Originally posted by Taylor14 In the article by photo zone, what does "you will basically have to live with an infinite depth-of-field even at max. aperture" mean? Does that mean that because the aperture of the 02 zoom is really equivalent to f/15.4-24.7 that you will not be able to focus on a subject and blur the background. I understand how to use aperture but I don't really understand what the "infinite" depth of field is. If so then the telephoto with the fixed aperture of F2.8 sounds more attractive. Also the telephoto includes a "large aperture that provides the photographer with a number of benefits including a bokeh (out of focus) effect" that is sounding pretty attractive.
Hi Taylor,
DOF is a confusing subject. The statement about the aperture of the 02 zoom being "equivalent to f15.4-24.7 is looking at things from perspective of Image Equivalence that has been promoted by some digital photographers. I feel that though there is validity to this view when comparing some gear in different sensor formats, it would be a lot more useful to mention the deep DOF and then suggest that there might be a way to work around it. For me, one of the more satisfying things about photography is to figure out how to get the images that I want with the gear that I have available to work with, and if I want narrower DOF with a small sensor, I know how I can get it.
For a given image (let's say a person's head filling the frame), you could use a wide FOV lens and get real close, a normal FOV lens from a bit farther away, or an even longer lens from quite a distance. Let's use the Q as an example to give us some numbers to consider.
With a 5mm lens (28mm EQ in 135mm format) I need to be @ 10 inches away from my subject. Using a DOF calculator (
Online Depth of Field Calculator ) I set the camera to "Pentax Q", FL to 5mm, f stop to f2.8, and distance to 10 inches, and I get a DOF of 2.87 inches. That's a lot thinner than infinite, but that's what happens when you are using any lens closer to it's Minimum Focusing Distance. That's right up in someone's face, and would create distorted features (exaggerating features that are closer to the camera), so let's back up.
When I go to 15mm (83mm EQ), I have to move farther away, let's say about 24 inches, and at the same f2.8, the calculated DOF becomes 1.8 inches. That's pretty thin, Most of the facial features will be in focus, but it's still pretty close. You'll be able to get away with background objects that are a bit closer to the subject.
We;re going in the right direction, so if I want even narrower DOF, I go with a 45mm lens, use the same f2.8, and back up to about 5 ft. Now the calculated DOF is a pretty thin 1.24 inches. At this DOF, you can pretty easily keep the tip of the nose as well as the eyes in sharp focus, plus everything in between. Certainly, this is not paper thin, but you can certainly get good subject isolation. If this is not enough control, or the change in perspective is not desirable, or if the working distance is too great, then you need a different combination of lens and camera to get what you want, or you'll have to compromise something if this is all you have at the time.
The distances here are approximations to get about the same framing, I just used my Q with the 02 zoom at both ends and a 50mm lens with a K to Q adapter and roughly framed my mannequin head that i use for testing lighting, then approximated the working distance. Please don't nitpick the test, the principle works.
The bottom line is, with a smaller sensor, if you want thinner DOF, then get a faster lens or shoot with a sufficiently fast longer lens and step back. This, of course, means that you will be shooting at longer distances, which changes perspective, but that's a different discussion. . In any case,.I usually find that most people prefer their protruding features flattened as opposed to exaggerated, and they are a lot more comfortable being shot from a distance as opposed to having a camera shoved in their face. If the DOF in a shot that I cannot reshoot is too great, I can blur the background in PP. If it's too thin, it's quite a bit harder to sharpen the Out of Focus areas. If there's a margin for error, I'll usually take too much DOF over not enough.
There are, of course, a lot of situations where we want deeper DOF, like macro where we never seem to get enough, and super tele at close distances where it's also usually way too thin, and that's where the Q shines, both with its extreme crop factor and deeper DOF..Ultra wide with a significant foreground object is another type of shot where deeper DOF is what you're usually looking for. The Q can accomplish all of these with the advantage that they can be done in lower light situations (because you can use larger apertures) than the larger formats without the need for a tripod, and that only emphasizes the small system concept.
Scott