Depth of field limitations

[RobbieW]

Hi,

Sorry a potentially silly question...

When shooting in Manual on my 350D I change the aperture to either 22 if I am shooting landscapes, or 5.6 for close up work. Where do the avaliable aperture settings come from?

I know my 18 to 55 mm lens has a max of 3.5 size of f3.5 at 18mm and f5.6 at 55mm, does my camera have any limits on it? or is it all set/restricted by the lens?

Thanks

Rob

 

[Lufbrarunner]

AFAIK the f number is the ratio of the aperture diameter to the focal length of the lens. So your max aperture will be restricted by the physical characteristics of the lens, ie barrel diameter.

 

[CT]

The maximum aperture comes from dividing the physical diameter of the lens front element into the focal length of the lens.

a 50mm lens having a front element width of 50mm would be an F1 lens.

The same lens with a front element 25mm in diameter would be an F2 lens... and so on.

So... when we describe a lens by it focal length and max aperture, we know at once whether it's a fast or slow lens. The longer the focal length, the bigger that front element has to be to obtain a given max aperture - a 200mm F2 lens would have a front element 4 times wider than a 50mm F2 lens.

Obviously, as the focal length increases, then so does the size of the glass to obtain a given max aperture, and lenses start to get very heavy and enormously more expensive.... unfortunately.

HTH.

 

[Arkady]

I know my 18 to 55 mm lens has a max of 3.5 size of f3.5 at 18mm and f5.6 at 55mm, does my camera have any limits on it? or is it all set/restricted by the lens?

It's all down to the lens you have fitted - an f/2 lens goes as wide as f/2 and an f/4 lens goes as wide as f/4.

Some lenses have a 'flaoting' maximum aperture, as your 18-55 does. More expensive lenses keep the widest aperture throughout the focal range.

Hope that helps - no such thing as dumb questions BTW.

 

[Darryn]

The maximum aperture comes from dividing the physical diameter of the lens front element into the focal length of the lens.

a 50mm lens having a front element width of 50mm would be an F1 lens.

The same lens with a front element 25mm in diameter would be an F2 lens... and so on.
snip...

Does the rule still apply to the Canon DO lenses?

 

[CT]

Does the rule still apply to the Canon DO lenses?

Absolutely. In recent years tele lenses have tended get considerably shorter, due to clever manipulation of the light path within the lens, resulting often in lenses shorter than their own focal length.

Mirror lenses are an easier example to understand, as they're often extremely short, but achieve the overall focal length by bouncing the image back and forth off mirrors within the lens, till by the time it reaches the sensor it's the required 500mm - (a common focal length for mirror lenses)

 

[siejones]

When shooting in Manual on my 350D I change the aperture to either 22 if I am shooting landscapes, or 5.6 for close up work. Where do the avaliable aperture settings come from?

Curious to know why you do this.

F22 will give you great depth of field but this isn't entirely necessary for every landscape shot. If fact I dont think I stop down this much for any landscape shot and would only consider it in the most extreme DOF shot. The reason for not doing so is that in the smaller apertures you start to suffer from diffraction problems. F22 will be suffer a lot. Have you been wondered why you landscape shots arn't very sharp?.

I recomend keeping your aperture setting to between F8 and F11 for landscape shots according to how close your foreground is and only in the extremes where you foreground is really close and you still want that distant mountain to be in focus do you stop down much further. F8 is where more or less any lens excels at and further up and down from there the quality tends to degrade. More expensive lenses deal with this better but at the widest and smallest apertures even these degrade.

The same applies for close up shots. F5.6 is great throw the background out of focus but not a neccesity because your subject is close up.

Depth of field is a little hard to get your head around and even when you think you have sussed it it jumps up and suprises you sometimes.

I recommed you do some reading on the apertures and how they effect you depth of field. Also how the extremes of both ends of the aperture will give you less quailty is your shots.

 

[oldgit]

The following is a very good article that graphically shows/explains the above.

Clicky