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Gear Talk => Lenses => Topic started by: EricPeeg on January 05, 2013, 09:26:05 AM

Title: Educate me about why 70mm <> 70mm...
Post by: EricPeeg on January 05, 2013, 09:26:05 AM
Alrighty, I should probably know this, but I guess I've never had the lenses to test this question before.

Yesterday evening, I received a copy of the new Canon 24-70 F4 IS lens. We can talk about why I selected that lens over the F2.8 and why I'm so crazy, but perhaps folks can explain to me something I observed this morning on my morning walkabout where I took the new lens out for a shakedown cruise.

The images below are uncropped and unretouched, other than being exported from Lightroom at smaller dimensions than the original RAW files from my 5d Mk II.

The first image (Img_4708.jpg) is the 24-70 shot at 70mm (at the top of the lens's rotational range) F4, 1/200th, 1000 ISO, -1/3 EV.
The second image (Img_4697.jpg) is the 70-200 Mk II shot at 70mm (at the bottom of the lens's rotational range) F4, 1/160th, 1000 ISO, -1/3 EV

Both images were handheld (left my tripod at home this morning), but I didn't move between switching lenses. Looking at the LCD, I thought "wait, is one of those more zoomed-in than the other?"

At home, thinking perhaps it was how I'd held the camera, I set up the tripod in the kitchen and repeated the test. This time, the tripod definitely didn't move between images.

The first image (Img_4735.jpg) is the 24-70, shot at 70mm, F4, 1/60th, 200 ISO, 0 EV.
The second image (Img_4736.jpg) is the 70-200, shot at 70mm, F4, 1/80th, 200 ISO, 0 EV.

As you can see, in both sets, there's a difference in the angle of view - not necessarily good or bag, but different. So, I can come up with two explanations for what I'm seeing, but I'm hoping someone can shed more light on this.

1. The difference can be explained by the fact that lens manufacturers paint numbers on the lenses which are approximations of the focal length, and two lenses of different design will inevitably have a different length (or angle of view) even at what's nominally the same setting.

or

2. This is the optical result of the difference in physical length between the 24-70 and the 70-200 lenses. At 70mm, the 24-70 is about 3 inches shorter than the 70-200. Because in both tests I tried to keep the focal plane of the image sensor more or less in the same place (roughly in the handheld setting, exactly when it was on the tripod), the front element of the 70-200 was always closer to the subjects, thereby shifting the scene.

My question to the collective wisdom of folks: Are either or both of these explanations accurate, or is there something else at play here? And also, what might explain the approximately 1/3 stop difference that the camera's choosing between shutter speed when it's working with the different lenses? Is this also due to the front element being closer to the subject (that whole "light falling off at the square of its distance" thingy I learned and forgot in my college photography class so long ago?)

Looking forward to learning from others here...
Title: Re: Educate me about why 70mm <> 70mm...
Post by: friedrice1212 on January 05, 2013, 11:12:43 AM
It could be the reasons you listed, but it could also be the fact that the same distance from the subject is not at the same spot on the lenses' focussing distances. On the 24-70 it might be at halfway between macro and infinity but on the 70-200 it will surely be closer to its closest focussing distance. And because of focus breathing, the angle of view changes. I don't know how much either of the lenses breathe, but the Nikon 70-200 gives an angle of 135mm at 200mm and closest focussing so there's lots of room for imprecision.
Title: Re: Educate me about why 70mm <> 70mm...
Post by: RS2021 on January 05, 2013, 12:16:49 PM
This effect is rather common... You will find this on the 70 end of the new 24-70 f2.8 II when you compare it to the old 24-70mm 2.8  with one framing very slightly wider.

The company designated numbers may still be correct optically, but I really dont know how that works. But this apparent variable framing is not something unheard of.

Go to this clip for a comparison...he reviews the two versions side by side, but this FOV comparison at 70mm happens around 4:37 mins into the clip...so fast forward :)

http://m.youtube.com/watch?v=rarKH78ObYw (http://m.youtube.com/watch?v=rarKH78ObYw)
Title: Re: Educate me about why 70mm <> 70mm...
Post by: sanj on January 05, 2013, 01:15:18 PM
You bring up an interesting point. I will make tests as well!
Thank you
Title: Re: Educate me about why 70mm <> 70mm...
Post by: scottkinfw on January 05, 2013, 01:47:54 PM
In both instances, the image on the lower pic is larger.  Did you measure from the focal plane of the camera?
Title: Re: Educate me about why 70mm <> 70mm...
Post by: traveller on January 05, 2013, 01:54:58 PM
1) All lenses are measured at infinity focus; most of what we call zoom lenses suffer from focus breathing, whereby the focal length changes as you change the focus: internally zooming lenses (i.e those with rear focus groups) are the worst culprits (actually, true zooms are parfocal).  Try repeating your experiment with each lens focussed at infinity. 

2) The focal length quoted is approximate and can vary between lens models; Canon's website should be able to give you the exact technical specifications if you're interested (sorry, I'm being lazy and can't be bothered to look them up!). 
Title: Re: Educate me about why 70mm <> 70mm...
Post by: RustyTheGeek on January 05, 2013, 02:00:26 PM
In both instances, the image on the lower pic is larger.  Did you measure from the focal plane of the camera?

+1  Notice the focal plane marking on the top of your camera.  Try a more precise test measuring from that point.

