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Author Topic: Educate me about why 70mm <> 70mm...  (Read 7093 times)

lodgepole

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Re: Educate me about why 70mm <> 70mm...
« Reply #15 on: January 05, 2013, 04:01:26 PM »
A bit off topic, but I'm also very curious about the apparent large differences in the depth of field of the two lenses. In both images, the 24-70 f4 seems to have much more DOF. Is this common between two lenses at the same focal length and aperture?

In part, it's due to the FoV seeming to be a bit less on the 70-200, but also very likely due to f/2.8 vs f/4. The larger the aperture (smaller f-number), generally the shallower DoF. Also influencing that is distance from lens to subject, distance between subject and background/foreground.

Apparently they were all shot at f4, though. I suspect much of the difference in DOF is due to a different vantage point relative to the fence, and due to focusing on the closer flowers with the 24-70 vs. focusing on the plant in the background with the 70-200.

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Re: Educate me about why 70mm <> 70mm...
« Reply #15 on: January 05, 2013, 04:01:26 PM »

TexPhoto

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Re: Educate me about why 70mm <> 70mm...
« Reply #16 on: January 05, 2013, 04:23:32 PM »
It's both.  The 70mm is an approximation on both. 

One thing you could do is shoot similar sets of photos, but shoot a mountain, or something a mile away or so.  Thus the issue of lens size will not matter.

Oh and you may also notice the lenses will zoom a little when focused.  So even the same lens is not the same focal length exactly at it's closest vs. infinity focus.  This is called focus breathing.

Oh, and it's not "<>" it's "!=".  != means not equal. -Of course this is the internet, so some people will argue about this, rather than the real topic.
« Last Edit: January 06, 2013, 09:45:56 AM by TexPhoto »

Drizzt321

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Re: Educate me about why 70mm <> 70mm...
« Reply #17 on: January 05, 2013, 04:55:19 PM »
A bit off topic, but I'm also very curious about the apparent large differences in the depth of field of the two lenses. In both images, the 24-70 f4 seems to have much more DOF. Is this common between two lenses at the same focal length and aperture?

In part, it's due to the FoV seeming to be a bit less on the 70-200, but also very likely due to f/2.8 vs f/4. The larger the aperture (smaller f-number), generally the shallower DoF. Also influencing that is distance from lens to subject, distance between subject and background/foreground.

Apparently they were all shot at f4, though. I suspect much of the difference in DOF is due to a different vantage point relative to the fence, and due to focusing on the closer flowers with the 24-70 vs. focusing on the plant in the background with the 70-200.

Ah, whoops, missed that part in the OP.
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bbb34

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Re: Educate me about why 70mm <> 70mm...
« Reply #18 on: January 05, 2013, 06:27:15 PM »
[...]

Oh, and it's not "><" it's "!=".  != means not equal.

Actually it is "≠". Just in case that one cannot find it on ones keyboard, there are a few more or less common alternatives, including "<>".

See http://en.wikipedia.org/wiki/Equals_sign#Not_equal .

Drizzt321

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Re: Educate me about why 70mm <> 70mm...
« Reply #19 on: January 06, 2013, 02:06:27 AM »
[...]

Oh, and it's not "><" it's "!=".  != means not equal.

Actually it is "≠". Just in case that one cannot find it on ones keyboard, there are a few more or less common alternatives, including "<>".

See http://en.wikipedia.org/wiki/Equals_sign#Not_equal .

I was actually going to just thank the OP for using the programmer/mathematical 'not equals' the way he did :)
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AudioGlenn

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Re: Educate me about why 70mm <> 70mm...
« Reply #20 on: January 06, 2013, 02:47:29 AM »
I've noticed this too when comparing my 24-70 II and 70-200 IS II
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MintMark

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Re: Educate me about why 70mm <> 70mm...
« Reply #21 on: January 06, 2013, 04:42:22 AM »
In both instances, the image on the lower pic is larger.  Did you measure from the focal plane of the camera?

This assumes some how that 70mm or any other focal length *must* have a certain framing...which may not be the case. The focal length designated might be just tied to the optical system...lets not forget these are multi lens designs.
So it may be perfectly ok to have slightly different FOV for a given focal length depending on the optical system.

As an anology, i am guessing (not certain) that  if one takes two refracting telescopes of exact same focal length but one scope has a larger objective say 100mm vs the other's 30 mm, in addition to their vastly different light gathering power, they may also frame the sky more or less, even though they have the same FL.

Hmmmm.... I think it's true that focal length, along with sensor size, is the very definition of angle of view (for a rectilinear lens)... http://en.wikipedia.org/wiki/Angle_of_view.

The other explanations seem more plausible to me.

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Re: Educate me about why 70mm <> 70mm...
« Reply #21 on: January 06, 2013, 04:42:22 AM »

sandymandy

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Re: Educate me about why 70mm <> 70mm...
« Reply #22 on: January 06, 2013, 04:43:27 AM »
You can also find this on the 35mm lenses. Samyang one was more like 37mm or such if i remember correctly. Afterall a few mm difference can have a big difference for example compare 24mm and 28mm. Just 4 mm but u can see the difference. I think its not about cheating but just the rules of physics.
Would be really confusing if we had lenses like "36.24mm" or "117.11" and such :D

EricPeeg

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Re: Educate me about why 70mm <> 70mm...
« Reply #23 on: January 06, 2013, 08:32:05 AM »
Many thanks to all who posted with thoughts - as I watched the responses roll in, my initial thought was "boy, if you wanted to demonstrate the shortcomings of your photographic knowledge to the world, this was the way to do it..."  :D

Anyhow, I think the primary note I took away from my initial post was that my sample photographs were incomplete in their construction - that I'd failed to control for focus, and hadn't considered the distinction between focusing to infinity, and something closer. I also had never encountered the term "focus breathing"...

