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Author Topic: Canon 2x extender --> shorter DOF?  (Read 5409 times)

Gregory

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Canon 2x extender --> shorter DOF?
« on: May 27, 2006, 11:20:28 am »

I've been using a Canon 2x II Extender with a 70-200 f2.8 non-IS lens for several months. the results are ok but not great. one thing that makes the photography difficult for this amateur is that the DOF at 400 f5.6 is incredibly short; roughly one centimetre.

I'm wondering. is the DOF shorter when using a 70-200 with a 2x extender at f5.6 than it would be when using a 400mm at f5.6?

my lens theory is basically zilch so please forgive my ignorance.

(note. I'm using a Canon EOS 350D so the focal lengths are essentially 1.6x longer than stated.)


regards,
Gregory
« Last Edit: May 27, 2006, 11:21:01 am by Gregory »
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Tim Gray

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Canon 2x extender --> shorter DOF?
« Reply #1 on: May 27, 2006, 03:02:05 pm »

200 with a 2x at 5.6 at 50ft will give you the same dof as 400 at 5.6 at 50' = roughly 1ft dof.

Here's a calculator - http://www.dofmaster.com/dofjs.html
plug in the 350D and that adjusts for the 1.6 so use the actual fl x extender.
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Gregory

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Canon 2x extender --> shorter DOF?
« Reply #2 on: May 28, 2006, 07:50:46 am »

Quote
200 with a 2x at 5.6 at 50ft will give you the same dof as 400 at 5.6 at 50' = roughly 1ft dof.

thank you. that was very educational.

I used the calculator to see what kind of DOFs I'm working with. based on ExpoImaging's DOF conversion table (basically, increase the actual focal length by 25% for 1.6x cameras), my EOS 350D + 200mm/5.6 + 2x Extender combination gives me a DOF of around 1/2" for distances between 10 and 15 feet. it's no wonder focusing can sometimes be very tricky!

today, I was photographing doves in one of our trees. the distance was only 6'. with the 200mm + 2x Extender, even at f8, that gives me a DOF of only around 1/8". ouch! (that's actually quite hard to believe... 1/8"?)
« Last Edit: May 28, 2006, 07:51:59 am by Gregory »
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Doug Kerr

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Canon 2x extender --> shorter DOF?
« Reply #3 on: May 28, 2006, 12:20:36 pm »

Quote
I've been using a Canon 2x II Extender with a 70-200 f2.8 non-IS lens for several months. the results are ok but not great. one thing that makes the photography difficult for this amateur is that the DOF at 400 f5.6 is incredibly short; roughly one centimetre.

I'm wondering. is the DOF shorter when using a 70-200 with a 2x extender at f5.6 than it would be when using a 400mm at f5.6?
[a href=\"index.php?act=findpost&pid=66724\"][{POST_SNAPBACK}][/a]

Depth of field is determined by:

1. Focal length. That is the focal length of "the entire lens". Remember, when we put an extender on a lens, we create a "new lens" of greater focal length, and that is the focal length of importance here.

2. Distance to the object plane of focus (to the subject, generally)

3. The aperture as an f/number. In the case of the "new lens" created by the addition of the extender, that is the f/number of the combination (for a 2.0x extender, that would be 2.0x the f/number to which the main lens proper was set). [Note that this isn't an "effective f/number" - it is the actual f/number of the "new lens".]

4. The circle of confusion diameter limit (COCDL) we adopt as our criterion of "acceptable blurring".

Thus, if we have a 70-200 mm lens set to 200 mm, with a 2.0x extender, and the "new lens" is set by the camera to an aperture of f/5.6 (that would mean that the main lens proper was set  to f/2.8 - the lens takes care of that itself once it learns that a 2.0x extender is in the circuit), and assuming that the subject distance is the same and that we use the same COCDL, then the depth of field should be the same as for a 400 mm lens at f/5.6.
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John Sheehy

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Canon 2x extender --> shorter DOF?
« Reply #4 on: June 03, 2006, 10:18:53 pm »

Quote
200 with a 2x at 5.6 at 50ft will give you the same dof as 400 at 5.6 at 50' = roughly 1ft dof.

