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Old Aug 10, 2006, 9:33 AM   #1
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ive been thinking of moving up to DSLR from my prosumer panasonic FZ7. its a big jump trying to understand lens focal lengths, sensor sizes,multipliers , converting sizes to that of a 35mm camera zoom factors and trying to stop thinking in zoom ( 2X , 12X etc) etc.

is there a difinitive guide or could someone post one, rather than us having to trawl through thousands of posts



i havnt got a DSLR yet so i cant learn through expirience
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Old Aug 10, 2006, 1:11 PM   #2
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Zoom mulipliers: 2x, 3x, 10x etc...

A fairly useless term - all it is is a ration of longest focal length over shortest focal length.

So 100-400mm lens and 25-100mm lens are both 4x zooms. So, just using the 4x term is really pointless - it doesn't tell you how wide or long the zoom actually is.

SLR lenses (and digital slr lenses) all have a focal length assigned to them. Keep in mind 50mm is approximately what the human eye sees. So, something with focal length less than 50mm is wider than the human eye while a focal length more than 50mm has more magnification than the human eye.

Now, SLR lenses were designed to produce a circle of light to illuminate 35mm film. Most digital SLR cameras have a sensor that's smaller than 35mm film. So, some of that light circle 'falls off' the sides of the sensor - the result is something similar to a cropped photo. This is where the 'crop factor' of the various DSLR manufacturers come into play. The factor applied is a result of the size of the sensor.

Canon uses full frame (1ds, 5d), 1.3x (1ds Mk II N, 10d) and 1.6x (30d,20d,350d,300d) sensors.

Nikon DSLRs are all 1.5x

Olympus DSLRs are 2.0x

KM / Sony DSLRs are 1.5x

To determine the EFFECTIVE focal length of a lens on a DSLR you multiply the focal length assigned to the lens (which you can find in any lens spec and printed on the lens itself) by the crop factor.

So, a 100mm lens on a Nikon DSLR would effectively be a 150mm lens. That same lens on a Canon 20D would be equivelent to a 160mm lens. Another way to think about it is this: If you put a 10mm lens on your Nikon DSLR and take a picture of an object, your picture would look almost identical to a picture taken on a FILM Nikon SLR camera that used a 150mm lens.

Even the new lenses being built specifically for digital SLRs still have their focal lengths quoted in 35mm terms. So you still apply the 'crop factor' to the focal lengths of those lenses to determine what the equivelent focal lengths will be on your DSLR.

For instance, Canon makes a 17-85 EF-S lens which is designed to work only on their 1.6x crop factor cameras. That lens has an equivelent focal length of 27.2mm - 136mm when used on the 1.6 crop factor cameras.

Hope that helps a little.
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Old Aug 10, 2006, 4:59 PM   #3
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An excellent Question (not so dumb) and a knowledgable reply.

Are you saying then that 70-300mm (you said these are quoted in 35mm terms) is actually a 112-480mm on a Pentax istD (1.6 crop factor).


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Old Aug 10, 2006, 5:04 PM   #4
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Yes, that's what I'm saying in general. Purists will argue the details but conceptually that is true.
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Old Aug 10, 2006, 5:07 PM   #5
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Thanks John - much appreciate sharing your knowledge.
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Old Aug 11, 2006, 7:56 AM   #6
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fantastic. exactly the answer i was lookig for. that clears up a whole lot.



so the size of the sensor determines the crop factor.



can you get something that will go between the lens and the camera ( a divider perhaps!) that will allow the lens to focus the full image on the sensor so no cropping occours?
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Old Aug 11, 2006, 8:22 AM   #7
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Not that I've ever heard of. I'm not an expert on optics but I would presume there are issues with attempting to do that or I'm sure manufacturers would have done it. Instead they have gone the route of creating lenses designed specifically for the smaller sensors - like 10-22mm lenses. These lenses have smaller optics because they don't need to create as lare of an image circle. Again, those lenses are still listed in 35mm terms so crop factor still applies (i.e. 10mm becomes 15 or 16mm). This was done to get people the ultra-wides they lost when they switched from film to DSLR (since most ultra-wide non fisheye lenses were in the 15-18mm range).

Again, perhaps one of the more optics savvy people here can explain the difficulties with the approach you suggested.
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Old Aug 11, 2006, 9:21 AM   #8
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so correct me if im wrong, so even though the numbers on the side of most lenses are in relation to 35mm cameras, so we have to apply crop factor to see what actual focal length is on a digital camera, we still get the full image as the optics in the lens are smaller.

this is all very confusing!
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Old Aug 11, 2006, 9:36 AM   #9
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schmintan wrote:
Quote:
so correct me if im wrong, so even though the numbers on the side of most lenses are in relation to 35mm cameras, so we have to apply crop factor to see what actual focal length is on a digital camera, we still get the full image as the optics in the lens are smaller.

this is all very confusing!
No - you don't 'get the full image'. Even with lenses designed for DSLRs you still get a cropped image.

Maybe this will help. The photo you capture will still be the photo you see throught the viewfinder (actually in most cases the viewfinder doesn't give you 100% of the photo - so you get a little more). So, when you look through the viewfinder you'll see the 'cropped' image.

The cropping ALWAYS HAPPENS. Remember, the focal length on a lens is a measurement of the optics of the LENS and is independent of the media recording the light. Here's a write-up:

http://www.mellesgriot.com/products/optics/fo_5.htm

So, the lens itself always has a set focal length or range of focal lengths - the lens hasn't physically changed just because you moved it from a film SLR to a DSLR - the optics are still the same.

BUT, because a smaller image circle is captured on the DSLR media the field of view of that image APPEARS as if it has been cropped. So, you have two separate discussion points:

1. What is the optical focal length of the lens - which remains constant regardless of what camera body the lens is mounted on. Even with the digital only lenses that have smaller diameter optics the laws of physics are still the same - you still measure focal length for the lens the same way.

2. What is the field-of-view generated on the particular media when that lens is used. The easiest way to see this affect is to take a lens and two bodies - one a film or full frame DSLR the other a crop factor DSLR. Switch the lens between the two bodies - you'll see the difference in the viewfinder.
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Old Aug 11, 2006, 9:38 AM   #10
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Here's another write-up in Luminous landscape - he does a much better job than I do of explaining this...

http://www.luminous-landscape.com/tu...dslr-mag.shtml


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