Apologies for such a basic question but I am still struggling to get my head completely around the interaction between these 3 elements.

I primarily shoot in low light settings so, from what I gleaned to date, I have the aperture as wide open as possible, boost the light available by pushing up the iso (as small an amount as possible to avoid grain) to view the subject properly and keep the shutter speed as a multiple of the frame rate (i.e. when I shoot at 25p I have a shutter speed of 1/50). Is this right?

When shooting in daylight how would this change? ISO should normally be left as there will be sufficient light but how should the aperture/shutter speed interact? I'm guessing it depends on what you are shooting and the effect desired but other than a larger aperture creating a shallower the depth of field, what else should I be consdiering (assuming I'm not in bright sun requiring any ND filters)

Looks like you got it down pretty good. Low ISO in daylight (that's really your only variable if you want shallow depth of field). From there, you will be adding ND filters.

Aperture changes if you want more/less dof or more/less light before you add ISO (noise).

You can change shutter speed to be faster than double frame rate (25p over 1/50th) if you want less motion blur. Note that the faster the shutter, the less light coming in also.

ISO - The lower the ISO, the less noise. In general, for sharper images, use the lowest ISO you can for situation.

Aperture - The larger the aperture, the more light goes through the lens. Hence, you can reduce ISO and/or increase shutter speeds.

If you want lots of depth in the shot, you'll need a smaller aperture, ie. f/16. To make up for that smaller aperture, you may need to decrease the shutter speed and/or increase the ISO. Larger apertures have less depth of field and allow for lower ISO and/or faster shutter speeds. So, basically each setting has a give and take with the other two settings. Basically, it's about finding the best combination for your situation.

As a note, lenses have sweet spots, meaning each lens has an aperture where it's sharpest at. For most lenses, this is in the f/8 range, although it can vary from f/8-f/11.

To answer your question, daylight simply allows for smaller apertures and lower ISOs, if desired. The more light, the more options you have. As a rule, I generally start with aperture to get the desired depth of field. However, there may be shots you want a faster or slower shutter speed and depth of field isn't an issue, so start with which setting suits your situation.

Thanks guys. So, for the sharpest pictures, I want the highest shutter speed, the lowest ISO and a high(est) aperture that I can get away for the available light? I had always assumed that a higher aperture would help with the picture as more light is coming in?

To get the most light in youcan set shutter speed at 1/25th second. The effect of shutter speeds on moving images is that the slower you set it the more blur it has, the faster it is the more "strobing" it can cause, making the picture too flickery. There is a generally accepted happy medium which is double the frame rate (so 1/50th sec at 25P) but it's not a solid rule, though you want to be caustious about straying too far from it (ie don't assume that 1/2000th sec will be OK).
For sharpest pictures it's a balancing act with trade-offs between apertures and ISO. The lower the ISO the less noise although there tends to be a certain point with all cameras where it makes little difference, so on the Nikon D3 for instance you gain virtually nothing worth worrying about going below 800 ISO in terms of noise etc. But if light's low then you'll either need to open the aperture OR increase the ISO OR both. Sometimes doing a combination of the two is wise as most lenses don't perform that well wide open. So for instance you might get beter results at ISO1600 and the lens at f2.8 than you would at ISO800 and the lens at f2. You can test your lenses or read reviews/test charts to see where you lens performs best (it's generally about 2 stops down and tails off again at f11 - so a 100mm f2 lens will probably be so so at f2, better at f2.8 and at its best from f4-f8).
Steve

Tim, The smallest aperture (f22) does not give the sharpest images, most lenses have a "sweet spot" of maximum sharpness. Instead of searching for your lens performance chart on the web to see where that particular lens sweet spot is, you can use a good rule of thumb of using between f5.6 to f8.

Also wide open apertures can give a softer image because shallow depth of field can make whats in focus very slim. For instance if I photograph someone at f1.8 and focus on their eyes, potentially their nose is slightly out of focus.

Very interesting. Are those charts on the web somewhere?

Just search for your lens as a review and most in depth reviews will show a mtf chart. Here is one as part of a review of the Canon 50mm f1.4 : Canon EF 50mm f/1.4 USM - Review / Test Report - Analysis (http://www.photozone.de/canon-eos/159-canon-ef-50mm-f14-usm-test-report--review?start=1)

For ISO I'd suggest that you take a look at this video...

Canon EOS 7D ISO Noise test on Vimeo

multiples of ISO 160 seems to be the sweet spot for the 7D... with ISO 160 being the best of any of the selections ... and the 640 setting looks workable, compares to ISO 100 even...

You should watch the video in a dark room with a good monitor... " FULL SCREEN "

to be on the safe side you could create your own tests of ISO noise and see exactly where your setup
will be acceptable to your project.

That is why some people were unhappy with the new canon T2i/550 because it didn't get the 160 multiples.

Some are speculating the noise on the Canon T2i/550 is because it does not have dual processors ???

Thanks guys!

As a newbie to the world of still photography (at least with a camera of the quality of the 7d) this brief thread has given me a crash course and allowed me to understand on a practical basis the relationship of iso setting, aperture, and shutter speed. Keep up the comments - have found them very helpful.

Norman- thanks for the link to the review on the 50mm f1.4 - I bought that as a second lens for low light situations.

[QUOTE=Norman Pogson;1517472]nstead of searching for your lens performance chart on the web to see where that particular lens sweet spot is, you can use a good rule of thumb of using between f5.6 to f8.
QUOTE]

The reason I suggested looking on the web is that different lenses definitely do bring up different results, and while your rule of thum is a good one, it's not foolproof. No harm in looking at mtfs and reviews as well as rules of thumb and doing your own testing.
Steve

I totally agree Steve, not everyone cares enough or has time to do lens research, so it's one of those rules of thumbs that work more times than it doesn't. When I started with photography I thought f22 was the best!

I'm embarrassed to say that the review I posted of the 50mm f1.4 took me by surprise to see f2.8 and f4 were the sweet spots. I own this lens!

For ISO I'd suggest that you take a look at this video...

Canon EOS 7D ISO Noise test on Vimeo (http://www.vimeo.com/10473734)

multiples of ISO 160 seems to be the sweet spot for the 7D... with ISO 160 being the best of any of the selections ... and the 640 setting looks workable, compares to ISO 100 even...

You should watch the video in a dark room with a good monitor... " FULL SCREEN "

to be on the safe side you could create your own tests of ISO noise and see exactly where your setup
will be acceptable to your project.

Wow, that really does show the difference, extremely useful. To a beginner it seems extremely odd that ISO 640 is comparable to 100 but no arguing with the proof!