Here's a calculator - http://www.dofmaster.com/dofjs.html
plug in the 350D and that adjusts for the 1.6 so use the actual fl x extender.
[a href=\"index.php?act=findpost&pid=66736\"][{POST_SNAPBACK}][/a]

I sometimes use a 2x and 1.4x stacked with 400mm f/5.6 (100-400 IS).  The DOF at 1120mm (rounded to 1200mm) and f/16 at 10 feet is about 0 according to the calculator!

It is certainly a challenge to decide when to use the stacked combo.  Chances of getting a sharp shot are not good with a close subject, as even your breathing can make the eye go out of focus., but when you get it right, it's a lot better than cropping and upsizing the demosaicing artifacts and noise.  You need bright light or flash, of course.  Even with the IS, I need 1/400, preferably 1/640, and I'm at f/16 wide open.

Usually, I do this if the subject is cooperative, I have shot it to death at 560mm, and can risk it flying or running away while I add the 2x and focus carefully.  Or, the subject is so far away and small that it's not worth shooting at 560mm.
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stever

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Canon 2x extender --> shorter DOF?
« Reply #5 on: June 06, 2006, 11:57:55 pm »

when i've tested my 20d with the 1-4 and 1.4x, i find it's worse than an equivalent crop without the extender (tripod, mirror lockup, etc.)

the 4.0 IS with 1.4x is marginally better than the 1-4 at 400, but when i put a 2x on, it's also worse than an equivalent crop with the 1.4x

i also tried a Tamron 200-500 and guess what - at 500 it's worse tha an equvalent crop from 400 (and still down on contrast and resolution compared to the Canon 1-4)

i recently rented a 5d and 300 2.8 as a sanity check.  the 20d with 300 2.8 + 2x was noticeably better than equivalent crops from the 300 4.0+1.4 or 1-4, but the equivalent crop from the 300 +1.4 was nearly as good and still slighty better than the 300 4.0 +1.4x or 1-4.  the equivalent crop from the 5d and 300 2.8 + 2x was the best of all by a small margin, but with any of the shorter focal length combinations it was noticeably worse - just not enough pixels left after cropping.

i was somewhat suprised how well the 1-4 held up - particularly at f8 (but not entirely as i've made a number of very good 13x19's including a couple at 5.6 and ISO 1600).  

the 20d with 1-4 will continue to be my choice for carryability, versatility, and hand-held action shots - until something really better comes along.
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PeterRobert

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Canon 2x extender --> shorter DOF?
« Reply #6 on: June 15, 2006, 05:56:28 am »

Stever referred to a Canon 1-4.  Could someone please tell me what that is please?
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John Sheehy

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Canon 2x extender --> shorter DOF?
« Reply #7 on: June 15, 2006, 08:14:13 am »

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Stever referred to a Canon 1-4.  Could someone please tell me what that is please?
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That's shorthand for the 100-400 f/5.6L IS, I assumed.
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John Sheehy

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Canon 2x extender --> shorter DOF?
« Reply #8 on: June 15, 2006, 08:29:16 am »

Quote
when i've tested my 20d with the 1-4 and 1.4x, i find it's worse than an equivalent crop without the extender (tripod, mirror lockup, etc.)

If you have a good copy of the lens, that shouldn't be.  With the 1.4x Kenko Pro, I can't even tell the TC was used without the 560mm fl in the EXIF.

I don't find any joy in cropping or upsampling the bare lens.  There are all kinds of artifacts in the images that show the pixel structure of the image, and zooming in on them is not too pretty.  Even if the TC image is a bit softer, pixel-to-pixel, you can remove noise with less detriment, and you are left with a subject with noise and pixels that are relatively very small.

Quote
the 4.0 IS with 1.4x is marginally better than the 1-4 at 400, but when i put a 2x on, it's also worse than an equivalent crop with the 1.4x[a href=\"index.php?act=findpost&pid=67589\"][{POST_SNAPBACK}][/a]

That shouldn't be, either.  The MTF charts for that lens look very good, and capable of significant magnification.

What are your standards for success with a TC?  Pixel-to-pixel contrast can be expected to drop a bit, even if more subject detail is being captured.
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