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05-25-2012, 03:08 PM - 1 Like   #1
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APS-C does not increase focal length over FF, it decreases field of view.

EDIT: Revised version here.

I'm sure this has been stated many times before but here it is once again anyway: APS-C does not increase focal length over FF, it decreases field of view.

Many people seem to confuse reduced field of view for increased focal length in their understanding of a cropped sensor (APS-C format: 23.7 x 15.7 mm) image vs. a full frame sensor (135 film format: 36 x 24 mm) image. The matter can be somewhat confusing so, there are many people who mistake less field of view for a longer focal length. This misunderstanding is perpetuated all over the internet and elsewhere. The perceived magnification from an APS-C sensor vs. a FF sensor only applies once the final image is viewed on a screen or when it is printed. It is not the case in the viewfinder of your camera, on the film negative, or when viewing the final image at 100% on a screen.

Contrary to what some incorrectly believe, you can't get any closer, i.e. have greater magnification, with an APS-C camera when doing sports photography, birding, or whatever other type telephoto work. This idea needs to stop being repeated as it is misleading. You won't be able to reach any farther with an APS-C camera than with a FF camera. Reduced field of view is what you're getting, not a longer focal length. If you want to reach farther, you need to get a longer lens not a smaller sensor.

I think that using the term EFL (equivalent focal length) is part of what helps perpetuate this misunderstanding. We may be better off using some term like EFOV (equivalent field of view) or EAOV (equivalent angle of view). EFL is a convenient shorthand for converting to the more widely known 35mm format FOV but it is also misleading to those who don't quite understand what is the real difference between the different sensor formats.

As it is stated in Wikipedia's Magnification factor section of their article on crop factor: "A given lens casts the same image no matter what camera it is attached to. The extra 'magnification' occurs when the image is enlarged more to produce output (print or screen) that matches a standard output size. That is, the magnification as usually defined, from subject to focal plane, is unchanged, but the system magnification from subject to final output is increased."

Similarly, in Wikipedia's macrophotography article it states that "when producing a 64 inch (1510 cm) print using 135 format film or sensor, a life-size result is possible with a lens having only a 1:4 reproduction ratio." This goes to show that magnification, in addition to being a result of a given lens' focal length and minimum focusing distance, can simply result from the enlargement of the image on a screen or on paper. My point is, the magnification is not coming from the cropped sensor; it either comes from the lens or the reproduction when printed or displayed on screen.

Say you take two pictures of a beetle from the same distance with a 100mm macro lens (a true macro lens capable of 1:1 reproduction, to be specific). Imagine that the picture you take with the APS-C camera has the beetle filling the entire frame. Now imagine that you us a full frame camera using the same lens and from the same distance to get the same 1:1 magnification. The beetle on the full frame image will not fill the entire frame. You will still get the very same 100% magnification from both cameras. What you will get from the FF image though, that you will not get with the APS-C image, is more of the beetle's surroundings. View both those images at 100% and the beetle is going to be the same size on your screen, as long as the pixel count is roughly the same. You will not have any reduction in terms of magnification with the FF image. To have the beetle be the same size on your screen, when your viewing mode is set to fit the screen, then you would have to crop the FF image. But at a 100% viewing size, given that the images have the same number of pixels, the magnification will be identical.

Imagine that you are taking these two pictures of the beetle above, again from the same distance and using the focal length, on film instead of digital. One image would be taken with a full frame camera on 135 film. The other image would be taken on an APS camera on 24mm film in the APS-C 3:2 aspect ratio. Imagine taking the negatives from both films and then placing them on top of each other: the beetle is going to be the same size on both images when lined up. The difference will be that with the 135 film the negative is larger so there will be more picture content surrounding the beetle.

I often times read opinions that a cropped sensor is advantages for telephoto work; I don't agree with that for all the reasons already outlined. The example with the beetle and the macro lens above could just as easily be made with a bird and a 200mm telephoto lens. If shooting from the same distance and with the same focal length, the bird is going to be the same size on either sensor, it's just that the size of the frame (i.e. the FOV) of the picture will be bigger with the full frame than with the cropped sensor.

In a way, the perceived magnification effect of using a cropped sensor is more akin to digital zoom than it is to a teleconverter. A cropped sensor gives you the "magnified" crop ahead of time, whereas cropping an image from a full frame photo accomplishes the same "magnification" but after the fact.

