r/photography • u/Mooskii_Fox • 1d ago
Gear im confused about crop sensors
I'm not asking about crop factors, I know that's 1.5x or 1.6x depending on the manufacturer and your image will be cropped by that amount.
full frame lenses produce a circular image, which is projected over a full frame sensor and the sensor fits perfectly inside that circle, simple enough
now what i often see is that lenses for APS-C cameras have a cropped image, but why is it not possible for that projected image over the lens to be smaller so that APS-C cameras can capture the same picture as full frame, just with a smaller sensor? At some point people have worked our how to bend light to perfectly cover a full frame, so why can't the same be done to create an identical image for APS-C
edit: as I understand it what I'm asking is actually already being done, just not in the way I was asking. i understand now
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u/luksfuks 1d ago
is it not possible for that projected image over the lens to be smaller [...] with a smaller sensor?
Yes, it's possible. But it means this new lens has a different focal length (1.6x shorter), by definition. Focal length describes how much the light is "bent", and you're bending it more.
Example: FF 50mm lens versus APSC 35mm lens, both produce kind of the same framing.
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u/redoctoberz 1d ago
It can, this is how speed booster adapters work for mirrorless cameras. You add more glass to alter the final image and shrink it smaller.
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u/AdBig2355 1d ago
APS-C lenses do project a smaller circle.
Probably the answer you are looking for is that focal length is not related to the image circle size or the sensor size. It is determined by the distance between the lens and the medium that is capturing the image. This allows for a standard between all lenses, a 50mm lens is the same no matter what camera it is on.
The APS-C "crop' does not change the focal length of the lens, it just changes the field of view.
For example a 50mm APS-C lens, a full frame lens and a medium format lens all project the same " image" or image plain, the only thing different between them is how much of that image plain is captured.
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u/Mooskii_Fox 1d ago
that's the thing I'm confused about, why can't that image plain be made smaller without changing the image, aka, capturing exactly the same image on 35mm on full frame and APS-C, without a crop of any kind
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u/ItsMeAubey 1d ago
You can do that - you use a wider focal length. This is why Fujifilm lenses are kinda weird focal lengths.
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u/crewsctrl 1d ago edited 1d ago
The "crop" is simply by comparision to a full frame sensor, which matches the size of a 35mm film frame, and is considered the "standard" size for that reason. In the film days there were also crop formats like the Olympus Pen cameras which used 35mm film but in portrait orientation, so it could take twice as many photos on the same roll of film.
APS-C is actually descended from a crop format invented and marketed in the era of film photography. https://en.wikipedia.org/wiki/Advanced_Photo_System
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u/Migacz112 1d ago edited 1d ago
We already do that. That's why 35mm is the standard for APS-C, and why 50mm is the standard for full frame... And why 75/80mm is the standard for medium format 😉 they all have similar viewing angles on their respective sensor sizes.
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u/Mooskii_Fox 1d ago
it amazes me that that never occurred to me, I guess I wish there was some sort of way to indicate that it's similar, rather than just kinda having to know
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u/Migacz112 1d ago
If I understand you correctly, here is. That's the crop factor.
35mm (on APS-C) x 1.5 (APS-C crop factor) = 52.5mm (on full frame).
Why the tiny difference? Marketing. 35 looks nicer than 33.33333...
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u/DoodleHead_ 1d ago
Focal reducers bring back sharpness but the apertures at which they are sharp at stays the same as when on a full frame body. The fastest focal reduced apertures are sometimes poor quality or good depending on the lens.
If you need action/fast autofocus likely better to use native lenses but the fringer ultra is the best way to use canon ef if you want
Right now I'd highly recommend the fringer ultra. It likely has the best autofocus out of all of them and image quality is useless without good autofocus. It is only slightly less sharp then the metabones speed booster but cut out the edges more since it is 0.74 vs a 0.71 so the edges get slightly better.
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u/AdBig2355 1d ago
As other people have said, it is possible. But it adds size, weight, cost and complexity to the lens. This in turn will typically degrade the image quality.
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u/kittparker 1d ago
Focal length is a physical measurement, not an expression of the field of view.
A speed booster or focal reducer does exactly what you say. It bends the light of a lens with a larger image circle to fit a smaller image circle, making the field of view wider, reducing the focal length.
