Why is a digital SLR zoom lens a different rating when compared to the old 35mm format? IE 55-200mm = 85-310mm

Answer 1

There are two types of DSLR those with full-frame CCD (the CDD is the same as film 35mm) and those with smaller CCD which happen to be ~75% the size. DSLRs that do NOT have full-frame sensors (the affordable ones ;) will inlarge your image because it isn't recording the outer edges.
So the lenses are actualy 1.5 times the distance.
Take a picture you have on the wall and take an inch or two from all the sides and then picture what's left as the whole picture and you've just croped it. Now make it the same size as the original and you've enlarged it. That is sort of how these smaller sensors cause the lenses to act differently than on a regular film SLR.

Answer 2

Here's the interesting reason. I have a film 35mm SLR Nikon N65 and have been thinking about purchasing a Nikon SLR digital. In my research I too found what you did and was confused by it. Talking to a pro photographer who has made the transition from film to digital I found out what's going on.

The area of exposure on a slice of 35mm film is a set size. The area of exposer on a digital camera however is a different size... it's actually smaller in square area than the film area. Thus the lens must 'compenstate' for the difference in exposure area. You can use the film lens for the same type camera (as I wanted to do) but when you compose your photo, you must take into account that the outside border around your focal area will not be included in the shot. Thus you have to 'zoom out' a bit more to get the same composition with the digital camera as you would on film.

Thus the difference you see between the lens is the compensation for the difference in exposure area.

Hope this answers the question and good luck!

Answer 3

Let's clarify one thing before we start; regardless of film or digital sensor - what you see is what you get - when shooting with an SLR.

DSLR's come in all shapes and sizes and with different sized sensors as well. Not all the sensors are the same size as 35mm film. Only the more expensive DSLR's have a "full size" sensor that is equal in size to 35mm film.

Without getting in to all the physics and math, suffice to say that when a sensor smaller than 35mm film is used,a converison factor must be applied to compensate for the change in the focal length of any lens in front of the sensor. For many cameras this conversion factor is 1.6. This implies that a lens that would be 100-300mm on a 35mm camera, would appear as a 160-480mm lens on a DSLR.

I say "Bonus!" With a 6Mp sensor you'll get stellar results and more mileage out of your zoom lenses.

The down side is if you are into close work and need to use wide angle lenses. To get results comparable to 35mm film, you pretty much need to spend large on a DSLR body with a full size sensor.

Answer 4

Because sensors are generally smaller than a full frame image on 35 mm film, the frame gets filled up sooner. Bad explanation... Here's a great explanation, if you are a visual learner. Scroll about 2/3 of the way down the page until you see the picture of the bird with 3 rectangles drawn around him. Read the associated text.

http://www.luminous-landscape.com/tutorials/understanding-series/dslr-mag.shtml

Answer 5

stehen m has about the best answer of all..it is nothing to worry about just understand yhat if you have a dslr with a 1.6 ratio which most are except high end ones you need to do the math.if you soot with a 28mm lens on your film camera it will amount to roughly a 50mm lems in digital.

Answer 6

two different kinds of cameras

they each need a different format to work