Originally posted by Cuthbert Thank you very much, I'm a mechanical engineer, I design cars and I know nothing about laws of physics, it's fine to find knowledgeable like you guys can can lecture me on the topic "it doesn't matter the design of a lens, number of elements etc... they all have the same DOF."...I wonder where you got your degrees, if you had any.
However, the bokeh pic is for you!
A 135mm lens is a 135mm lens is a 135mm lens. It is designed to project a set value of magnification on a subject, regardless of how its built. If it projects a wider or lesser amount of magnification
it is no longer a 135mm lens.
This is why you can have a 35mm ltd which is tiny and weighs next to nothing, or a Samyang 35mm which is built like a tank and probably weighs almost as much. Just because both are made differently, they're still both 35mm lenses because they both project a certain specific amount of magnification upon a sensor/bit of film/whatever. If they magnified more, they would be a longer focal length. If they magnified less, they would be a shorter focal length. This is Photography 101 stuff. The depth of field and all that stuff carries over due to that 35mm length and will not vary. It won't vary because if the lens is built exactly to specs and is exactly 35mm (or 135mm, or whatever) the depth of field can then be predicted as a result because of the math involved.
Of course this isn't the case, because lenses vary in focal length depending on design. That Pentax 50mm may actually be a 49mm. The Canon may be a 51.5mm. The Nikon could be a 49.5mm. All three will be stamped as a 50mm of course because thats what they (mostly) are to the casual photographer.
That's partly where the variance comes into play. The other comes when they measure the aperture values. Pentax's lens could actually be f/1.65, Canon's f/1.75, and Nikon's f/1.775. All three will stamp them as f/1.7 - again - for the ease of labeling and the fact that the difference isn't enough to quibble about.
The basic math involved isn't going to change. If you could increase or decrease depth of field by fiddling with the optics in a lens, it would have been done ages ago and would have been advertised as such. Of course this never happened because when you start fiddling with the optics you are either going to change the focal length or the aperture qualities of the lens or both.
Depth of field dependence on focal length is a constant. Its the fact that lenses may be slightly longer or shorter and have differently measured aperture values compared to whats stamped on them that changes things up and muddies the figures.
This is also why the depth of field is going to stay the same no matter what lens you're using.
A 28mm at f/4 will have the same DoF as a 50mm at f/4 as a 135mm at f/4 as a 300mm at f/4 (provided of course the lenses are actually the focal length advertised and the aperture values are measured to spec). The only difference is going to be how close or far from the subject you need to be to have it the same size in-frame for the camera.