
Without a doubt, your focal length choice affects the impact of your image, whether you are shooting video or stills. With the rise of digital imaging, there has been an uptick of available formats in which you can shoot. To help clarify the relationship between format and focal length, some have coined the phrase “35mm equivalent.” This term has value when you are planning a shoot, your lens package, and as an aid for anyone who began their careers using 35mm film and has transitioned to shoot in a new digital format. However, the final determining factor of your image is the perspective that your lens will impart—we've all heard that “a 25mm is a 25mm, is a 25mm” and that if you put a 25mm on a different format it will behave the same, no matter what. Or will it?
I have heard the arguments from all perspectives and decided to put it to the test for myself. I felt it important not to merely crop the image from a full-frame sensor, but rather to compare different formats with a lens native to each format. That is, correct native lenses to match the sensor size.
As it turned out, testing with different-sized sensors was very revealing, although not for the reason I set out to explore!
I enlisted the help of my friend, Justin Dise, a videographer who has written a few articles such as Selecting a Lens Package, in which he discusses what makes a normal lens normal. He brought along his Sony a7R II and a series of vintage Nikon AI-S lenses and a Voigtlander Nokton 40mm. I supplied a Panasonic G7 and the Blackmagic Pocket Cinema Camera (BMPCC from now on), plus 20mm and 14mm Panasonic prime lenses in MFT mount. We used a Nikon to Sony E, and Nikon to MFT Adapter to round out our camera-lens packages. For support, we used an E-image EJ60AAM Tripod package and a Magnus VT300.
The tests
We made a few tests utilizing a frame with 16:9 aspect ratio, shooting the highest resolution of each camera in that format, unless otherwise noted. With the a7R II and the G7 we shot stills, and with the BMPCC we shot HD video. I used a combination of DaVinci Resolve and Photoshop to compare images. Test 1 was shooting the 50mm Nikon in each format. For Test 2, we shot the 40mm on a full-frame sensor of the a7R II and the 20mm on the G7 with the cameras in the same position.
Test 1
This is pretty straightforward, with no surprises. As you record on a smaller format, your field of view gets smaller, which is exactly what you would expect, but important to establish as a ground rule. If you want to compare cropping your sensor to the images shot natively, please feel free. There were some small variations since the lens height was different from camera setup to camera setup.
Test 2
This is where things became more interesting, because it is all well and good to crop your image in post or use a smaller sensor, which only captures part of what your lens is delivering. The question is: how does that affect the image your lens produces? We set up the a7R II with the 40mm Voigtlander next to the G7 with the Lumix 20mm and matched lens center height and frame, and then had a subject (me) walk toward the cameras. While at first it appeared that there was a difference in perspective between the two setups, I was concerned that this may have been due to the cameras not being on the same axis. This led to repeating the test, only this time we used a single camera position, and were able to configure the two cameras so that their lens center was very nearly the same.
![]() |
![]() |
![]() |
![]() |
40mm on Sony a7S MarkII Full frame |
![]() |
![]() |
![]() |
![]() |
20mm on Panasonic G7 Four Thirds |
We took great care not to adjust the position of the tripod after setting it. Based on the 2x crop factor, a 20mm lens on a Four Thirds/MFT sensor would yield the same angle of view as a 40mm lens on a full-frame sensor, and it truly did. What I was expecting, however, was that the shots would look different, that the perspective would change and objects farther away from the lens would be significantly smaller as the wider lens would exaggerate the distance instead of compressing it. What makes this framing so appropriate to test the theory of focal length affecting perspective is that it includes a receding fence with lampposts that can be used to judge height and also recede to the vanishing point. Additionally, off in the distance, just off center is the Statue of Liberty. If I was correct, then with the 20mm on the G7, the fence and lamp posts would become noticeably smaller the farther they receded from the camera, compared to the 40mm on the a7R II. Likewise, the Statue of Liberty would be significantly smaller in the shot with the 20mm on the G7. As you can see below, that did not happen—MUCH to my surprise.
|
|
Okay, but what is really important is how the lens perspective affects the background behind a person. Enough with matching the field of view, I’m surprised the perspective is the same in this example, but what about if I’m specifically matching a human face between the two formats. How will lens perspective change then?
