Talk about Lenses, not Sensor Sizes!

 

The advice in the title is somewhat obvious now, and I have discussed the topic here several times. Yet, the discussion about sensor sizes comes up in YouTube videos continuously. Now, the frequency of topics in influencer content is not really a measure of what is important or not. Most of that noise can safely be ignored. But let me recap the fundamentals of equivalence nevertheless, and what it has to do with lenses.

The factor that a sensor is smaller than a so-called "full frame" 24x36mm is called the crop factor. APS-C has a crop factor of about 1.5, MFT a crop factor of 2, and the 1-inch sensor a crop factor of 2.7. A medium format sensor is even larger than 24x36 and thus has a crop factor of around 0.8.

Assume, we want to take the same picture using cameras with different sensor size.  For the pictures to look identical, we want the same angle of view, depth of field, background blur, image brightness, image noise, dynamic range, diffraction and shutter speed. We can achieve that in relation to full frame as follows:

• The focal length has to be multiplied by the crop factor. E.g., a 25mm MFT lens is equivalent to a 50mm full frame lens. You have to do this to get the same field of view.
• The f-stop had to multiplied by the crop factor too. E.g., f/2 on MFT is equivalent to f/4 on full frame. You have to do this to get almost the same depth of field and background blur.
• The ISO number has to multiplied by the square of the crop factor. E.g., ISO100 on MFT is equivalent to ISO400 on full frame. You have to do this to keep your shutter speed and still get the same image brightness.

The problem is that it is not always possible to achieve equivalent results. And the reason for that is the availability of lenses.

For a first example consider the 24-70 f/4 lens I use for my full frame Nikon Z5 II. An equivalent lens is the 16-50 f/2.8 for Nikon DX cameras. I have it for my Nikon Z30, and I can assure that it really delivers equivalent results. For an MFT camera, we'd need a 12-35 f/2 lens. Such a zoom lens does not exist. The closest one has f/2.8. You get less background blur and object isolation. Moreover, you might have to increase ISO one stop and get more noise. MFT, on the other hand, is very much lighter than full frame, and often has better in-body stabilization. This makes up for a small disadvantage.

For a more simple example, consider a 50mm f/1.8 full frame lens. You will be able to find equivalent APS-C lenses around 33mm with f/1.2. And there are 25mm MFT lenses with f/0.95, which is close to being equivalent. 

One also will observe that most full frame cameras start around ISO100, just like the MFT cameras. That makes a difference whenever you are able to use IS100 on full frame. On a tripod in a dimly lit scene, you are often able to use the lowest ISO and get a better dynamic range. In that case, the equivalence above is meaningless. There is no way to cope with the result on a smaller sensor.

There are more examples, where full frame has an advantage in noise handling and object isolation. But it does always come at the cost of more expensive and more heavy camera system, especially in relation to MFT. The difference is not so pronounced for APS-C which is sort of a sweeet spot. I should also mention that equivalent lenses have the same front diameter. Due to this fact, compactness can only come from length and weight.

The size factor, however, can be enormous. And that matters in wildlife photography or birding. A full frame 600mm lens is twice as long as an equivalent 300mm MFT lens and it can be three times heavier. This clearly speaks for the MFT.

For another example, compare the above mentioned Z5 II with the 24-70 f/4 to the Z30 with the 16-50 f/2.8. The difference in size is not enormous and mostly due to the missing EVF bulk. But the difference in weight is 1100g versus 700g.

The summary of what we learned is that you should look for equivalent lenses which help you get the same results in many cases, but with much less burden. Only when the full frame shooter can use ISO100 and noise matters, you are in a disadvantage. And you cannot cope with very fast, bulky and expensive prime lenses for object isolation.

We should also talk about resolution. Clearly, a smaller sensor is in a disadvantage because its content has to be magnified more. Moreover, it is possible to squeeze more megapixels to larger sensors, which also increases the resolution. On the other hand, it decreases the noise performance. Those full frame lenses achieve their maximal resolution in the center only. Your smaller lens might be able to achieve the same across a larger part of the frame. As to resolution, it is not at all clear that full frame is always better. That depends on the details of the lens and megapixel count.

The full frame lenses are often more "professional" like the whole full frame system, because they are more expensive and thus profitable. This increases the chances to find specialized or highly optimized lenses. But there are camera makers who put the same effort on APS-C. Fujifilm is a prominent example. I doubt that an affordable full frame lens is visibly better than an equivalent lens made for the APS-C cameras of Fujifilm.

As a summary, professionals will continue to use full frame. But semi-pros and professionals in the wildlife or macro section should really consider a camera with a smaller sensor to save bulk and money. If that is not an issue, full frame is still the most versatile option. It just does not guarantee better results.