Choosing between full frame vs. APS-C

I know some of us are choosing between a full frame camera and an APS-C camera, such as the D600 and the D7100.  The purpose of this post is to highlight some of the differences to help you reach an informed decision.

1. Low light.
One typical advantage of full frame cameras is better low light capability.  Some would say that you can use flash in low light.  Of course you can.  But low light capability can complement flash use, not only increasing the potential range of your flash but also making it easier to mix the flash and ambient for natural-looking results.

Nikon D600 with flash. ISO 12,800.

Moreover there are times when using a flash is difficult.  In the shot below, the background was cavernous, and the black ceilings would have been difficult to bounce from.  At the same time we were constantly moving, therefore a long exposure would have resulted in a blurry shot.  Thanks to the D3's high ISO capability, I could use a shutter speed as high as 1/160 (f/2.8, ISO 12,800) for a sharp shot.

Nikon D3 with Tokina 11-16 at 16mm, f/2.8, 12,800 ISO.

The better argument in my view is that a full frame camera no longer has an obvious advantage over APS-C for low light.  With new high-performing APS-C cameras, you would have to look at specific cameras to draw a conclusion.  For example, DXO scores notwithstanding, I believe a Nikon D7100 can have a comparable image quality to the Nikon D3, with both at 12,800 ISO, if the D7100 shot is reduced to the same resolution, especially if both are viewed at laptop viewing sizes. [UPDATE: I have confirmed that, as incredible as it sounds, the D7100 does indeed have the same high ISO performance as the D700/D3 when viewed at the same 12mp size.  See here.]

Nikon D7100 at 12,800 ISO (with flash). No noise reduction yet.

2. Sharpness.
All factors being equal, a full frame camera will be sharper than an APS-C camera.  I know there are skeptics out there so let's look at DXO scores for the Sigma 35 1.4, one of the sharpest lenses ever made.

On a 6mp Nikon D70, the Sigma 35 1.4 has a resolution of 6 perceptual megapixels (i.e. the lens outresolves the sensor).  On the higher resolution sensor of the 12mp Nikon D300S, the Sigma 35 1.4 has a resolution of 10 pmp, a nearly proportionate increase of total resolution.  On the 16mp Nikon D7000, however, the Sigma 35 1.4 only has a resolution of 11 pmp.

Although there is a significant increase in sensor resolution from the D300S to the D7000, the increase in total system resolution is marginal.  It appears the lens has reached its limit.

Or has it?  If we compare the Sigma 35 1.4 on the D7000 with the Nikon D4 which also has 16mp, the system resolution jumps to 14pmp.  This alone should tell you that all factors being equal, full frame cameras will give a sharper image than APS-C.  However, that's not the end of the story.  If you go still higher to a 24mp Nikon D3x, the total system resolution leaps to 20 pmp.  In fact, you may have heard that on the D800, the Sigma 35 1.4 reaches as high as 23 pmp.  So in fact, the Sigma has far greater potential resolution than the D7000 result would imply.

The explanation is simple: an APS-C camera has smaller pixels than a full frame camera, and a lens has to have a higher resolution in order to fully utilize the resolution of a sensor with smaller pixels.  Below a certain pixel size, a lens has to be extremely sharp to match the sensor resolution.  The Sigma 35 1.4 is already one of the sharpest lenses out there but even the Sigma cannot fully use the 16mp resolution of the D7000.  This is why I think that if DXO tests the Sigma 35 1.4 on the 24mp D7100 or D5200, the increase in resolution will be marginal.  Hopefully, in the future, manufacturers step up and design higher resolution lenses for APS-C (there are after all many high resolution Micro 4/3 lenses).  For now, though, there aren't any.  See the list of sharpest lenses for DX here.

Of course there are other factors that affect sharpness (duh!).  Nonetheless, if we stick to the issue of whether a full frame camera is sharper, the answer is that indeed, a full frame camera has a higher potential for sharpness than an APS-C camera.

3. Depth of field.

85 1.8G on Nikon D600

A full frame camera using a lens with the same field of view as an APS-C camera will appear to have a shallower depth of field, all other factors being equal (same distance to subject, same distance to background, same aperture).  However, one counterargument is that you CAN achieve a very shallow DOF on an APS-C camera as well, such as by using a longer focal length, moving closer to the subject, etc.

28-105 @ 105mm f/4.5 on Nikon D7100

Moreover, sometimes a very shallow DOF is not practical.  For example, if you use a 70-200 at f/2.8 on full frame to take a photo of a couple of people, the DOF is usually not deep enough to keep both faces in focus, unless they are exactly at the same distance to the camera.

On the other hand, at wider focal lengths, it is not easy to achieve a shallow DOF on APS-C, this gives wide-aperture wide angle full frame shots a distinctive appearance.

28 1.8G on Nikon D600

I personally believe that after we all have looked at many photos, we gain an intuitive expectation of the DOF of an APS-C camera as opposed to full frame, and when we see a shot with unusually shallow DOF at a wide angle, we recognize it as a full frame shot.  In fact, I would go even further. You know how some people say that full frame has a certain 'look' but they can't put it into words?  I think what gives full frame that distinctive appearance is the unexpectedly shallow DOF.

Sigma 50 1.4 on Nikon D600

4. Cost vs. versatility.

24-70 on full frame (Nikon D3)

One deterrent for getting a full frame camera is the cost of lenses.  It is true that full frame lenses generally cost more than their APS-C-only counterpart.  However, if you have an APS-C as a second camera, a full frame lens can function as two different focal lengths.  For example, the 24-70 2.8 is great as a standard zoom on full frame.  On APS-C it can work as a portrait lens.

24-70 on APS-C (Nikon D90)

5. Wider angles vs. reach.
The conventional thinking is that full frame is better for wider angles because at the same focal length, you'll have a wider field of view, while APS-C is better for longer focal lengths because at the same focal length, you'll have a longer equivalent focal length.

I think this is not a big factor.  There are wide angle lenses specifically designed for APS-C that will allow a wide field of view.  For example, the Sigma 8-16 can have a field of view as wide as a 12mm full frame lens.  That's just the same as the widest full frame ultrawide, the Sigma 12-24.

Sigma 10-20 @ 12mm on Fuji S5, an APS-C DSLR

As for reach, a full frame image can be cropped to have a smaller equivalent field of view (plus you have more flexibility for choosing the framing).  So I think the key issue is the resolution of the cameras you're comparing. For example, if you're comparing a D3 (12mp) and D7100 (24mp) then the D7100 will probably come out ahead because the DX-cropped D3 image will only have about 5mp, a much lower resolution than the D7100 (do consider other factors such as noise, etc.).  On the other hand if you're comparing a D800 (36mp) with a D70 (6mp) then the DX-cropped D800 image will have about 15mp, while the D70 has only 6mp, therefore the DX-cropped D800 image will probably be better (indeed at least one comparison of the D800 DX crop to the D7000 shows that the cropped D800 image is comparable to the D7000).

EPILOGUE
As you can see, the choice between full frame or APS-C really depends on what kinds of photos you take.  Personally, I like having the versatility of having both a full frame and APS-C camera.  When I want shallower DOF, such as on a standard zoom, I use full frame.  When I want a deeper DOF, such as an ultrawide or fast telephoto zoom, I use an APS-C camera.

RELATED POSTS:
Full Frame DSLR FAQ
Comparison of DOF between APS-C and FF in Depth of Field of Kit Lenses