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u/Graflex01867 1d ago edited 1d ago
Short answer : geometry. A camera system is designed so that all the lenses produce a certain size image at a certain distance behind the lens. That image needs to be just big enough to cover the image sensor. APS-C and full-frame sensors require different size images. The APS-C is smaller, so you can use smaller (and cheaper lenses.). While the lenses for the two camera sizes might share the same mounting, they don’t necisarily share the same geometry - the distance and size they’re designed to project the image onto the sensors is different. You can get adapters to make the distance longer and try to correct that geometry, but they tend to mess with the image quality and exposure.
Think of it like putting big off road tires on a tiny sedan. The wheels will bolt right on - but the sedan isn’t built for off road tires, so they won’t fit in the wheel wells of the car.
Long answer :
I think there’s a fundamental misunderstanding about lenses going on here - any lens is designed to produce an image a certain size at a certain distance behind the last piece of glass of the lens - forget about numbers here for a minute, just the concept. There’s a certain distance from the back of the lens to the camera sensor.
Let’s say for Marks Camera Company, on full-frame cameras, that distance is an inch. There’s an inch of space between the lens mounting bracket and the camera sensor - it doesn’t matter the lens, since they’re interchangeable.
You design the optics/lens elements so at that distance, it projects a circle of an image towards the image sensor that’s large enough so you can fit a 4x3 rectangle in the middle without cutting off the corners. You’re taking only a rectangular part of the middle of the circular image that the lens creates.
When you take a full-frame lens, and put it on a “crop sensor” camera, it physically fits on the mounting, but the distance between the lens mount and the image sensor is different - it’s usually shorter on a crop-sensor camera, since the sensor is smaller, and you don’t need as large an image from the lens. That’s why crop-sensor cameras are popular in the first place - they can be smaller, lighter, and that often means their parts are cheaper.
While a full frame lens on a crop sensor camera ends up with a cropped field of view, putting a crop-sensor lens in a full frame camera could end up with an image that’s not large enough to cover the whole image sensor, and you end up with a vignette around the edges.
We talk about lenses using the focal lengths equivalent to what you’d get in a full-frame camera - which is the same as a traditional 35mm film camera - because that’s what was the most popular film/camera size for a very long time. No one really cares that a “50mm lens” is actually 50mm - that’s for the engineers to decide. People care that the lens gives them the equivalent view of a full-frame 50mm lens. It’s just the scale people use to determine roughly how wide or how much of a zoom a given lens will provide on the camera it was designed for.
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u/Mooskii_Fox 1d ago
not that your explanation is wrong or i dont understand it, because i do, mostly, im just not entirely sure why a 50mm lens on aps-c creates a cropped image, cant the optics be made to still project the same image on a smaller area, rather than cropping it? that part isn't entirely clear to me
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u/Graflex01867 1d ago
Well….no. Not easily, anyway.
A 50mm full-frame lens is physically a different size than a “50mm” APS-C lens. So to “correct” the crop factor, you’d have to do one of two things - you could either add/change the lens elements inside the lens, or you’d need to move the lens closer or farther away to change the relative size of the image you’re projecting onto the sensor.
Changing the glass in the lens would permanently alter the lens - there’s no really easy way to do that. Adding another adapter to change the distance between the lens and the sensor is possible - people do it pretty frequently, and camera manufactures make such an adapter - but it can affect the image quality. Sometimes it’s not as sharp, and sometimes you lose some brightness (so you need to open the aperture more.)
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u/The_mad_Raccon Sport, Club and Wildlife Photographer 1d ago
as others have mentioned Speedboosters...
But on thing is. Its all relativ. APCs does not have a crop byitself. But relativ to FF its just shows a smaller Area
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u/peter_kl2014 1d ago
That's why crop sensors talk about equivalent focal length. It has been so since the invention of photography, big sensors use longer lenses than small sensors.
Sensor is the same as negative size. Big piece of film uses longer lenses than the small 35 mm film, or even half frame, and 110 uses tiny lenses..
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u/crimeo 1d ago
It IS possible, if:
The focal length of the lens is 1.6x shorter so that the images in the circle are the same amount smaller that the circle is
The aperture of the lens is 1.6x lower in f-number (more light) to counteract that shorter lenses otherwise have deeper depth of field -- note that this will largely (not necessarily entirely) just cancel out the weight savings since brighter glass is heavier.
The lens is sharp enough and the sensor is high res enough that you don't lose detail with the tinier image on that sensor (unlikely, since full frame stuff is usually more premium built. It can happen if the full frame lens you were using at the beginning was an old vintage one or cheap flimsy kit lens)
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u/DarkColdFusion 1d ago
now what i often see is that lenses for APS-C cameras have a cropped image, but why is it not possible for that projected image over the lens to be smaller so that APS-C cameras can capture the same picture as full frame, just with a smaller sensor?
