When do the differences between APS-C and full frame sensors matter, and why?

Question

I'm trying to decide on a higher-end digital SLR, and I'm down to choosing between an APS-C model and a full-frame model.

I understand that the sensors are of different sizes, and as such have an effect on the perceived magnification of the lens, with the smaller APS-C sensor having an effective focal length greater than what it otherwise would be with a full frame sensor. But why does this matter?

• What things should drive my choice between one or the other?
• In which situations is one better than the other, and why?

Answer 1

• One major difference is that a FF camera produces a depth of field that's around 1.3 stops shallower than an APS-C camera for the same subject & framing. This is most important when you have the aperture as wide as possible, e.g. for portraiture. To replicate the look of a 50 f/1.4 lens you'd have to use something like a 31 f/0.9 lens, which doesn't as far as I know exist!

Quick and dirty comparison image, APS-C Canon 30D left, FF Canon 5D right, same lens (FF image was zoomed in, however to give the same field of view), same composition, both f/2.8

• Another difference is that if you're using a lens designed for a full frame camera (like all Canon EF lenses) you are making full use of the image circle, which is less demanding of the optics and so you can expect a sharper image for the same number of megapixels. It's true that some lenses get softer toward the edge of the image, but you will still get higher average sharpness with most lenses, and telephotos will be sharper right across the frame. The crop factor of APS-C cameras takes the middle out of the lens and blows it up, losing sharpness in the process in a similar manner to a teleconverter.

• Larger formats allow for sharper optics. One of the driving forces for larger formats (other than the relatively constant resolving power per unit area of film) has been that it allows lenses to be produced which resolve a greater number of line pairs per picture height. Going full frame on a DSLR benefits from this to an extend - see: With all other things equal, in a DSLR, will a larger sensor produce a sharper image?

• A bigger sensor means bigger pixels, which in turn means you capture more light usually achieving lower noise levels in the process. Greater dynamic range goes hand in hand with this.

• You get a larger, brighter viewfinder on a full frame camera, which can be helpful composing shots. Having said that, I personally find the 5D viewfinder too large, I've not used a 7D but it has a very high spec 'finder.

• You have more mirror to move on a full frame camera. The larger mirror used to mean shooting speed is limited (the mirror on my 5D moves so slowly I can actually see the world slide sideways/up for an instant) however high speed full frame models are now available.

• Likewise the mirror box, focussing screen and pentaprism are larger, meaning the camera is larger and heavier.

• Lens hoods are designed for FF image circle and are therefore slightly more effective on FF cameras. This mostly applies to prime lenses, as zoom lens hoods are designed cut to accommodate the widest zoom setting, so everything else is already non optimal. If you're using an EF lens on a crop camera you ideally want the hood tighter (since the extra shading will lie outside the smaller sensor, a tighter hood won't vignette).

I have nothing against APS-C cameras but for any format it makes sense to use lenses designed for your sensor size. The range of EF-S lenses is smaller than the range of EF lenses. However for some uses (sports etc.) the smaller sensor size is helpful for the extra reach and speed it allows. Also the better noise characteristics of a FF sensor don't quite make up for the higher ISO you need to use get the same exposure when stopping down to match the DOF as a crop. So if you have to maximise DOF crop has a slight edge.

If there are EF-S lenses available for what you want to shoot then it won't be noticeably worse choosing this camera. However I feel full frame gives you more flexibility (speed aside) - as you can get the same deep DOF as a crop, but go narrower if you need to.

Answer 2

Remember that full frame is not explicitly better than APS-C, it's just 'different'.

It's perceived as better because shallow depth of field is very trendy, and that's the advantage of full frame, and for the portrait work I do it's invaluable, and even more important is the fact that I can shoot a scene at f2.8 and have it sharp, if I shoot the same scene on a crop and want the same bokeh then I have to shoot at f1.4 and it's significantly softer.

In fact, if you need maximum depth of field then full frame becomes a disadvantage. For example I shoot 3 or 4 models at a time and need the depth of field to cover them, this requires a very small aperture- like f8, this then requires a lot of light to fill the scene. If I shot the same scene with a crop camera I can get away with f5.6, and half the lighting power- the tradeoff being that the 5d files will be sharper, and allow me to crop in more, but if the shoot doesn't have the budget to cover the lighting costs then the small increase in sharpness is moot.

I shoot both full frame and APS-C because I need to use them both for different purposes. I'm even considering dropping down to micro 4/3 because I could have a use for that too (i.e. maximum depth of field at large apertures with minimal lighting)

Answer 3

Two very good answers already, but I want to chime in that isn't necessarily related to Crop vs FF, but rather your ultimate decision: 7D vs 5D Mark II.

While the 7D and 5D2 have different sensors, they are also geared toward different usages.

The 5D2 is not really designed with shooting action. It works (I use it), but the 7D is better with a higher precision AF, and machine-gun like burst shooting.

In general:

7D = sports, bird in flight, action
5D2 = studio and landscape

You can intermix the two with good results, that is each bodies strengths.

Answer 4

The main stylistic reasons revolve around the ability to capture wide-angle shots (although less of an issue with wider EF-S specific lenses) and that a larger physical sensor allows for a narrower depth of field (for a given aperture/focal length)

In additions, on a technical level, a larger sensor also allows for lower pixel densities, which can improve low-light performance.

Answer 5

A lot depends upon what type of photos you want to take and how you wish to display them.

Pretty much any camera, including most phones, can take a photo in bright light for relatively low resolution display on the internet. In the hands of the right photographer such an image can even be pretty good no matter what kind of camera is used.

Where the differences are the greatest and the limitations of lesser cameras become more apparent are under specific conditions that tax the light gathering capability of the camera the most, require very wide angle coverage, or challenge the resolving power of sensors and lenses the most.

