Why an APS-C Lens Might Be the Right Choice for Your Full Frame Camera

Mar 05, 2025

Alan Adler

A Sony Alpha 7R camera with a visible E-mount sensor. The camera body is black with textured grip on the left side and control dials on top. The brand logo is shown on the viewfinder area in white.

I often see questions on forums about the focal length of lenses and how they work on cameras of various sensor sizes. Is a 35mm lens really 50mm on an APS-C camera? No, it’s still 35mm, but the field of view (FOV) is narrower on the APS-C camera because the sensor is narrower.

What about Micro Four Thirds (MFT)? Can a 100mm telephoto lens on an MFT camera see like a 200mm lens? No, it can’t, it’s still only 100mm, but its FOV is approximately equal to a 200mm lens on a full-frame (FF) camera because of a narrower sensor. When a lens of a given focal length is designed for APS-C or MFT, the glass elements can be smaller than FF, reducing weight and cost.

One thing that doesn’t change for a given focal length regardless of sensor size is magnification. We normally speak of magnification with macro photography, but thinking in terms of magnification can inform us of the effect of focal length in various formats. At moderate and longer distances:

MAGNIFICATION = FOCAL LENGTH / DISTANCE

So for example, a 100mm lens and a distance = 2,000 mm (approximately six feet) has magnification = 100/2,000 = .05. An adult head is about 250mm (10”) tall, so .05 x 250mm = 12.5mm and that head will be 12.5mm tall on any format sensor.

Most full-frame cameras, like my Sony a7R V, accept APS-C lenses. Although full-frame lenses are engineered to cover all of the full-frame sensor, APS-C lenses cover most of the sensor. And the APS-C lenses are lighter and cheaper. The Sigma 56mm f/1.4 APS-C lens is razor sharp and much lighter than most full-frame lenses of similar focal length and aperture. Here is a flower shot with the Sigma 56mm APS-C and 55mm Sony Sonar full frame:

Side-by-side images of a single pink rose in bloom on a bush, set against a blurred street background. The left image has darker vignetting around the edges, while the right image appears brighter and more evenly lit. — Sigma APS-C 56mm (left) and Sony Sonnar 55mm full frame (right)

You can see that the flower is the same size in both frames, which is because both lenses are almost the same focal length and thus have the same magnification. But the APS-C lens produced a vignetted image. For this subject and many others, the vignetting is of no concern. I only want the center.

A vibrant, close-up photo of a fully bloomed pink rose with petals slightly curling at the edges. The background is blurred, highlighting the delicate texture and color of the flower's petals.

I also crop to the center for most of my bird photos.

A small brown bird with a speckled breast stands on the edge of a pink planter. It holds an insect larva in its beak. Green foliage is visible in the background. — Shot with Sony APS-C 70-350mm

My birding lens used to be the full-frame Sony G Master 100-400mm. A great lens. But the Sony APS-C 70-350mm is also great and is half the weight. I always crop my bird shots, and just crop less when taken with the APS-C lens.

If you mount an APS-C lens and don’t see the same magnification and the vignetting, your camera detected the APS-C lens and automatically cropped the image. That’s quite ok, but if you want to see the entire image set your camera to not crop with APS-C lenses and show the full frame with any lens.

Camera settings menu displaying APS-C/Super 35mm mode options: On, Auto (highlighted in red), and Off. Menu icons include a trash bin, question mark, and return arrow. — “Off” means automatic cropping is off.

Below are some more bird photos shot with the Sony APS-C 70-350mm.

A seagull with grey and white feathers, standing on the ground with its distinctive orange ringed eye and yellow-black beak. The background shows soil and a strip of green grass. — Note vignetting in full frame

Close-up of a seagull with speckled gray and white plumage. The bird has a striking orange and black beak and a distinct red eye ring, set against a dark, blurred background. — Cropped from above

So why not just walk with the full-frame lens? If you already own it, there may not be a reason. But the Sigma 56mm APS-C lens and many others are fast, sharp, and less expensive than full frame lenses of similar focal length and aperture. The Sigma 56mm sells for $430 and is f/1.4. The Sony 55mm sells for $998 and is f/1.8. Both lenses are about the same weight but if you want a full-frame 50mm f/1.4 lens, it is much heavier.

Sonyalphablog rates both the 55mm and 56mm “Outstanding” in the center at mid-apertures. He gives the FF 55mm 4 stars and the APS-C 56mm 5 stars.

Comparison chart for Sigma 56mm F1.4 DC DN and Sony FE 55mm F1.8 lenses, showing ratings from F1.4/1.8 to F16. Sigma is rated "Outstanding" to "Good" while Sony ranges from "Outstanding" to "Very Good. — Courtesy Marc Alhedaff sonyalpha.blog

Lenstip tested the 55mm Sonnar as resolving a peak of 63 line pairs and the 56mm Sigma as resolving 83 line pairs.

Weight and cost savings are great incentives to go APS-C with very fast lenses. For example, the Voigtlander Nokton 50mm f/1.2 costs $899. The TTArtisan 50mm f/1.2 APS-C lens costs $112 and is lighter and sharper. Sonyalphablog rates the TT lens sharper than the Nokton from f/1.2 through f/4 and equal beyond that. He gives the TTArtisan 4 stars and the Voigtlander 3 stars.

A table comparing "TTfiction Storm F1.2" and "Voigtlander 50mm F1.2 XMount" lens performance. Categories include center and corners, rated from "Bad" to "Excellent." TTfiction scores higher overall in the ratings. — Courtesy Marc Alhedaff sonyalphablog.com

Of course, even full frame lenses vignette. This is from the phillipreeve.net review of the Nokton. The reviewer absolutely loves the Nokton. I wonder if he has tried the TT Artisan 50mm f/1.2?

Four images showing the vignetting effect of a Voigtländer 50mm 1.2 Nokton E lens at different apertures (f/1.2, f/2.0, f/2.8, f/8.0) on a Sony A7s camera. Each image displays varying brightness towards the corners. — Courtesy of Phillip Reeve

Full frame f/1.0 or f/0.95 lenses are hugely heavy and expensive. The APS-C versions are much lighter and cheaper. For example, the Meike 35mm f/0.95 lens weighs 380g and costs $189. It is very sharp in the center. The Laowa full frame 35mm f/0.95 weighs 838g and costs $649.

There are clearly some great options in APS-C lenses.

About the author: Alan Adler lives in Los Altos, California. The opinions expressed in this article are solely those of the author. He has been an avid photographer for 60 years. He is also a well-known inventor with about 40 patents. His best-known inventions are the Aerobie flying ring and the AeroPress coffee maker.