Full-frame vs Crop-sensor comparison : Depth-of-field & Perspective
When the differences between full-frame and crop-sensor cameras are discussed, there is an inevitable question about whether the crop sensor multiplies the focal length. Whether a 50mm lens on a crop-sensor acts like a 75mm lens (on a 1.5x crop sensor) or 80mm lens (on a 1.6x crop sensor).
The answers given on the photography forums are confusing – yes, the focal length effectively increases. No, it doesn’t. Two polar opposite answers. The discussion (which tend to devolve into arguments) are convincingly made for both sides. The reason is because the topic is a complex one … and therefore the answer is (kinda) complex too.
One argument goes along the lines that the crop sensor is just that, a crop. An enlargement. That nothing changes – you just get less of the scene. And that there is no “equivalent focal length” when you go to a crop sensor camera. But what really happens is more complex than that.
With this article, I want to help analyze what happens when you change lenses between a full-frame camera and a crop-sensor camera. And we’ll analyze whether there is actually an equivalency between certain focal lengths, when using a crop-sensor camera. In other words, whether your 50mm lens becomes “equivalent to” a 75mm or 80mm lens when used on a crop-sensor camera.
Since this article ended up being a long meandering discussion, I thought it best that we start with the final summary. Just to save the impatient people some work.
Yes, a 50mm lens does indeed behave like an equivalent focal length of a 75mm lens (on a 1.5x crop sensor), or an 80mm lens (on a 1.6x crop sensor) for the same composition … however, the depth-of-field increases by about a stop.
Yes, a 100mm lens on a crop-sensor camera will give you the same perspective as a 150mm / 160mm lens (on a full-frame camera), if you don’t change position … however, the DoF increases.
But let’s discuss this with some images:
Notes on depth-of-field / DoF
Before we can go much further, we need to recap on Depth-of-Field
- shallow depth of field is NOT the same as bokeh.
The image above certainly has nice, smooth bokeh. But it also has shallow-depth-of-field. Two things which seemingly are the same, but aren’t. So on that note, if you are one of those who say things like “give it some bokeh”, then you need to stop. It is meaningless.
- You can not “zoom with your feet”, because if you change your position, your perspective changes.
With a zoom, the perspective does not change – you are merely enlarging the image. This distinction becomes an important point. If you put a 100mm lens on a full-frame body, and a 100mm lens on a crop body, and you want the same size for your subject in the frame, then you are going to have to move much further back to get the same image size with the crop sensor camera. And then your perspective has changed. And yes, then it will give you the same perspective as a 150mm / 160mm lens on a full-frame body when shooting at that new further distance.
- A tutorial on Depth-of-Field
Our friends at Cambridge In Color have written one of the clearest explanations on DoF that I’ve ever read. Spend some time there. The concept of “circles of confusion” is an important one. It is on this that everything that you need to know about DoF, hinges upon. An important note there – DoF is defined via circles of confusion, which is specified for a certain print size, at specific distance.
- The way that DoF is defined (via circles of confusion), means that viewing distance, and the size of the final image, affects how DoF is perceived. This then implies if you are comparing a 12 megapixel image and a 36 megapixel image, then you can’t judge the DoF of the image at 100% resolution. You are better off going to “full screen” on both those images, and comparing at an equal size. You have to “equalize” this for comparison, because if you view a 36 megapixel image at 100%, the DoF will seemingly be shallower than a sensor with lesser resolution.
- DoF changes incrementally. In other words, the only point at which something is most definitely *in* focus, is the actual plane of focus. From there it is an incremental change in sharpness to the foreground and background. By the time you are using a small aperture then, the depth-of-field extends deeper into the background and foreground. Even then, at the extremities of that small-aperture DoF, there is no exact point at which we can immediately say the image flips from sharp to un-sharp.
Direction & Quality of Light
I tried to distill the essence of what we, as photographers, work with – light! Before we can truly grasp on-camera flash and off-camera flash, and really, any kind of photography, we have to be aware of the direction and quality of light. We need to observe the light that we have, and then decide how best to use it, or enhance it.
With this book, I try my best to share those “aha!” moments with you, and I do believe this book can make a difference to your photography.
