There was a bit of a Facebook exchange recently in our camera club about how best to achieve a fill-the-frame image when your lens can’t provide enough reach. This wouldn’t even have been a discussion a few years ago – the only two ways to do it would have been to zoom with your feet (get closer) or to crop in post production. DSLRs offered no other options.
But today’s higher end DSLRs and all (most?) mirrorless cameras offer another option where you can isolate how much of the sensor read-out is captured in the RAW data for a particular image. When converted, that reduced RAW data then “fills the frame” appearing to create a zoom or magnified effect and brings you in “closer” to your intended subject.
The exchange on Facebook fell into two camps: those who called the result magnification and those who said that was misleading. Well, in fact, they are both right.
So let’s take it from the top.
You have a lens. You have a subject. You have a camera. The camera is a full-frame camera, when operated normally. Your lens may be a zoom or a prime, but regardless, it is not of sufficient focal length to be able to fill the frame with your subject of choice. What do you do?
If your lens allows it, you continue to zoom to a greater focal length, “magnifying” your subject and filling the frame. This is indeed true magnification, as it fully depends on the optics of the lens. The camera body and sensor have no contribution to the subject filling the frame.
But let’s say you don’t have the ability to zoom to fill the frame, but your camera has something called “crop modes”. Instead of your full-frame setting (36mm x 24mm), you select the APS-C equivalent (23mm x 15mm approx.), micro 4/3 equivalent (17mm x 13mm approx.) or something even smaller if permitted. What exactly is happening and what will your image look like?
First, to dispel a couple of myths, you are NOT zooming in when selecting one of these modes. Second, using one of these modes is NOT the same as using a longer focal length lens.
Instead, you are in effect saying to the camera:
- Let in all the light you did in full-frame mode and let all of that light hit the sensor as before.
- Convert all of that light into data as before.
- If I am shooting in RAW, discard the data that falls outside the area of the crop mode selected.
- If I am shooting in JPEG, discard the data that falls outside the area of the crop mode selected.
- When I import my RAW file into an editor, take all of the data kept and fill the frame with it. None of the data discarded at capture is available.
- When I import my JPEG file into an editor, take all of the data kept and fill the frame with it. None of the data discarded at capture is available.
And another myth, or maybe just a bit of related confusion about image size and quality. When using one of these crop modes, you are indeed discarding data from areas of the sensor that are outside the crop mode you have selected. That then means that your file size will be substantially smaller AND your image resolution will be substantially less than it would have been in normal full-frame mode.
For example, I have a Canon EOS R5. In full-frame mode, I obtain a 45 megapixel RAW file with a file size of approximately 39 MB. When using APS-C mode, the result is a 17.5 megapixel RAW file and a file size of approximately 11 MB.
Ok, then, the next question becomes – which is better, shooting in crop-mode in camera or shooting in full-frame mode and cropping afterward? Which provides a better result?
There will be times when getting the shot in final form in camera is necessary or even desired – perhaps for events where files have to be shared quickly or where the final result is going to social media with its low file size, low resolution standards. Why fuss with them in an editor when you don’t have to?
But for most situations, editing in post is expected and desired. We are always told to never throw away pixels until we have to, so it seems premature to use a crop mode in camera and eliminate the option of choosing your crop in post. Cropping in post provides much more versatility in terms of the positioning and orientation of the crop – and you can reverse it if needed and select a new one later.
What about that question of magnification? Strangely, dictionaries don’t seem to really distinguish between enlargements achieved optically (in this case, by zooming the lens) and enlargements achieved digitally (in this case, by discarding data), although there is a bit of a bias toward the former.
So, in my view, both optical and digital results are indeed magnified, but the result is very different. Having the same field of view in full-frame sensor mode vs crop sensor mode means that the full-frame version simply has more data to work with at the start. One is much better quality than the other (remember my 45 megapixel file vs my 17 megapixel file).
There is an excellent summary of crop mode by David Bergman on Adorama TV. In addition to quality differences, David also points out that depth of field doesn’t change when using crop mode in camera, whereas it would if changing the field of view using a zoom lens or zooming with your feet. So the captured elements of the image are also different in the two modes.
David also points out that there are other modes, at least for Canon, included in the same menu as the crop mode. For some reason, Canon chose to include different “aspect ratios” in this same menu, which is very confusing. Aspect ratio selections always capture the full sensor readout and simply select the identified aspect ratio to display the JPEG preview on the back of the camera. The full sensor readout is always available for image editing for RAW files.
Confusing? Just a bit. Sometimes providing a new feature such as crop modes just makes things a bit harder to understand and use. Hopefully this has cleared up some of the issue. Crop modes can be useful in some limited cases, but maybe not enough to offset the confusion of understanding them.