Question about 1.5x crop factor

Joined: 14 Jul 2004 Posts: 375 Location: Georgia

I realize that there is a 1.5 x factor for all nikon lenses on digital cameras but does this mean that i cant get a 10.5 mm lense? it would be a 15.75? Ive got a 50 mm 1.8 but its really just a 75 mm lense on my digital camera. Do they make lenses that are the true focal length on digital cameras?

Joined: 09 Apr 2006 Posts: 199 Location: Worcester, England

southernsky wrote:

Do they make lenses that are the true focal length on digital cameras?

All lenses are quoted as their true focal length.

southernsky wrote:

I realize that there is a 1.5 x factor for all Nikon lenses on digital cameras but does this mean that i cant get a 10.5 mm lens? it would be a 15.75?

A 10.5mm lens is a 10.5 mm lens is a 10.5mm lens.
It has the same field of view on a 1.5x crop camera as a 15.75mm would have on a 35mm (or full frame) camera.
The focal length of the lens does not change dependent on which form factor the camera has that is attached to. The only thing that changes is he Field of View ( or magnification in simplistic terms)

Joined: 18 Nov 2006 Posts: 288

To add to that, note that the focal lengh (more properly called the back focal length) is the distance between the vertex of the rear element and the focal plane (sensor) when the lens is focused to infinity. That is a distance measurement and has nothing to do with the camera in which the lens is used. The angle of view changes depending on the size of the sensor or film and the angle of coverage will change based on the design of the lens.

So, as an example, a 50mm lens is a wide angle on medium format, a normal lens on 35mm format and a modest telephoto on an APS digital format. All would be 50mm but the designer would need to be sure that he had enough field of view or coverage angle to fill the frame. That's why a 50mm wide angle for a medium format camera is larger than a 50mm normal lens for a 35mm camera.

The confusion stems from people talking about "equivalency" of one focal length from one format to the other. They should talk about the equivalency of the angle of view instead. It would be more accurate.

Joined: 18 Nov 2006 Posts: 288

I should also explain the angle of coverage a little. We often refer to it as the size of the image circle since, indeed, a lens always projects a circular image. As the focal length increases, subjects are magnified and as it decreases, the reverse is true. That 50mm lens will provide the same subject "size" on any format. The difference is that, on the wide angle medium format lens, you will see more of the surrounding area around the subject than you would with the normal 50mm on the 35mm format. On the APS digital you would see less of it. The surrounding area would be cropped and that is where the term crop factor (an unfortunate term in my opinion) came from.

The 10.5mm lens you are talking about has an angle of coverage designed for APS format. If you were to mount that lens on a 35mm camera, it wouldn't fill the entire frame because the angle of coverage would be too narrow. I guess you could refer to that as a reverse crop factor. :)

Joined: 14 Jul 2004 Posts: 375 Location: Georgia

Ok so I understand that it is not that the lense focal length is changing but my problem is that a 10.5mm lense on a digital body "looks" like a 15.75mm on a full frame camera. Im also wondering about the digital only lenses by nikon. Are these also going to give me a 15.75mm "look" due to the digital body?

Joined: 18 Nov 2006 Posts: 288

southernsky wrote:

Ok so I understand that it is not that the lense focal length is changing but my problem is that a 10.5mm lense on a digital body "looks" like a 15.75mm on a full frame camera. Im also wondering about the digital only lenses by nikon. Are these also going to give me a 15.75mm "look" due to the digital body?

Let me explain again. 10.5mm looks like 10.5mm. 15mm looks like 15mm. The 10.5mm lens is a "digital only" lens. That means that the image circle or angle of coverage is designed for APS format, not for 35mm format. If you were to mount it to a 35mm camera, it would not fill the entire frame. It would leave the corners black and, probably, the left and right edges of the frame.

The 10.5mm lens on an APS digital format is what we call a full frame fisheye. That means that it has a 180 degree angle of view across the bias rather than across the long axis. It does that on an APS digital format (any Nikon digital.) There is no crop factor because the lens is not designed to be used on a 35mm camera. A 15mm lens designed for a 35mm format will have the same subject size when used on a digital. The digital frame will crop the image so that you will see less of the area surrounding the central subject. That doesn't change the focal length. It changes the angle of view. If you put a 15mm lens designed for APS digital on a 35mm camera, the subject size will still be the same. But the lens will not cover the 35mm frame because the image circle is too small.

Using 35mm format lenses on digital works but crops the image. Using digital format lenses on a 35mm doesn't work because it doesn't fill the frame.

I've probably confused you enough. I'll let you think about it.

Joined: 14 Jul 2004 Posts: 375 Location: Georgia

Alright this will have to sink in but thanks for putting so much of your time in.