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05-15-2011, 11:27 AM   #1
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Lens Focal Length Rip Off.

When I use my DA*60-250 at 250, I don''t get the same magnification I get using my Sigma 70-300 at 250... or anywhere near the magnification I get with the Sigma 70-300 at 300.

I realize that the distance the birds were to the lens is emphasizing my point. But the difference is noticeable. The downside of this is when I bought the DA*60-250, I thought I was losing 50mm in focal length. But the loss in the magnification in the image size is way more than that.

So I was curious, what wouild I have to set my Sigma 70-300 to to be equivalent to 250 mm Pentax. The result.

250mm on Pentax DA*60-250



So am I to assume that what Pentax calls a 300mm lens is actually about a 200 in a Sigma meaning the Sigma 200 2.8 is actually a steal compared to the Pentax 300mm F4.

Sigma 70-300 @ 133mm



Sigma 70-250 at 240mm



I put my Pentax 18-135 to check what ti looked like and it seems to be consistent with the 60-250.

I have to say, when I bought the 60-250, I had absolutely no idea I'd be getting as short an equivalent as 133mm on my Sigma 70-300. How is this possible?


Last edited by normhead; 05-15-2011 at 11:37 AM.
05-15-2011, 11:35 AM - 1 Like   #2
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You are shooting at short distances. With IF lenses, the focal length is indeed shorter at near distances. A valid comparisson should be made at infinite distances. There are technical reasons behind it and you will find the same behaviour with any IF lens of any brand.

I personally find this more than outweight by the advantages an IF lens gives you, especially the constant length while focusing etc.

There are other current threads about the same topic, by the way.

Ben
05-15-2011, 11:36 AM - 1 Like   #3
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QuoteOriginally posted by normhead Quote
I have to say, when I bought the 60-250, I had absolutely I'd be getting as short an equivalent as 133mm on my Sigma 70-300. How is this possible?
Don't worry, this is perfectly normal. The maximum focal length of 250mm is measured at infinity. At the MFD, it's a little under 135mm. I believe it has something to do with the internal focusing (focus breathing?). I measured this a little while back on my 60-250. I'll look for the thread, but I'm pretty sure it's about 200mm by 10ft and about 225mm by 50ft. Again, don't quote me on the exact numbers, but it was something like that.

Edit: I guess Ben beat me to it
05-15-2011, 11:37 AM   #4
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Ah ha, back to the test facility.

05-15-2011, 11:52 AM   #5
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Still a noticeable difference for the Sigma, but the Sigma is out of focus so not a real comparison. Close enough I'm happy again though. Thanks.





Note to self, observation that Sigma 70-300 is better for magnifying than DA*60-250 for close small objects is confirmed and explained. I can now get back to life in general.
05-15-2011, 11:59 AM   #6
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QuoteOriginally posted by normhead Quote
Close enough I'm happy again though. Thanks.
Yeah, I wouldn't worry about it too much anyway. For near objects I imagine the need for a true 250mm focal length isn't really that important anyway.

I first noticed it when I took the shot below of my son. I was standing pretty much at the MFD and noticed that there was really no difference in the FOV as I pulled back from 250mm to 135mm. You can probably look through the VF, focused on a close object, and do the same thing and see how little change there is from 135mm to 250mm. Then repeat the same thing on something much further away and you'll see the difference pretty readily.

60-250/4 @ f/4.5, 250mm, near MFD
05-15-2011, 12:27 PM   #7
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QuoteOriginally posted by dgaies Quote
the shot below of my son.
Aaaaah, children at their best
05-15-2011, 12:59 PM   #8
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I had started a thread a while back for people to actually measure and report this for different lenses

To bad it died. This is something that unfortunately does not even show up in reviews and tests

05-15-2011, 01:02 PM   #9
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I had no idea it was so extreme.
05-15-2011, 01:42 PM   #10
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Wow. This is interesting. Its a constant learning experience this hobby!