The book is available on Amazon USA and Amazon UK, or can be ordered through Barnes & Nobles and other bookstores. The book is also available on the Apple iBook Store, as well as Amazon Kindle.
Figuring out a reasonable comparison method:
To take this tutorial away from the armchair to an actual shoot with comparison examples, the ideal would’ve been to have a full-frame camera that has exactly the same resolution as the crop-sensor camera. Just to keep things simple. And I’d have to have two lenses which are exact 1.5x versions of each other. For example 50mm and 75mm. Or 100mm and 150mm. Just to keep things even.
This doesn’t exist (that I know of), I tried a few ways of reasonably comparing the full-frame sensor and crop-sensor effect.
Then I had this brainwave – I could use these two Nikon lenses:
– Nikon 85mm f/1.4 lens (affiliate) on a full-frame camera,
– Nikon 58mm f/1.4 lens (affiliate) on a 1.5x crop sensor.
The 58mm focal length effectively acts like an 87mm lens on a 1.5x crop sensor camera. Close enough!
One obstacle here is that the two lenses display different optical qualities. For example, a harsh bokeh might make the background look sharper or crisper than a lens with smooth bokeh. And this might affect our perception of the DoF!
Then, if I use the 85mm lens on the Nikon D4 (affiliate) which has 16,16 megapixels,
and the 58mm lens on the Nikon D810 (affiliate) in crop mode (15,36 megapixels),
then I have two cameras which give me very similar resolution.
Two things which are affected when switching a lens between a full-frame camera body and a crop-sensor body, are:
– angle of view, and
And if you move your position, your perspective changes too. That makes a direct comparison difficult too.
I used these lenses and cameras in two different ways to visually try and explain:
- 1. D810 at full-frame (FX) and then set to 1.5x crop (DX crop), which effectively enlarges the image, but at a huge reduction in resolution.
- 2. Using a D4 (at 16 megapxiels), against a D810 (36 megapixels) at 1.5x crop, i.e., 15,36 megapixels, with “equivalent” lenses.
And then, finally, we’ll have a look at what a Depth-of-Field Calculator says, and whether this corroborates what we see.
1. Full-frame vs 1.5x crop, using a single camera and “equivalent” lenses
Both these images were shot with the Nikon D810 (affiliate), but with the two different lenses. I had the camera on a tripod, and didn’t move it at all. I just changed lenses, and the crop in the camera.
Click on the image to bring up larger versions.
On the left, the full-frame (36 megapixel) image with the Nikon 85mm f/1.4 lens.
On the right, the 1.5x crop image (15,4 megapixels) with the Nikon 58mm f/1.4 lens.
Both lenses were used at f/2.8
Notice how the FF image on the left has shallower depth-of-field in the background. The city appears more blurred than the image on the right.
Keep in mind that the 58mm lens’ bokeh might have had a slight effect here, if the lens’ bokeh itself is less smooth. However, the background does look sharper.
I did have to really reduce the 36 megapixel image on the left to bring it to the same size as the lower resolution image on the right.
Just to give you an idea, here is how the actual sizes would’ve compared, if I made this composite’s size the same as the height of the full-resolution image:
The different sizes here are largely not relevant. Remember, the circles of confusion is defined for a specific print size when viewed at a certain distance. So if we bring the two images to the same size, then we see the way the DoF actually appears in the photo.
Now, if this does not seem all that convincing since we had to play with the image size, let’s look at how the DoF appears when used with two cameras with similar resolution:
2. Full-frame camera vs 1.5x crop camera, using “equivalent” lenses
This is where I used two cameras with nearly the same resolution, and two lenses who were very nearly “equivalent” to each other for the sensor size.
Top image: the Nikon 85mm f/1.4 lens on the Nikon D4. The D4 has 16,2 megapixels.
Second image: the Nikon 58mm f/1.4 lens which gives an equivalent of 87mm on the Nikon D810 when it is set to 1.5x crop in the camera. The D810 then has 15,4 megapixels.
I wanted to use the cameras on a tripod, but the cameras are at different height, so this made the tripod very clumsy when I tried to match the exact framing and composition. In the end, I hand-held the cameras, and tried for compositions as close as I could. I tried to keep our model, Olive, the same size in the frame. So there is some difference to the images in that sense – the perspective is slightly different – yet, the DoF is noticeably shallower in the full-frame image.
