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Old Jul 23, 2004, 9:51 AM   #1
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I was wondering if the 1.6 magnification multiplier that happens on the Canon digital cameras effects the f stop as well as the focal length.

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Old Jul 23, 2004, 10:06 AM   #2
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No, it does not.

It all comes from the camera having a sensor smaller than a 35mm piece of film.

It effects DOF (because the sensor is smaller) the same way the larger negative on medium & large format cameras effects DOF for those cameras.

It effects the image produced. Imaging you had a 35-mm size sensor that was really high resolution. Imagine you took the resulting picture and cut out the middle portion of it. Now, what you cut (or cropped) is the resulting picture you would get from the 1.6x "magnification multiplier."

So you could directly simulate (except for the DOF changes) the effect of the 1.6x in photoshop if you had a camera with a larger sensor (like the Canon 1Ds.)

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Old Jul 23, 2004, 10:17 AM   #3
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eric s wrote:
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It effects DOF (because the sensor is smaller) the same way the larger negative on medium & large format cameras effects DOF for those cameras.
Would it?

Isn't it true that the distance from the lens mount to film plane is the same in both the 1Ds and the 10D/300D, ie the 'true focal lenght' is still the same?... and that the 1.6x factor is the result of just the cropping -> smaller field of view resulting from the smaller surface area of the sensor (the same lens still project the same image to the film plane on both camera)
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Old Jul 23, 2004, 12:08 PM   #4
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I'm not sure what you're asking. I didn't intend to say that the effect on DOF would be the same. But that the cause of the different DOF is the same. The picture recording medium is larger or smaller than 35mm. The larger the "medium" (film/sensor) the a smaller DOF.

This is why large format cameras often have tilt/shift planes in them, so you can really get that smaller DOF exactly where you want it (at least this is my understanding, I've never used anything but 35mm.)

Eric

ps. This comes from conversations with people who do large format photography and who know a lot more about photographery than I do. Take that as you will.
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Old Jul 23, 2004, 3:21 PM   #5
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Eric

My point is the 1.6x multiplier should not affect the f-stop, the 'true focal lenght', nor the DOF.

Think about it this way: the f-stop is a ratio between the diameter of a particular lens and its focal lenght. The lens barrel didn't change going from the 1Ds to a 10D/DRebel, and its f-stop also did not change -> the 'true focal lenght' can't change either and so is its DOF.
It's a crop just like Photoshop: The same image is projected to the back of the camera's film plane whether its a 1Ds or a 10D/Drebel, but the smaller 10D/DRebel's sensor "sees" only a narrower Field Of View approximating that of a 1.6x focal-lenght lens.

Now the DOF for the E1's is a different story. In this case, it's a true "format change" (same as your example of different APS vs 35mm vs 6x6cm negative size): The 4/3 lens design did change to account for the smaller sensor. This format allows them to use smaller lens while maintaining seemingly fast aperture from smaller diameters barrels (from the above ratio)! The DOF is larger with the E1 since it's 'true focal lenght' have changed, not by much, but its effect is similar to smaller sensors prosumer cameras.

This could be both good or bad, and several folks have already pointed them out: Excellent for macro, but bad when you want that 3D effect...
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Old Jul 23, 2004, 6:45 PM   #6
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I think we're arguing the same thing.
My first sentence was "no, it does not." in my original post. The 1.6x crop doesn't effect fstop. I never said it did (unless I've been taking drugs I'm unaware of.)

Then let me ask you this. Why do DOF calculators ask me for the size of the sensor I'm using? They ask me what the actual focual length of the lens is (that makes sense), they ask the fstop I'm using, and the distance to the subject. But they also ask for the size of the sensor. Because it makes a difference.

I just went through 3 of them on the web, every one asked for the size of the film/sensor.

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Old Jul 23, 2004, 7:58 PM   #7
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Eric

We are not talking about the same thing: I'm talking about DOF between 1.6x vs full-frame dSLR which are the exeption to all your web based DOF calculators!
"Depth Of Field (DOF): Many have contested that DOF inevitably increases with the 1.6 multiplier effect. But let's consider how the image is a simple crop of the original achieved through the lens, without any change in distance to subject. Being a simple cut-out of the original, no further "blur" is injected into the background (as commonly introduced by longer focal lengths and wider apertures). In that regard, DOF is preserved at the original aperture / focal length settings. However, one will essentially get an image resembling a longer focal length in overall size, but bearing a DOF akin to the original telephoto/aperture setting, which may seem shallower than normally envisioned with a non-digital SLR setup. Still, there is room for debate whether the Circle Of Confusion (COC) would add to the DOF."
http://www.hardware-one.com/reviews.asp?aid=236&page=3


The DOF calculators will work with the above E1, my D7, CP990 as well as all other digicams with smaller sensors. In fact I'm actually surprised that on one brought up the EF-s 18-55mm kit lens which is designed specifically for the smaller sensor which protrudes inside the camera mount of the DRebel smaller mirror box... My bet is this lens will have a larger DOF than a regular lens!
Someone with a Drebel should try the DOF between the regular 50mm and his kit lens set at 50mm :idea:
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Old Jul 23, 2004, 9:18 PM   #8
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I shall post this only for informational purposes.

http://www.photo.net/learn/optics/dofdigital/
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Old Jul 24, 2004, 12:14 AM   #9
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I bet that kit lens does have a larger DOF because it's "true" focal length is smaller than the equivalent true 35-mm lens.

I guess we just talked past each other in the night.
Why is it "the exception to all your web based DOF calculators"?

Unless this calculator is wrong:
http://dfleming.ameranet.com/dofjs.html
it shows that f8, 50mm 10foot distance shows under 4" of DOF on the 10D, with 35mm its just a bit under 7", & with 6x7 film its just under 17". So is that calculator wrong?

Maybe I just shouldn't have brought that entire line of discussion up, as it's really tangental to royy's question.

We both agree, it doesn't effect fstop (which is physical) or focal length (which is also physical.)

Eric
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Old Jul 24, 2004, 9:47 PM   #10
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my understanding is that the kit lens is a 'real' 18-55mm lens but it only covers an APS sized sensor. At 50mm on the EF-S lens the image is the same as with my 50f1.8 (disregarding qualitative aspects).

18mm-55mm is the same everywhere - but with different sized sensors the view is different.

The DOF is the same only we've cropped the photo.
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