'Crop' on APS-C vs. full-frame

[bradf]

Hi again.
I've read the excellent tutorials on this site - and I still can't quite understand one point.
I understand that if one put the same lens on 2 cameras ( FF and APS-C) that on the APS-C the image would appear as if a lens of 1.6 times greater focal length had been used.
My query is : if one took the image off the FF camera and then cropped off the periphery and enlarged it so the image exactly matched the size of the APS-C image is there any practical difference? Presumably you will have cropped off the 'excess' peripheral pixels off the FF image to end up with an image of the same size and pixels that you got with the APS-C. So in my mind the APS-C has not gained any advantage (for wildlife or sports photography). The 1.6 times extra magnification is 'illusory' - you've ended up with the same size image on the same number of pixels using either camera (its only the FF has not ended up with any advantage from having a larger sensor)??
Have I got it right ??
Thanks very much again for your help.
Cheers,
bradf

[Ray]

Hi again.
I've read the excellent tutorials on this site - and I still can't quite understand one point.
I understand that if one put the same lens on 2 cameras ( FF and APS-C) that on the APS-C the image would appear as if a lens of 1.6 times greater focal length had been used.
My query is : if one took the image off the FF camera and then cropped off the periphery and enlarged it so the image exactly matched the size of the APS-C image is there any practical difference? Presumably you will have cropped off the 'excess' peripheral pixels off the FF image to end up with an image of the same size and pixels that you got with the APS-C. So in my mind the APS-C has not gained any advantage (for wildlife or sports photography). The 1.6 times extra magnification is 'illusory' - you've ended up with the same size image on the same number of pixels using either camera (its only the FF has not ended up with any advantage from having a larger sensor)??
Have I got it right ??
Thanks very much again for your help.
Cheers,
bradf

You are essentially right, but only in circumstances where the pixel density of the FF camera is about the same as that of the APS-C camera. The telophoto advantage of the cropped format is useful only to the extent that it's pixel density is greater than that of the full frame.

A full frame 35mm Canon has 2.6x the sensor area of the Canon cropped format. The 21mp Canon 5D2 has approximately 2.6x the number of pixels of the 8mp 20D, therefore there would be no purpose in switching from a 5D2 to a 20D if you intended to crop the 5D2 image to the same size as the 20D image using the same lens. However, the new 7D should provide a noticeable advantage with its 18mp sensor which would be equivalent to a 46mp sensor if it were full frame.

Eventually, we'll probably get a 50mp full frame 35mm. When that time arrives, the 7D will serve no purpose in providing better resolution with lenses that are not long enough on full frame.

[EduPerez]

Hi again.
I've read the excellent tutorials on this site - and I still can't quite understand one point.
I understand that if one put the same lens on 2 cameras ( FF and APS-C) that on the APS-C the image would appear as if a lens of 1.6 times greater focal length had been used.
My query is : if one took the image off the FF camera and then cropped off the periphery and enlarged it so the image exactly matched the size of the APS-C image is there any practical difference? Presumably you will have cropped off the 'excess' peripheral pixels off the FF image to end up with an image of the same size and pixels that you got with the APS-C. So in my mind the APS-C has not gained any advantage (for wildlife or sports photography). The 1.6 times extra magnification is 'illusory' - you've ended up with the same size image on the same number of pixels using either camera (its only the FF has not ended up with any advantage from having a larger sensor)??
Have I got it right ??
Thanks very much again for your help.
Cheers,
bradf

Yes, you have it right; the only reason for a smaller sensor is the reduced price. Sometimes, camera manufacturers try to sell APS-C sized sensors as an advantage over FF; some of them even talk about "magnification factor", instead of "crop factor". Inexperienced users fall into this trap, because they like to think they can get cheaper telephoto lenses.

[Ed Blagden]

Brad,

Not sure I fully understand your question, but yes there is a practical difference between APS-C and FF.  Because FF sensors are bigger, for a given number of megapixels the pixel density (ie the number of pixels per square mm) on FF is lower.  This means that the pixels can be bigger, they capture more light, and the dynamic range and noise characteristics are improved.  So for a given generation of technology, a state of the art FF sensor will always be a bit better than a state of the art APS-C sensor, image quality wise.  But quite honestly, sensors are all pretty damn good these days so I wouldn't sweat it.

The bigger disadvantage of APS-C is that if you like making wide angle shots you have a problem.  I shoot a lot at 24mm for example, I really like that length for all kinds of purposes.  If I had an APS-C I would have to find myself a 15mm lens to get the same effect.  So do I:

1) go out and buy a 16-35 f/2.8 and annoy my wife still further.
2) go and get one of those little EF-S lenses for less money.  Cheaper, yes, but then I can't use it on a full-frame body.

Finally, the smaller the sensor the deeper the depth of field for any given aperture.  So if you like to use your fast lenses wide-open for creative reasons (eg subject isolation) then you have slightly less room for manoeuvre.

Hope that helps.


Ed (an avowed full-frame chauvinist and snob).

edit reason: for some odd reason when I typed in the letter b followed by a right-bracket I got a picture of a little yellow man wearing sunglasses.  Weird.  So I changed to numbers.

[Ray]

Brad's question is quite clear.

My query is : if one took the image off the FF camera and then cropped off the periphery and enlarged it so the image exactly matched the size of the APS-C image is there any practical difference?

Answer: There may be a resolution advantage if the pixel density of the cropped camera is significantly greater than that of the full frame, as indeed that of the 7D is compared with a 5D2.

