Question about magnification factor of Tamron and Sigma

[RayDream]

Just curious when I found out the specs of the following 2 lens:

Sigma 70-300 Macro, MFD = 0.95m
Tamron 28-300 VC Macro, MFD = 0.49m

The new tamron 28-300 VC has a shorter MFD, however strangely that lens has a lower macro magnification factor (1:3) than Sigma (1:2). Somebody please explain why!?

[sumball]

Shorter MFD doesn't mean higher magnification ratio. Probably it is due to its 28mm wide angle. Most WA lenses have short MFD but the mag ratio is not high.

[RayDream]

Thanks for the reply. However the review tests that I've checked were all tested at 300mm. How could both 300mm at shorter focus distance give opposite magnification factor?

[CY_OH]

The Tamron 28-300 might have an Internal Focusing design, which means that the actual focal length decreases as the focusing distance gets closer.

At 0.49m, the angle of view might only be similar to 200-230mm lens instead of 300mm.

[frederickykfoo]

In any lens the Minimum Focusing Distance has nothing to do with the magnification ratio of the lens. For example Macro lens of 90mm has shorter MFD than a 180mm macro. Both can do 1:1 magnification ratio. They are just features depends the design and requirement by the manufacturer. If they want greater magnification ratio, they will design for it. If they want low MFD, they will do it.

[RayDream]

Originally Posted by frederickykfoo In any lens the Minimum Focusing Distance has nothing to do with the magnification ratio of the lens. For example Macro lens of 90mm has shorter MFD than a 180mm macro. Both can do 1:1 magnification ratio. They are just features depends the design and requirement by the manufacturer. If they want greater magnification ratio, they will design for it. If they want low MFD, they will do it.

Heh your example doesn't say anything...

mag of shorter MFD 90mm = mag of longer MFD 180mm
Makes perfect sense to me... so what are you implying?

what makes greater magnification ratio then? Unless somebody come up with a comparism of 2 lenses of the same MFD, same focal length, diff magnification ratio that could explain it..

[Clockunder]

Generally, according to the law of optics physics, the magnification factor is directly proportional to focal length and inversely proportional to shooting distance.

This means that :
If you use a longer focal length and stand at the same shooting distance, the magnification is higher (i.e. your subject will appear bigger within the frame as the actual image on the sensor is bigger and this is also consistent with the result of a narrower field of view with a longer focal length.).

If you use the same focal length but stand closer (i.e. with a shorter minimum shooting distance), the magnification is higher too and the subject will appear bigger within the frame as the actual image on the sensor is bigger now.

Indirectly, it means that for the same magnification (such as 1:1), a longer focal length used would be at a further shooting distance. This is why the minimum shooting distance is supposedly further for longer focal length if the magnification factor is the same for both lenses.

What the threadstarter has noticed indeed appears to be an anomaly on the surface.

Notwithstanding the above, modern lenses (especially those wide range zooms) these days have complex designs which make direct magnification comparison not so easy, not only between lenses but also between different focal lengths of the same lens in different modes.

Actually, if we compare the Tamron AF28-300mm F/3.5-6.3 XR Di VC (Vibration Compensation) with another Tamron lens SP AF70-200mm F/2.8 Di LD (IF) Macro, there is also this anomaly.

AF28-300mm F/3.5-6.3 XR Di VC (Vibration Compensation)Macro Magnification Ratio 1:3 (at f=300mm, MFD=0.49m)

vs

SP AF70-200mm F/2.8 Di LD (IF) Macro
Macro Magnification Ratio 1:3.1 (at f=200mm, MFD: 0.95m))


Despite the AF28-300mm F/3.5-6.3 XR Di VC (Vibration Compensation) at a longer focal length (300mm vs 200mm) and standing nearer (0.49m vs 0-.95m), its magnification factor is only about the same (1/3 vs 1/3.1) as the SP AF70-200mm F/2.8 Di LD (IF) Macro.

The AF28-300mm F/3.5-6.3 XR Di VC (Vibration Compensation) at 300mm shooting at 0.49m has about the same composition as the SP AF70-200mm F/2.8 Di LD (IF) Macro at 200mm shooting from further 0.95m. This means if the former is at shorter focal length 200mm instead of 300mm and then standing further away to the same distance 0.95m as the latter (instead of 0.49m), it would produce a much lower magnification (i.e. wider "field" of view where subject becomes much smaller with the frame when compared to previously) than the latter. Indirectly, it means that the former at 300mm in macro mode must have had a much wider angle of view than the latter lens at 200mm and therefore, standing nearer at 0.49m does not produce a larger subject image on the sensor than the latter at further 0.95m.

Considering the reasoning from the "angle of view" of lenses, my only logical explanation I can come up with to threadstarter's question is, as someone has already mentioned earlier in this thread, the angle of view of the lens must have been wider at 300mm (i.e. looks very much less than 300mm) when in macro mode than when in telephoto mode. Due to the complexity of zoom lenses, the angle of view and magnification factor are not a simple comparison of focal length and shooting distance between lenses.

[p.s. the measurement of focal length is also a complex matter for lenses and the actual focal length may still be 300mm in macro mode for the aforementioned lens but the angle of view is no longer the same as in telephoto mode where a simple arc Tangent (diagonal of image sensor divided by focal length) easily calculates the angle of view at that focal length]

[RayDream]

Heh cool! Very comprehensive oberservations. Thanks for the info. Damn Tamron marketing strategy Got to read the specs carefully.