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Forum: Pentax SLR Lens Discussion
07-24-2015, 12:42 PM
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1) yes
2) No. But are there any particular reason why they should? Lenses for different formats are never truly DOF equivalent as DOF is dependent on focal length, aperture, focus distance and subject magnification.
3) No. And that's why some use MF cameras. They don't use them in order to be "equivalent" to a smaller format.
And this is the point of various formats. Equalize, to the extent it is possible at all, a cellphone with a 645 (to make the principle extreme) makes only sense for those who doesn't grasp what this is about.
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Forum: Pentax SLR Lens Discussion
07-15-2015, 06:25 AM
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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 ----------
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.....
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Forum: Pentax SLR Lens Discussion
07-11-2015, 02:24 PM
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True*
And what you choose of this is highly subjective and will change from shot to shot. Hence there's no universal law of equivalence to be had except for lens speed. ie 1.4 = 1.4. You can't get away from that one. It is also the industry standard since the invention of photography. For a reason.
*(except that ponit 3 above is only true as long as you are within the subject magnification available on FF; APS has larger magnification and hence thinner minimum DOF at the same aperture and angle of view for comparable lenses).
---------- Post added 07-11-15 at 11:29 PM ----------
Lenses doesn't get equivalent by cranking up the ISO. ISO is a property of the sensor, not the lens. Cranking up ISO or not is a subjective decision. Which one of the parametres of reciprocity you use while using different formats is totally subjective. Hence you cannot derive some objective principles from it. That is, if you're a photographer.
If you do follow the "principle of eqiuvalence" you are loosing most the advantage of FF. You been better stickin to a smaller format.
I've used different formats and I don't crank up the ISO unless I have to.
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