Anyhow, this morning I went out and made a new set of awe-inspiring images, trying to make sure I incorporated some of the advice. I've attached them below:

  • Image_4818.jpg is the 24-70 F4 IS, shot at at manual, 70mm, F4, 1/320th, ISO 200
  • Image_4819.jpg is the 70-200 F2.8 mk II IS, shot at manual, 70mm, F4, 1/320th, ISO 200

Both images were made with the camera mounted to the tripod via a baseplate on the tripod mount of the camera (I didn't use the tripod ring of the 70-200, in other words). The tripod and camera didn't move between images, so the focal plane (image sensor) was stationary. Obviously, because of the difference in lens size, the front element of the 70-200 is three inches "closer" to the scene than the 24-70.

The center focus point of the camera was placed, not on the green park sign, but on the trees/house immediately to the left of the green park sign. I visually confirmed that the lenses appeared to be focused all the way to the infinity mark.

The images are uncropped and unretouched, just downsized from the RAW file.

To me, the results continue to be interesting for what they show, even if I can't really explain fully why. To my eye, the 70-200 is slightly narrower (more "tele") in its "70mm" position than the 24-70. I now can't see the more significant depth-of-field distinction that the original (less-carefully-constructed) images of the Christmas lights seemed to show. At 100% in Lightroom, I can see some differences in image quality, but that wasn't what I was testing originally. And most puzzling to me, there seems to be about a 1/3 stop of difference in light gathering - on the second image, the camera reported the scene to be +1/3 EV, while the first image was +0. There are literally only a few seconds between the two images (long enough to do the lens swap), and it's a grey overcast morning - I didn't observe any shift in the ambient light conditions that would have explained the difference. (How's that for scientific? No? Not scientific? Well, fine! So it's anecdotal. Whatever.)

Anyhow, thanks again for everyone who offered knowledge - I appreciate it very much. Now I just have to decide whether I'm keeping the 24-70, or returning it for the 2.8 non-IS version...  ;)
« Last Edit: January 06, 2013, 08:39:36 AM by EricPeeg »

EricPeeg

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Re: Educate me about why 70mm <> 70mm...
« Reply #24 on: January 06, 2013, 08:37:16 AM »
Oh. And also - the thread about "<>" vs "!=" vs "≠" made me smile. I do a fair bit of SQL coding at work right now, so that's the explanation for the title ...  ;D

tcmatthews

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Re: Educate me about why 70mm <> 70mm...
« Reply #25 on: January 06, 2013, 09:43:32 AM »
Hello long time lurker.  It seems that there is a general confusion that the focal length and field of view contribute everything to the framing of the pictures.  There is also a very slight contribution by front objective (or something in the complex optics of a zoom) of the lens. 

In general a larger front objective  will have a larger apparent field of view. If two different (ie a 24-70 and 70-200) lens of the same focal length are placed on a tripod the same distance from a subject you should not expect the same framing.  It should be close, but not the same.  If you look at it this way field of view angle of each of the lens is 35 deg at 70mm if one lens has a front element that is 82mm and one has a front element of 77mm the one with 82mm should have gathered more of an image.  Very near to the camera this will not be apparent but a infinity is should be noticeable.   

I say in general because the focal length are approximate and some lens change focus length slightly during focusing. 
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TexPhoto

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Re: Educate me about why 70mm <> 70mm...
« Reply #26 on: January 06, 2013, 09:52:50 AM »
Don't get too wrapped around the axle about this.  When was the last time you looked a great photo and said: "Man, that's pretty good, but it looks like it was shot at 64.2mm.  70 mm would have been awesome!".  Back when 70-200s were 80-200s, I had a friend tell me his 24-70 and 80-200 set did not seem complete.  After all, what if he needed a 75mm?  I punched him.

Zooms tend to cheat more than primes in this area.  Some very technical reviews will tell you the actual measured focal length.

EricPeeg

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Re: Educate me about why 70mm <> 70mm...
« Reply #27 on: January 06, 2013, 09:59:55 AM »
You're right - I haven't done this in the past. I'm thinking, though, that I could really make a pest of myself in the future...

"Hi, I'm Henri Cartier-Bresson."
"OMG, you're Henri Cartier-Bresson! OMG OMG OMG! That picture of the guy, jumping over the puddle? OMG OMG OMG! What lens did you use?!!!"
"Uh. I dunno. 35mm?"
"Oh. Well, that's ok, I guess. Too bad you didn't have that new Shorty Forty. Hack."

Yeah, that could be good fun... or a recipe for being punched. Or worse.   :D

Don't get too wrapped around the axle about this.  When was the last time you looked a great photo and said: "Man, that's pretty good, but it looks like it was shot at 64.2mm.  70 mm would have been awesome!".  Back when 70-200s were 80-200s, I had a friend tell me his 24-70 and 80-200 set did not seem complete.  After all, what if he needed a 75mm?  I punched him.

Zooms tend to cheat more than primes in this area.  Some very technical reviews will tell you the actual measured focal length.

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Re: Educate me about why 70mm <> 70mm...
« Reply #27 on: January 06, 2013, 09:59:55 AM »

vmk

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Re: Educate me about why 70mm <> 70mm...
« Reply #28 on: January 06, 2013, 10:32:45 AM »
Could be lens size?
is the 24-70 matches the length to 70-200 when you zoom in 24-70 to 70mm???

wild guess  ;)

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Re: Educate me about why 70mm <> 70mm...
« Reply #28 on: January 06, 2013, 10:32:45 AM »