Are you confused with the conversion factor of 1.4 to 1.6 (and probably other conversion factors I don't know about) applied to most lenses designed for the 35mm format but used on DSLR using an APS-C sized sensor? 
Well, maybe this will help. Using AutoCAD and PhotoShop, I was able to make the sketches herein.
What seems to confuse a lot of people is the perception that a lens
designed for a 35mm film format, used on a APS-C sensor, becomes 1.5 times more powerful. I have heard, as example, that a 300mm lens designed for 35mm film format becomes a 450mm on a DSLR with an APS-C sensor (as 300mm x 1.5 = 450mm). That would be magic. A 300mm lens is a 300mm lens is a 300mm lens. Got it? What changes, is that because the APS-C sensor is 
smaller than the full frame 35mm format, the image is cropped. It appears to be a longer telephoto, but it is not. The angle of view changes, because of the smaller size sensor of 23.5 x 15.7mm. It's no different that cropping an image in PhotoShop. See figure below for the APS-C crop.
Now, if you were to use a lens designed for a 35mm formal and use it on a 4 x 5 inch view camera, the image circle would not be big enough to cover the entire 4 x 5 frame. What you would get is a round image in the middle. See figure below.
Using a wide angle lens would have the same effect. As example, a 20mm lens designed for the 35mm film format would give a angle of view equivalent to a 30mm lens. Again, you are just cropping the image.
Owners of APS-C sized DSLRs claim that they get more tel
ephoto for the money. Similarly, the owners of full frame DSLRs claim that they get more wide angle for the money. Nobody can win at that game, it all depends what you shoot more often. Wildlife and sport photographer will have an edge with longer telephoto, while keeping the same widest aperture, for a lot less money. On the other hand, Landscape and architectural photographers will benefit from the true wide angle of their lenses. If you take all kind of pictures, you decide what's more important to you. 
The sensors have evolved a lot in the past ten years or so. Most cameras were using CCD sensors a while back. Now, with technology advances and production costs going down, CMOS sensors are being used more and more. The bigger the sensor, the less noise is introduced and the higher ISO settings can be used. At this time, CMOS full frame sensors seem to be the fashionable sensors because they produce less noise. What about all those lenses I bought for my Pentax K10D? If Pentax joins the current trend and use full frame sensors, all my DA lenses won't work with full frame. The image circle will be too small. I am confident that the evolution of sensors will be such that APS-C sensors will produce less and less noise. I can use my lens designed for the 35mm format on my APS-C sized sensor DSLR, but cannot use the DA lenses on full frame DSLRs. Beside, it costs less to manufacture APS-C lenses and we stand to gain from that.
Just my two cents worth.
Thank you for reading my post. Let me know what you think.
Yvon Bourque
Well, maybe this will help. Using AutoCAD and PhotoShop, I was able to make the sketches herein.All lenses designed for the 35mm film format project an image circle on the film plane of roughly 43mm in diameter, regardless of the focal length of the lens. I am showing a 300mm lens in the figures herein, but all lenses designed for the 35mm film format project the same size image circle on the 35mm film plane. A 35mm film frame measures 36mm x 24mm and therefore the image recorded is not round, but measures 36mm x 24mm. The portion of the round image outside the 35mm frame is wasted.
See figure above for an overlay of the image circle, the 36 x 24mm film or full frame , the APS-C sensor and a 4 x 5 inch film sheet. See figure below for the actual 35mm film full frame. It is full frame 35mm film format because it captures the rectangular image of 36 x 24mm within the image circle.
What seems to confuse a lot of people is the perception that a lens
designed for a 35mm film format, used on a APS-C sensor, becomes 1.5 times more powerful. I have heard, as example, that a 300mm lens designed for 35mm film format becomes a 450mm on a DSLR with an APS-C sensor (as 300mm x 1.5 = 450mm). That would be magic. A 300mm lens is a 300mm lens is a 300mm lens. Got it? What changes, is that because the APS-C sensor is 
smaller than the full frame 35mm format, the image is cropped. It appears to be a longer telephoto, but it is not. The angle of view changes, because of the smaller size sensor of 23.5 x 15.7mm. It's no different that cropping an image in PhotoShop. See figure below for the APS-C crop.
