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October 11th, 2009, 11:10 AM | #1 |
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Smaller sensors do not have deeper DOF
Before I start, let me clarify the subject line. When smaller sensors are used at the same f-number (e.g. f/2.8), they do indeed have deeper DOF. But large formats can always stop down to get the same DOF, so small sensor are not capable of deeper DOF than the large sensor, and that is what I mean by the subject line. Now, on to the meat.
When comparing two different sensor sizes, it makes sense to assume that all the other variables will be equal:
It so happens that the 5D2 and 7D have very similar technology, so it's very easy to compare them in these conditions. In ample light, the large sensor is capable of:
In low light, the large sensor is capable of:
General conclusions:
If you shoot in low light and can not handle thinner DOF, then the large sensor will be more expensive with no noise benefit. (There may be other benefits, such as contrast.) But at least it will be able to do everything the small one did, including the same DOF. On the other hand, if you shoot in ample light or you can handle thinner DOF, then the large sensor will provide a benefit. For example, the 7D with 50mm 1.8 ISO 160 has the same DOF and noise as 80mm f/2.9 ISO 400. This is easy to prove to yourself with a simple experiment using any raw camera, such as a DSLR. Take picture A with whatever settings you want. It will simulate the smaller format. For example:
Then decide on the crop factor that you want to simulate. For example, 5D2 -> 7D is a crop factor of 1.6X. And take a second picture with the same camera in the same position with the same focus distance and same lighting:
Now you have one picture at 50mm f/2.8 ISO 100 and another at 80mm f/4.5 ISO 250. Now:
What you will find is that they are the same. Again, this simple experiment can be done by anyone with a single raw digital camera. The sensor technology is the exact same, so we know that isn't a factor. One way to look at it is to consider just two variables: sensor size and f-number. 1. Sensor size:
2. F-number:
Here's what I think happens when one or both of the factors are changed in some way:
F-number scales with sensor size. What about lens weight? This one is much trickier, because every lens design tends to be unique. But if you if assume the exact same lens design, then you find that larger formats do not, in fact, have heavier lenses. For example, compare the 300mm f/2 lens on Nikon FX (FF35), which has the same angle of view as 200mm f/2 on Nikon APS-C (~S35):
Then consider that you only need 300mm f/3 to get the same DOF, diffraction, and light gathering power as the 200mm f/2 on ASP-C. The 300 f/2.8 has the same weight! Here's another example, again with Nikon (because their crop factor of 1.5X just happens to align very closely with their lens selection):
Here we see it is 10% heavier, but not significantly. (The difference may be due in part to the fact that the 600mm only needs to be f/4.2, not f/4.0, to get the same DOF, light, diffraction, etc.) The reason why I'm comparing these expensive superteles is because they have optical designs that are similar. When you compare other focal lengths, it is very hard to find a lens in one format (e.g. APS-C) that has the same design (just scaled up) for another format (e.g. FF). It's true, of course, that larger-format lenses *tend* to be heavier, but that's because they tend to have the same f-number. And as established, they don't need to have the same f-number in order to get the same results. How what happens when the conditions/assumptions vary?
The same scene should always be given. There are many times when perspective cannot be changed, such as when a cliff prevents forward movement or a wall prevents backward movement. Other times, it is possible to change the perspective, but it is undesirable for artistic reasons (e.g. distortion/compression). In any case, changing perspective is like changing angle of view, scene, or lighting: it's a fundamental element of composition that must be kept constant for any comparison to make sense.
The field of view is just as important in composition as perspective, and can't be compensated other ways for the same reasons.
Modern sensors have been within 1/3 stop of the same sensitivity for the last few years. Read noise, on the other hand, has more variety among manufacturers and models. Generally, the smaller the sensor, the less read noise per area (at low ISO). The same is true for FWC. So the amount that a given sensor is better in this area will change the results, even as much as 1/3 stop just for a lower read noise.
Raw recording is important, because underexposure only works if response is linear (or curve is well tuned). A nonlinear response (like film) will lose more than 4 stops of information if it is 4 stops underexposed, because it is nonlinear. The same processing is pretty obvious, as sharpening, demosiac, etc. can have a big effect on DOF.
If a sensor has larger capture resolution, and displayed at a larger size and resolution, then it will be capable of thinner DOF. For that reason, a smaller sensor with the same aperture and higher resolution actually has thinner DOF than a larger sensor. The reason why this is true essentially comes down to apparent iris diameter. (I am not using the word "aperture" because it is often confused with f-number.) When the per-area performance of the sensor is the same (as in assumptions above and many real life situations), then the DOF, light gathering ability, and noise all depends on only one thing: the aperture of the lens. The iris diameter of 32mm f/1.2 is 26.6mm. The iris diameter of 50mm f/2.0 is 25mm. They both provide the same FOV on S35 (e.g. 7D) and FF35 (e.g. 5D2), respectively, and the DOF is the same because the physical aperture is also the same (~25mm). Light gathering ability and noise, too, are the same: one focuses the light in a smaller space with more intensity, the other spreads it out over a larger space with less intensity. In either case, the total amount of light is the same. This is true of all formats when given the above assumptions. From 1/3", 2/3", S35, FF35, 645, etc. Larger sensors get the same DOF/light/noise by through longer focal lengths, narrower f/numbers, and smaller reproduction ratios. Smaller formats have the same DOF/light/noise through shorter focal lengths, wider f/numbers, and larger reproduction ratios. For example, with a 16.4-foot subject distance, all of the following camera/lens combinations will have the same 40 degree horizontal AOV and 6.7 feet DOF (using h/CoC=1200): Code:
2/3": 14mm f/0.9 4/3": 24mm f/1.6 Super35: 34mm f/2.2 Still FF35: 49mm f/3.2 645: 76mm f/5.0 617: 220mm f/14 Code:
2/3": 15mm iris diameter, 14mm focal length 4/3": 15mm iris diameter, 24mm focal length Super35: 15mm iris diameter, 34mm focal length Still FF35: 15mm iris diameter, 49mm focal length 645: 15mm iris diameter, 76mm focal length 617: 15mm iris diameter, 220mm focal length The f/number is inversely proportional to the reproduction ratio for any amount of DOF or diffraction for any sensor size. So as you can see, the iris diameter is a big factor in a lot of things. Many photographers tend to focus on f/number ("relative" aperture), whereas other fields (e.g. astronomy) use the word "aperture" in the correct sense, which is physical aperture. F-number scales with sensor size. |
October 11th, 2009, 02:51 PM | #2 |
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In practice, the smaller sensor will show more DOF. Two parameters impact the depth of focus: the lens focal length & aperture setting. For the same field of view, the smaller sensor will have a shorter focal length, therefore the resultant DOF will be larger, using the same f-number. BTW, The crop factor does not apply to the lens aperture.
