I have a Canon 7D which has an APS-C camera sensor and gives a crop factor of 1.6. Some literature I read says that as a consequence, a standard lens will provide a perceived zoom. So for example, if I have a 100mm lens, the camera sensor will provide the equivalent to a 160mm lens (100x1.6 = 160 mm). Other literature just says that it just provides a reduced image from that which a full sensor would provide. In other words a reduced image.

I can't see how a standard lens will provide a perceived zoom. I think it will just in essence crop the picture that would be provided by a standard lens. A lens will just do what it is designed to do and cannot zoom beyond its normal range ... or have I got it wrong?

In short - help!!!

That's not entirely true.

We actually did a FF versus APS-C test some years ago... comparing images from an 18MP APS-C to similarly cropped images from a 21MP full frame. We used the same lens from the same distance to shoot the same subject, then cropped the full frame image down to the size of the APS-C image. In every case, the APS-C image was very clearly superior... sharper and with more detail. And that was using an APS-C camera known to have an unusually strong anti-alias filter (which rob some fine detail from images). There have been improvements to both types of cameras since then, so I'm sure the results of a similar wouldn't be all that different now.

Plus, adding a teleconverter to a lens always costs some image quality... Exactly how much varies wildly and depends upon the quality of both the lens and the teleconverter. Loss of IQ is greater with stronger teleconverters, too. Not to mention, teleconverters also "cost" light lost to them.... one stop with 1.4X, two stops worth with 2X. This can and does effect autofocus in many cases.

There are advantages and disadvantages to crop cameras such as yours. There are also advantages and disadvantages to full frame.

For example, there are now quite a few to choose among, but there didn't used to be many truly wide lenses for APS-C. That's changed, but most wide angles for APC-S are zooms, such as Canon's own EF-S 10-22mm USM and EF-S 10-18mm IS STM. There simply aren't many primes (i.e., not zooms... single focal length lenses) that are wide angle on crop cameras. There's not the selection that's available for full frame and the full frame lenses are "less extreme", so can be better "corrected". A 10mm wide angle lens for use on an APS-C will have more inherent distortions than a 16mm will on full frame. In other words, a 10mm focal length is more difficult to "correct" than a 16mm lens.

But at the other extreme... with telephotos, an APS-C camera has an advantage. The crop format "leverages" telephoto focal lengths to "be more powerful". For example, for sports and wildlife photography I use a Canon EF 300mm f/4L IS USM lens a lot on my 7D-series cameras. That's an approx. 3 lb., <$1500 lens that uses 77mm filters and is easily handheld for a full day of shooting. In order to have the same speed and "reach" with a full frame camera, I'd need to get out an 8 lb., $9000> 500mm f/4 IS USM lens that has a 130mm diameter front element (so it uses rear drop in filters)... and a sturdy tripod to sit it on since I can only hand hold it for a few minutes. Sure, the full frame camera and big lens would make great images... more "enlargeable" than what I get from my APS-C cameras. But I'd also be less mobile with the FF camera and much larger lens... and I'd have to lay out a whole lot more money.

Full frame sensors might be less crowded than the smaller APS-C. That may make for less heat gain, which is one of the causes of "noise" in digital images. In addition, there may be more space between the individual pixel sites, reducing "cross talk" between them, another cause of "noise". Plus the individual pixel sites on the FF sensor might be larger and have a weaker or no anti-alias filter, making for better light gathering ability and greater detail capture in images.

However, APS-C sensors are the most common type and have improved greatly in their high ISO and image quality capabilities in recent years. Unless you make really large prints (say, bigger than 16x20").... and/or use super high ISOs... you're unlikely to see much difference in finished images. The vast majority of people will see little or no benefit in images quality switching from APS-C to FF. They just think "FF is better" because they evaluate their images at ridiculously high magnifications on their computer monitors... when they look at their images "at 100%" on a computer monitor, with a 24MP camera that's like looking at a five foot wide print from 18 or 20" away! It's absurd because most will never, ever make prints anywhere near that large. With higher resolution cameras, it's even more extreme and unrealistic. Back off to 25% or 33% to do more sensible evaluation of image quality and judge things like sharpness, focus accuracy, the appearance of noise, etc.

