[nostatic]

Assume a spherical cow...

[lunelson]

Right -- what I was trying to get at is this question: Will a 50mm lens on a 1.5x crop sensor give the same image as a 75mm lens on a full-frame sensor? I know the field of view will be the same due to the crop factor -- how about DoF?

[MattGunn]

Are FOV and Focal length hopelessly connected?

The Field Of View (FOV) is dependant on the focal length (f) of the lens, the dimensions of the sensor (x) and the distance to the object (O). This can be expressed mathematically as:

FOV=x*O / f

and so if the dimension of the sensor is reduced and the focal length and object distance remain the same, then FOV will reduce proportionately. A longer focal length lens at the same object distance will also reduce the FOV

Would a picture of the same subject, at the same distance, taken with a 50mm lens at f/4 on the 1.5x camera look exactly the same as a picture taken with a 75mm lens at f/4 on the full frame camera? Or would there be some difference in DoF, distortion, or something else?

Pictures taken at a fixed distance of the same object with a 35mm camera and 75mm lens, and an APS_C camera with a 50mm lens would have roughly the same field of view. The Depth of Field is dependant on many things including how big you intend to view the final image, the lens focal length, object distance, sensor resolution etc and so with both lenses set to the same aperture the DoF may not be identical. Distortion in the image is due to the lens and so could well be different.

Edit: An APS-C sensor has approximately a 1.5X crop factor

[Marc Sabatella]

Anyhoo, long story short, if I want to use the 50/1.7 (or any other "film" lenses) on a DSLR, I shouldn't expect the images to look different in terms of FOV or magnification than they would on film. But, because of the sensor size compared to a 35 mm film cell, the digital image may be less sharp (with all other factors - shutter speed, EV compensation, aperture, etc. - being equal).

No. The fact that the sensor is smaller means the image is cropped, which in turn means the FOV is narrower than it would have been on 35mm film - *exactly* as if you shot with a 75mm lens. Whether or not it is less sharp or not depends on which 50mm and which 74mm lens you are comparing, which film and which digital sensor you are using for the comparison, etc. There's basically nothing at all you can conclude about sharpness just based on the crop factor.

I guess the only other question is what to do with the aperture ring. A few of the lenses I have do have the "A" setting that the K2000 manual mentions, but not the 50/1.7 or the 40/2.8 ("pancake").

See the sticky thread in the beginners forum on how to use manual lenses with a DSLR.

[Marc Sabatella]

Thanks for clarifying Marc, but there has to be an element of magnification involved.

Of course there is. I'm simply pointing out that without also know the particular resolution of the lenses you are comapring and of the film and sensor you you comparing with, there isn't anything you can conclude about the matter. For instance, let's take your example:

for example the human eye has an almost 180 degree FOV, changing that FOV to say 60 degrees will not make our eyes resolve at a greater distance just restrict the amount of information our eyes see.

Correct. But now let's say that a given specimen of one species naturally sees 180 degrees and a given specimen of another species naturally sees 60 degrees, because that's how their eyes are designed. Which sees with greater resolution? It's impossible to say just from the difference in FOV - there are too many *other* variables involved when comparing between different species. And if you try to compare a given specific 50mm lens on a given APS-C digital sensor against a hypothetical 75mm lens on a given type of 35mm film, that's the equivalent of comparing two different species. We really don't know which will have more resolution. So there's really no point in talking about it. All we know for sure is that the FOV is narrower when using a 50mm lens on APS-C than when using it on 35mm film.

Are FOV and Focal length hopelessly connected?

Yes. For a given sensor/film size, they are directly proportional. Or, stated another way, for a given focal length, sensor/film size and FOV are directly proportional.

[Lazaruscomeout]

Somebody, please correct me if I'm wrong, but this is how I understand it.

If I have a specific subject that I want to capture from a set distance with the same field of view, ie: a flower filling the frame from 4 feet. A 50mm lens on a 1.5 crop DSLR at an aperture of 2.8 will create an identical image including field of view and depth of field as a 75mm lens on a full frame at an aperture of 2.8.

The only way to change depth of field at a given aperture is by changing my physical distance to the subject. So a 50mm lens on a crop DSLR will give a different depth of field than a 50mm on a full frame if I want to achieve the same field of view, because I have to move physically closer to the subject with the full frame.