I was going to post this into the other thread, but decided it deserved to be highlighted on it's own.

A lot of D-SLR's have what is referred to as a focal length magnification factor. This effectively makes all lenses mounted to the camera "longer" than they would be on a film camera.

That works to an advantage on the long end of the spectrum, since a 200mm lens, is effectively acting like a 300mm lens.

HOWEVER, it hurts D-SLR users when it comes to wide angle. An 18mm lens on a D-SLR has essentially the same effect as a 27 mm lens on a film SLR.

Let's take a quick look at the magnification factors and then I'll explain where it is really coming from.

Canon (300D/Digital Rebel, 20D, D60, D30) :: 1.6x (22.5mm by 15mm sensor)
Nikon (all D-SLR) :: 1.5x (23.7mm by 15.5mm sensor)
Canon 1D :: 1.3x (29mm by 19 mm sensor)
Canon 1Ds and 1Ds Mark II :: 1x (36mm by 24mm sensor)

Why the difference in magnification factors?

The difference in magnification factors comes from the size of the sensor used in each camera. For the 1Ds and 1Ds Mark II, the sensor is the same size as a 35mm frame of film would be. Thus, no magnification factor.

ALL the other sensors are smaller than 35mm. Thus, when you use a "regular mount" lens on a D-SLR with a sensor smaller than 35mm, some light (image) that would have been captured on a full frame camera is outside of the area of the D-SLR sensor. This is referred to as a field of view CROP.

That smaller sensor cropping is the condition causing the magnification factor. There is nothing magical about a D-SLR that causes lenses to get longer, just the sensor size.

Trigonometry Answer

Effectively we have a triangle.

The lens focal length sets the base of the triangle (not the hypotonous).
The height is established by the size of the sensor.

The angle from the base to the hypotonous dictates your field of view.

It follows then, that as the sensor gets smaller, the ANGLE from the base to the hypotonous gets SMALLER. Thus, even though the base of the triangle remains the same (the focal length of the lens), you are looking at a tighter field of view with a D-SLR that has a smaller sensor.


When I get a chance I'll add some diagrams to this explanation, but I hope that helps in clarifying where the magnification factor is coming from.

Dave

 

Good post! I see lots of misinformation out there and this will help a lot. Bob Atkins also wrote a good article on that topic.

Mark

 

Thanks Mark.

It took me a while to wrap my head around this one too. You're right there is a lot of misleading information that helps to create the confusion.

IMO, the main source of the confusion is the phrase "digital zoom" that is used a lot in the marketing of digital P&S cameras. People are familiar with digital zoom causing magnificiation, so when we then talk about Digital SLR's having a magnification factor, it only seems natural that it must come from the fact that it's digital. I fault the manufacturers and the marketing people for this source of confusion.

 

Yeah, I find a lot of the marketing information to be really confusing. I think some of it is deliberate, too. I think I need to get one of those fancy new digital tripods!

Mark