[Ken Bennett]

My query is : if one took the image off the FF camera and then cropped off the periphery and enlarged it so the image exactly matched the size of the APS-C image is there any practical difference?

Assuming that both sensors have the same pixel pitch, and that the crop of the full frame sensor provides an image with exactly the same number of pixels as the 1.6x sensor, then from a purely technical standpoint, there is no practical difference.

Many full frame snobs (sorry, Ed :-) use this to say that one should always shoot full frame, since one can simply crop to the tighter composition when necessary. Again, from a purely technical standpoint, using the above assumptions, they are correct.

However, from a shooting standpoint, I think there is a huge difference. I don't "see" cropped images when I shoot -- I see the whole image area in the viewfinder. Having been trained to fill the frame, I hate shooting loose and cropping later. That's just not how I see. I would much rather put a 1.6x camera on my 300, and get an effective 480mm/2.8 lens, than shoot with a full frame camera and have to crop after the shoot.

There is another practical consideration. There just aren't that many camera comparisons that meet the assumptions made above. So one often has to decide between having more pixels from the crop camera, or larger pixels from the full frame camera (for example, a D300 vs a D700, both 12 megapixels.) This is NOT always an easy or obvious choice.

My personal recommendation is to have both. I often shoot with two cameras, and it's practical to have a full frame camera with a wide angle zoom, and a crop camera with a tele zoom. A good combination would be the D700 with a 24-70, and a D300 with a 70-200. That way you are covered from 24mm to ~300mm with two lenses, shooting high quality 12 megapixel images from both cameras. It's a little harder for Canon shooters like me (and in truth I just split the difference and carry two 1D Mark II cameras most of the time, with their 1.3x conversion. I'll only use the 40D when I need the extra reach.) But I think the 5D Mark II and the 7D would make a good combination with the appropriate lenses. The 24-105 and the 70-200/4 come to mind.

[stamper]

Quote

However, from a shooting standpoint, I think there is a huge difference. I don't "see" cropped images when I shoot -- I see the whole image area in the viewfinder. Having been trained to fill the frame, I hate shooting loose and cropping later. That's just not how I see. I would much rather put a 1.6x camera on my 300, and get an effective 480mm/2.8 lens, than shoot with a full frame camera and have to crop after the shoot.

Unquote

I take it you mean lens? Sorry to nit pick. I was on the Nikon Cafe earlier and a poster was talking about differences between FX and DX and the "greater reach" DX has. It is a myth? The effective 480mm/2.8 lens is also a myth? This marketing disinformation that has to be cleared up for the benefit of photographers?

[Ken Bennett]

Quote

However, from a shooting standpoint, I think there is a huge difference. I don't "see" cropped images when I shoot -- I see the whole image area in the viewfinder. Having been trained to fill the frame, I hate shooting loose and cropping later. That's just not how I see. I would much rather put a 1.6x camera on my 300, and get an effective 480mm/2.8 lens, than shoot with a full frame camera and have to crop after the shoot.

Unquote

I take it you mean lens? Sorry to nit pick. I was on the Nikon Cafe earlier and a poster was talking about differences between FX and DX and the "greater reach" DX has. It is a myth? The effective 480mm/2.8 lens is also a myth? This marketing disinformation that has to be cleared up for the benefit of photographers?

No, I mean camera. A Canon APS-C camera has a so-called 1.6x "lens conversion factor." That's so because the sensor is smaller than full frame -- it is a *different format* than the full frame camera.

This is no different than in the days of film -- different film formats gave very different results with the same focal length lens. For example, I still own three 90mm lenses -- the 35mm version is a nice portrait short tele lens, while the 6x9cm version is a normal lens, and on my 4x5inch camera it is a wide angle. With film cameras it was much easier to understand this concept because all three of those 90mm lenses were different -- I had three separate lenses with the same focal length, each giving a wildly different result on its matching camera system. Why? Because the smaller the film size, the more apparent magnification you get from the same focal length.

With digital cameras, we're usually talking about the same lens. Say, for example, a 300mm f/2.8 lens. The cameras look the same, too, which adds to the confusion. So we take the same lens and put it on two different cameras that look the same, and get different results, and then we argue about it.

It's much easier to talk about this when we have cameras that are similar in all other respects than sensor size. The Nikon D300 and D700 fit this bill nicely -- they are 12-megapixel cameras that look very similar in size and shape and user interface, but one is DX and one is FX. Now put your 300mm lens on each camera, and take a photo of a distant object. Notice that the photo from the D300 (with its smaller sensor) shows a more magnified view. Why? Because the smaller the sensor size, the more apparent magnification you get from the same focal length. If we start comparing Medium Format Digital cameras to the "35mm-style" DSLR cameras, nobody would think twice about the difference in field-of-view between an 80mm lens on the MF camera and an 80mm lens on either FX or DX digital bodies. They are different because of the different formats (i.e., sensor sizes.) But we have endless discussions about this with DSLR bodies.

So getting back to that "480mm lens" discussion. It's not really a 480mm lens -- it's a 300mm lens on a *different format.* If I put a 300mm lens on an 8x10 camera, it would be a normal lens, not a telephoto. If I put a 300mm lens on a 35mm film body or a full frame DSLR, it's a nice telephoto lens. On an APS-C camera with the smaller sensor, it has a narrower field of view, and provides more apparent magnification. So yes, it does have more "reach," but it's not a "480mm lens." We have adopted the shorthand of calling it a "480mm equivalent" because most photographers have the 35mm film camera system as a shared point of reference, and it lets us compare lenses across different formats.

Does this help?