Lost horizons
Setting up an actor, we proceeded to see how the format affected the image produced by the lens. The results, from a testing standpoint, were disappointing and, in the end, revealed certain weaknesses in the testing procedure. I've included those here, for full disclosure.
The first part involved shooting the 50mm in all four formats—full frame, APS-C, Four Thirds, and Super 16. The goal was to frame our actor so that he was the same size in all four frames. This would involve moving the camera farther away from the actor as we changed formats.
As you can also see from the images below, the amount of visible environment in the frame around the actor clearly changes.
For the next pass, I chose to compare a 28mm lens on a full-frame sensor to a 14mm lens on a Four Thirds sensor, because this would prove that there is a difference between the perspectives that each lens delivers. So, we mounted the Nikon 28mm on the a7R II and the Lumix 14mm on the G7. The results are below, and check out the chart for the distance between the camera and the subject for each lens/sensor combination.
The image from the a7R II is a frame grab from a full-frame video in HD. With the G7, I shot a still so as to maintain the 2:1 crop factor for comparison (the G7 crops its Four Thirds-sized sensor in 4K video). Both were shot in 16:9 mode. Look at the composite of the two below, where the 14mm is overlaid at 45% opacity on top of the 28mm full frame. Note the lamp post and the peak of the building.
There’s something rotten in Denmark
After careful scrutiny, I determined that something was not right—that the actor didn't maintain a constant position and the ground was too uneven, so tilt was introduced. It was obvious that far too many unanticipated variables affected the test, but I was determined to try again.
New premise: depth of field—aye, there's the rub
I wanted to simplify the test to compare a lens on a full-frame camera to a lens of half that focal length on a Four Thirds (MFT) camera when photographing a person, to see how the image changes when you have matching frames. To remove as many variables as possible, I used a wig head on a stand so it wouldn’t move, and I placed various registration stickers on it so I could compensate for faulty registration (when the camera's sensors and lens axis do not match). I shot indoors and didn’t tweak the camera position for the test.
I wanted to push the test as much as possible, so I used a 28mm on the full-frame camera and a 14mm on the Four Thirds camera, hoping to see a difference in the amount of distortion each image delivered. For full frame, I shot with a Sony a7S in full-frame mode and in 3:2 aspect ratio. The 12MP sensor would better match the Panasonic G7's sensor also shot in 3:2 mode. I took a few test shots, and saw that the images were basically the same—except they weren't. With everything matching between the two setups (distance, f-stop), the perspective of the lenses matched, but there was a noticeable difference in depth of field.
So I shot with each camera, racking through the f-stops while changing shutter speed and ISO for consistent exposure. By now I'd completely accepted that “35mm equivalent” is a valid term, and that perspective is really only affected by distance from the sensor plane to the subject. Two cameras with different-sized sensors will produce images with a matching perspective, as long as the lenses used produce the same angle of view in their respective formats. This is where equivalent focal lengths come into play: a 28mm on a full-frame camera delivers the same image as a 14mm on a Four Thirds camera. Granted, there may be slight differences due to different optical designs, distortion, and any lens corrections performed by the camera.
However, the real difference is evident in the depth of field. To get roughly the same depth of field with the full-frame camera as with the Four thirds, I had to narrow the aperture 2 to 3 stops more—f/5.6 or f/8 on the a7S, compared to f/2.8 on the G7. See the images below, so you can reach your own conclusions.
All's well that ends well
The camera’s sensor format has a decided effect on the field of view of the lens, causing the need to move the camera closer or farther away from your subject to get the desired framing. Also, the closer the camera is to the subject, the more exaggerated the distance between subject and background will appear to be; the farther away the camera is, the more compressed distance will appear. This isn’t a focal-length issue—it’s a distance-to-subject issue.
