Please explain like I am 5 -Crop factor DX

mona525

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Apc-s crop factor is really confusing me and the more I read I just get more confused :/

If I purchase a 17-50mm lens for my DX camera will it actually be what a 25-75mm will look like on a FX? or will it be 17-50 and it's designed for DX?

I've posted before about wanting a 24-70 but when I think about it with my DX I think it will not be "wide" enough to take landscape photos. Is this correct?

I am not a professional photographer nor will I ever be. This is a hobby of mine and I just would like to take nice photos of friends and places I go. I just want to make sure the lens I purchase is a good fit.
 
Lay a business card on top of an index card. The index card is a "Full Frame" sensor. The business card is a "Crop Frame" sensor. The amount of image covered by the crop sensor is significantly less than the amount of image covered by the full-frame sensor simply because it is physically smaller.

The focal length of a lens WILL NOT CHANGE between sensors. The ONLY thing that change is the field of view, or how much of the image is covered by the sensor.
 
Yes, if you purchase 17-50mm DX lens it will look like 24-75 on FX. However, if you don't know what 24 looks like on FX, it doesn't matter.

The only time the company publishes in "equivalent" lens length, I believe, is on fixed lens cameras or cell phone cameras.

Some people use 24 equivalent for landscapes, and some feel it isn't wide enough. But I think it's definitely in the landscape territory.

Do you have a kit lens? How does it look to you at 17mm (or however close it gets?) If that focal length is useful to you, that's all that matters.
 
Apc-s crop factor is really confusing me and the more I read I just get more confused :/

If I purchase a 17-50mm lens for my DX camera will it actually be what a 25-75mm will look like on a FX? or will it be 17-50 and it's designed for DX?

I've posted before about wanting a 24-70 but when I think about it with my DX I think it will not be "wide" enough to take landscape photos. Is this correct?

I am not a professional photographer nor will I ever be. This is a hobby of mine and I just would like to take nice photos of friends and places I go. I just want to make sure the lens I purchase is a good fit.
My "kit lens" is the Nikkor 18-55mm zoom to go with my "DX" Nikon D5000, and I think the 18mm wide end of that is pretty good for most landscape shots.

If you want REALLY WIDE, then look at wider lenses, such as the 16mm's or the 14mm's, but you're going to have to spend quite a lot. The 16-85 f/3.5 is fairly "affordable at a list price of $700, and of course they go up from there.

Then there are some wider lenses, at high list price, but frankly, I wouldn't spend that kind of money for landscape photography. Especially on a DX-format sensor.
 
I made a thing to help visualize.

Crop.jpg
 
This may make more sense

The lens focal length is what it is. 50mm is a 50mm.

You have to first understand what size of sensor you are buying in the camera. This varies dependent upon the camera. Since lenses are interchangeable they would have to have a chart on them for the perceived Field of View (the focal length for the sensor size).

The size of the sensor determines the "crop"
35mm FullFrame sensor = 1 to 1
APS-C sensor = 1.5x (or 1.6x) to 1
other smaller sizes increase that amount ==>https://lensvid.com/wp-content/uploads/2014/01/Sensors-size-01-01.jpg
as below - crop factor is the 4th row.
Sensors-size-01-01.jpg


You then take that crop factor, such as 1.5x for APS-C and multiple it to the lens focal length.
So a 18-55 on a APS-C sensor would
18 x 1.5 to 55 x 1.5 = 27 to 82.5 in perceived field of view

with a 24-70
per the chart for an APS-C of say 1.5x
24x 1.5 = 36
70 x 1.5 = 105
Thus on a APS-C sensor it would have a smaller sensor size. If you see the image above of what PORTION of the IMAGE circle from the lens it is capturing, or in essense creating a "fake" zoom, it's 1.5x of the focal length of the lens, which is 24-70. So the Field of View (or perceived focal length) is 36 - 105 of a 24-70 lens.

If you are using an 18mm all the time for wide angle shots.
Then you'll probably want to stick to a lens that can do 18 or wider for your camera body.
 
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I like mine better.;)
 
This may make more sense

The lens focal length is what it is. 50mm is a 50mm.

You have to first understand what size of sensor you are buying in the camera. This varies dependent upon the camera. Since lenses are interchangeable they would have to have a chart on them for the perceived Field of View (the focal length for the sensor size).

The size of the sensor determines the "crop"
35mm FullFrame sensor = 1 to 1
APS-C sensor = 1.5x (or 1.6x) to 1
other smaller sizes increase that amount ==>https://lensvid.com/wp-content/uploads/2014/01/Sensors-size-01-01.jpg
as below - crop factor is the 4th row.
Sensors-size-01-01.jpg


You then take that crop factor, such as 1.5x for APS-C and multiple it to the lens focal length.
So a 18-55 on a APS-C sensor would
18 x 1.5 to 55 x 1.5 = 27 to 82.5 in perceived field of view

with a 24-70
per the chart for an APS-C of say 1.5x
24x 1.5 = 36
70 x 1.5 = 105
Thus on a APS-C sensor it would have a smaller sensor size. If you see the image above of what PORTION of the IMAGE circle from the lens it is capturing, or in essense creating a "fake" zoom, it's 1.5x of the focal length of the lens, which is 24-70. So the Field of View (or perceived focal length) is 36 - 105 of a 24-70 lens.

