Hey there
Originally posted by Ploki - that means the effective focal length for old lenses is whatever is written on the lens x 1.5.
No! Stop right there!
The focal length is a lens property and the medium (as in, sensor or film) does not affect it!
The only thing is that the crop sensor is effectively a smaller canvas, a smaller window, so the field of view is smaller. The edges get cut off, and effectively the lens appears to be less "wide."
Equivalence using that multiplier is only important if you use a film/FF camera and a crop camera, and you want the exact same field of view. Keep in mind that using different focal length lenses means that even if the field of view is the same, there will be different depth of field and distortion and space compression.
Originally posted by Ploki 35mm x crop factor won't look the same as 50mm despite the fact that image should technically look the same size.
You are right that they will not look the same. But you are wrong about the "technically" part. Equivalence means nothing, it doesnt exist, it is a made up concept. It merely helps a photographer visualize the field of view of a certain lens on a certain camera. If I tell you to think of a 25mm lens on the 645D or a 5mm lens on the Q7, you won't know what field of view you get. But if you use equivalence and multiply it, you will be able to compare it to the lens on your APS-C camera and easily visualize it. Of course, the field of view will be the only thing. Actual rendering could be quite different.
Originally posted by Ploki That would mean APS-c actually brings nothing good to the table at all, at least I can't see it.
Well, some people would agree with you, and they want a FF (or bigger!). But crop sensor is much easier to produce (costs less), takes less space, allows for more compact camera designs and you can put more electronics in there (like in body image stabilization). And some people like that, since it usually has bigger pixel density, it can achieve higher apparent magnification or make lenses appear longer (more tele). So this can be useful for some macro and wildlife photography, as well as for hikers.
Originally posted by Ploki Why didn't they bring the lens closer to the sensor so it wouldn't crop?
There is a whole science of optics. The distance between the lens and the sensor is precisely defined, and you cannot simply move that focal plane. The register distance is defined by the K-mount specifications. The other problem is the mirror, which needs some space between the lens and sensor. This is why only mirrorless cameras can have a lens mounted very close to the sensor. So if you want to change the register, you might as well change the whole mount. And putting the lens "closer" does not increase/decrease the image circle, but it would mess up the focusing and make everything less bright. Its a complicated issue
Originally posted by Ploki Are the lenses made for APS-C (that show a circle crop on a full frame) designated with crop factor being taken into account, meaning that 50mm for APS-c would technically not be the same as a normal 50mm?
No, a 50mm lens from the 1970s made for film will show the exact same field of view as the DA 50mm, on the same camera. Focal length is focal length, serious photography brands do not write "equivalences" on the lens, but the actual physics. But if you mount a crop-only lens on a bigger sensor (like full frame or medium format), it might produce an image circle that is too small, and you will get dark edges. But again, do not buy an older/FA 50mm expecting it to behave differently than your new 50mm. The focal length is the same. And if you use it on the same medium/camera, the field of view will be the same.
Edit: Basically, the problem is that 35mm film became the standard and now all other mediums are judged by it. Crop sensor is not really "worse" but it is "different." But I am sure some will disagree and say that bigger is better. Ive edited the post instead of doubleposting, so there might be some more text since you read it.
And thanks for looking at my 500px page