Not to mention.... FF cameras and the lenses they require are bigger, heavier and typically a lot more expensive.

Why are FF cameras more expensive? The reason comes down to the basics of the manufacturing process. The typical silicon wafer used to make image sensors can accommodate 80 APS-C size sensors. Or the same wafer can be used to make 20 FF sensors. Plus, there are often flaws on those sensors which cause some of the sensors to fail quality control. If there are 4 flaws on a wafer that cause 4 sensors to fail QC, with APS-C that's a loss of 5%. The same wafer with the same flaws used to make FF sensors might mean a 20% loss instead! It's probably less dramatic, but there are likely other savings among the various other components in the camera, with the smaller format. As a result... a high-end APS-C camera rarely costs more than $1500 to $1800... while a high-end FF typically costs $3000 or more.

There are some other relatively subtle differences. For example, the larger full frame format offers a little bit more control over various Depth of Field effects: Potential for a little stronger background blur AND the ability to use approx. 1 stop smaller apertures for greater DoF. Actually, both these are indirect results of other differences.... Seemingly stronger blur effects from any particular large aperture are because with full frame you either need to use a longer focal length lens or move closer to your subject. And the reason smaller apertures for greater DoF is possible before "diffraction" is a concern is because an image from a full frame camera will be less magnified to make any given size of print. To make an 8x12" from FF is about 8X magnification... the same size print from an APS-C camera requires about 13X magnification.... assuming no cropping in either case.

Full frame cameras also may have a slower continuous shooting rate than APS-C. FF shutter actions are louder in many cases, too. And it's common for FF to have slower flash syncs.

I use both formats... full frame for some purposes (about 10% of my work) and APS-C for others (about 90% of what I shoot).

For most peoples' "real world" uses of their cameras, the images they make with them and how they use those images, a modern APS-C format is usually more than enough.

I have a Canon 7D which has an APS-C camera sensor and gives a crop factor of 1.6. Some literature I read says that as a consequence, a standard lens will provide a perceived zoom. So for example, if I have a 100mm lens, the camera sensor will provide the equivalent to a 160mm lens (100x1.6 = 160 mm). Other literature just says that it just provides a reduced image from that which a full sensor would provide. In other words a reduced image.

I can't see how a standard lens will provide a perceived zoom. I think it will just in essence crop the picture that would be provided by a standard lens. A lens will just do what it is designed to do and cannot zoom beyond its normal range ... or have I got it wrong?

In short - help!!!

That's not entirely true.

We actually did a FF versus APS-C test some years ago... comparing images from an 18MP APS-C to similarly cropped images from a 21MP full frame. We used the same lens from the same distance to shoot the same subject, then cropped the full frame image down to the size of the APS-C image. In every case, the APS-C image was very clearly superior... sharper and with more detail. And that was using an APS-C camera known to have an unusually strong anti-alias filter (which rob some fine detail from images). There have been improvements to both types of cameras since then, so I'm sure the results of a similar wouldn't be all that different now.

Plus, adding a teleconverter to a lens always costs some image quality... Exactly how much varies wildly and depends upon the quality of both the lens and the teleconverter. Loss of IQ is greater with stronger teleconverters, too. Not to mention, teleconverters also "cost" light lost to them.... one stop with 1.4X, two stops worth with 2X. This can and does effect autofocus in many cases.

There are advantages and disadvantages to crop cameras such as yours. There are also advantages and disadvantages to full frame.

For example, there are now quite a few to choose among, but there didn't used to be many truly wide lenses for APS-C. That's changed, but most wide angles for APC-S are zooms, such as Canon's own EF-S 10-22mm USM and EF-S 10-18mm IS STM. There simply aren't many primes (i.e., not zooms... single focal length lenses) that are wide angle on crop cameras. There's not the selection that's available for full frame and the full frame lenses are "less extreme", so can be better "corrected". A 10mm wide angle lens for use on an APS-C will have more inherent distortions than a 16mm will on full frame. In other words, a 10mm focal length is more difficult to "correct" than a 16mm lens.

