 Jeff Millard |
Focal Length vs Zoom Value
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Alan N. Marcus |
Hi Jeff, When it comes to magnification and cameras, most text books proclaim 50mm focal length to be magnification 1 and 100mm as magnification 2, 200mm=4x, 400mm=8x etc. However magnification, when it comes to cameras, is not that cut and dried. The text books are reciting magnification as related to the venerated 35mm film camera. The 35mm is generally outfitted with a 50mm focal length lens. Now focal length functions as to magnification, for any format, is based on the dimensions of the format. For the 35mm film camera the format was set by Leitz (Germany Camera Company) that marketed the first 35mm in 1924. Their chief engineer set the format dimensions at 24mm by 36mm. Now what is telephoto and what is wide-angle and what is normal is determined by the diagonal measure of the format. In this case that dimension is 43.27mm. Mr. O. Barmack (Leitz chief engineer) mounted a 50mm as this is a logical rounding up of the curious diagonal measure. Thus 50mm became the yard stick. Actually the lens of choice should be 43mm ~ 45mm as this fixes the angle of view at 53°, consider “normal” by opticians. Astronomers embraced the 35mm film camera and mounted them to their telescopes, referencing the 50mm as magnification 1. Thus if a telescopic lens or telescope is functioning at say 1000mm than that authority references the magnification as 1000 ÷ 50 = 20 written as 20x. The actual value should be 1000 ÷ 43 = 23x. Note the tiny difference is insignificant. Now if a camera has a different size format than the divisor is the actual diagonal measure. Your Canon is normally marketed with a kit zoom lens. In this case the lens zoom range is 18mm ~ 55mm. Odds are, the center of this zoom range matches the diagonal measure of this camera’s sensor chip. Most of these cameras are fitted with a chip smaller than the 35mm full frame. Likely you camera has a crop factor of 1.5 meaning all that pertains to a full frame must be divided by 1.5 to make the correlation. Additionally this correlation is made from the 50mm rounded up diagonal, and not from the actual 43.27. Thus 50mm ÷ 1.5 = 33mm. Let’s say the diagonal and thus the normal for your camera is 30mm, That being the case, 30mm = magnification 1 – 60mm = magnification 2 – 120=4x – 240=8x – 480 = 16x etc. When the optical ability of mounted lens to zoom in is exhausted, the electronic zoom continues. Thus if you mount a lens capable of zooming out to 300mm, the magnification would be 300÷30= 10x. Now if the electronic zoom is set to 2x then the final effect is 20x. Keep in mind that camera makers almost most always talk in terms of the 35mm equivalent. Thus a 35mm with a 1000mm mounted would be references as 20x. Magnification is actually more complex because we are viewing an enlarged image on our monitor as are the prints we make. As an example a 4x6 print measures about 100mm by 150mm. The 35mm frame is 24mm by 36mm. The print is a 4.2x enlargement i.e. 24mm x 4.2 = 100mm and 36mm x 4.2 = 151mm No body said it’s easy! Alan Marcus (marginal technical gobbledygook)
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- Gregory LaGrange Contact Gregory LaGrange Gregory LaGrange's Gallery |
Can depend on who you're talking too and what they're talking about. Cameras like your panasonic have a lens with an actual focal length, optical zoom if it's a zoom lens, and a digital zoom. The digital zoom is merely enlarging the center area of the view finder. Think of it as taking an actual image, making into an 8x10, and then cutting it down to a 4x6 so that the final image is a tight shot. For slr's, you have some that have sensors that are the same size as what you see as a film negative. More kinds have sensors that are smaller. So to match, the view finder is smaller, has a narrower field of view. Some people call it a telephoto effect, which it isn't, as if it gives you more focal length to a shorter lens. It's merely just a narrower view, no different than if you had a view from a window and taped cardboard around the edges of the window, blocking what was to the far left and right. It doesn't bring what you see out there any closer, just what you see is predominantly what's in the middle of view. Since longer focal lengths have narrower fields of view than short focal lengths, you'll see it expressed relevant to that. If you want to get a tight shot of an airplane, fill the frame, then depending on how high it is, you may need something from 300mm on up. I'm guessing that was an air show, so it can depend on what kind of trick they're doing at the moment. Something relatively close, maybe a 200 will fill the frame, but I thing most of what you'll see you'd need something longer. Otherwise, you'd have to take your picture, and crop to a tight frame. Which is essentially what your panasonic did. But the should be better with a slr and a lens long enough to get a decent shot of the plane.
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Jeff Millard |
No- not easy but I think this is why photography has grabbed me-- equally packed with technical and artistic. Perfect answer Alan. I couldn't quite figure out why the 'cheaper' cameras got more with less lens out there. (I guess 'lens size envy' is really a factor of 'sensor size envy' as well ;-) I'll do the math and see what that Panasonic was getting at 70mm to see what I'll need to do similar ranges with the DSLR. I bought the DSLR because the fixed lens camera had no aperature range but I didn't realize the glass I would need to reach out would be so much bigger with the 'better' camera.
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- Gregory LaGrange Contact Gregory LaGrange Gregory LaGrange's Gallery |
You put better in quote marks as if you may think you've made a mistake. If that's the case, then just crop your photos with the slr to the same perspective as the panasonic. That's essentially what it was doing.
