DX lens on a DX body. Will crop factor apply?
I want the answers a simple to understand as possible. Here's my doubt.
I have a Nikon D5100 and that's a DX camera. I have a 18-55 mm DX Lens. So will this DX lens on D5100 equivalent to 27-82.5mm? Or the same 18-5mm because it's a DX lens?
Will the crop factor apply to any lens used on D5100 whether it's DX or FX.
Your question is not totally clear, but then the issue of crop factors is something that the camera manufacturers have made FAR more complicated than necessary.
The focal length of a lens is what it is. So if you have an 18-55mm lens, that is the focal length regardless of what camera it is used with. The focal length is a measurable dimension and not subject to manufacturer's rating games.
Crop factor is a concept that comes about when the manufacturer of the camera uses a sensor that is physically smaller than the standard 35mm film frame.
When you are using film, the lens projects an image on the film frame that is based on the design of the camera and lens combination. In the case of 35mm, that image is a circle that is about 44-45mm in diameter. Now, if you choose to use only a portion of that image, you have 'cropped' the image. You can also use lens with a longer focal length that will project a 44-45mm circle but that contains exactly the same image. Therefore, using a longer focal length image can effective lead go cropping of the image.
What is done with digital cameras is to use a sensor that is physically smaller than the 24x36mm dimensions of the standard 35mm film frame, but then couple that with use of a SHORTER focal length lens so that you capture the same image on that smaller sensor that you would capture on film using a longer lens.
That is, the 18 - 55mm lens that is furnished with Nikon DX digital cameras such as the D5100 or the D100 will create the same image on the sensor in those digital cameras as a 27 - 82.5mm lens on a 35mm film camera.
The absolute focal length is 18-55mm.
This focal lengths together with the DX sensor gives you the same perspective (angle of view) as 27 - 82, 5 lens would give you on a larger FX sensor (or on 35mm film).
The 35mm (FX) equivalent (27 - 82, 5 with your lens) is often mentioned with those lenses because the 35mm equivalents are still much more in our consciousness and most SLR photographers know what to expect.
So the crop factor does not depend on the lens but on the size of the sensor. So while your lens is a wide angle to short tele (portrait) lens on your camera (DX) on a camera with a full sized sensor = 24x36mm like the FX sensor your lens would be an extreme wide angle to normal lens.
It will be "equivalent" to a 27-83mm lens on an FX body.
Or, you can just know what field of view an 18-55mm lens gives you on your DX camera, and go with that. That's all "crop factor" refers to, the field of view a lens of a certain focal length gives on a sensor of a particular size.
No matter if the lens is built for DX or FX the crop factor will apply on your camera or any other aps-c sensor camera.
The crop factor will reduce the angle of view of the lens, giving you the angle of view of different focal length, it will also make your image look bigger because of the magnification factor of the crop sensor but the focal itself will remain. 18mm will always be 18mm no matter if you're on a FX or aps-c sensor camera.
Look at these example.
The angle of view of the 55-200mm (DX lens) at 200mm is 8°.
The angle of view of the 70-200mm (Fx lens) at 200mm on a crop sensor camera is 8° while mount on the D800 FX camera the angle of view will be 12°.
If you add the crop factor 1.5x to the 55-200mm at 200mm we get 300mm.
Now let's look at the Nikon 300mm f/2, 8 (FX). The angle of view is 8° when mount on a full frame body.
The 55-200mm at 200mm will be the equivalent of using the 300mm on a full frame camera, you will get the same reach.
Although the focal length remains the same, your 18-55mm zoom will have the equivalent angle of view of a 27-82.5mm lens on a 35mm film camera or full-frame DSLR. So you were correct in your math.
Yes the 1.5x crop factor will apply to any lens used on your D5100.