70mm vs 70mm

mmcp42

mmcp42

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:thinking:
I have three lenses whose focal lengths nicely overlap:
17 - 40 L
28 - 85
70 - 200 L

the middle one isn't L (had it for ages)

so I thought I'd do comparitive tests
I took 20 shots
17 - 40 @ 40mm
28 - 85 @ 40mm
28 - 85 @ 70mm
70 - 200 @ 70 mm

each @ f/4, f/8, f/11, f/16, f/22

apart from the expected DOF change, the first 10 are actually almost indistinguishable (damn means I don't have an excuse to get a new mid-range)

the odd thing is (we're getting close - hang in there)
is the framing for the 2nd and 3rd sets are substantially different :thinking:

why is that? - I expected one 70mm focal length to give the same (framing) results as another

or is it that the bigger zoom puts the front element sufficiently closer to the subject to make the difference?

here are the pics:

LensTest.jpg
 
Or is it that you couldn't adjust the 28-85 to exactly 70mm?

What focal length is in the EXIF data?
 
nice try :)
70mm exactly - thought of that one already!
 
Probably because 70mm ain't necessarily 70mm. Just because the lens reports it's at 70mm doesn't mean it is exactly that and two different lenses might be at slightly different focal lengths even if they both report the same. Seen a few reviews of long lenses where they've said the rated max length is a bit "optimistic", so while it may quote 500mm, it's actually more like 450mm.
 
Its the image size on the sensor that matters - i'm not sure the distance of the object lens from the sensor will affect that.........

Each lens will have varying magnification factors though:

17-40 is 0.24, i guess the 28 thingy will be about 0.2ish and the 70-200 2.8 IS is 0.17
 
Thanks Vertigo
I hadn't thought of that as a reason, but it sounds all too plausible

does that mean I can get a new mid-range now then - please - please - please? :naughty:
 
Interesting, I'm curious about the answer to this one too. I had similar results when taking test shots with my Sigma 70-300 APO (when I still had it) and the Nikon 18-135 Kit lens. Portraits taken with both lenses set to 135mm gave significantly different framing results.

You've prompted me to test my 3 Nikons at 105.
 
Jonnyreb said:
Its the image size on the sensor that matters - i'm not sure the distance of the object lens from the sensor will affect that.........

Each lens will have varying magnification factors though:

17-40 is 0.24, i guess the 28 thingy will be about 0.2ish and the 70-200 2.8 IS is 0.17
Click to expand...

er - the words make sense, but the sentences make my head hurt!
shouldn't a 70mm with the same sensor = another 70mm with the same sensor?!?
or did I miss something?

(or are you agreeing with me that the front of the lens of the big fellah is closer (?!?) enough to make a difference?)
 
kendo said:
Interesting, I'm curious about the answer to this one too. I had similar results when taking test shots with my Sigma 70-300 APO (when I still had it) and the Nikon 18-135 Kit lens. Portraits taken with both lenses set to 135mm gave significantly different framing results.

You've prompted me to test my 3 Nikons at 105.
Click to expand...

and publish I hope!
 
mmcp42 said:
(or are you agreeing with me that the front of the lens of the big fellah is closer (?!?) enough to make a difference?)
Click to expand...

lol - no, I'm hypothesising that the lenses each have different magnification values which will affect image size.
 
This was covered in another thread recently but, 70mm is only 70mm if the lens is focused at infinity. As you reduce the focal distance the focal length will also shorten, how much depends on the design of the lens - different lenses = the differing results you've experienced.
 
yes - I remember seeing that thread
interesting

so can I buy a new mid range now ?!?!?!?!?!?
 
Sure and seeing as it's your round I'll have a 135L, cheers!
 
OK. Here's the 3 shots with each Nikkor all at 105mm


Nikon105Micro.jpg


NIkon70-200VR.jpg


Nikon18-135.jpg


I know the focus is off, but it was done rather hastily and it the framing we're interested in.
 
well I'm having a 50 1.2L with a 600 L IS chaser!!!!!
(oh is it time to wake up already!)
 
kendo said:
OK. Here's the 3 shots with each Nikkor all at 105mm


Nikon105Micro.jpg


NIkon70-200VR.jpg


Nikon18-135.jpg


I know the focus is off, but it was done rather hastily and it the framing we're interested in.
Click to expand...

and they're all exactly



different

well stap me vittles!
thanks for taking the time
 
pxl8 said:
This was covered in another thread recently but, 70mm is only 70mm if the lens is focused at infinity. As you reduce the focal distance the focal length will also shorten, how much depends on the design of the lens - different lenses = the differing results you've experienced.
Click to expand...

Spot on.
A macro lens that does 1:1 will have significantly less magnification at 1:1 than it does at infinity. The same applies to normal lenses too, just a bit less.

... also assumes that the factory/designers have put the 70mm mark where its actually 70mm and not just as a rough and ready guide.
 
so the bottom line seems to be
the numbers give a vague indication of what you may or may not get
just as well they have viewfinders innit!
 
mmcp42 said:
and they're all exactly



different

well stap me vittles!
thanks for taking the time
Click to expand...

I'm sure the results from my 18-135 and my old Sigma were more obvious than these ones.

Not a problem sir, you're more than welcome :thumbs:.
 
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