At the end of the day, I feel satisfied that the phrase “a 25 is a 25 is a 25” isn't accurate once you start changing formats. Put that theory to bed, I'm convinced. The format affects not only the perspective of your lens, but the depth of field of your image, and now I’ve got the images to prove it.
17 Comments
It was nice to see your side-by-side images and tests, however informal and error-prone they might be. I can always respect someone who will do their own testing and try things out to find what really works best for them. The reason I think MFT is the best format (particularly for video) is the depth of field, as you saw in your tests. Outside of portraiture and very specific filmmaking needs, isolating the subject and blowing the background away is usually more of a detriment than an asset. I wrote a whole article about why large sensors suck (which you can judge the correctness of for yourself), and the excessively shallow DOF is the biggest reason by far.
It's also important to remember that equivalence is more of an academic guideline than a rule; if I'm standing in a place and taking a picture with my MFT gear, I'm not necessarily going to use the equivalent focal length to a full-frame camera in the same situation. There's also the problem of lens limitation: if I'm standing there with a 25mm prime on MFT and that's the only lens I brought, I'm definitely not going to do the same thing as if I brought a full-frame with a 35mm prime or even a 24-70mm zoom. The lens in use affects composition choices just as much as the desired compostition affects the way the lens is used; it's not realistic to say "25mm MFT is 50mm FF, and if you had these two combinations of gear, you'd definitely compose this photo the exact same way." The equivalence is only a field-of-view equivalence, with literally everything else being affected by the differences. Equivalence isn't an apples-to-apples comparison so much as "if this is what you're going for and you're used to dealing with a particular format, you can kind of get a similar result if you start with this in the shooting format." Far too much value is placed on equivalence.
Hi Jody, thanks for reading and your comments. Best format is subjective of course, and dependent on the needs of the project, but I do like it myself. I was just trying to prove/disprove the "A 25 is a 25 is a 25" statement, any other conclusions drawn are incidental. I shared the flaws I found in the early tests and images because I felt it best to be transparent about my testing and conclusions, Tests and methodologies evolve over time and I find it useful when I read about improvements that others see to make in their own testing procedures, so I wanted to include mine. Best
Sorry, I meant that I do like the MFT format myself. Thanks again for commenting. Best
Focal length affects the resolution of the image. Sensor size affects the field-of-view.
It's very simple.Equivalent focal length is nonsense. Reducing the size of the sensor does not increase the resolution as does increasing the focal length. Let's look at an extreme example. Compare a 50 mm lens and a 1000 by 1000 pixel sensor to the same lens using the sensor cropped to one pixel.In the "equivalent focal length" jargon the equivalent focal lengthwould be 50,000 mm.
But the resolution is exactly the same, which is the size of 1 pixel/focal length.
Hi wayne,
35mm Equivalent focal length the term the industry has adopted to maintain a consistent description for an expected field of view for a particular camera when the digital revolution began. The equivalent focal length makes sense to veteran photographers, but often serves to just confuse those new to the art. This article was more about clearing up some of the confusion about the relationship of the format size to the focal length of the lens and what affect that had on the lens perspective using native-size sensors and not just cropping the image from a full frame sensor.
Thanks for reading!
Question - Whe I use an adapter to mount Hassleblad 6x6 lenses (eg, 80 mmf2.8) on my Nkon Df, is the focal length (info called for on the "non CPU lens " space on the Nikon menu) changed? How about the widest f stop?
Hello R, thanks for reading. I haven't seen an effect on f/stop, as long as the adapter is maintaining the proper flange focal distance, so your lens functions properly. On the Nikon, I would expect your 6x6 lens will effectively longer than it was on your Hassleblad. How much longer would depend on the ratio of the image area on your nikon compared to that of your Hassleblad. I'm really a video writer, and this was intended to be for video, however the principles are the same, I just don't know the ratios between the two cameras you mentioned. On the Panasonic G7 (Four Thirds sensor) the 14mm gave the same perspective as the 28mm on the Sony A7, at the same subject to camera distance. Hope this helps.