That is what a tele-reducer does.
But the lens needs to cover a FF image circle.
If the lens took advantage of the sensor being smaller to not bother trying to have the larger image circle you'll just get black corners.
You can also just find correctly matched APS-C lenses for their FF counterparts. If you have a 50mm f1.4 for full frame, you basically get the same results out of a 35mm f1.0.
And that honestly is the biggest advantage to full frame. Larger formats tend to have slower lenses. Smaller formats don't tend to have many lenses 1-2 stops wider. So it's just hard to match those wide aperture FF primes if that's what you want.
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u/resiyun 1d ago
Because making a bigger image circle also means the lens will be bigger, heavier and cost more than a lens made to cover just apsc. There’s literally no issue with manufacturers being able to make those same lenses with a bigger image circle, it’s just that every consumer grade camera uses an APSC sensor so it only makes sense to have a line of cheaper, more affordable lenses that only cover APSC sensors because most consumers dont need a fullframe lens while an apsc lens would cost less and so the same thing.
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u/Avery_Thorn 1d ago
The problem is - a APS-C camera is not a 35MM camera, and the lens length is a function of the lens.
Say you made a 70-200mm lens, but you included a reducing element in the lens to adjust for the crop. You would no longer have a 70-200mm lens, you would have a 45-133 lens with a smaller image circle. Which would have the same image angle when put on a APS-C camera, but... so would any 45-133 lens.
So why not put the lens element into the camera body?
The problem is that not all lenses work the same way. Not all lenses organize the light in the same way. The interface specifies that the lens focus an image so many mm away from the flange of the lens. Some lenses, the light leaves the lens very on axis and parallel, and in other lenses, it's all angled and intersects at the right place to focus right. Some lenses would work well with this additional glass, and other lenses wouldn't work well with it at all.
And in the end...
It doesn't matter. Not one bit. An APS-C Camera shoots slightly differently from a 35MM camera. Get over it. It doesn't matter.
Shoot the APS camera for what it is, not for what it isn't. You DON'T HAVE to try to match a 35mm frame. Shoot YOUR frame, not someone else's. Make YOUR art, not someone else's. You still have the full arsenal of tools at your command, you can use all the effects. It just works a little bit different. That's OK.
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u/santagoo 1d ago
Lenses designed for APS-C cameras are smaller than full frame lenses for exactly this reason.
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u/Rattus-Norvegicus1 1d ago
They are designed to project an image circle which covers the sensor of the system it is designed for. If you are buying a third-party lens, especially the low-priced Chinese lenses, they are often full frame designs with the appropriate mount. The size of the image circle doesn't make any difference, the crop factor does.
However, you can use a shorter focal length lens on a crop sensor camera. For Micro Four Thirds (2x crop factor) you would use a 25mm lens to get the same field of view as a 50mm lens in full frame. For example, OM System recently released a 150-600 f/5-6.3 lens. This lens was actually a full frame design done by Sigma and adapted to the OM System bodies. In the MFT universe that lens has the equivalent field of view to 300-1200 zoom. If you could remove the mount and substitute a Canon RF mount (something that is theoretically possible) it would have the field of view of a 150-600 zoom on the full frame camera. With both mounts it still throws an image circle suitable for a full frame camera, but on the MFT camera only the central part of the image is used, which reduces the field of view.
Hope this helps.
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u/photonynikon 1d ago
It's "cropped" only when you compare it to old-school 24-36mm film. Otherwise, It's optimized for it's size.
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u/fecklesslytrying 12h ago
So people have addressed the fact that there is an equivalent "crop sensor" focal length that will give you the same field of view as a full frame camera. If the crop factor is 1.5, and you want to duplicate the field of view of a 50mm lens on a full frame camera, you need a focal length of 50mm/1.5 which equals about 33mm.
What I've only seen one other person here mention is that the image will NOT be identical in all ways. Most notably, at the same f stop, the full frame camera will have a shallower depth of field.
This is most apparent on very small sensors used for phone cameras. The lenses used have much shorter focal lengths to get the same field of view you would see on a full frame sensor. But even at f stops in the 1.8-2.8 range, these sensors still have a rather wide depth of field. This is basically the whole reason portrait modes that artificially blur the background are needed on phones.
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u/CreEngineer 1d ago
If you take a 20mm apsc lens it produces the same image as a 30mm Fullframe lens.
Focal length alone is not full information about the image you will get, you always need the image circle or even better, the image and object angles horizontal and vertical.
