focal length, and maximum magnification

DreDaze wrote in post #14837168
so it leads me to think that some number is wrong...probably the case of the focal lengths not actually being 85mm at the MFD...

That's probably the reason. The focal range of most zooms is specified when focused at infinity, not a close subject. If i take a picture at 15 feet away with my 15-85, Canon 18-200, and Tamron 18-270 PZD all set to the same focal length of 50mm for example, the results from all three lenses will be different sizes. And the difference can be significant. I recall seeing the results of a test on this subject a while back. When the lens in question was zoomed in at infinity the resulting picture was 15 time larger than when zoomed out at infinity. Using the same lens at its MFD, zoomed in the image size was only 5 times the size of the image zoomed out.

DreDaze wrote in post #14837168
but also just using the numbers provided like i stated before the 17-40L at 40mm at it's minimum focus distance would provide an image that encompasses 6.2" horizontally...and the 15-85mm at 85mm at it's minimum focus distance would yield an image that's 3.4" horizontally....

That harks back to your original post:

DreDaze wrote in post #14831215
... plugging the minimum focus distance, and longest focal length into a dimensional field of view calculator shows the 15-85mm would photograph a much smaller area 6.2" vs. 3.4"

Which dimensional field of view calculator? It seems likely to me that it is taking into consideration the crop factors reasonably to be expected to apply to an EF and EF-S lens, so while the magnifications could actually be the same, the fields of view will be different.

You have to be careful of your wording as it is confusing. I don't really think that you mean "focus closer" here at all, but "magnify more".

The 17-40 focuses closer; it also magnifies more.

It seems that this is because the 15-85 must be further away in order to focus, and at that distance, its focal length is not 85 mm at all, but, if my calculations are correct (if not, somebody do it right, please), about 44 mm!

At the end of the day, the only way you are going to be convinced is by trying it, so a visit to a camera store might be the answer!

xarqi wrote in post #14838002
Which dimensional field of view calculator? It seems likely to me that it is taking into consideration the crop factors reasonably to be expected to apply to an EF and EF-S lens, so while the magnifications could actually be the same, the fields of view will be different.


You have to be careful of your wording as it is confusing. I don't really think that you mean "focus closer" here at all, but "magnify more".

The 17-40 focuses closer; it also magnifies more.

It seems that this is because the 15-85 must be further away in order to focus, and at that distance, its focal length is not 85 mm at all, but, if my calculations are correct (if not, somebody do it right, please), about 44 mm!

At the end of the day, the only way you are going to be convinced is by trying it, so a visit to a camera store might be the answer!

this one:
http://www.tawbaware.c​om/maxlyons/calc.htm

and yes i do mean more magnification...i caught myself in the original post writing 'focus closer' you figure i wouldn't do it again, but apparently i did

DreDaze wrote in post #14838038
this one:
http://www.tawbaware.c​om/maxlyons/calc.htm

There ya go - I'm betting that you put in 1.6 for the poorly named "focal length multiplier". Less of the scene is captured as a result of the smaller sensor, so of course the dimensional field of view is smaller. However, the magnification is no greater as a result as that smaller field is spread over a correspondingly smaller sensor.

This is just the great "crop sensor gives more reach" myth in a slightly different guise.

I'd hazard that it won't be the last time either!

xarqi wrote in post #14838766
There ya go - I'm betting that you put in 1.6 for the poorly named "focal length multiplier". Less of the scene is captured as a result of the smaller sensor, so of course the dimensional field of view is smaller. However, the magnification is no greater as a result as that smaller field is spread over a correspondingly smaller sensor.

This is just the great "crop sensor gives more reach" myth in a slightly different guise.

I'd hazard that it won't be the last time either!

but i used 1.6 for both examples...

DreDaze wrote in post #14838777
but i used 1.6 for both examples...

(scratches head)

Instead of all this mathemagical back-and-forth, how about someone with a 17-40 and a 15-85 take test shots of the maximum magnifications and see what's what?

armis wrote in post #14839047
Instead of all this mathemagical back-and-forth, how about someone with a 17-40 and a 15-85 take test shots of the maximum magnifications and see what's what?

