Before discussing the details, let me get the answer to his question out of the way, "It won't!"
What my friend was talking about is the "multiplier effect." Today's expensive professional level digital single lens reflex cameras (DSLR) have full sized sensors, meaning their sensors have the same size as a 35mm negative. Other professional DSLR's, as well as consumer oriented DSLR cameras have smaller sensors. Nikon calls their smaller sensors, DX sensors and their full size sensors, FX. When comparing lenses mounted on a DSLR with a DX sensor to a 35mm film camera, or a DSLR with an FX sensor we "multiply" the lens' focal length by 1.5, so a 300mm lens becomes a 450mm lens, or does it?
What's really happening is the field of view is being reduced or cropped. Cropping is the process of cutting part of an image out of a photograph to make the final photo. In the case of a Nikon D80 with a DX sensor, with a 300mm lens attached, the effective field of view is that of a 450mm lens. The photo is cropped, not magnified.
On the right is a view of Monument Valley through the lens of a DSLR. Inside the white rectangle is the image picked up by a full size, FX sensor. Inside the yellow rectangle is the image picked up by a smaller, DX sensor.
Note that the DX image is a cropped portion of the FX image. The field of view has been narrowed along both axises, horizontal and vertical. There is less of the scene in the DX image than the FX image.
Look at the two resultant photos on the right, below the lens view. Both are printed to the screen at the same size. The FX image is at the top, and the DX image is on the bottom.
If you examine them carefully you will find that the DX image appears more "magnified" than the FX image. That's happened because to print both photos to the same physical size, the smaller DX image was enlarged, to increase its size to the FX's image size.
This is where I believe the misunderstanding comes from, when discussing the "multiplier effect." Personally I think we would all be served better by eschewing the "multiplier" terminology and changing it to the "crop effect" and the "crop factor." If we used these terms from the beginning, I don't think anyone would think they could get more magnification from a 300mm lens than from a 400mm lens.
Now that we understand the crop (multiplier) effect, I can hear you ask, "So does the crop factor affect travel photography?" Yes it does, according to the situation. It depends on the type of photograph you're taking while traveling.
- If you're taking portraits, wildlife or macro photos then the crop factor won't be an issue.
- If you're taking landscape photos, or architectural photos in a crowded city, or indoor photos, the crop factor will impact your travel photography.
This means that a landscape photographer using a DX DSLR needs to use extremely wide angle lenses to capture the same scenic panoramas as FX cameras. I use a Sigma 10mm-20mm zoom lens for my wide angle landscape photographs. It has the equivalent angle of view to a 15mm-30mm zoom lens which is a wide angle zoom lens specification in anyone's book.
If you've researched typical DSLR DX sensor based camera kits, I'm sure you've noticed that most come with a lens such as an 18mm-55mm zoom, which can provide a good angle of view for landscapes, architectural, and indoor photos. Now you know why.