Understanding Exposre - Questions

[pandabean]

Hi all,

I have recently got a EOS 400D and Brian's book. I am half way through Brian's book and I have a few questions as well as someone to say whether I understand it or not based on what i have said. Basically I don't want to think of something based on the book and then find it is the wrong thing at a later point.

One of the basic things he talks about is how ISO, Aperture and Shutter speed act as a triangle. So each increase/decrease of one of these is a stop?

So if I increased my shutter speed from 1/500 to 1/250 I would have to then change the aperture by 1 stop from f8 to f11 to compensate and make it a correct exposure? If it was ISO instead of aperture I was changing would it be from say 200 to 100?

Is it basically if you add something to one of them you have to subtract it from the other so that they balance out?


Regarding DOF, he mentions that on some cameras you can change the distance on the camera to focus on a particular point in a landscape. I am unfamiliar with this feature.

He also mentions using the exposure needle on the camera to determine if what you are looking at is correctly exposed and then later explains about locking the exposure and recomposing the scene. How do you know when to do this and when not to?
Would a situation be similar to his red sunset picture with the bridge be an example of using the Exposure Lock where if you focused on the bridge the camera would try and expose it correctly but would end up making the sky overexposed?



Hope you dont mind me asking these questions but I am wanting to see if I understand it a bit more.

Andy

[marnel47]

With Regards to the DOF topic what he was saying was that if you have an object in the forground of your landscape shot you should manually focus the lense for the distance of that object since your shooting for maxium DOF you'd want to make sure your focus was on that front object because naturally everything behind that point will be in focus because of the DOF. Assuming your Aperature is set right usually f16 or higher for me.

Using Exposure lock will only be effective if your not using evaluate as your metering method. For example on your 400D if you switched the metering mode to partial you could then point your camera at the subject you'd like to expose for lock that exposure and recompose your scene. (I'm not 100% sure if that statement is 100% correct with regards to that not being effective with evaluate turned on, so i'll let somebody else chime in) :)

[lungdoc]

I think you have the right idea but wrong terms - a change from 1/500 to 1/250 is a usually called a decrease in shutter speed (though it's an increase in exposure time i.e. time shutter is open). This will let in more light therefore aperture needs to be smaller to give same exposure: which as you correctly indicated means an increased F stop ( terms are confusing) OR "film/sensor" needs to be less sensitive meaning a reduced ISO speed.

Focus on any DSLR can be changed manually, or use autofocus and select the point you want, or use "focus recompose" by autofocus, lock focus and move the aiming point.

Exposure question at the bottom is more complex since it depends on the metering mode used - Camera will attempt to do this for you in evaluative metering. Simplest is to imagine using a spot meter and in that case your example is correct - if you want a well exposed bridge versus a silhouette bridge the exposures will be different, using exposure lock or exposure compensation (or manual mode with deliberate under-or-overexposure of the part you are metering off/aimed at). In practice I often "cheat" by just using evaluative and looking at LCD and adjusting exposure compensation, after a while you get to know when to add or subtract; this doesn't work in a fast-moving situation.

[pandabean]

Thanks guys. :)

I guess it does take a bit of time to get used to the terms. So "decrease in shutter speed" = longer length of time the shutter is open for? And vice versa for an increase. Also is an increase in aperture mean a smaller f number?

I think I asked at some point about DOF in a previous topic I made but I was wondering how much blurring do you get out of the XTI kit lens at the full 50mm and at f5.6? I know it wont be as much as a 100mm lens with the same aperture.

I havent got round to playing with the metering modes. I know from the manual what they basically do but I am not sure when to use them. Is there a page in the book that talks about it?

[Technophile]

yes, a decrease in shutter speed means a longer time the shutter is open. widening or opening up the aperture means to use a smaller f number, which lets more light in. in my opinion, "increasing the aperture" sounds confusing, since it could be read as "increase the f number" or "increase the area the aperture opens up", which are opposites.

also note that not all cameras have the same increments for shutter, aperture and ISO. for example, on the rebel XT, the shutter and aperture can be adjusted by 1/3 stop, while the ISO can only be adjusted by 1 full stop. on a 30D, all three settings can be adjusted by 1/3 stop.

[DrPablo]

F/stop is a confusing number for people new to photography, but it helps to understand exactly what it comes from.

First of all, to calculate it, f/stop = focal length divided by diameter of the aperture opening. So as aperture gets larger, the f/number gets smaller; it's a simple reciprocal.

As this equation reveals f/2.8 on a 20mm lens permits the same amount of light as f/2.8 on an 80mm lens, but the opening on the 80mm lens is 4 times as large. Because of light fall-off with distance you need a larger aperture to achieve the same film / sensor illumination with longer lenses.

Now the aperture scale is measured in stops. A 'stop' is a doubling of light. Now it's the area of the aperture opening that determines the light entry, not the diameter per se. The area, of course is pi x radius squared. So to double the light entry (or increase it by one stop) you double the area of the aperture opening; but because f/stop represents a linear dimension and not area, each f/stop is measured in intervals of 1.4 (which is the square root of 2). So a 1.4-fold increase in f/number is one stop, and a 2-fold increase in f/number is 2 stops.

This is why the aperture scale goes as follows (in full stops):

1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 - 32 - 45 - 64 (etc; yes, some lenses go to f/64 and above)

There are intermediate stops as well, with most cameras allowing you to vary aperture in (roughly) thirds of a stop. I say roughly because the scale is designed for convenience, and frankly being accurate to within 1/10 of a stop is seldom important for anything.

Knowing the math behind this is important when you're shooting manually and when you have to think about exposure compensation. Like when you're shooting with a filter that you can't meter through, like an infrared filter. Or if you attach a bellows to your camera for extreme macro shooting you'll need to calculate bellows factor.

When you're starting off, though, it's often enough to know in general that f/stops are like golf scores, i.e. the opposite of what you expect at first.

[MT]

Link to an online DOF calculator

http://www.dofmaster.com/dofjs.html

[thrumyeye]

In my opinion, "increasing the aperture" sounds confusing, since it could be read as "increase the f number" or "increase the area the aperture opens up", which are opposites.

My opinion agrees with yours! Many times I've heard "increase aperture" and I sit wondering - increase my f number or my opening?

Thank you for making this point!