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10-06-2009, 10:18 PM   #1
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minimum focussing distance of Macro lens

Hello folks...I have been researching on which Macro lens to go in for....but I need to determine

1) what 'minimum focusing distance' constitutes a macro lens?
2) I have an MC Auto Focal (do not go by the name - it is a manual focus lens) 28 mm with minimum focusing distance of .20m. Does this qualify as a macro lens by present days standards
3) my pentax 18-250 has a minimum focusing distance of .45 m but at 250mm I seem to be able to focus as much lesser distance than .45m. How is this possible?

Any inputs are welcome...btw I recently posted my work on flickr and picasa...this is from my older P&S...will start adding my K200D shots shortly

Picasa Web Albums - Kamalpreet S. Saw...
Flickr: Kamalpreet S. Sawhney's Photostream

10-06-2009, 11:50 PM   #2
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Focusing distances are normally measured from the "film plane" (ie, from the sensor in a digital camera). When you get into macro distances this becomes a lot less useful, so magnification ratios (1X, 1.5X, etc.) are used instead, where the ratio is between the actual subject size and the size as rendered on the sensor.
10-06-2009, 11:58 PM   #3
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Most people, when discussing macro lenses, primarily talk about magnification (the ratio of the size of the image on film/sensor vs the size of the real object). Of course magnification ratio is related to focusing distance.

Most fixed-focal lenses (forget about zoom lenses for now) have max. magnification about 1:7 (the size of the image on film/sensor is 1/7 the size of the real object). At that magnification, the (minimum) distance from the film/sensor plane (NOT from the lens) to the object is about 8X focal length. A 'normal' 50mm lens has minimal focus distance of about 40cm.

Strictly speaking, 'macro' lens is defined as a lens that can achieve at least 1:2 magnification. Most current macro lenses nowadays can get 1:1 magnification. To get to the macro magnification, the minimal focusing distance is much less than that of a non-macro lenses. For example, a 50mm macro lens, at 1:1, has focusing distance of 19cm; at 1:2, 22 cm. A 90mm macro lens at 1:1 has focusing distance of 29cm and at 1:2, 35cm.

The 28mm lens you mention has minimal focusing 20cm. At that distance, the magnification ratio is about 1:6 or 1:7, not quite 'macro.'

Later, the designers of zoom lenses figured out a way to achieve magnification much better than 1:7. 1:4 is not uncommon. Some zoom lenses can even go all the way to 1:2. Initially such lenses were call 'close focusing' but marketing deparments of the lens manufacturers decided to call these zoom lenses 'macro,' adding more to the confusion.

IIRC, the 18-250mm lens can do 1:3, pretty impressive for a superzoom. But I don't think it can focus closer than 45cm. Measure again to make sure. Note that the distance is from the sensor plane, not from the front of the lens.

It's not just magnification. A proper macro lens has other characteristics not found in non-macro lenses: flatter field, minimal distortion, ....

Also, note that the aperture markings ('nominal' apertures) are when the lens focuses at infinity. At close distances and particularly at macro distances, the effective aperture is slower. At 1:1, the effective aperture can be 2 stops slower than the nominal aperture.

To add to the confusion, some people use the term 'micro' lenses. It's the term used by Nikon.
10-07-2009, 04:58 AM   #4
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Focal lenths also matters

To add more confusion many zooms change their apparent focal length when they focus on a subject closer than infinity. So while the 50-200mm may be 200mm at the long end when focused closer it may give the same image as a 150mm or 180mm. Additionally most true macro lenses also are optimized for closer focus distances.

(NOTE: I may be picking on this lens, it is an example, as I have no idea what it appears to be).

10-07-2009, 06:10 AM   #5
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QuoteOriginally posted by skamalpreet Quote
Hello folks...I have been researching on which Macro lens to go in for....but I need to determine

1) what 'minimum focusing distance' constitutes a macro lens?
2) I have an MC Auto Focal (do not go by the name - it is a manual focus lens) 28 mm with minimum focusing distance of .20m. Does this qualify as a macro lens by present days standards
3) my pentax 18-250 has a minimum focusing distance of .45 m but at 250mm I seem to be able to focus as much lesser distance than .45m. How is this possible?
As Sean said, "Minimum focus distance" is measured from the image plane,not the lens. The distance from the lens is called "working distance".

Minimum_Focus_Distance=Focal_length*((1+m)^2)/m where m=magnification.

Working_Distance ~ Focal_length*(1+1/m)

The ~ sign means "approximately equal to"; it is approximate because it is true only for an ideal "thin lens"and does not account for any hoods, lens recesses, etc..

To answer your question about the 28mm lens, consider that to qualify as a Macro lens, the magnification should be 1/2 or more. Substitute 1/2 into the Focus distance equation,

28mm((1+.5)^2/(.5)) = 126mm, so it is not a macro lens.

Dave
10-07-2009, 06:53 AM   #6
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To simplify this, a macro lens will hit a reproduction ratio of ~1:2 or larger. This means that the subject would be about half the size on the sensor as it is in real life.
Once you start getting larger than life size, you are getting more into the "micro" range.
10-07-2009, 02:16 PM   #7
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Summarizing / restating / augmenting what's been said so far:

- The shorter the focal length, the shorter the focal length you need to get into macro territory. This is not some sort of arcane science, but common sense - a longer focal length magnifies thigs more from a given distance, so you need to get close to it with the shorter focal length to get the same magnification.

- Because of this, we don't typically compare lenses in terms of minimum focus distance. We compare in terms of magnification ratios: how big you can make an object appear on the sensor. Since the sensor is basically about an inch across, there's a very easy to measure this for any lens you have access to: focus on a ruler from as close as the lens allows. However many inches fit across the the width of the viewifnder, that's the magnification ratio. That is, if you can see three inches across, it's 1:3. Two inches and it's 1:2. And if your lens allows you to fill the viewfinder with a single inch, then it's 1:1. That's really where most people would say you need to be to be considered "true" macro, although 1:2 gets you close enough that many will claim that's macro too.
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