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| 10-29-2008, 04:38 PM | #46 |
 Originally posted by jeffkrol  You people are reinventing the wheel here........ Granted, it takes some ability with abstract thought to digest... Now sometimes this sort of thing hurts my brain, so I just refer to Julian's Lens Calculator and see what changes affect the DOF and Hyperfocal calculations. Now let me see...change the media size and...what? The CoC changes. Change the CoC and the DOF changes. Well, IBD... Now according to Julian's popup help, CoC is dependent on enlargement, viewing distance, and visual acuity. For convenience, these have been standardized for the various media format sizes assuming standard final image size, visual acuity, and viewing distance. (Read your Wikipedia article a little more closely...it talks about this.) Now maybe Julian does not know his stuff and neither do the folks at DOF Master or any of the hundreds of other calculator Web pages. Ditto for the camera manufacturers who have printed different scales on MF lenses than those used on 35mm lenses of the same focal length. Steve (Steam finally subsiding from ears...) | |
| 10-29-2008, 07:27 PM | #47 |
| Originally posted by stevebrot This topic has been discussed and chewed on and spat out over and over again on this forum and elsewhere. There are adequate resources on the Web and in various reference books. The formulas are published and explanations have been given. Granted, it takes some ability with abstract thought to digest... Now sometimes this sort of thing hurts my brain, so I just refer to Julian's Lens Calculator and see what changes affect the DOF and Hyperfocal calculations. Now let me see...change the media size and...what? The CoC changes. Change the CoC and the DOF changes. Well, IBD... Now according to Julian's popup help, CoC is dependent on enlargement, viewing distance, and visual acuity. For convenience, these have been standardized for the various media format sizes assuming standard final image size, visual acuity, and viewing distance. (Read your Wikipedia article a little more closely...it talks about this.) Now maybe Julian does not know his stuff and neither do the folks at DOF Master or any of the hundreds of other calculator Web pages. Ditto for the camera manufacturers who have printed different scales on MF lenses than those used on 35mm lenses of the same focal length. Steve (Steam finally subsiding from ears...) The common values for CoC may not be applicable if reproduction or viewing conditions differ significantly from those assumed in determining those values. If the photograph will be magnified to a larger size, or viewed at a closer distance, then a smaller CoC will be required. If the photo is printed or displayed using a device, such as a computer monitor, that introduces additional blur or resolution limitation, then a larger CoC may be appropriate since the detectability of blur will be limited by the reproduction medium rather than by human vision; for example, an 8″×10″ image displayed on a CRT may have greater depth of field than an 8″×10″ print of the same photo, due to the CRT display having lower resolution; the CRT image is less sharp overall, and therefore it takes a greater misfocus for a region to appear blurred. Guess we just look at it differently...  Kind of like being able to 1) stand closer or farther to change DOF, 2)Change lenses (effectively standing closer or farther away), 3)changing a display type. 4) viewing distance ect. So in effect the sum of the things you can do to create a specific DOF is less related to sensor size then a plethora of other factors.. Calculators of DOF are based on assumptions which may or may not apply... At least that is how I see it.....Of course if ALL assumptions are identical then the size matters... http://www.largeformatphotography.info/articles/DoFinDepth.pdf p4: Values for full-frame 35 mm images are less consistent: assuming the standard viewing distance and 8× enlargement gives an acceptable CoC of 0.025 mm; however, commonly cited CoCs are 0.025 mm to 0.035 mm, probably representing different assumptions of final-image size. It should be obvious that the choice of CoC is somewhat arbitrary, and dependent on assumed reproduction and viewing conditions. A comparatively large CoC may suffice for a billboard, but a CoC smaller than the standard value will be required if one intends to critically examine small areas of large images at close distances. Many hand-camera lenses incorporate depth-of-field scales to facilitate setting the focus and f-number to obtain the desired depth of field. Depending on how closely the viewing conditions assumed in generating these DoF scales match the actual viewing conditions, the DoF obtained using these scales may or may not be appropriate. For example, some 35 mm camera manufacturers assume only 5× enlargement of the original image when generating DoF scales; if the actual final image is an 8× enlargement, the DoF obtained using the lens DoF scales may be insufficient. Think about it....... Last edited by jeffkrol; 10-29-2008 at 07:39 PM. | |
