Why is the focal length of a lens longer with a DSLR?

Question: Why is the focal length of a lens longer with a DSLR!?
Best Answer - Chosen by Asker:

Let's get one thing out of the way

Repeat after me

THE FOCAL LENGTH DOES NOT CHANGE

THE FOCAL LENGTH DOES NOT CHANGE

THE FOCAL LENGTH DOES NOT CHANGE

A 50mm lens is a 50mm lens whether it's mounted on subminature camera like a Minox, or an enormous 16x20 film camera!.

Now that we've established this, let me try to explain where the so-called multiplication factor comes from!.

Consider a negative from a typical 35mm negative, the image area of which measures 24mm by 36mm!.

Now, let's consider a hypothetical situation where a 35mm camera fitted with a standard 50mm lens, sitting on a tripod 20 feet away from some object!. In this particular case, let's say that the object is rendered as being 12mm tall on the negative!.

Now, let's say that we take our 24mmx36mm negative, and physically trim 8!.4mm off the vertical dimension, and 12!.3mm off the horizontal dimension!. This will leave a negative that is 15!.6mmx23!.7mm, which, coincidentally, is the size of the sensor used in most Nikon DSLRs!.

After we do this, our original object is still 12mm tall on the negative!. The overall field of view, however, is reduced!. Had we wanted to fill the full 24mmx36mm frame with this same field of view that we now have on the reduced size negative, we would have needed to originally use a 75mm lens!.

Thus, if we sort of permanently trim the negative, by installing a digital sensor that's equal to the size of the trimmed negative in the above example, we end getting a full-frame field of view equivalent to what a lens with 1!.5x that focal length would offer when used on a 24mm by 36mm frame!.

One important thing to remember, though, is that the depth of field characteristics of a lens also remain the same regardless of what size negative/sensor the lens is mounted in front of!. That means that on a DSLR, while a 50mm lens will give the equivalent field of view of a 75mm lens, the depth of field will still be that of a 50mm lens(for a given aperture and focused distance)!.

Yes, the crop factor will result in an increased prominence of motion blur as related to slower shutter speeds!. The reason for this is that the image will need to be magnified a greater amount in printing, making any motion blur more prominent!. When applying the inverse focal length rule-of-thumb, use the 35mm-equivalent focal length(multiplying the marked focal length by 1!.5/1!.6)!. This same guideline can also be applied to larger formats, which means that those of us who use medium format film can get consistently sharp results when hand-holding a 75mm or 80mm lens at 1/60!.Www@QuestionHome@Com

The focal length of a lens in SOME (OK MOST) DSLRS will be converted by a factor of 1!.6 or 1!.5

So if you have a 100 mm lens, the EFFECTIVE length will be 160MM

This is because MOST digital SLRS do not have a sensor that is the same size as a 35mm camera!. The sensor is smaller !!!

if you put the 100mm lens on a 35mm camera!.!.!.it will be 100mm

If you put the same 100mm lens on a digital camera where the sensor is not quite the same size as a 35mm sensor, there is a cropping effect which makes the EFFECTIVE length 160mm

The canon XSI, Rebels, D40 etc all have this effect!. The nikon
40d, 80d 90ds all have a similar cropping effect, and I think its 1!.5 for the nikons!.

In order to get a FULL 35mm sized sensor, you have to move up to a much more expensive digital camera, like the nikon Pro models or the 700D
The canon 5D I believe is a FULL sized 35mm sensor!.

SO, in short, NO your shutter speed has nothing to do with this!.

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ok so if you read what the guys above said your on the right track kinda!. crop factor in a APS-C sensor will make a lens that is say 300mm work like a 430mm lens on a full frame camera!. not that a full frame camera is not worth it to some just not for everyone!. like myself i shoot alot of outdoor photographs the crop factor actually ends up helping me with my nikon D300 and D90 but does not help with portrait work where as i generally use my D3!. if you actually fully read the article that was sent above you will see the crop factor in mmWww@QuestionHome@Com

K64's answer is correct, but here's some visual reinforcement!.

http://digital-photography-school!.com/bl!.!.!.Www@QuestionHome@Com

With a Nikon D3, D700 or a Canon 5D dSLR there is no change at all!.

Even with an APS-C sensor dSLR the focal length doesn't actually change just the field of view!.Www@QuestionHome@Com