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Old Jun 3, 2009, 10:29 AM   #1
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Default Lens Multiplier Question

I'm comparing Digital SLRs. Generally they have lens multipliers of 1.5 or 1.6. Some of the cameras Im looking at have body/lens packages. If the camera says it includes an 18-55 and 55-200 lens do I have to multiply these by the lens multiplier or has this been taken into account and the lenses will actually be 18-55 and 55-200 on the digital body they come with?
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Old Jun 3, 2009, 11:05 AM   #2
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The focal length of a lens is the focal lenght of a lens; it doesn't change depending on what camera body it's mounted on. The "lens multiplier" is for people that are familiar with 35mm film and full frame digital SLRs. Since many dSLRs have images sensors that are smaller than a 35mm film exposure, the same lens will result in a narrower angle of view when mounted on one of these APS-C dSLRs. So a 50mm lens on an APS-C dSLR will have the same angle of view as a 75mm or 80mm lens on a 35mm film or full frame digital SLR.

If you aren't familiar with the angles of view certain lenses will provide on a 35mm film or full frame digital SLR, then the "lens multiplier" is meaningless to you.
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Old Jun 3, 2009, 11:12 AM   #3
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Here's how it got explained to me:

Full Frame Image Sensor Size: the sensor is 24mm tall and 36mm wide= a diagonal measurement of 43.3mm
Canon XSi Image Sensor is: 14.8mm tall and 22.2mm wide = a diagonal measurement of 26.7

Now divide the diagonal measurements, the larger by the small to get your ratio:

43.3/26.7 = 1.6

This means your lens focal length is increased by this factor due to the size of the sensor.

A 100 mm lens on the smaller sensor will deliver photos as if it were 160mm, not 100mm because of the sensor size issue (100 x 1.6 ratio= 160)

This phenomna is called sensor cropping.

Image cropping is a real problem when taking close up photos, the shortest lenses are 50% longer than their stated focal length due to image cropping.

Somehow, seeing how the math worked made it make sense to me.

Last edited by FaithfulPastor; Jun 3, 2009 at 11:28 AM.
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Old Jun 3, 2009, 11:19 AM   #4
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Quote:
Originally Posted by FaithfulPastor View Post
This means your lens focal length is increased by this factor
A 100 mm lens on the smaller sensor is 160mm, not 100mm.
The Focal Length of a lens is a physical property of that lens. What changes when you move the lens from one camera body to another, is the angle of view, not the focal length. If you only have one camera body, and you don't have any significant experience with lenses on a camera body that has a different size image sensor than the one you're considering, then the "lens multiplier" doesn't mean anything to you.

A 100mm lens is a 100mm lens even if it's not mounted on a camera.
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Old Jun 3, 2009, 11:56 AM   #5
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Quote:
Originally Posted by TCav View Post
The Focal Length of a lens is a physical property of that lens. What changes when you move the lens from one camera body to another, is the angle of view, not the focal length. If you only have one camera body, and you don't have any significant experience with lenses on a camera body that has a different size image sensor than the one you're considering, then the "lens multiplier" doesn't mean anything to you.

A 100mm lens is a 100mm lens even if it's not mounted on a camera.

TCav: I lost my 7 iron golf club. I couldn't find a 7 iron for sale by itself. But I did find a 6 iron. I bought the 6 iron and bend it 3 degrees, so it matches the loft of a 7 iron.

So in my bag, you'd look in and see two 6 irons. But I don't have two six irons, I got a six that plays like a six and a six that plays like a seven.

I know the lens's focal length doesn't change when it's moved from one camera to another. But to the user, it acts like it does. So even though it says 100mm on the lens, if you put it on a cropped sensor camera, it'll act like a 160mm, regardless of what you call it, that's how it acts.

From a technical standpoint, what you've written is true. From a practical application standpoint, I believe it's much easier for people to think in terms like "altered focal length" because that's the easiest way to do the conversion.
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Old Jun 3, 2009, 12:00 PM   #6
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TCAV is correct. The way to re-state Faithful Pastor's information is:

A 100mm lens on an aps-C canon DSLR (1.6 factor) has the same FIELD OF VIEW as a 160mm lens does on a full frame camera.