| 10-29-2008, 09:39 PM | #48 |
| Originally posted by jeffkrol Please explain to me how DOF is ever really comparable when you can, for a intents and purposes, pretty much define it on your own The common values for CoC may not be applicable if reproduction or viewing conditions differ significantly from those assumed in determining those values. If the photograph will be magnified to a larger size, or viewed at a closer distance, then a smaller CoC will be required. If the photo is printed or displayed using a device, such as a computer monitor, that introduces additional blur or resolution limitation, then a larger CoC may be appropriate since the detectability of blur will be limited by the reproduction medium rather than by human vision; for example, an 8″×10″ image displayed on a CRT may have greater depth of field than an 8″×10″ print of the same photo, due to the CRT display having lower resolution; the CRT image is less sharp overall, and therefore it takes a greater misfocus for a region to appear blurred. Guess we just look at it differently... Kind of like being able to 1) stand closer or farther to change DOF, 2)Change lenses (effectively standing closer or farther away), 3)changing a display type. 4) viewing distance ect. So in effect the sum of the things you can do to create a specific DOF is less related to sensor size then a plethora of other factors.. Calculators of DOF are based on assumptions which may or may not apply... At least that is how I see it.....Of course if ALL assumptions are identical then the size matters... http://www.largeformatphotography.info/articles/DoFinDepth.pdf p4: Values for full-frame 35 mm images are less consistent: assuming the standard viewing distance and 8× enlargement gives an acceptable CoC of 0.025 mm; however, commonly cited CoCs are 0.025 mm to 0.035 mm, probably representing different assumptions of final-image size. It should be obvious that the choice of CoC is somewhat arbitrary, and dependent on assumed reproduction and viewing conditions. A comparatively large CoC may suffice for a billboard, but a CoC smaller than the standard value will be required if one intends to critically examine small areas of large images at close distances. Many hand-camera lenses incorporate depth-of-field scales to facilitate setting the focus and f-number to obtain the desired depth of field. Depending on how closely the viewing conditions assumed in generating these DoF scales match the actual viewing conditions, the DoF obtained using these scales may or may not be appropriate. For example, some 35 mm camera manufacturers assume only 5× enlargement of the original image when generating DoF scales; if the actual final image is an 8× enlargement, the DoF obtained using the lens DoF scales may be insufficient. Think about it....... In the most basic terms, the crop factor allows for the same size image at a lower magnification. Steve BTW...you touched on the central assumption that is the most difficult to manage. The calculations assume infinite resolution. | |
| 10-30-2008, 12:30 AM | #49 |
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Do you find this a bit ironic? It must be remembered that these DOF tools assume that a photographer is going to produce a print of a specific size (usually 8" x 10"). Most of us end up printing larger than this at some point. Therefore, many photographers determine the aperture that the DOF tool indicates will produce the desired DOF. Then, they stop down the lens one stop to add a bit of a safety margin. Guess if you have an aps-c you can just eliminate the safety margin...... http://www.ronbigelow.com/articles/depth-of-field-1/depth-of-field-1.htm Depth of Field -- Part II Digital Photography FAQ * DOF decreases with aperture size. Remember, however, that smaller F-stop numbers mean larger apertures. * DOF increases with distance to the subject. This is why you can focus at infinity and get everything beyond a certain distance in focus. This is also why macro shots have such shallow DOF. * DOF increases as focal length decreases. This is why wide angle lenses have deep DOF, but telephoto lenses have shallow DOF. * DOF stays the same or decreases as image sensor (sensor size or film size) decreases. This is the one that confuses a lot of people. Do the following thought experiment: Keep everything about your camera exactly the same, but replace the sensor/film with one that is smaller. The image coming off the lens is exactly the same, so the depth of field cannot change. If you assume that you'll be making enlargements to the same size from two different size originals, then the DOF will be shallower for the smaller original, i.e., more enlargement decreases DOF. The effect of small sensors, such as those found in compact digital cameras, on DOF has been a source of great confusion for many. If we think about sensor size in isolation, then it