Lens crop factor confusion

[I know nothing]

I have a DSLR with an APS-C sensor and a 17-50 zoom lens that is digital format specific.
Now, I want to acquire a faster 50mm prime lens.
However, I'm confused over whether or not the 50mm prime will give me the same FOV as the zoom when it is extended to 50mm.
I'm aware of factoring in the crop when multiplying the mm of a prime on my DSLR, however, I've seen some people make multiplication on digital specific lenses too, and that has left me confused.

[Prospero]

The calculations have to be made on digital only lenses as well. The focal lengths listed on lenses is the 'real' or 'optical' focal length. It does not refer to the field of view.

Thus, the 17-55 on a 1.5x crop body will give the same field of view as a 25.5-82.5 lens on a full frame SLR.

A 50mm prime on a crop body will have the field of view of a 75mm lens on a full frame SLR. This is the same field of view as the 17-55 would give when zoomed in at 50mm.

[coldrain]

The calculations have to be made on digital only lenses as well. The focal lengths listed on lenses is the 'real' or 'optical' focal length. It does not refer to the field of view.

Thus, the 17-55 on a 1.5x crop body will give the same field of view as a 25.5-82.5 lens on a full frame SLR.

A 50mm prime on a crop body will have the field of view of a 75mm lens on a full frame SLR. This is the same field of view as the 17-55 would give when zoomed in at 50mm.
And that is of course if you take 1.5x as APS-C crop factor. The "real" APS-C crop factor is 1.6x (like the Canon sensors) (APS-C film has that size) and then 50mm will become 80mm.
And if you happen to have a Sigma APS-C DSLR, the 50mm would become 50 x 1.7 = 85mm.
But for that Sigma you would have to convert a Canon EF 50mm lens :eek: .

[I know nothing]

So, I might as well just buy all my lenses designed for full frame rather than digital, if it's going to give me the same focal length.
At least the lenses for full frame can be used on both systems.

[coldrain]

Well, the weight will be higher. And it is only valid if you actually have full frame.

[Prospero]

Also, with full frame lenses it can be difficult to get the range you may want with a zoom lens. For instance, a popular range for fast lenses (f/2.8) on a crop camera is 18-50. This range will give you a wide angle, but also a moderate telephoto. Such a range is very desirable when shooting certain events (weddings, parties, etc.).

Furthermore, there is only one ultra wide angle lens available for a crop body that also work on a full frame lens, namely Sigma 12-24. Due to the high weight, and its limits when using filters, the APS-C only lenses (Tokina 12-24, Sigma 10-20, etc.) may be a better alternative.

[Ray Schnoor]

The "real" APS-C crop factor is 1.6x (like the Canon sensors) (APS-C film has that size)
I'm not sure where you get that information.

APS-C film had a negative size of 25.1 x 16.7 mm
Nikon APS-C 1.5x crop factor has a sensor size of 23.6 x 15.5 mm
Canon APS-C 1.6x crop factor has a sensor size of ~22.5 x ~15.0 mm

Not that it matters all that much. They are "all" considered to be "real" APS-C size.

Ray.

[kgosden]

I guess it is too bad the only 'real' digital APS camera was a dead end since it would have really added to the confusion. Anyone remember the Minolta RD3000? It used the Vectis lenses from the Minolta APS format film SLR so there a lens was a lens. Although I seem to recall not all Vectis lenses worked since the sensors, it used two to get 2.7MP, and the light path was strange.

[DonSchap]

Since you never specified what Camera body you were shooting, you have the crowd guessing your digital crop factor (DCF). These DCFs vary all over the place, from 1.3x to 2.0x and goodness knows what else.

Please specify the camera type so you can get an "apples-to-apples" comparison and eliminate any corn-fusion.

Whatever is stamped on the lens is multiplied by the DCF... and provides you with the "effective focal length" on the glass when it is on your camera.

APS-C lenses are designed to work on APS-C sensor cameras ... and if placed on full frame sensor bodies usually result in heavy vignetting at (wide) shorter focal lengths. Canon's EF-S lenses simply cannot be attached to Full Frame Sensor bodies, due to physical design of the lens collar. If you did, somehow, manage to attach the EF-S lens to the FF body, you'd probably wind up damaging the mirror mechanism, as the EF-S inner element protrudes into the body of the camera. The APS-C sensored bodies have smaller mirrors, so they miss this lens protrusion when they lift and move.