If someone has the 17-40 and a camera with a sensor the same as mine (Kiss X4 aka 550D aka T2i) then I can do the 15-85 half of the test. We can use the same subject which will show it (e.g. a millimeter ruler). If anyone with a similar camera/sensor and a 17-40 lens is interested let me know.

clarnibass wrote in post #14839478
If someone has the 17-40 and a camera with a sensor the same as mine (Kiss X4 aka 550D aka T2i) then I can do the 15-85 half of the test. We can use the same subject which will show it (e.g. a millimeter ruler). If anyone with a similar camera/sensor and a 17-40 lens is interested let me know.

You can verify part of it yourself. See how many mm of the ruler you can fit in the frame horizontally at MFD and 85 mm, divide the width of the sensor in mm by that, and the result is the maximum magnification, cited above as 0.21.

xarqi wrote in post #14839532
You can verify part of it yourself.

So I just did... and the magnification is actually just under 0.24. The MFD was also almost 31cm and not 35cm like the specs say. I assume the specs are not so accurate anyway because they say the MFD is 35cm but 1.2' and there's approx 1.5cm difference between those two lengths. Still 4cm closer than specs is a bit more of a difference.

clarnibass wrote in post #14839826
So I just did... and the magnification is actually just under 0.24. The MFD was also almost 31cm and not 35cm like the specs say. I assume the specs are not so accurate anyway because they say the MFD is 35cm but 1.2' and there's approx 1.5cm difference between those two lengths. Still 4cm closer than specs is a bit more of a difference.

Most things made outside of the US are measured in metric. The approximate conversion to 1.2 feet is as close as they can get with one decimal point. I've achieved focus at between 32 and 33 cm after careful measurement with my son assisting. 31cm seems very close. Assuming your measurement is correct, I wonder if there are individual differences between examples. I can't see how there could be without other things being affected. Maybe the specs on consumer lenses aren't as tight as we would hope.

mwsilver wrote in post #14840206
Most things made outside of the US are measured in metric. The approximate conversion to 1.2 feet is as close as they can get with one decimal point. I've achieved focus at between 32 and 33 cm after careful measurement with my son assisting. 31cm seems very close. Assuming your measurement is correct, I wonder if there are individual differences between examples. I can't see how there could be without other things being affected. Maybe the specs on consumer lenses aren't as tight as we would hope.

We use mostly metric here, except a few things that are in inch measurements, I guess because it is used to be this way. I'm used to mm a lot more but inches too because I'm used to working on musical instruments where there are often both systems.

The website says 35cm and 1.15' which is very close. The lens itself says 1.2'. My point was that they didn't try to be very accurate in matching the mm and inch measurements, so I don't assume either of those is an exact measurement.

The 31cm is pretty accurate but not super accurate. I'd say approx +/- 2.0mm or maybe 3.0mm. I can measure to the closest 0.1mm if interested... but that would be pretty pointless considering the sensor mark alone is about 1.0mm thick

clarnibass wrote in post #14840538
We use mostly metric here, except a few things that are in inch measurements, I guess because it is used to be this way. I'm used to mm a lot more but inches too because I'm used to working on musical instruments where there are often both systems.

The website says 35cm and 1.15' which is very close. The lens itself says 1.2'. My point was that they didn't try to be very accurate in matching the mm and inch measurements, so I don't assume either of those is an exact measurement.

The 31cm is pretty accurate but not super accurate. I'd say approx +/- 2.0mm or maybe 3.0mm. I can measure to the closest 0.1mm if interested... but that would be pretty pointless considering the sensor mark alone is about 1.0mm thick

And do we really know how accurate the placement of the sensor mark is. And does it represent the front of the sensor as we probably would assume. The sensor mark could easily be a mm or two off with its thickness adding additional inaccuracy .

mwsilver wrote in post #14840206
Most things made outside of the US are measured in metric. The approximate conversion to 1.2 feet is as close as they can get with one decimal point.

This sort of thing makes me grin. What are they doing using ANY decimal points in the Imperial system at all?

The conversion from 0.35 m (who uses cm apart from hat makers?) to Imperial gives 1' 1 50/64" more or less.

Great system you have there guys!