Yes, I agree the most significant difference is in the depth of field. I have some closeup shots of flowers, some with a bee on it, and they came out sharp with a good depth of field at its maximum aperture of f8 on a camera with a 1/4 inch image sensor. Maybe a good macro setup would be to use the smaller format cameras.
Hi Jim, good point. Back on days when it involved an optical process to make movies, or prints. Part of the depth of field calculation involved circles of confusion. The smaller your imager Film) the smaller your circles of confusion - essentially because when blowing up a smaller format (say Super16mm to match 35mm you would have to magnify it many more times, so you needed a sharper image to begin with,
Nowadays I'm noty sure that applies, as now we are displaying based on number of pixels.So 1920 x 1080 on a full frame sensor and 1920 x 1080 on a Four Thirds, is still 1920 by 1080. The differences now are sensor design, pixel sensitivity - generally lower with smaller pixels, and of course lens and also diffraction off the iris - the smaller the imager the quicker diffraction and image degration occurs.
Thanks for reading and commenting.
Superb and fascinating analysis. Some DOF surprises for me. Thanks for sharing it.
My pleasure, glad you found it useful.
I wish you would not use the words "full frame" when I think you are referring only to 35mm. Here you say:
"With the rise of digital imaging, there has been an uptick of available formats in which you can shoot."
This sentance is correct because there are many differen sizes. The only real crop format is the APS-x formats because they were designed to use a 35 format lenses and the sensor crops the image area. Other sizes that exists, specifally mentioned is m4/3, does not crop the lens image area. It literally covers the entire frame of the sensor.
Other full frame formats include Hasselblad - 36 × 48mm - for the same reason: the lens covers the entire sensor and none is cropped outside of the sensor. Anther is Pentax with a sensor size of 43.8mm x 32.8mm. Full frame is simply the idea that the lens covers the sensor and therefore m4/3 is also full frame. Today there are also many lenses designed for APS-x formats, making it also now a "full frame."
What people mean by 25 is 25 is 25 means is the focal length only, hence the 25 is 25.... It is correct because I have also tested it and researched it. F2.8 is also the same. If not, why would they not be sued for false advertising? When you shoot at 2.8 the ratio of light hitting the sensor is the same whether 35mm or 4/3. The depth of field is also identical IF you keep the same focal length--not equivalents which is what many people try to do.
The real and only difference comes down to field of view. If a consistent field of view is what you are after then the differences will be much greater and confusion ensues for all sensor formats and lenses and creating "equivalence".
For comparison testing I would rather see using consistent Focal Lengths and apertures. This truly shows the difference in sensors and what you will really get in end results with real use. It will show that 35mm is a great studio format and will show advantages and disavantages to other formats too, whether it be m4/3, APS-c or what is known as medium format.
Hi Bryce, thanks for your comments.
Full Frame is a standard term referring to 24 x 36mm or the stills aperture. If you want to say a lens covers full frame, it usually means it covers the still frame aperture and it is unnecessary to explain further.
The idea behind a “25 is a 25 is a 25” is that the perspective (distortion) of the lens doesn’t change no matter what format it is on. This is a theory I wholeheartedly believed in, but many people disagreed with me, so I chose to test it out and prove I was correct. If you look at the four images of the actor next to each other, these were all shot with the same 50mm lens on four different formats, adjusting the camera position to match the size of the actor in frame. You can see there is quite a bit of difference between the images as far as the background covered is concerned. If the lens acted the same no matter what the format, then adjusting the subject to camera distance for the subject to be the same size in frame would have yielded the same background behind the subject in each shot. It didn’t, so that is why I no longer accept that a “25 is a 25 is a 25” is correct once you start changing formats. In essence I proved myself incorrect.