One of the main reasons I hope to be able to upgrade to a full frame system eventually is to have the advantage of greater magnification in the viewfinder relative to FOV. For example, when using a normal 50mm lens on FF I would get the approximate field of view of my 31mm lens on my K-5 but with the actual magnification in the viewfinder of a 50mm lens. That will make a big difference with manual focusing. Of course, in addition to field of view, the difference in sensor size between FF and APS-C affects depth of field, low light performance, and dynamic range.

Anyway, there's my long winded rant about what could admittedly just be described as a matter of semantics. I'd propose becoming more familiar with the different fields of view by focal length and sensor format, though that would be a bit more complex.


Last edited by TomTextura; 06-03-2012 at 09:31 PM. Reason: Add link to revised version
05-25-2012, 03:17 PM   #2
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You are correct on the facts.

BUT, APS-C does give greater reach in one practical sense, which is that you're going to be using a denser sensor than you would with an FF (without spending an extra gazillion). In other words, the APS-C area is just a crop of the FF area, but the APS-C sensors actually used aren't just cropped versions of FF sensors -- you're getting more pixels in that APS-C area than you would with a FF sensor, therefore *effectively* somewhat greater reach. Not as good as a longer lens, but in the real world with the real sensors that actually exist and their real price tags, I'd still rather use an APS-C body for telephoto work for that reason.
05-25-2012, 03:34 PM   #3
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Good effort, but there will always be (loud, argumentative) people that think a 25/1.4 lens on a m4/3 sensor is "equivalent" to a 50/2. There are worse examples of even stranger math, but I'm sure they'll show up soon if this thread gets lots of comments.

I do have to agree with vonBaloney, though. Pixel density makes a big difference when cropping or using a smaller sensor, but only in the case of a lens sharp enough to outresolve the full frame sensor. This is a pretty good real-world test: https://www.pentaxforums.com/forums/pentax-q/177309-pentax-q-da300mm.html The DA300 is very sharp, so the Q can get some extra detail, but if the same experiment were done using the DA50-200, you'd probably get a soft blobby mess.

Also, getting rid of the 35mm focal length equivalence stuff would be great. The focal length (and aperture, and thus f/stop) is a fixed property of the lens and does not magically change when you put a different sized sensor behind it.
05-25-2012, 03:47 PM   #4
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I agree with TomTextura completely.

@vonBaloney: Would an APS-C sensor with 1.5x denser pixels give the same result as a FF sensor? DOF would still be different. Also denser sensor would typically generate more noise, more CA.

05-25-2012, 03:59 PM   #5
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QuoteOriginally posted by Doanh Quote
I agree with TomTextura completely.

@vonBaloney: Would an APS-C sensor with 1.5x denser pixels give the same result as a FF sensor? DOF would still be different.
I'm not comparing the APS-C sensor to the full FF sensor -- just to an APS-C sized area cut out of it. So DOF, etc would be the same. Everything but pixel density (and what goes with it) would be the same.

QuoteQuote:
Also denser sensor would typically generate more noise, more CA.
Probably, but to get same magnification from the APS-C sized slice of the FF sensor, you'd have to enlarge it, degrading quality. If you looked at those Q vs APS-C comparisons linked above, the Q clearly had greater detail with no noise problems, although I'd like to see it with a newer APS-C model. I don't want to side-track the discussion here started by the OP, just pointing out that although yes, you don't really get greater reach; yeah, you do, sort of.
05-25-2012, 04:03 PM   #6
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I won't go into resolution because 1) it's fluid, and 2) I'm lazy. But we can approximate AOV+DOF equivalence by multiplying both the focal length and aperture by the format-faktor. With nominal APS-C (sensor diagonal 30.1mm) the factor is 1.44. With my K20D (diagonal of 28.1mm) the factor is 1.54. So a 50/1.2 lens on my K20D has about the AOV+DOF of a 77/1.8 lens on a 135/FF camera.
05-25-2012, 04:05 PM   #7
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QuoteOriginally posted by vonBaloney Quote
You are correct on the facts.

BUT, APS-C does give greater reach in one practical sense, which is that you're going to be using a denser sensor than you would with an FF (without spending an extra gazillion). In other words, the APS-C area is just a crop of the FF area, but the APS-C sensors actually used aren't just cropped versions of FF sensors -- you're getting more pixels in that APS-C area than you would with a FF sensor, therefore *effectively* somewhat greater reach. Not as good as a longer lens, but in the real world with the real sensors that actually exist and their real price tags, I'd still rather use an APS-C body for telephoto work for that reason.
This is truly the amount of "magnification" you get with APS-C versus full frame. Obviously, if you are using a K5 with 16 megapixels versus, say a D700 with 12 megapixels, you do have a little bit of extra reach. At the same time, if you have a D800 and a K5, your effective reach is exactly the same.