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October 11th, 2009, 03:53 PM | #3 | |
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You are talking about using both at the same f-number. This thread explains that it's also possible to use them at different f-numbers to arrive at the same DOF, and increasing gain (ISO) to get the same brightness and noise.
Quote:
The surprising truth is that it does! May I kindly suggest that you try the very simple experiment outlined above to prove it for yourself? |
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October 18th, 2009, 11:05 PM | #4 |
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I went ahead and did some comparison shots to demonstrate the principle:
Images demonstrating how f-number scales with sensor size This image was taken with 70mm f/4 ISO 640 on a 1.6X sensor similar to the 7D: And this one was taken at 111mm f/6.4 ISO 1600 on a FF35 sensor (5D2): Follow the link above for the rest of the shots. |
October 19th, 2009, 03:55 AM | #5 |
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This has to be the most confusing, convoluted and misconstrued explanations of DoF and the relationship to sensor size I have ever read.
Usually, threads on this subject mention circle of confusion, subject to camera distance, field of view, f/stop and focal length. I don't think I've ever seen "lens weight" described as a contributing factor... |
October 19th, 2009, 05:36 AM | #6 |
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"...This has to be the most confusing, convoluted and misconstrued explanations of DoF and the relationship to sensor size I have ever read..."
Succinct and too the point, Liam! :) |
October 19th, 2009, 09:47 AM | #7 | ||
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Quote:
Quote:
Let me try to clarify. There is a common misconception that lenses for larger formats have to be heavier than smaller formats. The post explains why that is only true if the lens is built to be capable of thinner DOF. When DOF is the same, weight, too, tends to be the same. |
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October 19th, 2009, 11:18 AM | #8 |
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Daniel, I fully understand depth of field and all of the determining factors.
I find it extraordinary that you write, "In order to learn something new, sometimes it's necessary to stop disqualifying new information solely on the basis of being different from your old information." The laws of physics don't change because Canon bring out a new consumer camera! I know the point you are trying to make, but with respect, your opening post is a confused mess. Yes, you can stop down to maintain a similar DOF between formats, but the simple truth is all things don't remain equal and there's a hell of a lot of difference between shooting at f/1.4 on one camera and f/5.6 on another. Anyway, here's a simple explanation with a rather neat calculator to show the relationship between sensor size and depth of field. http://www.cambridgeincolour.com/tut...ensor-size.htm It is worth noting that even though the numbers add up, the images will not look the same. |
October 19th, 2009, 11:58 AM | #9 | |||
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Quote:
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Images demonstrating how f-number scales with sensor size |
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October 19th, 2009, 12:26 PM | #10 | |
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Quote:
I'm not mistaken at all. Stopping down to match depth of field will affect lens contrast, edge sharpness and bokeh. It can result in a similar depth of field, but there's more to it than that. Look at your own test, they're not the same at all. If you don't believe me go shoot at test. |
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October 19th, 2009, 12:37 PM | #11 |
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What exactly is this point to this thread?
To say X sensor/film size with Y focallength and Z F-stop with W light conditions with respect to ASA sensitivity will give different looking images but that if you adjust any of these variables you can get the same DOF and same grain for each image? Isn't that a DUH? |
October 19th, 2009, 01:10 PM | #12 | ||||
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Quote:
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That large sensors are capable of everything that small sensors are when it comes to DOF, diffraction, and noise. I ran into a DP that wanted to switch from the 5D2 to the 7D because he thought it had an advantage in deep DOF. This thread explains why that is incorrect. Quote:
Perhaps it should be, but based on the number of people who are shocked and amazed when they find out it's true, I'd say it's not an obvious fact to most. For example, many have a very hard time believing that 112mm f/6.3 ISO 1600 on the 5D2 has the same noise as 70mm f/4 ISO 640 on the 7D, just as the image above shows. |
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October 19th, 2009, 01:37 PM | #13 |
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Uhm.. the 5Dm2 image is at least a 1/2 stop brighter.
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October 19th, 2009, 01:51 PM | #14 | |
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The new way of being a "pro" photographer is(and I see this all the time): shoot, look down at LCD, spin a wheel, shoot, look down at LCD, spin a wheel, look at subject really confused since the screen isn't showing what they see in real life, spin a wheel, shoot, give up and put it on P and put a flash on the camera. |
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October 19th, 2009, 01:52 PM | #15 |
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When I open the a2 PNG file in my image editor, the neutral 3.5 patch (the one below black) has an average RGB value around 87. The same patch in the a1 PNG file has an average around 87. Are you looking at different files? Or a different part of the file?
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