Does a full frame with a teleconverter perform better than an APS-C without a teleconverter?

Question

Context

Putting a teleconverter on a full frame lens magnifies the center of the lens but gives more reach due to that with some drawbacks. I've rediscovered focal reducers that do the opposite, collect the light into a smaller image circle for a smaller sensor.

Question

In terms of image quality, does a full frame with a teleconverter (1.4x to be fair) overall perform better than an aps-c without a teleconverter? Or, the related comparison: is a full frame with a teleprime better than an APS-C body with a focal reducer in between.

To make this fair, assume that the bodies are of the same generation.

Reference

• Should I get a tele-converter or a camera body with more megapixels and crop?
• How does a teleconverter affect depth of field?
• Will there be any difference in photos taken using full frame and crop lenses on a crop camera?

Answer 1

A good teleconverter or extender works better than cropping if the lens can outresolve the resolution of the sensor by an amount greater than the degradation of the TC and the resulting loss in lens speed is not a determining factor in the viability of the shot.

It depends entirely upon the teleconverter in question, the lens in question, and the full frame and APS-C cameras in question. The same is true when comparing a full frame camera + 1.4x to an APS-C camera when both use the same lens. Optical quality is never better than the weakest link in the equation. Adding a low grade TC to a high grade lens and camera should be expected to degrade image quality worse than cropping with a bare lens. But not all extenders are optically inferior to high grade lenses.

I've seen in-depth analysis that has gone one way with a particular camera/converter/lens combo and other comparisons with different cameras/converters/lenses that swung the other way. There are way too many variables from one camera/teleconverter/lens combination to the next to usually be able to say for sure one way or another which is best.

Consider just one such variable: the needed resolution of the final image. If the final use of the image only requires a resolution of, say, 2400x1600 pixels (3.8MP) then it doesn't matter at all if you crop a 22MP FF image down to an 8.6MP image to get the same angle of view as a 22MP APS-C camera would deliver with the same lens. If, on the other hand, you need all 22MP then the TC on the FF or the APS-C camera with the same bare lens becomes much more attractive.

Let's look at another variable: diffraction. If you need to shoot at f/11 which camera will have an advantage? The FF camera with a DLA of f/10.1 or the APS-C camera with a DLA of f/6.6? Using a smaller sensor does not reduce the size of the wavelength of light at a specific frequency.

Along with the transmission loss and potential aberrations due to the additional glass a teleconverter places in the optical path one needs to also consider the differences in efficiency of the two sensors being compared. Due to the way the ratio of linear edges to area increases as a pixel gets smaller, a sensor with larger pixels and otherwise the same technology will be more efficient. The properties of light in terms of wavelength size don't change with the size of the sensor. Even with so-called "gapless sensors", smaller pixels mean a lower percentage of the light falling on a sensor actually makes it to the bottom of a pixel well and is measured by the sensor.

When using an extender or teleconverter with a telephoto zoom lens, one should also consider the positive contribution a TC can have on geometric distortion. Most telephoto zoom lenses demonstrate mild pincushion distortion at the long end (which is where you'd generally use a zoom lens with a TC). Most quality TCs introduce a little barrel distortion that counters the pincushion to one degree or another.

Then there are considerations not directly related to pure optical image quality. If you are shooting action or sports, for example, it doesn't matter how great the image quality you get is if your AF system is too slow or too inaccurate to nail focus on the intended subject at the critical moment. With lower end lenses (i.e. narrower apertures) and teleconverters, focusing tends to be slower and less accurate with a TC if AF even works at all. Due to the narrower baseline allowed by the size of an APS-C lightbox and mirror, similarly designed PDAF systems, which measure light coming from opposite sides of the lens, will perform better in terms of accuracy and shot-to-shot consistency in a FF camera that allows a wider baseline. With premium lenses and the best TCs, focusing speed takes much less of a hit and the advantages in terms of accuracy and shot-to-shot consistency of the wider AF baseline allowed with a larger sensor and mirror often outweigh the difference in speed.

Having said all of that: in general when the best cameras are coupled with the best lenses the best teleconverters will usually give better image quality of static subjects shot from a tripod mounted camera than using the same camera without a TC and then cropping away. The Canon EF 70-200mm f/2.8 L IS II + Canon EF2X III yields better IQ than the EF 400mm f/5.6. Not by a whole lot, but it is better. The combo is also more expensive. However, even with the same camera/TC/lens, if the stop of light lost with a 1.4X is more crucial than the loss of resolution from cropping in such a case cropping might be the better the way to go.

Also generally speaking, a crappy lens coupled with a crappy TC will almost always look worse than just cropping with the same lens or using an APS-C camera instead of a full frame camera. Remember what we said at the beginning:

A good teleconverter or extender works better than cropping if the lens can outresolve the resolution of the sensor by an amount greater than the degradation of the TC and the resulting loss in lens speed is not a determining factor in the viability of the shot.

Answer 2

It's difficult to be sure. The lens-camera system is a complex one. Making any changes to it can have unexpected consequences.

Using a APS-C camera on a FF lens does throw away some of the useful light gathered. But interposing an alien lens cluster in the form of a focal reducer or teleconverter may or may not do more damage to the overall image quality.

First party teleconverters are matched to a specific number of lenses, but focal reducers are, as far as I know, all third-party.

One reviewer found that using the Canon 16-35 FF lens on a Sony FF body gave markedly worse results. This might indicate that making guesses without testing is folly.

Answer 3

Let's assume that both sensors have the same pixel count and that the crop factor equals the teleconverter magnification. Then in theory, the answer seems simple.

Both do the exact same thing: simply use the centre portion of the image.

The smaller sensor does it by placing the same number of pixels in the central image area.

The teleconverter does it by optically magnifying the central area to cover all pixels of the larger sensor. Optical magnification is imperfect: it introduces additional optical aberrations and some of the light is absorbed by the added glass elements.

Thus the teleconverter loses.

If we forget about the imperfections of the optical magnification, then both methods do the same thing: they spread the same light flux (i.e. same number of photons → same shot noise in the image) over the same number of pixels.

If the crop factor does not equal the teleconverter magnification, the sensors don't have the same pixel count, or they don't have the same noise performance, then the story changes.

Image quality aside, the smaller sensor camera will have the advantage of a fully functional auto-focus system. When using teleconverters, autofocus performance will generally be reduced.

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No, I haven't actually used teleconverters in practice. Above I presented theoretical arguments that I found reasonable and may help you make a decision, but I may have missed a few things. If so please point out what it is exactly.