A camera is a tool. Tools come in different shapes and sizes. Some are better suited for using one way for certain tasks. Other tools are better suited for using other ways to do other tasks.

In general larger sensors allow the possibility of better image quality, but they in no way guarantee it. Sometimes the difference in quality between different cameras taking the same photo will be incremental and hardly even noticeable. Sometimes it will be quite obvious. Often the other things needed to allow using the larger sensor to get that quality means spending a lot more money on things such as lenses in addition to the higher initial cost of the larger sensor.

Where larger sensors have greater advantages:

• The magnification needed to view the image, as projected onto the sensor, at a specific size is smaller for a larger sensor compared to a smaller one. This means the flaws projected by the lens aren't as magnified when the image is viewed. To get the same sharpness with a smaller sensor, the lens must also be sharper.
• Since exposure is measured as the amount of light per unit area, larger sensors collect more total light for the same exposure value. This tends to make larger sensors less noisy. The age of the sensor's technology can also affect this to a significant degree. But if two sensors from the same manufacturer use the same generation of technology the larger one will almost always be less noisy when shooting the same scene with the same settings and then viewing both at the same size.
• Larger sensors tend to have larger photosites which are also known as pixel cells or sensels. Because a larger photosite has more surface area, it can collect more photons before it reaches full well capacity. This gives sensors with larger photosites more dynamic range. DR is the difference between the brightest value a sensor can record and the dimmest value that can be recorded and still be discriminated from noise. This is one reason why larger sensors are less noisy than smaller sensors.
• Since lenses of the same focal length will give a wider angle of view when used with a larger sensor, it is easier to make lenses that give wide angles-of-view for larger sensors.

Where smaller sensors have greater advantages:

• The magnification needed to view the image, as projected onto the sensor, at a specific size is larger for a smaller sensor compared to a larger one. This means a lens with a specific focal length will give greater "reach" with a smaller sensor than a larger sensor. Things the same distance from the camera will appear larger with a smaller sensor than on a larger one when the same focal length is used with each and the respective images are then viewed at the same display size.

• Since aperture is expressed as a ratio of the focal length to the diameter of the entrance pupil, shorter focal length lenses can achieve the same exposure or f-number with a smaller aperture diameter than a longer focal length lens. If a larger sensor needs a 100mm focal length for a certain angle of view and a smaller sensor only needs a 50mm focal length to give the same angle of view, the smaller lens can have an entrance pupil half as wide and give the same f-number as the longer lens.

• Since lenses of the same focal length will give a narrower angle of view when used with a smaller sensor, it is easier and less costly to make telephoto lenses for smaller sensors. The same telephoto lenses will give a narrower angle-of-view (more "zoom") when used with smaller sensors.

In the end, the quality of a tool is only as good as the skill of the one who is using the tool. Only when the tool is limiting the skill set of the user will a better tool make a difference.

For more, please see:

What Do I Gain from Moving to a Full-Frame DSLR?
Why does it seem like large sensors are necessary for good low-light performance?
Do full frame sensors have a higher exposure?
Does sensor size impact the diffraction limit of a lens?
Why are larger sensors better at low light?
What is the visual difference between Full Frame, APS-C and Micro Four Thirds pictures?
When do the differences between APS-C and full frame sensors matter, and why?
Full Frame Vs Crop Frame
Is the low-light advantage of larger sensors attributable to the sensor itself or to the larger aperture of the lenses?
Do I need a full-frame camera for low-light photography?
How does the smaller mirror in APS-C cameras offer these advantages?
What significant improvement will I get if I upgrade to Canon EOS 6D from EOS APSC 600D?
How does sensor size impact depth of field and diffraction for macro photography?
6D or 80D for upgrade from 100D?
Big sensor and landscape photography, DoF
Can we always talk about noise difference between crop and full frame?
Fancy technology cropped vs old technology full frame - Which will give better images?
Is small sensor always a bad thing?
Does sensor size affect lens distortion?
Which sensor "full frame" vs. APS-C (1.6 crop) gives more distortion?
This recent answer to this older question: Are full-frame cameras bad for sports photography?

Answer 6

@Matt Grum has already given a rather comprehensive answer talking about the advantages of going full frame. But there's a flip side to using APS-C, which for certain uses gives it an advantage over FF.

Precisely because FF cameras use the whole image circle, images can suffer from vignetting, even when using otherwise excellent lenses. This can of course often be eliminated by simply stopping the lens down, but doing that negates the DOF advantage that a FF body provides.

While FF is great when looking for wide-angle shots, a crop-sensor body gives you more bang for the buck when buying long telephotos. Even if you were to buy the Canon EF 1200 mm f5.6 L lens to maximise your telephoto range on an FF body, all other things being equal, an APS-C body would still turn it into an 1920 mm f5.6 lens. This means that I could buy the 400 f4 L IS instead of the 600 f5.6 L IS to get the equivalent framing, and save a whopping $7000 or so! This is why most of my wildlife photographer friends end up choosing APS-C bodies.

Answer 7

@Clark and @Matt Grum's answers are very good, but they are both missing one of of the most important points pro smaller sensor sizes:

• The camera and all comparable lenses can be smaller and thus more light-weight.

Answer 8

The main problem with the 7D is not the APS-C sensor, it's the 18 million pixels squeezed onto it that's it major weakness, meaning soft images unless very good technique / fast shutter speeds are used.

Answer 9

APS-C versus FF is all time debate. I try to use simple camera with simple function. This gives me time to concentrate on composition. I feel i should use FF if really they want big printouts, otherwise APS-C format is good. Buy good "L" lens, for good sharp results.