Apaflo Loc: Anchorage, Alaska

The "real" numbers are usually very close to the marked numbers, and they are not estimated. Most are within 5%. The point is that if a 300mm lens is actually 285mm, does anyone really think it should be sold only as a 285mm lens? In real life, who cares!

Robert Bailey Loc: Canada

I haven't heard a clear answer yet to the question "Is one a full-frame lens, and one a crop-sensor lens?".

leftj Loc: Texas

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leftj Loc: Texas

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Retina Loc: Near Charleston,SC

I just thought he meant the older Canon lens is labeled with its actual focal lengths and the other labeled as the 35mm equivalent when used on a crop sensor, which would make 300mm on the Tamron something closer to 200mm.

Apaflo Loc: Anchorage, Alaska

It would not make any difference at all. A 300mm lens doesn't change focal length if it is marked FX or DX nor if the camera is FX or DX.

But in fact both of those lenses cover a full frame sensor.

waegwan Loc: Mae Won Li

No, they are both FF lenses but the comparison photos I posted were shot on a crop-sensor camera. Hopefully tomorrow I will do some more shots outside using a full frame and a crop at various distances.

waegwan Loc: Mae Won Li

That is a good point and although I have a crop sensor camera I don't have any crop sensor (EF-S) lenses.

Retina Loc: Near Charleston,SC

Thank you. I didn't catch that.

Apaflo Loc: Anchorage, Alaska

Tamron (and all other manufacturers) labels the lens with it's actual focal length. At least for interchangeable lenses.

Lenses that are permanently attached to a camera are generally the only ones that might be marked with an "equivalent" 35mm full frame focal length.

BboH Loc: s of 2/21, Ellicott City, MD

Its the angle of view. Primary magnification (PMAG) is determined by the horizontal width of the angle of view of the lens divided by the horizontal width of the sensor.

Retina Loc: Near Charleston,SC

I appreciate that.

blackest Loc: Ireland

I think i may have managed it for a lot less with an enlarger lens it is very much external focusing only f4.5 - f16 though.
http://www.uglyhedgehog.com/t-431652-1.html#7333145

So for an 80mm lens infinity focus is at 80mm
a near object at 800mm (10x focal length) the lens must move out to 88.8 mm
for 400mm object distance (5x focal length)it would be 100mm 1.25 (80mm x 1.25)
and 160mm object distance (2x focal length) it would be 160mm 2 (80mm x 2) which pretty much explains why extension tubes are used for macro

Lens equation
1/ object distance + 1 / image distance = 1/focal length

Distance of the object in front of the lens Position of the image behind the lens
in units of the focal length in units of the focal length

10 1.11
100 1.01
1,000 1.001
10,000 1.0001

This table pretty much explains why there is such a difference between near focusing and far focusing
if we use a 100mm lens for easy math we can convert to actual mm

10x 1.11 1meter 111mm from the sensor
100x 1.01 10meters 101mm from the sensor
1000x 1.001 100meters 100.1 mm from the sensor
10,000x 1.0001 1000m 100.01mm from the sensor

At 10meters the lens barely needs to move 1 mm is about the width of the 1 in this post

dof and hyperfocal distance tables and calculator
Hyper focal distance for nikon dx body 100mm lens
f4 125.1 meters
f5.6 88.5 meters
f8 62.6 meters
f11 44.3 meters
f16 31.4meters

using the lens equation

1/ object distance + 1 / image distance = 1/focal length
or
1/focal length - 1/object distance = 1/ image distance.
f4 1/0.1 -1/125.1 = 9.9992 =0.10008 meters or 100.08mm

f5.6 9.9988 0.100113m 100.11mm
--
f16 9.9681 0.100319m 100.32mm

So aperture and sensor size makes a difference to where infinity effectively begins but still a tiny difference in where the lens needs to be in front of the focal plain.

TL:DR
For focusing nearer than infinity the lens needs to move further away from the sensor the nearer you focus. Around 10x the focal length is relatively easy to achieve, the distance the lenses have to move is 10% of the focal length, although that can be significant and hard to engineer on a long focal length lens. At infinity focus the lens will be within a fraction of a mm of its actual focal length (which is measured at infinity anyway) So you see very little change in magnification at infinity but a lot more at close focus distances. It also makes sense that you will see this more on a long focal length lens than a short focal length lens. Making a Zoom lens is much much harder than making a Prime.

It pretty much explains why a nifty 50mm is so much cheaper than a long telephoto, it is a lot easier to make :)

Screamin Scott Loc: Marshfield Wi, Baltimore Md, now Dallas Ga

Floyd & Gene beat me to the answer. It is due to IF. I shoot a lot of macro & I see this phenomenon frequently.

waegwan Loc: Mae Won Li

Yes I believe we have the answer. What I hope to do tomorrow is just to post some example for other folks who are thinking about leses in this range and what to expect.