise.

Generally, when I am photographing a scene I will decide what I need by way of aperture first (do I need high or low depth of field?).

Then I will consider what sort of shutter speed I need - slow shutter to blur motion (e.g. water fall) or fast.

If I cannot achieve the kind of combination of shutter and aperture I want, I will then adjust the ISO sensitivity to bring the combinations into line.

For different shooting situations, the sequence may change.

If I feel that I need a low-noise image, I might set the ISO first (200 or lower), then compromise on desired shutter or aperture settings to achieve the exposure.

Perhaps a way of thinking about the problem is like this:

Your camera has "aperture priority" (Av) and "shutter priority" (Tv) settings/programs that allow you to set your preferred axis and have it calculate the other.

If you modify this thinking to include "ISO priority" ahead of both, then (at least to me) the picture starts to make sense.

For the camera to calculate an exposure "firing solution", it must have the freedom to vary by itself at least one parameter axis. In other words, you set one or two parameters, and the camera calculates the remaining.

In the way that the camera software is designed, there is no provision to have the ISO as the "floating" axis that the camera can calculate.  I actually don't have a problem with this situation as this axis has a "good" and a "bad" end unlike the shutter and aperture axes where the limits are simply different - each is valid in its own creative context.

With digital cameras, I am not aware of any valid reason for wanting to use a high ISO for the intention of adding noise to the image.  Note that this is different to film where high grain from fast film was used as a creative option by some photographers.

If you want to have a noisy image, then you will achieve a better result adding noise/artificial grain during digital post processing.

Hope this ramble is of some use...

Andrew
Logged

jule

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 738
    • http://www.juliestephenson.net
Best ISO to use?
« Reply #3 on: March 20, 2006, 05:26:48 pm »

Thank you for your replies.
Dark Penguin, I have a bit of a void in my practical application of my understanding of getting the best parameters for an image. I think I need tutorial "Applying ISO 101",    

I am still a little stuck in the film 100, 200, 400, 800 thing, and need a bit of help to get around that. From what I understand, unless one is photographing under controlled lighting situations, nearly every image would benefit from having its' own custom ISO setting.

I would normally set my ISO to the 'old ASA' amount, and generally then my aperture, and if the Tv calculated and histogram looked ok...that was the image I settled with. If I wanted a specific shutter speed, I would do the same if the Av was acceptable enough. I'm sure I could do better than that, with the ability to customise and control settings more appropriately, in particular the ISO - so that is what I was asking.

Andrew, thank you so much for your detailed reply. It helped me to understand that I am on the right track with my understanding. I probably need to just keep pushing myself to experiment changing the ISO, (because I was not used to changing it very much), so I get a better feel for the effect it has with different combinations of aperture and shutter speeds, and get my mind set out of the way I am accustomed to thinking. I didn't know if there were 'tricks of the trade' for ascertaining and fine tuning the best ISO for an image.  

When I have combinations of Aperture and shutter speed that aren't quite right, I'll just have to be more mindful to dial in changes with the ISO to get the parameters which suit best.

Thank you again.
Julie
« Last Edit: March 20, 2006, 05:28:06 pm by jule »
Logged

BernardLanguillier

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 13983
    • http://www.flickr.com/photos/bernardlanguillier/sets/
Best ISO to use?
« Reply #4 on: March 20, 2006, 10:09:58 pm »

The question makes a lot of sense in many situations:

- is it better to have a faster speed with a higher ISO to reduce hand shake, even if the noise is more visible?
- is it better to keep a low ISO, but then risk motion blur?

There is no universal answer to this, since it depends on:

- the presence or not of moving elements in the scene, and how these should be rendered,
- one's ability to remain steady at low shutter speeds,
- the noise behaviour of the camera used in the conditions where the image is taken (the light level does impact a lot),
- ...

When I want an image to be sharp, I typically prefer to set a slightly higher ISO in order to secure shutter speed.

Regards,
Bernard

Peter Jon White

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 88
Best ISO to use?
« Reply #5 on: March 24, 2006, 08:51:06 pm »

Use the lowest ISO that allows you to use the shutter speed you need to either stop action or show motion, and the aperture you need for the depth of field you want in the image.

The reason to use the lowest possible ISO is that you'll get the highest possible image quality with the lowest ISO, providing it's a real ISO, not some extended setting, like the 50 ISO Canon tacks onto the bottom end of the 5D. Some fool may pipe in with the notion that high ISO lends a certain "character" to an image. Well, that "character" can always be added in later in Photoshop. You want to start with the best possible file you can. If you want to distort it later, all well and good. But by starting with the best possible image, you have more creative options open to you later, in the darkroom, digital or otherwise.
Logged

Jonathan Wienke

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 5829
    • http://visual-vacations.com/
Best ISO to use?
« Reply #6 on: March 25, 2006, 01:11:16 pm »

Peter has nailed it. The best ISO setting to use is the lowest one that still allows acceptably wide DOF (determined by aperture) and acceptably fast shutter speed (to avoid motion blur and camera shake). In many cases it's a tradeoff between motion blur and graininess or a tradeoff between graininess and DOF. For slow-moving or static subjects, a slow shutter speed and low ISO works best, but for rapidly moving subjects, raising the ISO and using a faster shutter speed will deliver better results; the increase in noise/graininess is less undesirable than the motion blur you would get at low ISO. It's all a judgment call and dependent on the shooting conditions and subject matter.
Logged

jule

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 738
    • http://www.juliestephenson.net
Best ISO to use?
« Reply #7 on: March 25, 2006, 05:40:15 pm »

Thanks everyone for your replies and guidance. Practice....experiment.....pratice..practice.
Julie
Logged
Pages: [1]   Go Up