Now, if you were to use a lens designed for a 35mm formal and use it on a 4 x 5 inch view camera, the image circle would not be big enough to cover the entire 4 x 5 frame. What you would get is a round image in the middle. See figure below.
Using a wide angle lens would have the same effect. As example, a 20mm lens designed for the 35mm film format would give a angle of view equivalent to a 30mm lens. Again, you are just cropping the image.Owners of APS-C sized DSLRs claim that they get more tel
ephoto for the money. Similarly, the owners of full frame DSLRs claim that they get more wide angle for the money. Nobody can win at that game, it all depends what you shoot more often. Wildlife and sport photographer will have an edge with longer telephoto, while keeping the same widest aperture, for a lot less money. On the other hand, Landscape and architectural photographers will benefit from the true wide angle of their lenses. If you take all kind of pictures, you decide what's more important to you. 
The sensors have evolved a lot in the past ten years or so. Most cameras were using CCD sensors a while back. Now, with technology advances and production costs going down, CMOS sensors are being used more and more. The bigger the sensor, the less noise is introduced and the higher ISO settings can be used. At this time, CMOS full frame sensors seem to be the fashionable sensors because they produce less noise. What about all those lenses I bought for my Pentax K10D? If Pentax joins the current trend and use full frame sensors, all my DA lenses won't work with full frame. The image circle will be too small. I am confident that the evolution of sensors will be such that APS-C sensors will produce less and less noise. I can use my lens designed for the 35mm format on my APS-C sized sensor DSLR, but cannot use the DA lenses on full frame DSLRs. Beside, it costs less to manufacture APS-C lenses and we stand to gain from that.
Just my two cents worth.
Thank you for reading my post. Let me know what you think.
Yvon Bourque
9 comments:
You had a good start, but then fell on your face a little there. As you originally/correctly noted, the focal length, i.e., the MAGNIFICATION, of a lens does NOT change when you go to an undersized sensor. You "gain" ABSOLUTELY NOTHING. You only LOSE something, that being the portion of the image between the APS-C frame and the 35mm (full) frame. This applies to BOTH wide angle and telephoto. As for the "multiplier" factors, they speak ONLY to "angle of view," so while the "angle of view" of a telephoto lens of 300mm focal length may be the equivalent of a 450mm with an APS-C sensor, you don't have a 450mm image - just a PIECE of a 300mm image. The so-called "benefit" to tele shooters is pure marketing BS. I can get the same "multiplier" with a full frame sensor or film any time I want - simply by cropping down to an APS-C frame size. If you're stuck in APS-C sensor land, however, YOU CAN'T RECOVER THE MISSING PART OF YOUR IMAGES. You CANNOT take the same picture, all you can do is take a picture with a shorter focal length lens, which changes magnification and perspective, which is NOT the same thing. Again, you gain NOTHING with APS-C sensors, you only LOSE something.
Mr. Anonymous, I have not fallen on my face, it's you that has not read my post correctly. This is what is written in the post:
I have heard, as example, that a 300mm lens designed for 35mm film format becomes a 450mm on a DSLR with an APS-C sensor (as 300mm x 1.5 = 450mm). That would be magic. A 300mm lens is a 300mm lens is a 300mm lens. Got it? What changes, is that because the APS-C sensor is smaller than the full frame 35mm format, the image is cropped.
No wonder you sign "Anonymously,I would too if I was in your shoes. You were so quick to criticize my post, that you forgot to read it correctly. That is when you fell on your anonymous face!
Don't take life so seriously. Take time to smell the flowers and take a picture of them as well. :)
Anonymous:
If you were to use a full frame 10mp sensor and crop the image of a 450mm lens down to a 300mm telephoto equivalent you would be left with a 6 to 7 mp image.If you use a 300mm lens on a APS-C 10mp sensor, you get the 450mm lens equivalent picture still at full 10 mp. Think of that a little. It's not brain surgery, just a little understanding of digital imaging.
