lens aperture question

[Pat H]

I know that lenses have aperture values on them (ie: f/4-6). I thought that this was the min and max for that particular lens, but what about people saying they use apertures of 20 or so for more DOF? This is a really stupid question, but what lenses support apertures like that?

...I know I probably have this totaly misunderstood, but I'm pretty new to SLRS.

[ScottE]

The apertures named in a lens description is the maximum aperture, such as f/3.5 - 5.6 for a zoom lens. The number represents the denominator in a fraction lens focal length/diameter of opening in aperture. Thus the smaller the opening, the larger the the number. Thus and aperture of f/4 has twice the diameter of an aperture of f/8. However the ability to transmit light depends on the area of the aperture opening, so in order to double the light, you need to increase the aperture size by the square root of 2 (approximately 1.4). As a result you get a progression of aperture sizes from largest to smallest of 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, etc. with each stop halving the amount of light transmitted by the previous.

Your lens may have a maximum aperture of f/5.6, but unless it is a mirror lens it will be able to "stop down" to smaller apertures.

Scott

[Pat H]

Thanks for the explanation. :)

[tim]

ScottE's explaination seems too complex for me, so i'll have a go.

An 18-55 F4-F5.6 lens has a maximum aperture of F4 at 18mm, and a maximum aperture of F5.6 at 55mm. You can stop the lens down to whatever you like, F8, F16, up to the limit of the lens, but you can't go wider.

Aperture primarily controls depth of field, and does it by letting more or less light in, from different angles. If you halve the light coming in (eg moving from F2.8 to F4) you have to double the shutter speed. If one changes, the other has to too. If you add ISO speed into the mix, that gives you three interlinked variable to play with.

Someone should be able to give you a link to some good information, personally I recommend doing a local photography class to pick up the basics - worked for me :)

[lostdoggy]

If you double the light coming in (eg moving from F2.8 to F4) you have to halve the shutter speed.


That doesn't sound right??? double the light when changing from f/2.8 to f/4??? Shouldn't it be half the light???

[Pat H]

ScottE's explaination seems too complex for me, so i'll have a go.

An 18-55 F4-F5.6 lens has a maximum aperture of F4 at 18mm, and a maximum aperture of F5.6 at 55mm. You can stop the lens down to whatever you like, F8, F16, up to the limit of the lens, but you can't go wider.

Aperture primarily controls depth of field, and does it by letting more or less light in, from different angles. If you double the light coming in (eg moving from F2.8 to F4) you have to halve the shutter speed. If one changes, the other has to too. If you add ISO speed into the mix, that gives you three interlinked variable to play with.

Someone should be able to give you a link to some good information, personally I recommend doing a local photography class to pick up the basics - worked for me :)

I may consider a photography class, I apparently have a lot to learn. ;) But i will be majoring in film in college, so it may help me a little, but i do realize that film making and still photography are pretty different.

[tim]

That doesn't sound right??? double the light when changing from f/2.8 to f/4??? Shouldn't it be half the light???

Yes, sorry, I mis-spoke, i've edited the post.

[FlyingPhotog]

I may consider a photography class, I apparently have a lot to learn. ;) But i will be majoring in film in college, so it may help me a little, but i do realize that film making and still photography are pretty different.

Not if you intend to get into the technical side. DPs (Director's of Photography) on movie shoots are all about light, lenses, apertures, film speed (ISO) etc...

Even directors (well, the really good ones) have a solid working knowledge of how light levels will affect a given scene and whether or not they will have the DOF to shoot the scene the way they want.