Digital camera's Lens aperture value

[anoop_rajagopal]

Hello,

 

Why is it all digital cameras come with a nice aperture value like

f/2 or f/2.8 at an affordable price? Is there a conversion factor for

aperture value, similar to the focal length multiplier , that the

manufacturers not specifying?

Similarly, the minimum aperture is limited to f/8 in most of the

Digital cameras.

Also in digital, one cannot get the background blur possible with a

similar aperture in a film SLR. In other words the DOF is always high

in digital.

Considering all these, I feel there exists a conversion factor for

the aperture values of digital cameras, similar to the focal length

multiplier.

 

Thank you for your answers.

[basscheffers]

No there isn't. It's just easier to make small lenses with large apertures. The same is true in the other direction, a standard (fixed) lens on a medium format camera is quite slow, 2.8 or 4 is quite normal. (as opposed to f1.4 for 35mm)

 

And yes, they always have lots of DOF.

[peter_foiles2]

NO, there is not a conversion factor for the aperture. Camera manufacturers have created a great deal of confusion by taking the "factor" approach. All they are trying to convey is the angle of view of the lens. NOTHING about the optical characteristics of the lens changes.

 

To answer some of your specific concerns about DOF. At a given shooting distance ther are only two things that affect DOF, aperature and focal length. Digital P&S type cameras have very small sensors and therefore have very short focal length for anormal lens. A typical P&S digicam has a focal length in the range of 7-20 mm. Lenses in that focal length range have a large DOF no matter what the aperature. This is not really a digital effect at all but a format size effect. The smae is true if you compare 35mm film to medium format film. To shoot the same composition with both formats you will use a longer focal length lens (less DOF) for MF than for 35mm.

[todd peach seattle, washi]

<i>

Similarly, the minimum aperture is limited to f/8 in most of the Digital cameras. </i>

<p>

As I understand it, this is due to diffraction effects. To some extent, the degree to which diffraction degrades the image is inversely proportional to the physical aperture size. I'm probably screwing up the math somewhere, but it's something like this: an 8mm lens at f/8 has a 1mm physical aperture. A 35mm lens at f/32 has (about) a 1mm physical aperture.

<p>

As the others have said, this theory extends 'the other way' into large format as well. It's not uncommon for a 210mm lens for a 4x5 camera to have f-stops out to f/64. 210/64 is still about 3.28mm for a physical aperurture. Or even f/128, for a physical aperture just under 2mm.