Originally posted by starbase218 My wants are different, but that wasn't really my point. My point is, once you put a lens on a smaller sensor camera than the one it was intended for, you ARE losing f-stops relative to that camera. It's math, and it's caused by all the light falling not on the sensor, but beside it.
Wrong.
---------- Post added 07-23-15 at 12:58 AM ----------
Originally posted by surfar my K50 has 1.5 crop factor....so does that mean my DA/L lenses(designed for aspc)....are the correct F designated speeds?.....and my FF designed lenses are actually not those spEeds on the k50?....PLEASE EXPLAIN?
Aaargh this equivalence nonsense.
All you lenses have correct designated speed. Everyone saying that an F:4 lens is equivalent to an F:5.6 lens is talking nonsense as F:stops are defined in a manner that make exposure constant. Anyone why bring DOF into this simply does not understand the theory of exposure. DOF has no more to do with this than subject blur has to do with shutter speed. No one would define shutter speed for subject motion....
A lens have the same speed regardless of what format you use it on.
When you use different formats but the same angle of view you must use lenses with different focal lengths. Bigger formats therefore means longer shutter speeds at the same DOF than smaller formats. The problem starts when some insist that lenses must have the same DOF wide open at medium focusing distances, and not at all at shorter focusing distances. Without, of course, explaining why.
Sensor size is immaterial when it comes to a lens aperture; aperture is a property of the lens. Hence, the concept of equivalent lenses is meaningless as long as it demands different external factors.
It is comparing apple to oranges
---------- Post added 07-15-15 at 03:30 PM ----------
Here's something I've written before on the subject.
The 2.8 is not defined by DOF but exposure. What makes it nonsense is not understanding this simple point. DOF doesn't enter its definition and therefore shouldn't be used as marker for equivalency.
The law of reciprocity gives freedom from what parts that make up the exposure you want to change due to changing conditions to achieve the same goal exposurewise. Note that reciprocity does not "care" about DOF or motion blur or image noise; only exposure. This freedom mean that it makes no sense insisting on constant DOF in the image in order to compare lenses. It is possible to do that, sure, but that just an opinion; it makes no sense making into a rule because different subjects and circumstances demand different solutions, and besides, DOF equivalent lenses cross formats only exist in theory - not in real life. No one shoots after the "law of DOF wide open equivalency at moderate distances" anyway either. You do not shoot your FF camera at 200ISO so that it is equivalent to an APS body. It is not mandatory for an APS shooter to choose his/hers lenses from FF DOF wide open and shoot accordingly. Most will use fast lenses in order to use their cameras to their best advantages in low light regardless of format. In this way you maintain the benefit of your larger sensor camera.
The only thing that you can't get away from is the lens speed; at 1.4 at a certain light level and at a certain sensitivity will give you a certain shutterspeed regardless of format. This is the only equivalency there is.....