How to calulate the focal lenght of digital SLR lens

Actually, it depends on the Crop factor or FLM (Focal length multiplier).
Crop factor is the ratio of format size of a camera compared to a reference format; most often, this term is applied to DSLR cameras, relative to full-frame 35mm film as a standard.
This ratio is also commonly referred as a focal length multiplier ("FLM") since multiplying a lens focal length by the crop factor or FLM gives the focal length of a lens that would yield the same field of view if used on the reference format. The term format factor is sometimes also used.
Most DSLRs on the market (especially entry-level cameras) have nominally APS-C-sized image sensors, smaller than the standard 24x36 mm (35mm) film frame. For example, most of Canon's consumer-level DSLRs use a sensor that measures 15x23 mm. The result is that the image sensor captures image data from a smaller area than a 35mm-format camera would, effectively cropping out the corners and sides that would be captured by the 24x36 mm 'full-size' film frame.
Because of this crop, the effective field of view (FOV) is reduced by a factor proportional to the ratio between the smaller sensor size and the full-frame 35mm (reference) size.
For entry-level DSLR cameras, this factor is between 1.5 to 2.0X. For example, a 24mm lens delivers a moderately wide-angle FOV on a full-frame camera, but on a camera with a 1.6X crop factor, an image made with the same lens will have the same field of view that a full-frame camera would make with an approximately 38mm lens (24 X 1.6 = 38.4).
Look at your camera's user manual and it clearly states the crop factor.
P.S. Full frame DSLR has no crop factor (i.e. crop factor = 1)

2007-04-03 09:50:01 補充：
Depends on the particular DSLR crop factor, a 24mm lens could be converted into:1X : 24mm1.1X :26.4mm1.2X: 28.8mm1.3X: 31.2mm1.4X: 33.6mm1.5X: 36mm1.6X: 38.4X

2007-04-03 09:50:38 補充：
sorry should be:1.6X: 38.4mm

2007-04-03 09:58:03 補充：
Or you can do the following calculation to get the crop factor:The square root of ﹛864 (24X36) ÷ (length X width of the CCD/CMOS sensor)﹜e.g. Nikon D1(sensor size 15.6 X 23.7)The square root of ﹛864 (24X36) ÷ (15.6 X 23.7)﹜= 1.53X

Friend,
I am a Canon guy, I will try to use Canon system as an example.
http://www.the-digital-picture.com/Canon-Lenses/Field-of-View-Crop-Factor.aspx
With the advent of Digital SLR Camera Bodies, the term Field of View Crop Factor has come into our world. The source of this term is the smaller-than-35mm sensor present in many of Canon and other manufacturers' DSLR sensors. Canon's EF Lenses still focus the image on the same plane as before, but sensors smaller than 35mm sensors do not capture the entire image. Thus, the image is "cropped". The Field of View Crop Factor (FOVCF from here on) refers to the amount of the image that is cropped.

The following table illustrates sensor and viewfinder differences across the current and recent Canon Digital SLR models.





Model
FOVCF
Sensor
Pixel Size
Pixels/Megapixels
Viewfinder

Canon EOS 400D Digital Rebel XTi
1.6x
22.2 x 14.8mm
5.7µm
3888 x 2592
10.1
.80x
95%

Canon EOS 350D Digital Rebel XT
1.6x
22.2 x 14.8mm
6.4µm
3456 x 2304
8.0
.80x
95%

Canon EOS 300D Digital Rebel
1.6x
22.7 x 15.1mm
7.4µm
3088 x 2056
6.3
.80x
95%

Canon EOS 30D
1.6x
22.5 x 15.0mm
6.4µm
3504 x 2336
8.2
.90x
95%

Canon EOS 20D
1.6x
22.5 x 15.0mm
6.4µm
3504 x 2336
8.2
.90x
95%

Canon EOS 10D
1.6x
22.7 x 15.1mm
7.4µm
3088 x 2056
6.3
.88x
95%

Canon EOS 5D
1.0x
35.8 x 23.9mm
8.2µm
4368 x 2912
12.8
.71x
96%

Canon EOS 1D Mark II
1.3x
28.7 x 19.1mm
8.2µm
3520 x 2336
8.2
.72x
100%

Canon EOS 1D Mark II N
1.3x
28.7 x 19.1mm
8.2µm
3520 x 2336
8.2
.72x
100%

Canon EOS 1DS Mark II
1.0x
36.0 x 24.0mm
7.2µm
4992 x 3328
16.6
.70x
100%

A nice point about all of Canon's DSLRs is that they maintain a 3:2 aspect ratio. A 4x6 will print uncropped, 5x7 and 8x10 prints will need to be cropped. There is no need to be concerned about which FOVCF DSLR was used to take the picture.

As of today, Canon uses APS-C 1.6x sensors in the consumer xx0D and prosumer x0D lines. Canon's 1D line uses 1.3x sensors, and the 1Ds and 5D lines use 1.0x full frame sensors.

After the information, the final answer ...
a 24MM EF-lens in 35mm camera, in a 350 or 400D = 24x1.6 = 38.4 mm
the same lens in 1D = 24x1.3 = 31.2 mm
the same lens in 1Ds + 5D with 1:1 sensor, will be 24mm :)
Hope all help la :)