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Alan N. Marcus |
I did a little math for you: The imaging chip of the Panasonic Lumix DMC-FZ8 is 17% of the size of a full frame 35mm that calculates to a crop factor of 6. This camera is normally supplied with a zoom 6mm ~ 72mm. Multiply by 6 = 35mm equivalent 36mm ~ 432mm Thus the aircraft shot at 78mm zoom focal length correlates to 78 x 6= 468 Realistically you will be viewing an image on the monitor or a 4x6 inch print. One more item: Opticians view discussions on magnification as claptrap. They sight different camera formats and different viewing distances and different enlargement sizes, all of which confuse the outcome. They use angle of view as the solitary method to make this measurement. A 35mm full frame with a 28mm wide-angle mounted has an angle of view of 46° height - 65° width - 75° diagonal A 35mm full frame with a 43mm normal mounted has an angle of view of 31° height - 45° width - 53° diagonal
Note the manufacture only quotes the diagonal angle because in the realm of wid-angle values this will be impressively big, like TV’s are measured and sold by their diagonal measure. Alan Marcus (more technical gobbledygook)
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Jeff Millard |
Your math is perfect (resulted in the same crop factor quoted by the manual) ;-) All this is making more and more sense except... the sensor size on the panasonic is larger than the canon SLR but yet supports more effective zoom than the same physical length Canon lens. Why do the SLRs seem to need a much longer lens to achieve the the same zoom factor. Greg suggested cropping but I'm not sure I understand that-- Isn't it better to use optics to pull the image in rather that digitally enlarge? I suppose there is a balance between the two tools but I don't think the Cannon with the 18~55 lens all the way out 55/(88) and THEN cropped/elarged would have had the same image quality as the Panasonic with the lens out to 78 (468). And-- to achive the same 10x zoom on the Cannon I'd need > a 300mm lens. Am I wrong?
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W. |
Hi Jeff, A Panasonic Lumix DMC-FZ28 has a sensor size of (6.13 x 4.60 mm. A Canon D40 has a sensor size of 22.2 x 14.8mm. So how is "the sensor size on the panasonic larger than the canon SLR"?
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Alan N. Marcus |
Hi again Jeff, You are basing your conclusion; regarding which is bigger on the crop factor. The crop factor of the Panasonic is 6. The crop factor for the Canon is 1.6. What does the crop factor tell us about the size of the sensor chip? Let’s first look at the Panasonic. Crop factor 6 means, if we are presented with data pertaining to a full frame 35mm, we must divide that data by 6 in order to rearrange it and make it applicable to the Panasonic. Another way of saying that same thing is to write this value as 1/6 = 0.17 or stated anther way, the chip in the Panasonic is 17% of a full frame. Thus 24 x 0.17 = 4 and 36 x 0.17 = 6. Thus the chip in the Panasonic is about 4mm by 6 mm. As to the Canon, its stated crop factor of 1.6. (actually 1.62). So 1/1.6=0.63 meaning its chip is 63% of the size of a full frame. Thus 24x.0.63=15mm by 36x.0.63=22. Thus the chip size is about 15mm by 22mm. That’s much bigger than the Panasonic. Larger chips have the advantage. The chip is covered with light sensitive sites. Tiny chips have tiny sites. Large chips have larger sites. Now when the shutter opens, light in the form of photons stream through the optical system and bombard the chip’s surface. The larger a site the more likely it is to be hit by a photon. The ISO setting of the chip determines how many photon hits will be required to register an exposure. All the while competing radiant energy in the form of heat and UV and IR are also hitting the chip. The bottom line is small chip sites are more susceptible to being tripped off by stray stuff. Stray signals are ‘noise”. Noise is the electronic equivalent of grain. Thus larger chips are better. A 10x magnification achieved on a larger chip is better. The larger chip will require a longer lens to achieve 10x magnification. Longer lenses are generally sharper because the curve requirement (lens shape or profile of the lintel seed) is less steep. Long lenses are thus easier to make and generally sharper. Alan Marcus (more marginal technical gobbledygook)
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Jeff Millard |
Lights of light (flash?) bulbs going off... but as to the Panasonic sensor size... they state sensor is: [10.1 megapixels, 1/2.33" CCD]. Perhaps I mistook that for 1 inch x 2.33 inches or 25.4mm x 59mm vs the 15mm/22 you reasoned? Hoping that is the error as otherwise everything you have generously explained makes sense. I think what you are also saying is that if the panasonic is zoomed in at 10x and the canon at 8x there is still more info on the Canon when you crop the subject to the same size due to the surface area/size of the sensor? After 'enlarging' some of my Canon shots I think I am seeing this first hand. Thanks for so much of your indulgence on such a newbie question...
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Alan N. Marcus |
Hi again, The history of the CCD sensor contained in our modern digital cameras has its roots in TV. Television preceded the digital camera and if you think about it, TV is an electronically produced image. In the 1950’s the TV cameras were giant cameras filled to the brim with vacuum tubes and associated non-solid-state electronics. The heart of the cameras of that era was a vacuum tube that generated the antilog electronic image. It was a cylindrical tube and its top was a flat screen. The camera lens focused an image of the outside world on this screen. The tube is called a Vidicon. As time passed TV cameras evolved into Digicams that sported small sensors. These sensors are the ancestors of today’s CCD (charged coupled device) sensors. Now the Vidicon was a round tube that fit into a very compact space. They were labeled by their diameter. Thus the 1/2.33” CCD you spoke of is a label attached to the diameter of the tube. This is an archaic nomenclature that has no logical reason to exit in today’s CCD’s. The number pertains to the size of the blank chip that will, after manufacturing, accommodate a much smaller rectangular sensor area.
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W. |
"if the panasonic is zoomed in at 10x and the canon at 8x there is still more info on the Canon when you crop the subject to the same size due to the surface area/size of the sensor?" Correct.
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