Now, after all this, you may still not be entirely convinced that there is an “equivalency” in the focal lengths if you simply multiply the focal lengths by the crop factor. There were uncertainties perhaps in both the ways I set these comparisons. You might still wonder what happened to the argument that a crop sensor just gives you an “enlargement” of the full-frame image.
So let’s go back to the “armchair method”, and look at what a Depth-of-Field calculator says.
3. Depth-of-field calculator
For these, I used the Simple DoF Calculator app on my iPhone. I like it. But there are others available too. In any case, you should have a DoF calculator on your phone. It just makes sense.
This is an exercise that you can play with on your own, using DoF tables, or a DoF calculator.
It works this way every time – the full-frame camera gives us shallower DoF, for the same field of view, than the crop-sensor camera does.
We bring it right back to where we started:
- Yes, a 50mm lens does indeed behaves like an equivalent focal length of a 75mm lens (on a 1.5x crop sensor), or an 80mm lens (on a 1.6x crop sensor) … but, the depth-of-field increases by about a stop.
- Yes, a 100mm lens will give you the same perspective as a 150mm / 160mm lens, if you don’t change position … but, the DoF increases.
So, yes, a 50mm does (kinda) act like a 75mm / 80mm when used on a crop-sensor camera. The focal length is effectively increased on a crop-sensor camera. But at the loss of the shallow DoF that a larger sensor gives you.
How important that change in depth-of-field is to you, is something you need to decide for yourself. For me, I like that extra control that shallow DoF gives me in creating a specific look to photographs. While the change in DoF might seem incremental when compared in a final image, it is an element that I do want to be available to me – the shallower depth-of-field which full-frame cameras allow.
A little bit of homework #1 – The armchair version
Let’s change it around now. What would the effect be on:
– the field of view
– the composition of your image
– the depth-of-field
if you used the same lens on a FF camera and a crop-sensor camera, but you moved your position to keep your subject the same size in your viewfinder?
A little bit of homework # 2 – Practical exercise
Better yet, do it as a practical exercise:
Shoot the same portrait of someone (who doesn’t move),
using a FF sensor camera, and then a crop-sensor camera,
while keeping to the same framing of your subject.
and also keeping the same perspective. (i.e., You can’t move.)
Now compare the DoF between the two images, for the same resolution.
Let us know what you find.
Addendum & clarifiction – as a reply to many of the comments
I do agree about the focal length remaining the same. As mentioned several times here, a 100mm lens remains a 100mm lens. It’s physics.
Anyone who follows this website will know I’m also a big supporter of using correct terminology. It helps with clarity if the same things are named the same things, and not obscured by everyone making up their own phrases.
The counter-point to this – and the main reason I wrote this article – is that it is easier to understand what happens with the FF / crop switcheroo, if you think in terms of the effective focal length changing. It really is, even if this makes you grind your teeth.
So while we could remain sticklers for the mathematics of it all – e.g., a 100mm lens remaining a 100mm lens – it is easier to understand what happens if we think in terms of the field of view changing = effective focal length being different.
If we remain in one spot, then using the same focal length lens (say a 50mm lens), on the FF camera and on a crop-sensor camera, makes little sense to a portrait photographer. You just get a crop version. This of course makes sport photographers and wildlife photographers very happy.
Now if we think of the difference in using a FF camera and a crop-sensor camera, and that this effectively brings in that 50mm / 75mm “change”, and we frame our subjects the same way on the FF camera, and with a crop-sensor camera … then there is this effect as described in the article. Call it a change in field of view … call it a change in effective focal length … there is something happening here that is more than just a crop of the image.
I honestly think that thinking of this as the focal length effectively changing on you, is more meaningful in a practical shooting sense, than thinking purely in terms of the mathematics of the optics.
I hope this makes my intent with this article more clear.
Filed Under: bokeh, digital imaging, Olive, techniqueTagged With: full-frame vs crop-sensor comparison
Since I got my Nikon D7000 camera 6 years ago I’ve used it almost everyday. That is a lot of shutter clicks, 148,558 to be exact. It looks like I will be in the market for a new camera soon as the D7000 is only factory tested to 150,000 clicks. My dilemma is should I go full frame, or stick with my cropped frame?