I’m not sure I follow what you feel is incorrect or inaccurate about the testing.
As far as f/stops and depth of field: yes f/stops are a ratio, a mathematical calculation that helps you mathematically determine depth of field. T Stops are more accurate for determining how much light is passing through the lens and so are used for exposure. Of course with prime lenses there is not often a significant difference between f/stop and T stop. Within the same format, when getting your subject the same image size at the same f/stop, and focusing on the subject, the depth of field will be the same (in general) with comparable lenses of different focal lengths. However wider lenses will reveal more background (and the background will be less magnified) than with longer lenses, therefore a wider lens will have more “apparent” depth of field than a longer lens. This is of course within the same format/imager size. Once you start comparing different formats however, this no longer holds true as illustrated byt the images of the wig head.
Thanks again for reading and the comments.
>>I’m not sure I follow what you feel is incorrect or inaccurate about the testing.<<
You answer this in your paragraph with this statement:
>>50mm lens on four different formats, adjusting the camera position to match the size of the actor in frame. <<
You moved the tripod to have the same Field of View. This is not the correct way to test--having too many variables.
35mm, APS-C, M4/3, and every other sensor size will have the same DOF as long as the same aperture with the same focal length is used when shooting the same distance to the subject. The only difference in the results will be the captured field of view. This applies to my m4/3 or my Canon.
In your tests you changed the subject and/or the distance from the camera to the subject, therefore invalidating the tests.
I do appreciate the work you did go to because you certainly did demonstrate what happens when a person uses a different sensor/lens combo and they expect to have the same image. It's not happening no matter the format. Only some things will be the same.
Hi Bryce, thanks for clarifying. First I should point out that what I was testing was for lens perspective, and not for depth of field. So I don’t feel that my test is invalid. I was testing to see if a lens (I used a 50mm in the test) would give the same image on different formats. This necessitates moving the camera to have the subject be the same size in the frame. I readily acknowledged the issues that moving the camera caused. As far as the statement a 25mm is a 25mm is a 25mm, for this to be true adjusting for the subject’s size in frame the field of views would then have to match. Clearly – even with the inaccuracies introduced - there are significant differences between the same lens on different formats when matching the size of the subject in frame.
As far as depth of field, this is always a subjective topic. If you take two lenses for the same format, one with less resolving power (less sharp) than the other, you could test and conclude that the lens with lower resolving power had higher depth of field, just because it is harder to discern what is in focus and what isn’t with a lower resolving lens compared to a sharper lens.
I’m not sure about your statement “35mm, APS-C, M4/3, and every other sensor size will have the same DOF as long as the same aperture with the same focal length is used when shooting the same distance to the subject.”, merely because I haven’t spent much thought on it, nor have I tested it. However, it is a moot point with me, because what I care about is how much depth of field I have when the subject is a certain size in frame. Also something else to consider is what I refer to as “apparent depth of field”, which means that a wide-angle lens seems to have more depth than a longer lens when the subject size in the frame is the same with both lenses on the same format – because the longer lens magnifies and seems to bring the background “closer”, while the wide-angle lens shrinks the background and appears to move it further away. The smaller an item is in frame the sharper it will appear.
Hope this clarifies what I was testing for, and thanks for the discussion.
This topic has been discussed to death in various Internet forums, and I'm happy to see that someone took the time...and the discipline...to produce a nice set of images. Thank you. But it still won't convince some of the folks who "know" better. (grin)
Hi Tom, thanks for reading and the kind words. I used to be one of those folks who "knew better", but I wanted to test what I "knew". The four images with the 50mm lens are compelling, even if the methodology was slightly flawed. You can clearly see that when the subject is the same size in frame and the camera has moved the background shown is different, which blows a hole in the “a 25 is a 25 is a 25” theory. It seems so obvious now.