Originally posted by shardulm The thing that throws me off with this calculation is that.. the focal length spec and FOV are disjoint if what you say is true. So an FA 77 will focus @77mm regardless of crop or not and is specified so. A 50-135 will focus in that range crop or not right? But FOV for 70-200 and 50-135 is similar when put on respective bodies but both lenses are different focal length spec. Or is it that this 50-135 is really a 70-200 but with a smaller image circle and hence specified so for crop bodies?
Please enlighten.
Gosh - there are a lot of articles about this but I will try.
First putting both the DA* 50-135 and the FA 77 Limited at 77mm and using them on the K-3 you get the same image in the viewfinder as far as the angle of coverage and the magnification seen. This is assuming no focus breathing (another topic) etc. but in all practical ways the images are identical. If you then mount the two on the K-1 and do the same test the FA 77 - shows the same part of the image inside the crop frame - but now shows more outside of it in clear and acceptable focus and quality. The DA* 50-135 still at 77mm will show the same image inside the crop frame but outside of that it will severely vignette and or show a black area where the lens does not provide adequate coverage of the Full Frame image circle.
So both project at a 77mm focal length - but one has a limitation that it only projects a smaller diameter circle that can't fully cover the larger sensor. The objects in the circle that it is projecting are the same size and orientation as the FA 77 makes inside that same circle. The difference is that the FA 77 has a larger circle - including things outside the circle that the DA* 50-135 can project. This larger circle is totally ignored on the APSC camera - it's just wasted hitting the internal walls of the camera's shutterbox.
Another way to conceptualize it is that you are looking out of your house onto a scene through a window. Someone comes along and masks off part of the window with plywood. Your view is reduced of the outside world. You can't see all the same stuff but the stuff you see hasn't changed. The smaller masked off window is APSC, remove the extra plywood and restore your entire window and now you have Full frame. Now imagine that your plywood was removed but there are bushes in the way that block the view - that's like putting an APSC (DA) lens on the camera - you can't get a good view of the area outside of the APSC image circle in most cases - a few lenses are technically APSC but have been found to be quite good or at least acceptable in the Full Frame area. The DA 70 is one of these as is the DA 40. The DA* 60-250 when modified by the removal of a baffle in the rear of the lens seems to work very well on Full frame also.
So taking this back to the angle of view - all lenses have a focal length. That focal length is a strong predictor of the actual field of view for a given sensor size. This is fixed and a property of the lens. The image circle that the lens covers tells you what size sensor you can use with the lens. Too small and the edges won't get any light or will get distorted light.
Take a 300mm designed for a Hassleblad or an 8x10 camera. Put that on the APSC camera - compare that shot to the shot from the DA* 300 - they should be the same roughly. Then try to use the DA* 300 on the large 8x10 camera - a large part of the sensor (film or digital) will be outside the image circle but the central portion will look the same as if you were using the large format lens.
Does this help? If not there are articles that have been written that do a better job that I can do. The important thing to recall is focal length is not changed by the camera sensor size; sensor size affects what you see with the lens; lenses that project smaller image circles can't provide the full view on the larger sensor; and it is dang confusing to discuss.