Edit: Just to be clear, this is the amount of "digital zoom" you can do with a given camera. If you take a 12 megapixel full frame image, and crop it to APS-C you have about 6 megapixels left. Therefore, you have more room to crop on the APS-C with any of the standard sensors out there right now (ranging between 16 and 24 megapixels).

Last edited by Rondec; 05-25-2012 at 04:29 PM.
05-25-2012, 04:10 PM   #8
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QuoteOriginally posted by Rondec Quote
This is truly the amount of "magnification" you get with APS-C versus full frame. Obviously, if you are using a K5 with 16 megapixels versus, say a D700 with 12 megapixels, you do have a little bit of extra reach. At the same time, if you have a D800 and a K5, your effective reach is exactly the same.
Right, for $2000 more. Not really a fair fight. But if I went and spent that money on longer lenses, APS-C wins again I think.

05-25-2012, 05:02 PM   #9
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So actually to end the arguments, regardless of what camera, lens or Sensor size. Give or take the viewfinder % you
will get what you see !
05-25-2012, 09:42 PM - 5 Likes   #10
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It took me the better part of three years to get personal closure on this subject, and it didn't happen until I bought a 645 and developed that first roll of 120.

Here's the plain truth: APS-C doesn't "do" anything; it is a different format than 135 (a.k.a. full-frame).

That, in most cases, you are afforded the convenience of using 135 mount lenses on APS-C cameras is simply that - convenience.

Advanced Photo System (APS) format is the brain child of Eastman Kodak. It debuted in 1996 and is comprised of three sub-formats:
H - High-definition 16:9 aspect
C - Classic (not crop!) 3:2 aspect
P - Panorama 3:1 aspect
Calling APS-C format sensors "cropped" is quite simply marketing mumbo-jumbo.

As to magnification, well that's a matter of perspective.
I can safely argue that it isn't the lens that magnifies, but rather the recording medium - the sensor or film. 135 is approximately 2.4 times larger in area than APS-C, so doesn't it follow that the recorded image on 135 is of higher magnification than APS-C?
Or perhaps magnification doesn't pertain to the recording format at all?
Maybe it's the size of the print that defines magnification.
Or maybe it's the number of pixels in the raw digital image.
As Rico said earlier, magnification is fluid.

110, 126/127, APS-C/H/P, 135, and 120/220 are all fully independent formats. They require no legitimizing or justification to exist in the photographic world.

Once you can wrap your brain around the concept that APS-C is simply a different format, that happily often shares lens mount compatibility with 135, you will likely find more enjoyment in your photography.

More fun facts that provide zero qualitative measurement of what makes a great photograph:
  • Field of View is relative to format and distance.
  • Angle of View is relative to format and focal length.
  • Image Scale is bound to distance and focal length. It is constant across formats.
  • Snickers really satisfies.
05-25-2012, 10:43 PM   #11
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+1 Excellent overview! My only slight quibbles:
QuoteOriginally posted by Venturi Quote
135 is approximately 2.4 times larger in area than APS-C...
* 135 is 35mm-wide film in a Kodak-designed cartridge; it's not a camera format. The sprocket holes leave frames 24mm across. The full-frame format of 36x24mm is 135/FF. Other formats using 135 carts: 135/HF (half-frame, 24x18mm); 135/P1 (58x24mm) and 135/P2 (65x24mm) panoramic; 135/SQ (24x24mm) square; and others, less common. And some cameras with these and other frame sizes can use 35mm-wide rollfilm in long (100ft etc) rolls, which can't really be called 135. Picky, picky...

* A nominal APS-C frame is 25.1x16.7mm and has 49% the area of a 135/FF frame. The sensor on my K20D is 23.4x15.6mm and has 42% the area of a 135/FF frame. So 135/FF is about 2x larger than nominal APS-C, and about 2.4x larger than APS-C as implemented by some sensor makers. IIRC some of our dSLR sensors are slightly different sizes.

Other than those trivialiaties, you're dead-on. Yes, each format is independent, with various strengths and weaknesses, and it behooves toggers to learn how to use their cameras. Back in the day, I might shoot 135/HF, 135/FF, 6x6/MF, and 9x12/LF on the same day, sometimes with the same lenses. My colleagues and I didn't think of crop factors and equivalences. We learned what different lenses and films would do with each format we used.