But at the other extreme... with telephotos, an APS-C camera has an advantage. The crop format "leverages" telephoto focal lengths to "be more powerful". For example, for sports and wildlife photography I use a Canon EF 300mm f/4L IS USM lens a lot on my 7D-series cameras. That's an approx. 3 lb., <$1500 lens that uses 77mm filters and is easily handheld for a full day of shooting. In order to have the same speed and "reach" with a full frame camera, I'd need to get out an 8 lb., $9000> 500mm f/4 IS USM lens that has a 130mm diameter front element (so it uses rear drop in filters)... and a sturdy tripod to sit it on since I can only hand hold it for a few minutes. Sure, the full frame camera and big lens would make great images... more "enlargeable" than what I get from my APS-C cameras. But I'd also be less mobile with the FF camera and much larger lens... and I'd have to lay out a whole lot more money.

Full frame sensors might be less crowded than the smaller APS-C. That may make for less heat gain, which is one of the causes of "noise" in digital images. In addition, there may be more space between the individual pixel sites, reducing "cross talk" between them, another cause of "noise". Plus the individual pixel sites on the FF sensor might be larger and have a weaker or no anti-alias filter, making for better light gathering ability and greater detail capture in images.

However, APS-C sensors are the most common type and have improved greatly in their high ISO and image quality capabilities in recent years. Unless you make really large prints (say, bigger than 16x20").... and/or use super high ISOs... you're unlikely to see much difference in finished images. The vast majority of people will see little or no benefit in images quality switching from APS-C to FF. They just think "FF is better" because they evaluate their images at ridiculously high magnifications on their computer monitors... when they look at their images "at 100%" on a computer monitor, with a 24MP camera that's like looking at a five foot wide print from 18 or 20" away! It's absurd because most will never, ever make prints anywhere near that large. With higher resolution cameras, it's even more extreme and unrealistic. Back off to 25% or 33% to do more sensible evaluation of image quality and judge things like sharpness, focus accuracy, the appearance of noise, etc.

Not to mention.... FF cameras and the lenses they require are bigger, heavier and typically a lot more expensive.

Why are FF cameras more expensive? The reason comes down to the basics of the manufacturing process. The typical silicon wafer used to make image sensors can accommodate 80 APS-C size sensors. Or the same wafer can be used to make 20 FF sensors. Plus, there are often flaws on those sensors which cause some of the sensors to fail quality control. If there are 4 flaws on a wafer that cause 4 sensors to fail QC, with APS-C that's a loss of 5%. The same wafer with the same flaws used to make FF sensors might mean a 20% loss instead! It's probably less dramatic, but there are likely other savings among the various other components in the camera, with the smaller format. As a result... a high-end APS-C camera rarely costs more than $1500 to $1800... while a high-end FF typically costs $3000 or more.

There are some other relatively subtle differences. For example, the larger full frame format offers a little bit more control over various Depth of Field effects: Potential for a little stronger background blur AND the ability to use approx. 1 stop smaller apertures for greater DoF. Actually, both these are indirect results of other differences.... Seemingly stronger blur effects from any particular large aperture are because with full frame you either need to use a longer focal length lens or move closer to your subject. And the reason smaller apertures for greater DoF is possible before "diffraction" is a concern is because an image from a full frame camera will be less magnified to make any given size of print. To make an 8x12" from FF is about 8X magnification... the same size print from an APS-C camera requires about 13X magnification.... assuming no cropping in either case.

Full frame cameras also may have a slower continuous shooting rate than APS-C. FF shutter actions are louder in many cases, too. And it's common for FF to have slower flash syncs.

I use both formats... full frame for some purposes (about 10% of my work) and APS-C for others (about 90% of what I shoot).

For most peoples' "real world" uses of their cameras, the images they make with them and how they use those images, a modern APS-C format is usually more than enough.