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Forum: Pentax SLR Lens Discussion
08-17-2017, 05:31 PM
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and just to continue the thought, this ties into my post above - the 70 as a longer lens has a shallower depth of field than the 50, so the "blur", the areas going out of focus, will be more pronounced sooner, and as you move further or closer from/to your subject, you will increase or decrease the depth of field, respectively.
I think what makes this challenging is the number of variables involved and tracking how changing just one component can have a trickle down effect that's greater than what is first anticipated.
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Forum: Pentax SLR Lens Discussion
08-16-2017, 12:14 PM
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OK, let's just look at this from an optical standpoint with the lens and subject and aperture all at the same location/same setting. If you were shooting on the K1 in crop mode with the 70, an APS-C lens, the optical viewfinder shows a black outline of the crop image size, but overall, you see the full frame, and depending on the image circle of the lens, you may or may not have vignetting. You can change the mode of the K1 from crop to full frame. When you do this, the black outline goes away. Now, did anything come into or go out of focus for doing that? No. That's because optically, we made no change. Let's say we focused on the subject's eyes, and the tip of the nose was out of focus and the back of the ear lobes were out of focus. Did changing it from crop to full frame mode suddenly make these go in or out of focus? No. It just changed how much of the image circle was captured. To continue with this thought, let's assume the 70's image circle was big enough to fill the full frame (don't think it is, but let's pretend for our example), and you had it on a K1, but you wanted the equivalent framing of being on a crop body to fill the full frame view, you'd have to get closer to the subject to do so. When you move closer to a subject with any lens, the depth of field will get shallower. Again, this has nothing to do with sensor size, but just optics. The closer a lens is to the subject, the thinner the plane of focus. So moving closer to the subject to have the exact same framing will make the DoF shallower or thinner because of the optics of the lens, not the sensor size. Now maybe the cheeks and chin aren't as crisp as the eyes you were focusing on, but when further back, they might have looked pretty good. Now, this is where we get into another knot. As was mentioned right after my first post, Some view DoF as what was defined as sharp on an 8x10 print from 35mm film. This is a subjective application of measurement. It was decided that a point that still looked to have a certain level of fineness at a certain level of englargement as being in focus would be the front/back of your depth of field. Is that point really as sharp as the exact plane of focus? No. Will making the print bigger reveal the focus change more obviously? Yes. So when you start looking at different size prints from different formats, you start introducing other subjective perceptions of what was in focus or out of focus. The focus of the light converging on the plane of film or sensor hasn't changed by printing it bigger or smaller, but what the human eye can see and what any one person might define as acceptably sharp or in focus is affected, and here's where I think we go back down the rabbit hole and get all wrapped around the axle on DoF changing when changing from crop to FF. And finally, you mention changing from a 70 to a 50 to maintain a field of view. Well, changing the optics will change the DoF - longer lenses have a shallower range of focus in front/behind the exact plane of focus than wider lenses, so swapping the 50 for the 70 will bring in a whole new set of optical characteristics that are unrelated to the sensor size, but again, how much you enlarge it etc. will determine what you perceive as in focus and the DoF.
Hope this is making sense.
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Forum: Pentax SLR Lens Discussion
08-14-2017, 04:00 AM
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I think there's a lot of confusion regarding DOF on FF versus crop. The DOF of a specific lens at a certain aperature at a certain distance from the subject is going to be the same regardless of the image sensor size. Where I think people get hung up on this is if you're moving the lens forward or backwards to frame the same area of the subject to account for the difference in what you see framed with FF versus crop, you will change the DOF in the process. If you stay in one position and change lenses to have the same framing or field of view going between FF and crop, you'll change the perception of DOF because you've changed the lens, and the DOF of the different focal lengths will be different. So while the actual DOF of an object 10 feet away for a 70mm at 2.4 will always be the same regardless of the sensor size, if you start changing the distance of the lens to the subject - get closer with the 70 on FF to get the same field of view as the 70 on crop let's say, then you'll "change" the perception of DOF, and this case, it will get "narrower".
Hope I said that in a way that makes sense ;)
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