Quilb wrote:So essentially i would get same picture but the one taken with 40mm lens would be brighter?
Given the same shutter speed and ISO was used, no.
I think that where you may be getting some confusion, is that I suspect that you're trying to relate a given aperture value to a the physical "opening" that any aperture value might represent.
Let's go back a step, if we can, and just understand that, in any given situation, there's a finite amount of light available for you to shoot with. Doesn't matter if it's outdoors, indoors, under water, sunny, raining .... or you're going to be using flash.
The deal is that, at the moment that you're going to make your actual exposure, there is only a finite amount of light.
To make a good exposure, you, as the photographer, need to understand exactly how much light that might happen to be, so you need two things: a way of assessing how much light there is, and a means of expressing the result of your assessment.
Most cameras today have built-in light meters, or you can buy a handheld one. That takes care of the assessment part of the process.
The problem lies in how the results of this assessment are expressed: mostly, it's expressed in terms of a combination of a set of values, those values being a shutter speed, and aperture, and an ISO.
What's happened is that everything has been laid out on a platter for you: "here's the settings you need and you'll get a great photo" is what the camera seems to be saying to you.
Wrong!
What the camera's actually saying to you is "Here's a set of values that seems to be reasonable for the current available light value. Please use these as a starting point, and now make some intelligent judgments about the actual scene you're shooting, your composition, and how you want the final image to look, and then you can make an image".
The problem is, as I said, the camera is giving you one (of many potential) final result. The step of telling you the actual EV (Exposure Value) has been omitted.
Mostly, that's probably ok, but if you're anting to understand this stuff .... well, you need the data, don't you?
And an EV is exactly that: it's just a measurement of light. THAT is the means of expressing your assessment of the value of the available light in any given situation. It's always expressed in the same way, and it always has exactly the same meaning.
EV 5 is always EV 5, just as saying you're driving at 45kph expresses a certain constant, so too does saying that the current light is EV 5. Or EV 11. Or whatever.
Now - and this is where it gets tricky - there's no need to know too much about specific EVs or how much light they represent. What's more important is to understand the concept that, like driving at a given speed, any given EV simply represents the amount of light available to make an image.
Hand in hand with that concept goes the simple fact that any given aperture and shutter speed combination results in the passage of a certain amount of light. No more, and no less.
And, for any given combination of shutter speed and aperture values, there's a myriad of other combinations of shutter speeds and aperture values that will result in exactly the same passage of exactly the amount of light.
No more, and no less.
This is all very easily expressed mathematically, and quick look at the range of shutter speeds available on a camera will show you some of this: 1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/125, and so on.
The only other major factor is that of emulsion/sensor sensitivity. This tells you how receptive to light the medium that is going to record your image is, it's it's measure in ISO.
So .... we can take a particular value of measured available light, apply to it the sensitivity of the recording medium, and we can then deduce a set of shutter speeds and aperture values and (hopefully) come up with a well exposed image.
And this is true regardless of the camera we're using (coming back to the point of your question).
I can use a handheld meter, see an EV of ... whatever ... maybe then set ISO 100, 1/250 @ f/8, and apply those values across a broad range of cameras, and obtain a set of correctly exposed images from each of those cameras.
Those cameras could range from a simple compact film PHD, a more complex but still simple digital PHD, a consumer DSLR, a pro DSLR, a medium format SLR or DSLR, and then even a LF sheet film camera.
Your confusion lies in that fact that, for each of these cameras, because of their actual sizes, and because of the relative size of their componentry (image sensing area, lenses and focal lengths etc) the physical size of some elements discussed, such as the aperture, will be very different.
That's the nature of the beast: aperture is based upon a number of complex relationships, and while its physical attributes are governed by those complex relationships, the end result - its expression as a simple value - is, to all intents and purposes, constant across all the different cameras that we're likely to encounter.
Posted: Sat Jun 02, 2007 7:49 pm