I keep asking myself, is a full frame camera really worth it? I took a Nikon full frame D610 and a Nikon cropped frame D7100 on a test drive around Paris to see the real world differences.
What exactly is a “full frame” camera?
Film was the unchallenged king for a century and the most popular format was 35mm film. It was sometimes referred to as “small format” to differentiate it from “medium format” or “large format” cameras, but for most of us it was the standard film we used. Although called 35mm, it actually measured 36x24mm.
When they started making digital cameras they used a smaller sensor than their film counterparts, roughly 24x16mm, but the bodies still took advantage of the same 35mm lens. The smaller sensor size meant there was a part of the image that never made it to the smaller sensor. About a decade ago, developers in Japan decided to increase the size of their sensors to the equivalent of 35mm film and as a very cleaver marketing ploy, called them “full frame” renaming their current cameras as merely “cropped frame.”
Makes you feel like you’re missing out right? Lets take a look at some of the key differences.
The first thing to understand is what is cropped on a cropped frame camera. A 35mm lens on a full frame camera will look wider and capture more image area than the same lens on a cropped frame camera. Basically, the optics of the two cameras work the same way, an image passes through all of the elements of glass in a lens, but when it hits the sensor, by having a smaller sensor in a cropped frame camera, only part of the image will make it to your sensor. The result will look as if you took a full frame image and then cropped in.
Using the same lens whilst shooting the Column at Bastille, I was able to make the angel larger in my shot in the cropped frame image. This extra zoom is great for wildlife or sports photographers.
To test out the high ISO performance I thought I would slip into the dark underbelly of Paris and visit the Catacombs. Flash and tripods are not permitted in the museum, and with only some low intensity lighting to illuminate the skulls of the over 6 million dead that lay rest there, it was the perfect testing ground.
I shot the two images with the same settings and changed the focal length to mimic the same field of view. From the offset, the D610 looks to have better ISO performance. The D7100 has a pink cast, especially in the shadows as a result of the noise of the image.
The enlargement below is from one of the shadow areas. The full frame is on the left and the cropped frame on the right. I brought up the shadow details and blacks in the processing (the same amount for both images) because grain really starts to look ugly when we do this and I wanted to push them to their limits to check the results.
Whilst they are close at normal exposure, by bringing them up like this you can see clearly that the full frame outperforms the cropped frame here.
Dynamic Rage is the amount of luminance that can be captured in an image. If a scene has bright whites and dark blacks, you want your camera to have a large enough dynamic range to capture both parts of your image without blowing out to pure white, or clipping to complete blacks.
I went down to Port de l’Arsenal for this test and I must admit they look incredibly similar. I feel the full frame image here is a little more contrasty. Looking at the windows and the side of the boat, and then looking at the shadows created by the trees I honestly get confused as to which camera is which.
Then I went into the heart of the Latin Quarter to the Pantheon. At first glance, I thought the same as the port. The full frame appeared to be more contrasty and the cropped image was flatter… unless I looked at the shadows of the column on the doors.
Whilst still very dark, you can still make out the detail of the patterns in the full frame image, whereas they are almost entirely black in the cropped version.
Full frame equivalent
We often hear that a 50mm lens on a full frame camera is the equivalent of a 75mm on a cropped lens. What we are talking about here is field of view.
Field of view of the 50mm on the cropped frame gives me 4 floors of this building in the Latin quarter. If I were to use a 50mm lens on the full frame, I would see the entire 6 floors of the building. So to get the same 4 floors, I need to use a 75mm on my full frame to get the equivalent field of view.
Whilst it has the equivalent field of view, a 50mm lens is still a 50mm lens on either camera. In this GIF below you can see the difference between lenses whilst shooting the same field of view. The full frame was shot with a 21mm lens, and the cropped frame with a 14mm lens. Both give the same field of view, but because of the nature of the lens, the 14mm on the full frame distorts the perspective more.
Does a full frame camera have better Depth of Field?
Yes and no. Technically, the depth of field between a full frame 50mm lens and a cropped frame 40mm lens should be identical, their field of view will differ as explained above. So, to get our foreground subject (which, in our example, is our coffee cup) to be the same size within the image we will need to either zoom in, or move closer to our subject.