shouldn't have any effect on DOF since a using a smaller sensor is just like cropping a piece of a larger sensor. However, if we want to do a comparison between different systems then we typically want to compare two images that have the same composition and size. To do this, we will need both a shorter focal length lens, and a bigger enlargement for the system with the smaller sensor. The former increases DOF, while the latter decreases it. The effect of using a shorter focal length lens dominates and you get larger DOF in this comparison. See below. My own way of looking at this: 2 cameras 1)ff @12MP 1) APS-C at 24MP Shoot an image w/ the FF camera that fills the sensor. Shoot the same image w/ the APS-c that fill 50% of the screen, effectively using 12mp out of the 24. Using the exact same lens and f-stop. Now enlarge both images to the same size, say 8x10. Both will have the exact same resolution (12mp upscaled) The APS-c will have to be enlarged 1st to fill the aps-c frame, then to match the FF, then to match the 8x10. Which one will have the "perceived" shallower DOF? Note above: more enlargement decreases DOF... Second note: You would have to be farther away thus increasing DOF in the APS-C photo. I believe the aps-c print will have equal or shallower DOF then the FF yet have the same pixel count, same image............... Don't mind being wrong though. Granted the 2x res increase w/ an aps sensor is extreme but the point is you can't really ever say anything about DOF unless all things are totally equal, which is rare and sensor size IS only one of many considerations except as convenience to those that have used it for eons in FF. AND you have to compare a specific FF camera to a specific APS-C camera. Sorry for rambling. Just trying to get my head around this stuff..... Last edited by jeffkrol; 10-30-2008 at 12:43 PM. | |
| 10-31-2008, 11:50 AM | #50 |
| Originally posted by stevebrot Urban Legend...NOT!! There were several extensive discussions of this issue within the last couple of months here on this forum. I am sorry that I started a discussion I actually tried to avoid in the first place. Yes, it was discussed so many times, and sometimes I actually participated. But not now. I confine myself in saying that with everything said since my original posting, it still holds true. Just one little note... For anybody preferring an online DoF calculator over the actual formulas: Please be aware that you have to adjust 3 parameters when doing the comparison (at constant FoV and physical aperture diameter in mm): focal length, f-stop number, CoC (e.g., multiply all three by the crop-factor when going from APS-C to FF). Larger sensors are believed to lead to narrower DoF because they are typically used with wider glass. Where the urban legend is coming from. The true core in this legend is: wider glass enables narrower DoF. | |
| 10-31-2008, 01:11 PM | #51 |
| Originally posted by falconeye Urban Legend...YES!! I am sorry that I started a discussion I actually tried to avoid in the first place. Yes, it was discussed so many times, and sometimes I actually participated. But not now. I confine myself in saying that with everything said since my original posting, it still holds true. Just one little note... For anybody preferring an online DoF calculator over the actual formulas: Please be aware that you have to adjust 3 parameters when doing the comparison (at constant FoV and physical aperture diameter in mm): focal length, f-stop number, CoC (e.g., multiply all three by the crop-factor when going from APS-C to FF). Larger sensors are believed to lead to narrower DoF because they are typically used with wider glass. Where the urban legend is coming from. The true core in this legend is: wider glass enables narrower DoF. Depth of field: DOF vs. format size - Wikipedia, the free encyclopedia For the less technically inclined: Bob Atkins on DOF and Small-Sensor Digital Steve (The end of my participation as far as this thread hijack is concerned...) | |
| 11-01-2008, 09:19 AM | #52 |
| Originally posted by stevebrot Normalize all factors so that there is only one variable (format)...and... [...](The end of my participation as far as this thread hijack is concerned...) I will terminate my contribution by giving an example with a precise set of parameters for everybody to verify of falsify: APS-C, choose: Pentax K20D (CoC=20µm), focal length = 50mm, f-stop = f/2.4. FF, choose: Canon 5D (CoC=30µm), focal length = 75mm, f-stop = f/3.6. (Why these are the correct choices was the topic of other threads already.) FoV is 27 degrees (horizontally), physical aperture in mm is 20.8 mm ((f=50)/2.4 or (f=75)/3.6). Subject distance, choose: 10 meters. E.g., go to Online Depth of Field Calculator and obtain: Near limit: 8.41 m Far limit: 12.3 m (in either case). So, please, read my lips one more time: DoF (for a given FoV) is a function of the physical diameter in mm of the aperture -- ONLY!Enough said. | |
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