(Even more trivial trivia: We can class 120/220/620 together, but 126 and 127 are quite different. 120/220/620 frames are all 56mm wide; 126 frames are 26mm wide; 127 frames are 40mm wide. Picky, picky...)

Now, besides nominal and approximate APS-C, and Canon's APS-H, and those bloody expensive too-many-megapickles FF-D cams and the 645D (actually 44x33) and more monsters, we have numerous smaller formats, from m4/3 (frame size about the same as 110 film) to all those wee tiny P&S sensors that may be barely 5mm across (similar to Super-8 film). Again, each has strengths and weaknesses; are different tools for different tasks. There are no 'better' nor 'worse', just more or less appropriate for some purpose.
05-25-2012, 11:26 PM   #12
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Apologies to the class, and thanks for clarifying & correcting Rico.

Hmm, since generally speaking digital formats correlate to one of the many film formats/factors would it be fair to say that the 44x33 medium format sensors are the only real "cropped" sensors?
05-26-2012, 12:30 AM   #13
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QuoteOriginally posted by Venturi Quote
Hmm, since generally speaking digital formats correlate to one of the many film formats/factors would it be fair to say that the 44x33 medium format sensors are the only real "cropped" sensors?
HA! If only it were that simple...

Many film formats are different 'crops' of standard film stocks. Take a look at the history of cine formats -- if you dare. (See List of film formats - Wikipedia -- I count over 150 formats there). I have a database of many still-film and digicam frame sizes, but I won't include all the cine formats. Many are fairly obscure, and even the majors are misleading. The dominant 35mm format for many years was 24x18mm, what still toggers call half-frame but cine folks call full- or single-frame; while the 36x24mm 135/FF size in cine is/was called double-frame.

On the other side, various P&S and phone.cam sensors are sized based nominally on old vidicon (image orthicon) TV camera tubes, which didn't have direct film correlations. I've noted about 18 different sizes there, all marked in fractions of inches, but which indicate nothing about the actual frame size. OOPS, my database needs updating. I just found this page Image sensor format - Wikipedia which now lists 32 sensor formats. I'm almost afraid to look for TV formats.

So almost everything is a chop'n'crop. Ay yi yi...

Last edited by RioRico; 05-26-2012 at 08:07 AM.
05-26-2012, 01:15 AM   #14
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QuoteOriginally posted by Venturi Quote
Snickers really satisfies.
At least there should not be any argument there!
05-26-2012, 07:07 AM   #15
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QuoteOriginally posted by vonBaloney Quote
APS-C does give greater reach in one practical sense, which is that you're going to be using a denser sensor than you would with an FF
QuoteOriginally posted by Rondec Quote
This is truly the amount of "magnification" you get with APS-C versus full frame. Obviously, if you are using a K5 with 16 megapixels versus, say a D700 with 12 megapixels, you do have a little bit of extra reach. At the same time, if you have a D800 and a K5, your effective reach is exactly the same.
QuoteOriginally posted by vonBaloney Quote
Right, for $2000 more. Not really a fair fight. But if I went and spent that money on longer lenses, APS-C wins again I think.

I think all three points are correct. I suppose I was a bit quick to write off any advantage of APS-C for telephoto work, especially once taking cost into consideration. Truth be told, I don't do any photography beyond what I consider to be a medium telephoto length since my longest lens is a DA* 50-135mm and I don't use it as much as my other lenses. The majority of my work is between the wide-angle to "normal" range so, I honestly don't know a lot about getting the most out of telephoto. If I were to have a FF and an APS-C body, and the prior didn't outnumber the later in terms of MP, I probably would sometimes favor using the smaller frame when taking telephoto shots.

Just to reiterate though: What you see in the viewfinder with APS-C vs. a FF with telephoto lenses of equal fields of view though won't be the same. That is, if I use my DA* 50-135mm zoomed out to 135mm on my K-5, I may be able to print at a FOV equal to roughly a 200mm on FF, but what I see in the viewfinder at the time of taking the picture will have the magnification of a 135mm FL lens, not a 200mm one. In this sense, the FF will have a "magnification" advantage and get me "closer" to the subject as it will be using the longer lens to accomplish the same FOV. I imagine that with advances in EVF technology this difference could eventually be made mostly moot.
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