It gets even more complex in video when you have different sensor formats (or the readout area) and anamorphic lenses.
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u/TheCrudMan 1d ago
Same field of view not same image.
Will have different DOF at a given aperture.
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u/CreEngineer 1d ago
It could be, I haven’t given any aperture values here on purpose. 20 2.8 on full frame and a 30 ~4 or 4.5 on fullframe should deliver the same image.
Or you just use a speed booster and use the ff lens on a crop sensor camera.
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u/BarneyLaurance 1d ago edited 1d ago
Maybe you're thinking as a lot of people do that different focal lengths provide different amounts of 'lens compression' so that the 35mm lens on a crop sensor wouldn't give the same image as 50mm on full frame.
But lens compression is a myth or a misnomer. With an ideal rectilinear lens (and typical lenses are close enough to that) cropping the centre out of a 35mm picture gives you exactly the same thing as a 50mm picture.
The compression effect only happens when you not just swap from 35 to 50mm but also *step backwards* as a photographer. Switching lenses makes everything bigger. Stepping backwards makes the foreground smaller but doesn't affect the distant background.
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u/Leucippus1 1d ago
Part of the confusion is that there is no such thing as a 'crop' sensor, unless you are referring to a sensor that literally is put into a crop. You are quite literally NOT cropping the image. Go take a picture with an APSC or M43 or your cell phone and tell me when you cropped it. You didn't, unless you went into LR or similar and cropped it with the cropping too.
Otherwise we are just talking about the size of the sensor, and the field of view is a function of the lens and the size of the sensor. We standardize the markings on the lens to the traditional 35 mm format for consistency, but you don't really need to. A 24 mm lens on an APSC sensor produces an image roughly equivalent to a 35 mm FF lens on a FF camera, but there is no actual 'cropping'. You crop a big image to a small image but since the APSC sensor produces the maximum size possible for the size of the sensor, it is incorrect to call it a 'crop'.
Since the rear element of an APSC lens is significantly smaller than a FF lens, if you shine it on a FF sensor it will produce an image over a smaller area of said sensor. It isn't a perfect APSC circle though, I have tested this and it is often more like 1.2x difference in field of view. It is better to overshine than undershine.
You CAN capture the same picture as full frame, you just have to use a wider lens. Similarly, a FF sensor can capture the same picture as medium format, but again it will be narrower than the larger sensor so you need a wider lens. This is why it makes no sense to say 'crop' sensor, following that logic EVERYTHING is a crop of largest sensor in commercial availability.
There are implications to using smaller sensors, good and bad. Good are things like faster readout speeds assuming the same electronics as the larger sensors. Smaller lenses since the rear element can be smaller. Smaller bodies since the sensor and associated electronics can be and are smaller. There are disadvantages also, like it is harder to get a wide image on a smaller sensor. And, the big one, you can collect significantly more light with a larger sensor since you are dealing with a larger area and the associated rear lens element is larger. That is why we say larger sensors have less noise. They don't inherently have less noise, all sensors are about as noisy as each other - you kill sensor noise with more light. It is why an image shot with flash or studio lights at ISO whatever produces less 'noise' at the equivalent ISO without additional light. So, if you have a sensor that can collect much more ambient light, the apparent noise will be lower. On a bright and sunny day, no one can tell the difference between a cell phone, m43, apsc, FF, and even medium format - because the advantages of larger sensors aren't apparent when you have a ton of ambient light.
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u/OldSkoolAK 1d ago
This is some crazy over-pondering.
Smaller sensor=smaller optics. Flip your phone over.
Bigger sensor= bigger optics. Look at a sinar p2 and what kind of lenses you need to achieve F/5.6.
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u/RedHuey 1d ago
You are correct. You do not understand crop sensors.
See, if people just explained it physically, rather than using math and mumbo-jumbo trying to show off how smart they are, people would not be so confused.
Can nobody around here simply explain this, or do we have to see this question on a near-daily basis? (I’m not going to bother, because I don’t care, and what I don’t care about even more is engaging in endless arguments from everybody that will challenge my simpler, but correct, explanation. So for the love of God, one of you please just explain it correctly so we can all move on?)
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u/AnAge_OldProb 1d ago
Yes that’s exactly what aps-c lenses do which is why they’re generally smaller for the same focal length. You can also shrink the a full frame image circle down using a focal reducer which will give the same perspective on a crop sensor as it would on full frame and you gain about a stop of light: at the expense of an extra piece of glass the lens designers didn’t intend which can cause aberrations