correctly picked up

[drewdunnrespect]

hiya gang
over the last few nights while on 12 hour night shifts i have been reading this book
http://www.waterstones.com/watersto...igital+photography+-+an+introduction/7246522/
now then the book is split in to chapters and each page/secetion of the chapter deals with a diffrent aspect of photography
now the first chapter is called the fundermentals and the third aspect is called focasing and depth of field
now while i have been reading this i have been taking notes on what i have learnt now i just want to check that i have picked up what he (the auther) is saying correctly, so what i am going to do is right my notes out and i just would like to know if they are correct or i have miss understood.

now

aperture: means the size of hole in the lens which light has to travel though in order to get to the sensor and its recognised measure is F now if the above is correct the smaller f is the further away a subject appears I.E the further you have zoomed out this also means that the hole in the lens will be alot bigger because you have zoomed out and f is smaller.
i.e the hole in the lens which the light has to pass through is bigger

now if all of that is true and i really have learnt it correctly that must mean that depth of field means the distance between the top blurry bit of the picture and the bottom blurry bit of the picture. Now i appreciate that not all pictures have a top blurry bit and a bottom burry bit but i find that, that is the easiest way to understand depth of field, if i am correct.

is this correct so far

drew

 

[timf2009]

Hi Drew

You're getting there. Aperture is the size of the hole in the lens and it is referred to as an f-number. The smaller the number the more light gets into the camera. This doesn't make the subject look smaller or bigger though.

The size the subject appears in the picture is related to how far it is away if you have a fixed lens. If you have a zoom lens you can zoom in and make the subject appear bigger. The zoom is referred to as focal length and is measured in mm. A typical kit lens will zoom from 18mm (wide angle - subject looks small) to 55mm (subject looks bigger).

Depth of field is how much of the picture is sharp and how much is blurred. Pick something close to you and take a picture of it with your lens set to f3.5 or however low your lens goes. Then without moving change the aperture to f22 and shoot again. You'll see that at f22 more of the picture is sharp and at f3.5 the subject is in focus but most else isn't. This effect is most noticeable if you zoom in as far as you can.

Keep at it !

 

[TomBartlett]

You're pretty much spot on with depth of field. It's the part that's in focus. As timf2009 says, the smaller the f number, the smaller the area that will be in focus.
Camera: -------blurry-------{in focus}-----blurry---- = small f number.
Camera: --blurry--{.............in focus............}--blurry-- = big f number.
Hope this helps

 

[drewdunnrespect]

yes i get depth of field now cheers guys

i also understand aperture which is measured in f numbers and the smaller the fnumber the more light gets in to the camera. and bigger f number less light gets in to the camera but it also means that there is more in focas in the picture

now zooming in and out i also understand but what i dont understand is how to keep f the same and still be able to zoom in and out because when i zoom in and out on my kit lens f automatically changes

 

[pteron]

now zooming in and out i also understand but what i dont understand is how to keep f the same and still be able to zoom in and out because when i zoom in and out on my kit lens f automatically changes

That's because f number is a ratio of the focal length of the lens to the effective diameter of the lens. So at a focal length of say 18mm, to achieve f2 the effective aperture needs to be 9mm - the lens would therefore need to be wider than this. But at a focal length of 55mm, the f2 aperture would be 27.5mm and the lens at least this wide. A wide lens costs way way more to make well than a narrow lens, so the cheaper zooms limit the f number at higher zooms.

Your kit lens is probably only 60 or 70mm in diameter and limits at maybe f3.5 at wide angle and f5.6 at telephoto? So if you set f3.5 at wide angle then zoom, the lens has to reduce the f number as you go because it is already at the maximum aperture it can do. My 17-55 lens is f2.8 all through it range. It is probably twice as wide as the standard kit lens and costs nearly a grand.

 

[drewdunnrespect]

so andy

going on what you are saying if you wanted to zoom right close in on a bee on a flower you would need a small fnumber like 3.6

and your depth of field would be dead small and the amount of light that gets in through the lens to the actuall camera would be huge incomparrison to a large fnumber like 5.6

 

[TomBartlett]

If you want to zoom close on a bee, really you want a high f number otherwise your area of focus will only be a couple of mm. The problem is, as you've picked up, the bigger the f number, the less light comes in.
The issue with your lens is, as pteron said, it automatically changes when you zoom, you can't do anything about this. If you want a lens that doesn't change aperture when you zoom, you'll have to splash some cash.

 

[pteron]

so andy

going on what you are saying if you wanted to zoom right close in on a bee on a flower you would need a small fnumber like 3.6

No not really. The f number is not relevant to how close you want to zoom. You pick the f number for two reasons - initially for light gathering as a large aperture gathers more light so you can use a faster shutter speed and reduce the effect of camera shake. Secondly, the f number sets the depth of field which is the range of distances that are in focus. Say you focus on a flower at 6ft and you have an aperture of f2.8. On a typical SLR the foreground at 4ft and shorter will be out of focus and the background above about 10ft will be out of focus. The same shot at f16 will be in focus from about 40cm in front of you to infinity.

So with your bee on the flower you can throw the background out of focus by selecting a small f number. But the zoom is selected by the focal distance of the lens.

Hope this makes sense!

 

[drewdunnrespect]

will read the replys once been to sleep cos try to comprihend that lot after a night shift is not going to happen