Not that it necessarily matters (unless you want the close focus magnification!) but is this a normal feature of IF lenses (both zoom and prime)?
05-15-2011, 02:01 PM   #11
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QuoteOriginally posted by kiwi_jono Quote
Not that it necessarily matters (unless you want the close focus magnification!) but is this a normal feature of IF lenses (both zoom and prime)?
Yes, this can occur on any IF lens, prime or zoom.

That said, it doesn't reduce the amount of magnificantion in the sense that the maximum magnification reported for a given lens is measured at the MFD and so the fact that the actual focal length is smaller at the MFD is already taken into account.
05-16-2011, 02:53 AM   #12
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QuoteOriginally posted by Lowell Goudge Quote
I had started a thread a while back for people to actually measure and report this for different lenses

To bad it died. This is something that unfortunately does not even show up in reviews and tests
Actually you can get an idea what happens at closest focusing range from the manufacturers' specifications even though it is never stated explicitly.

As a random example take a Sigma 70-200mm f/2.8. Here the specifications tell us:

Closes focusing range = 140 cm = 1400 mm
Maximum magnification = 1:8

From this we can actually compute the focal length at closest fcusing distance using the general formula relating focal length (f), subject-to-lens-optical-centre-distance (s) and lens-optical-centre-to-sensor-distance (b):

1/f = 1/s + 1/b, and
M(agnification) = b/s


For the chosen example we have, based upon the information in the brochure for that lens:

s = 8*b
9*b = 1400 and thus b = 155.56 mm and s = 1244.44 mm

finally leading us to:

1/f = 1/155.56 + 1/1244.44, or f = 138(.3) mm

Note that these formulae are indepenent upon details in the design. We usually don't know the position of the optical centre but we do know, or can always measure, the sum s+b. Further, knowing the size of one's sensor, it is quite easy to measure image sizes and hence, determine M as in the examples with the ruler above.
05-17-2011, 12:11 PM   #13
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QuoteOriginally posted by Ben_Edict Quote
You are shooting at short distances. With IF lenses, the focal length is indeed shorter at near distances. A valid comparisson should be made at infinite distances. There are technical reasons behind it and you will find the same behaviour with any IF lens of any brand.

I personally find this more than outweight by the advantages an IF lens gives you, especially the constant length while focusing etc.

There are other current threads about the same topic, by the way.

Ben
This is an interesting thread, I don't own any IF lens so I am not at all familiar with the issues surrounding IF lenses. I only see disadvantage to the lens, mainly close focus will be at somewhat less than the stated FL. Maintaining a constant length while focusing, how much an advantage is this over a lens that do not?

Thanks,
05-18-2011, 11:47 AM   #14
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QuoteOriginally posted by excanonfd Quote
This is an interesting thread, I don't own any IF lens so I am not at all familiar with the issues surrounding IF lenses. I only see disadvantage to the lens, mainly close focus will be at somewhat less than the stated FL. Maintaining a constant length while focusing, how much an advantage is this over a lens that do not?

Thanks,
The advantage grows with the focal length. In a short focal length lens, IF does not make much sens. But the longer the FL, the longer the threads in a conventional lens design must be, in order to achieve close focus. So, I am for instance very happy, that my Sigma 50-500 has IF, because it extends more than enought with increased focal length.

Also, when focusing at near distances, standard lenses can be compromised quality-wise, as they are usually designed for infinite distances (the obvious exception being macro lenses). IF lenses have internal lens elements, which allow focusing and can be used as correction elements for near distances.

Another point, which I simply guess from theory, but can't remember if I have read it somewhere is, that at short distances, the lens extension can already lead to light loss, which is not the case with an IF lens.

Ben
05-18-2011, 11:57 AM   #15
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This sort of thing is pretty common, even in high end lenses. I believe Nikon's $2400 70-200 VRII is only 135mm at MFD at 200mm.
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