Honestly, the big thing about the APS-C vs. Full-Frame, at least for me, is this:
I currently use a Pentax *istDL, which has a 6.something mp APS-C CCD. All of my DA lenses are for that format. While they become an interesting novelty because of the vignetting that happens when you zoom out on a 35mm camera, I wouldn't want to have to either buy a new set of lenses for a full-frame camera, or use my manual-focus lenses for my K1000 with the new digital.
I'm perfectly fine with the quality you get from a 14.something mp APS-C CMOS sensor (which is what you will find on the K20D, I believe). If I want to get quality much higher than that, it usually calls for either 120/220, or just breaking out the good ol' 4x5 and taking my time with the shot.
I'm just a lowly student, but I can at least appreciate the expense of reinvesting in new equipment. I'm perfectly fine with the results I get with the APS-C sensors, even with CCD.
I had warned Pentax people for this:
http://www.pentaxuser.co.uk/forum/viewtopic.php?p=19671#19671
So, the day for Full Frame will come and the Photo companies managed to do what they had tried with APS film format in the past. TO EARN MONEY!!!
All this war was for the companies to make us buy a new format and after 5 years we will need a new equipment. So, If you are Ok with APS size body, keep it and you will have no any problems!!!
I would like the full frame standard, that's why I got Full Frame lenses. And if Pentax didn't give me what I needed, I took it from Tamron.
So the Full Frame body will be for us not for you, who are already happy with APS, aren't you?
teogin
Stop fighting guys. Give us a clear view with your respectful knowledge. Most of readers are new to these technical understanding which is confusing. Thanks for both of you.
For the record, there are two different anonymous posters here, I was the intial one. I referred not to what you said initially (as I said, you had it right at first), but rather, what you said later, i.e., "Wildlife and sport photographer will have an edge with LONGER TELEPHOTO" (emphasis mine). You do not get a "longer telephoto" with an APS-C sensor; you get a smaller angle of view with the SAME telephoto, which is not the same thing.
As for the second comment regarding my post, I understand the pixel pitch difference, I said nothing about that. What I did say was I can crop a FF image (pixel differences noted, but notwithstanding) to get the same framed image at the same focal length as I can get by putting the same lens on an APS-C camera. The APS-C user, on the other hand, cannot shoot the same image as the FF shooter because in order to frame it the same, the APS-C shooter must either use a shorter focal length (which provides less magnification) or (if possible) move back, which changes perspective. Thus, FF always includes the composition possibilities of APS-C, but the reverse is not true; you have gained nothing in "telephoto" but lost something (in angle of view).
Clear now?
For me the time has come to buy a new camera, and choose between full-frame and APS. Having read all of the above, I would like to add something. Comparing two cameras with the same amount of pixels, same (zoom)lense, same amount of light and thus aperture. Most of the time, I shoot outside, and with limited room to move. So I choose my framing, and adjust the zoom's focal length accordingly. Say, that for the FF, I choose a 75 mm focal lenth, then for the APS, I will arrive at around 50mm. In that situation, I will have longer field of depth, the hyperfocal distance will be closer by, with the APS. For my type of photography (scenes outside) that is mostly what I prefer, though not always. And I will have to deal with somewhat more noise. Personally, I don't see a larger picture area as a real advantage; at longest range, things are most of the time still smaller than the picture area; at shortest range, the APS camera lenses normally start at 18 mm, and the FF lenses at 24 or 28 mm, which give comparable scene dimensions.
@ Anonymous about "losing" and "winning".
You discard the amount of pixels, but this aspect is very important.
With a 300mm f2.8 on a 10MP crop body, you can not take the same 10MP image on a fullframe sensor, you can pull out more detail, you can reach further en you can fill you image with your subject at 450mm (bird, lion etc....).
For the same image on a FF, you'd have far less MP's, or you'd need to buy a very expensive 450mm f2.8.
That's not a loss, that's a benefit.
APS-C does "loose" flexibility at the ultra wide angle end, but more en more UWA lenses are comming out...
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