I think you have an excellent question.  What follows is my opinion and I also look forward to a more educated answer.

Refer back to your option #1.  The numbers painted on the lens are indeed somewhat approximated.  Like all other things in the real world, nothing is totally exact.  There are always slight variations to the established range because the range designation is meant for reference, not perfection.  It should be somewhat identical to all other lenses of that design, but not exactly the same when compared to other designs.  You will find that the aperture is also not exact when compared to the spec, if you could measure it exactly that is.  This variation from spec is seen in many reviews (esp when super-tele-zooms are reviewed) where the advertised apertures and zoom ranges are tested.  The advertised numbers are often a bit inflated to make the lens seem a bit better than it really is.

I don't think there is anything wrong, it's just a good observation on your part.  I look forward to the discussion...
Title: Re: Educate me about why 70mm <> 70mm...
Post by: curtisnull on January 05, 2013, 02:00:52 PM
I have noticed by looking at the patents that we see detailed here on Canonrumors.com sometimes that the REAL focal length and the REAL aperture are not the same as the MARKED focal length and MARKED aperture.

What I mean is you see a patent for a 50mm f 1.4 lens and the details on the patent will say something like it's really a 51.2 mm and f 1.51. (not real numbers here, just an example I made up)

So maybe "70" on the 24-70 is really 68.2mm and "70" on the 70-200 is 73.1mm.

Don't know this for sure, just something I have seen from the patents and am speculating on.
Title: Re: Educate me about why 70mm <> 70mm...
Post by: RS2021 on January 05, 2013, 02:02:13 PM
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.
Title: Re: Educate me about why 70mm <> 70mm...
Post by: tron on January 05, 2013, 02:14:18 PM
They cheat ... a little.

Canon's 100-400 at 400mm is reported to have less magnification than fixed focal length EF400mm f/5.6L.

The reported focal length is 360 to 370mm. So it is about 10% less!

10% less at 70mm means 63mm (I guess this is worst case).

All the same the 16-35mm f/2.8L (version I) is reported to be wider that 16-35mm f/2.8L II.

Title: Re: Educate me about why 70mm <> 70mm...
Post by: EvilTed on January 05, 2013, 02:47:29 PM
This happens with the 24-70 F/2.8 II and the 70-200 F/2.8 II.

Details here:

http://www.the-digital-picture.com/Reviews/Canon-EF-24-70mm-f-2.8-L-II-USM-Lens-Review.aspx (http://www.the-digital-picture.com/Reviews/Canon-EF-24-70mm-f-2.8-L-II-USM-Lens-Review.aspx)

ET
Title: Re: Educate me about why 70mm <> 70mm...
Post by: Radiating on January 05, 2013, 02:50:53 PM
Focal length is rated at INFINITY and on top of that focal length is typically rounded within a range of +/- 7%

Canon usually only rounds a little on most pro lenses. 

Your photos are the result of focus breathing from the two different lens designs though. Focus breathing as mentioned earlier can have a huge effect, resulting in 135mm @ 200mm at MFD.

Title: Re: Educate me about why 70mm <> 70mm...
Post by: lodgepole on January 05, 2013, 03:05:37 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?
Title: Re: Educate me about why 70mm <> 70mm...
Post by: Drizzt321 on January 05, 2013, 03:36:46 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.
Title: Re: Educate me about why 70mm <> 70mm...
Post by: Marsu42 on January 05, 2013, 03:43:32 PM
This happens with the 24-70 F/2.8 II and the 70-200 F/2.8 II.

... imho in this case it's a rather severe difference, because a) given the short zoom range a few mm more or less do make a difference, esp. a difference on the wide end matters and b) the sharpness is always compared the "wide end", but in reality it should be like 24mm mk2 vs 26mm mk1 which might make some sharpness difference.
Title: Re: Educate me about why 70mm <> 70mm...
Post by: lodgepole 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.
Title: Re: Educate me about why 70mm <> 70mm...
Post by: TexPhoto 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.
Title: Re: Educate me about why 70mm <> 70mm...
Post by: Drizzt321 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.
Title: Re: Educate me about why 70mm <> 70mm...
Post by: bbb34 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 (http://en.wikipedia.org/wiki/Equals_sign#Not_equal) .
Title: Re: Educate me about why 70mm <> 70mm...
Post by: Drizzt321 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 (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 :)
Title: Re: Educate me about why 70mm <> 70mm...
Post by: AudioGlenn on January 06, 2013, 02:47:29 AM
I've noticed this too when comparing my 24-70 II and 70-200 IS II
Title: Re: Educate me about why 70mm <> 70mm...
Post by: MintMark 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 (http://en.wikipedia.org/wiki/Angle_of_view).

The other explanations seem more plausible to me.
Title: Re: Educate me about why 70mm <> 70mm...
Post by: sandymandy 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
Title: Re: Educate me about why 70mm <> 70mm...
Post by: EricPeeg 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:


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...  ;)
Title: Re: Educate me about why 70mm <> 70mm...
Post by: EricPeeg 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
Title: Re: Educate me about why 70mm <> 70mm...
Post by: tcmatthews 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. 
Title: Re: Educate me about why 70mm <> 70mm...
Post by: TexPhoto 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.
Title: Re: Educate me about why 70mm <> 70mm...
Post by: EricPeeg 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.
Title: Re: Educate me about why 70mm <> 70mm...
Post by: vmk 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  ;)