Whenever you see focal lengths on a lens or in a review of a lens etc. they are always referencing the physical focal length of the lens - NOT it's field of view.

So even though digital Olympus DSLR lenses are ONLY used on cameras with a 2x crop factor, lens specifications are still physical focal length. A 100mm Oly lens is physically 100mm focal length. It has a field-of-view of 200mm on the Oly dslrs though.
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Old Jun 3, 2009, 12:07 PM   #7
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Quote:
Originally Posted by FaithfulPastor View Post
TCav: I lost my 7 iron golf club. I couldn't find a 7 iron for sale by itself. But I did find a 6 iron. I bought the 6 iron and bend it 3 degrees, so it matches the loft of a 7 iron.

So in my bag, you'd look in and see two 6 irons. But I don't have two six irons, I got a six that plays like a six and a six that plays like a seven.
And if someone who's never played golf before, reaches into your golf bag, pulls out a club, hits the ball, and likes where the ball went, he's going to keep using that club. It doesn't matter to him that it's a six that plays like a six or a six that plays like a seven. The ancestry of your collection of golf clubs is meaningless to him, much like the "lens multiplier" is to someone that's never used a film SLR

Last edited by TCav; Jun 3, 2009 at 12:39 PM.
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Old Jun 3, 2009, 12:50 PM   #8
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I tend to agree with everyone here in a way (technically speaking, everyone is right). I had this problem originally too. As I've never shot 35mm SLR, I too came to the realization that the "crop factor" is meaningless. My 70-200mm @ 200mm is 4 times longer than my 50mm. That's all that really ends up mattering.

I've only ever actually bothered to multiply things out was when I was using an online calculator for DOF or AOV that didn't support the "sensor size" value (I had to remember that if I wanted 10 degrees horizontally, though the calculator said I needed a 200mm lens, in actuality, I needed only 135mm.

The other case is when seeing a photo and the photographer says "20mm lens", you think "wow, i'd love a lens like that", you have to realize that you might actually need 14mm or 30mm depending on your camera body and the original photographer's...

The last case is when comparing point and shoot cameras that do (or don't) have the "35mm equivalent" values listed, or when comparing one to an SLR with a known "crop factor".

In short, the big point you need to remember is that when comparing two cameras with crop factors of 1.5 and 1.6 respectively, the camera with the crop of 1.6 will have a slightly narrower angle of view (it will see about 6% further) when compared to the other using the exact same lens. Rather than compare all the possible sensor sizes to each other, since the most common constant is 35mm film, its most common to compare everything back to 35mm.
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Old Jun 4, 2009, 1:58 AM   #9
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As you indicated its really only useful when comparing cameras. If your comparing a 4:3 camera to an APSC its useful, or even to P&S units.

Earlier this year I picked up a Panasonic LX3 P&S with a focal length of 24 to 60mm (35mm equivalent). This relates to my Pentax dSLR's (crop factor of 1.5) lenses of about 18mm to 40mm (or almost the equivalent of the 18-55 kit lens).
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Old Jun 4, 2009, 6:10 AM   #10
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To put it in the simplest terms, it's easiest to see the effect.

If you have a 28mm lens on a "full frame" DLSR(e.g. Nikon D700) or a 35mm film camera, you will get what you see here: (minus the "box" of course!!)



If you use the SAME lens on a DSLR with a smaller APS-C sensor(about 1.5x or 1.6x crop, for example a Nikon D300), you will get less of the total image thrown by the lens, because the sensor is smaller, and doesn't capture all of it. (just the part inside the box)

So in the end, the D300 will capture this:



You can see you are missing the outside edges. So although the lens is 28mm IN BOTH CASES, the field of view CAPTURED by the D300 (1.5x crop sensor) looks like a 42mm lens. When people talk about "equivalent focal length", it really means the "equivalent field of view", because the outside is cropped.
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