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Old Mar 13, 2005, 2:09 AM   #1
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First question I am pretty sure I know the answer. . .but just wanted to make sure:lol:. If I were to have a 300mm lens on a Canon DSLR with a 1.6 mag factor, giving me a "480mm" lens. The DOF that I get off of that lens will only be equal to the 300mm lens, and not a 480mm lens, correct? (if that makes and sense:-))

Second question
Would the EF 300mm (for example) f2.8L give me a faster shutter speed stopped down to f5.6 as would the (for example)EF 100-300 f4.5-6.6 @ f5.6? Naturally the 300mm 2.8 is a much larger lens, bringing in much more light than the 100-300, but is this irrelevant when both lenses are used at the same aperture?

Those are just two questions that have been bugging for the past few days. . .Thanks!
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Old Mar 13, 2005, 8:13 AM   #2
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KM_krazy wrote:
Quote:
First question I am pretty sure I know the answer. . .but just wanted to make sure:lol:. If I were to have a 300mm lens on a Canon DSLR with a 1.6 mag factor, giving me a "480mm" lens. The DOF that I get off of that lens will only be equal to the 300mm lens, and not a 480mm lens, correct? (if that makes and sense:-))
Correct! - It's only a crop with an equivalent FOV...



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Would the EF 300mm (for example) f2.8L give me a faster shutter speed stopped down to f5.6 as would the (for example)EF 100-300 f4.5-6.6 @ f5.6? Naturally the 300mm 2.8 is a much larger lens, bringing in much more light than the 100-300, but is this irrelevant when both lenses are used at the same aperture?
Correct! - this is irrelevant when both lenses are used at the same aperture.
F/5.6 is the ratio of the focal lenght over the effective opening of the lens -> when you close the f/2.8 down to f/5.6 you are reducing the lens opening and less light is coming through...:?
This is why constant f/2.8 lens are so much larger in diameter than the variable aperture lenses and why when you zoom out, i.e. the focal lenght increases while the skinny lens barrel stayed constant -> aperture ratio increases!

BTW there's other benefit - even when you closed down the f/2.8 lens is brighter because it stays wide opened during focus helping the camera focus easier, and there's also more defocus info for a camera to work with: http://konicaminolta.com/products/co...x7/ope01b.html

-> hence more precise focusing
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Old Mar 13, 2005, 10:15 AM   #3
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Ok, NHL. Time to bring up something and see what you think. It's a slight digression from what KM_krazy asked, but I got wondering...

Let me pull up a DOF calculator.
10D + 300mm f2.8 + 100 ft to subject = 3.61 ft DOF
35mm film + 300mm f2.8 + 100 ft to subject = 5.7 ft DOF
35mm film + 380mm f2.8 + 100 ft to subject = 2.21 ft DOF

So while the 1.6x crop factor of the Canon Camera's doesn't give you the same DOF as film with the "equivalent" lens, it does produce a smaller DOF than if you had the same lens on a film camera.

Calculated by this really nice DOf calculator:
http://www.dofmaster.com/dofjs.html
(if that link does work, go to the main page and follow the link to the Online calculator.)

Eric
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Old Mar 13, 2005, 11:42 AM   #4
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eric s wrote:
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Ok, NHL. Time to bring up something and see what you think. It's a slight digression from what KM_krazy asked, but I got wondering...
Good question... 'distance' is the key :idea:
"3. If you use the same lens on a EOS 10D and a 35mm film body and crop the 35mm image to give the same view as the digital image, the depth of field is IDENTICAL"

FYI - http://www.photo.net/learn/optics/dofdigital/
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Old Mar 14, 2005, 12:33 AM   #5
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Then that DOF calculator I used is wrong? I don't know enough to correct either your source or the calculator. But clearly one is wrong as our sources don't agree. In my example, the DOF was not the same and the distance and lens were. The sensor size, just like film size, effects DOF.

And I don't understand why your quote mentions that "...and crop the 35mm image to give the same view as the digital image..." Croping a 35mm image does absolutely nothing to the DOF. You do that after the image is created. That clause seems pointless to me.

I think I'm missing something....

Eric
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Old Mar 18, 2005, 10:54 AM   #6
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KM

I think the answer to your first question is NO and NO; neither of the options you present are correct.

Rule 1: With a constant focal length the DOF is greater the smaller the sensor size.

The DOF of the 300mm lens without a crop on 35mm film is shallower than the DOF of the same lens on a reduced size sensor. It is shallower in direct proportion to the crop factor multiplier.

e.g. if DOF of an object 10m from the camera on the 300mm lens = 10cm on the 35mm camera it will be 16cm on the smaller sensor camera.


Rule 2: With a constant angle of view the DOF is greater the shorter the focal length.

So your second option is wrong too: the DOF of the 300mm lens on the APS-C sensor is not equal to the notionally equivalent 480mm lens' DOF on a 35mm camera either. It is also out by the crop factor multiplier but in the other direction.

e.g. if DOF of an object 10m from the camera on the 300mm lens on the reduced sensor camera = 10cm (as above) the DOF on a 480mm lens on 35mm would be 10/1.6 = 6.25cm.


NHL's link explains it clearly.

http://www.photo.net/learn/optics/dofdigital/

However I think he misunderstood your initial question when he said that you were correct in your initial assumption.

Eric

You do lose DOF when you crop - in direct proportion in fact to the amount of the crop. This is just another way of stating Rule 1. This is very intuitive when you consider a wide angle shot with everything in focus, as you crop you reduce the amount of "stuff" in the picture and therefore are reducing the DOF accordingly. It's less intuitive when you have a narrow DOF in a picture but is still true.
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Old Mar 18, 2005, 2:26 PM   #7
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I'm having a hard time internalizing that. I hope that you're not also going to tell me that the perspective is compressed equivalent to a 480mm lens?
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Old Mar 18, 2005, 3:53 PM   #8
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eric s wrote:
Quote:
Croping a 35mm image does absolutely nothing to the DOF. You do that after the image is created. That clause seems pointless to me.

I think I'm missing something....

The assumption is that after you crop you resize the image to be the same as before cropping, thereby losing some detail. The more detail you lose, the shallow the effective DOF.
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Old Mar 19, 2005, 5:51 PM   #9
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Eric is right. Don't forget that DoF is defined by the "circle of confusion" which is the point at which a point becomes a circle and therefore "fuzzy" and seen as out of focus.

If you crop and enlarge, then what was a point, may well become a circle and therefore be seen to be out of focus.



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Old Mar 21, 2005, 11:43 PM   #10
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Eric,

I haven't tried to digest what's going on here, but was the 380 example supposed to be 480? Don't mistake my question for me having a clue. I just noticed this and wondered if it was a typo in the message or possibly in the calculation.
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