does focal length/aperture stops affects exposure?


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seezhijie

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#1
i recently read an article which said if you take the focal length divided by the Aperture stops, it would be how wide open the aperture is.

eg. focal length 100mm divided by Aperture f/10 means the opening is 10mm wide.
so at the same time, if focal length is 200mm, divided also by Aperture f/10, the opening will be 20mm wide.

does this affects the exposure in any ways? i've done practical tests but couldn't find any differences
 

photoart

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#2
i dont know if I am understanding your examples correctly but

if you are on aperture priority mode, the shutter speed will adjust itself so you get proper exposure most of the time
 

catchlights

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#3
the physical size of the aperture is different on different focal length lenses, but the amount of the light enter the lens thru different focal length lens with the same aperture value will be the same.

eg, you shoot a 50mm lens with f5.6, 1/125s, dismount the 50mm lens and mount a 200mm lens, shooting at f5.6, 1/125s, these images from two different lens should be have same exposure.
 

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seezhijie

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#4
i dont know if I am understanding your examples correctly but

if you are on aperture priority mode, the shutter speed will adjust itself so you get proper exposure most of the time
well of course i made sure that didn't happen. I was in Manual mode
 

seezhijie

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#5
the physical size of the aperture is different on different focal length lenses, but the amount of the light enter the lens thru different focal length lens with the same aperture value will be the same.

eg, you shoot a 50mm lens with f5.6, 1/125s, dismount the 50mm lens and mount a 200mm lens, shooting at f5.6, 1/125s, these images from two different lens should be have same exposure.
so i suppose the extra focal length makes up for the wider opening? sorry i can't learn something if i don't see the logic within
 

JJLoke

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#7
i recently read an article which said if you take the focal length divided by the Aperture stops, it would be how wide open the aperture is.

eg. focal length 100mm divided by Aperture f/10 means the opening is 10mm wide.
so at the same time, if focal length is 200mm, divided also by Aperture f/10, the opening will be 20mm wide.

does this affects the exposure in any ways? i've done practical tests but couldn't find any differences
The article u read only applies to telescopes!

In camera optics, there are multiple group of optics to to 'compensate'



In telescopes, the aperture is the size of the optics, either mirror or lens. The bigger it is, the more light it can capture, thus able to see more distant objects.

The focal length of the lens is affected by the curvature of the mirror or lens.

Focal ratio is important in astrophotograph. f/4 sporting scopes is the fastest and >f/12 are usually for bigger scope with high magnification capabilty.

Simply put, how much time can photons captured with an aperture of x millimeters travel along the focal length of y millimeters to reach focus. The formula u read is therefore

focal length / aperture = focal ratio

magnification is = telescope focal length / eyepiece focal length

A 2000mm scope with a 10mm eyepiece will give u 200x magnification
 

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Diavonex

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#9
i recently read an article which said if you take the focal length divided by the Aperture stops, it would be how wide open the aperture is.

eg. focal length 100mm divided by Aperture f/10 means the opening is 10mm wide.
so at the same time, if focal length is 200mm, divided also by Aperture f/10, the opening will be 20mm wide.

does this affects the exposure in any ways? i've done practical tests but couldn't find any differences
Watch this video:

http://www.youtube.com/watch?v=hyadfAkihAs&feature=channel
 

JJLoke

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#11
I don't really get this part. Mind elaborating?
Lets take a camera lens as an example.

When a look the the schematic diagram of a camera lens, u will see a big lens at the front aka objective lens, follow by many other smaller ones in the middle and the rear. It is states a #of elements in #of groups. Example 11 elements in 6 groups.

If u rid of all the internal lens except for the objective lens, the objective lens will become dependent on the diameter of the lens (light gathering) and focal length, effectively becoming a telescope lens

What the internal lens does is to correct for aberrations arising from the main objective lens, flatten the field, zooming in zoom lens, etc.

A biconvex lens will bend light inwards, shortening the light path, thus the focal ratio will become smaller. Known as 'focal reducer'.

Example:

80mm diameter lens, 480mm focal length, f ratio is 6

with a convex lens, my focal length is shortened

80mm diameter lens, 400mm focal length, f ratio is 5

A biconcave will bend light outwards, increasing the light path, thus the focal ratio will become larger.

Taking the above example, 80mm diameter lens, 960mm, f ratio is 12

A tele-extender multiplies the focal length. 2x tele-extender multiplies by 2 times.

Known as 'barlow' in telescope terms

Hope u understand ;)
 

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seezhijie

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#12
Zzz nvm no need for me to know since it's telescopes. Thanks anyway~ I'll be closing thread in a few hours' time to allow some extra comments
 

JJLoke

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Aug 2, 2009
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#13
Guess u really read the wrong article.

Since u have experiment it urself, more or less u understand whats happening already.
 

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