If we zoom, we will be compressing the background (and the blur with it) making it look more blurry. If we move physically closer to our subject, we will shorten our focal distance and consequently decrease our depth of field.
Availability of Lenses
Canon and Nikon both make lenses specific for cropped frame cameras. Nikon call theirs FX for full frame and DX for cropped. Canon EF and EF-S. Whilst full frame lens will work with both of the different formats, the cropped body lens won’t always work. They are specifically designed for the smaller sensor and therefore have a smaller area to cover. They consequently use less glass and are usually cheaper to manufacture and will weigh considerably less too.
You can mount a cropped frame lens on a full frame camera, but is it worth it? When you shoot your image, the centre of the image will look perfect, but at the edges you will find black fall off from the smaller lens at the edge of the barrel.
Nikon has a DX mode feature in their FX cameras that essentially just crops your image back to a DX sensor size. On Canon you will just need to crop the image yourself.
When you choose to operate this, way your full frame camera will basically become an expensive cropped frame camera and behave as such with everything else mentioned here. The drawback will be the reduction in megapixels.
The D610 has 24mp but only in FX mode. In the DX mode it will drop down to about 10mp. At this point you are far better off using the DX lens on the D7100 and taking full advantage of the 24mp.
The quality of full frame lenses will be better. Both Nikon and Canon don’t make “professional grade” lens in their cropped frame series. But they will be more expensive and heavier for their quality.
When choosing a lens you should really consider this. Even if you only have a cropped frame camera now, if you think you might upgrade one day or use it on a 35mm film camera, you might want lenses that will continue to be useful with your new camera rather than having to buy a new arsenal of lenses as well.
Is a ‘cropped’ image smaller?
Whilst the field of view is “cropped” there is no cropping of your actual image. Depending on your camera the physical image size might not be smaller at all. The image size is determined by the megapixels in a camera, not the physical sensor size.
So for our test, the 24mp D610 has 24mp, practically the same file dimensions as the 24mp D7100 camera, which, as you guessed, also has 24mp. If you wanted to print images from your camera, they would both be the same maximum size because the dimensions of your image in pixels between this full frame and cropped frame camera are the same.
So which format should you choose?
I really enjoyed running these tests and analyzing the results from the images around Paris. There were a few surprises in there for me. I expected the full frame camera to outperform the cropped frame hands down, but in reality the differences weren’t as large as I thought. It does appear that the full frame has won the competition, but at what cost? The selection of lens that work with the cropped frame camera is far greater, but the quality of them is far lower. Full frame bodies are by in large more expensive but even this is changing with new cropped frame cameras becoming more professional—it is really hard to make the justification I need to make the switch.
The cropped frame can zoom 1.5 times further, and when I was out shooting the fireworks at Bastille Day, the difference between a 200mm on a cropped frame camera and the same lens on a full frame camera really makes all the difference to my shot. But if you prefer wider for street photography, then maybe you prefer the 1.5 times wider side of your lens.
Whilst these two sensors we tested were pretty equally matched, I expect the new nikon D500 cropped frame camera will have a much better sensor than both of them and the results from that will outperform the full frame camera we used here in ISO and dynamic range. Sensors will continue to become better and ISO and dynamic range are both elements that have seen massive improvements. The first full frame cameras would have terrible ISO and dynamic range compared to the modern cropped frame cameras.
So, ultimately, this only leaves Depth of Field as an inherent change in the performance of the two formats. The rest comes down to what feels good in your hand. What other bells and whistles do you want with your camera? The cropped frame D500 offers 4K video whereas the full frame D610, which costs the same price, doesn’t. The D7100 performed remarkably against the D610 and it is half the price.
For now, I think I might keep the cropped format, and spend the extra money in some new higher quality lenses to really get the best from both worlds.
About the author: Alexander J.E. Bradley is the founder of Aperture Tours (formally Paris Photography Tours) and heads up the tours in Paris. A professional photographer for over a decade Alexander enjoys shooting the surreal by mixing dreamlike qualities into his conceptual images.
You can find more photos and articles like this on the Aperture Tours website, or by following Aperture Tours on Facebook, Twitter, and Instagram. This post was originally published here.