If you are using an 18mm all the time for wide angle shots.
Then you'll probably want to stick to a lens that can do 18 or wider for your camera body.

I am not looking to just do wide photos but basically a general lens that I can leave on my camera without having to lug a few lens around. I like the "bokeh" effect and my kit lenses do not have that. I do have a 50mm prime lens which I love but sometimes I do not have the ability to get closer/further when taking photos.
So when looking at this lens online is it really 17-50 or should I factor the crop and think of it closer to 25-75mm? or is it Sigma 17-50mm f/2.8 EX DC OS HSM Zoom Lens for Canon 583101 B&H
 
The lens is really 17-50mm.
It will be the same at 50mm as your current 50mm prime - they are both 50mm and that doesn't change. If you've a current zoom lens that covers a similar range of focal lengths then the view will still be the same. 17mm on a kit lens is the same as 17mm on any other. *


The only reason the whole "crop focal length change" convention exists is because during the film era the 35mm film size was KING for most amateurs and many semi to full pros. Thus as a result when crop sensor digital cameras came out everyone was "converting" to get an idea of what the focal lengths were comparable to on their 35mm cameras.
I've you've never used another camera then it really doesn't matter at all.
 
I am not looking to just do wide photos but basically a general lens that I can leave on my camera without having to lug a few lens around. I like the "bokeh" effect and my kit lenses do not have that. I do have a 50mm prime lens which I love but sometimes I do not have the ability to get closer/further when taking photos.
So when looking at this lens online is it really 17-50 or should I factor the crop and think of it closer to 25-75mm?
You really don't have to consider the "crop factor" unless you are comparing that lens and what the field of view is on TWO DIFFERENT CAMERAS; a "DX" and an "FX". Since you are dealing with just your one camera, you can ignore all that crop factor stuff because what you see is what you get.

As to throwing the background out of focus, and achieving a smooth bokeh while doing so, you need to look at the entire lens construction. So to get that, you basically need a longish lens with a wide(ish) maximum aperture, and excellent rendering of the out-of-focus areas. All that means you will be looking at a fairly expensive lens. Yes, you can own one, but in the zooms that produce excellent bokeh you will need to spend some serious cash.

If you stick to a prime lens, you can get that creamy-smooth out-of-focus for less money. I'm not up on Canon lenses, but ask that specific question and some of the Canon people will steer you toward a good choice.
 
mona525 said:
Apc-s crop factor is really confusing me and the more I read I just get more confused :/

If I purchase a 17-50mm lens for my DX camera will it actually be what a 25-75mm will look like on a FX? or will it be 17-50 and it's designed for DX?

I've posted before about wanting a 24-70 but when I think about it with my DX I think it will not be "wide" enough to take landscape photos. Is this correct?

I am not a professional photographer nor will I ever be. This is a hobby of mine and I just would like to take nice photos of friends and places I go. I just want to make sure the lens I purchase is a good fit.

The 17-50mm f/2.8 or 16-50 f/2.8 or 17-55mm f/2.8 lenses designed for APS-C cameras are designed to fill the role that the film-era 28-70mm or 24-70mm type zoom lenses filled--specifically a wide-angle, semi-wide, normal, and short-telephoto, utility zoom lens for general assignments where multiple angles of view are needed on one, single zoom lens. So, yes, think of a 16-50 or 17-50 or 17-55mm f/2.8 lens as being the "utility zoom" lens for modern APS-C cameras.

I've used the Canon 24-105mm f/4 L IS USM zoom on a 1.6x Canon...it gives a pseudo-normal to telephoto range of picture angles, and it is in a semi-wide-angle lens when shot on an APS-C body. Since 24mm multiplied by 1.6 is 38.4mm, the angle of view is what I call a "semi-wide"...it's not very wide in angle of view, but it is close to the 35mm focal length that was at one time very popular, and which is actually somewhat useful for many types of situations--but it is NOT "wide-angle" on APS-C.

At one time, the 35mm to 105mm zoom lens was considered a useful hobby lens, suitable for many walk-around scenarios; a 24-70mm zoom lens when used on a Canon 1.6x camera would be a 38.4mm to 112mm zoom lens equivalent--which would in many ways, make a pretty capable picture-taker for many people, but it would come up short when true wide-angle views are needed.

Lenses are specified and sold in their true,measured focal lengths for both APS-C (Nikon) and FX and m4/3; it is up to the buyer to do the math to get the 135 film format, "traditional" equivalent picture angles figured out and mentally pictured.
 
Like you're 5?

Ok. Here goes.

When a Mommy sensor and a Daddy sensor love each other very much they make a little crop sensor.

Now this little crop sensor has a very good understanding of what's going on around him, but he's just a little guy so he doesn't get the whole picture.
 
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The idea of a certain focal length being "converted" on a crop sensor camera relates to field of view, and not actual focal length.

Putting a lens on a crop-sensor camera does not change its focal length. The "conversion" relates to the comparable field of view on a different camera.

A 35mm lens on a crop-sensor camera gives the same (more or less) field of view as a 50mm lens on a full-frame camera. It's still a 35mm lens, so its behavior related to depth of field at certain apertures is what would be expected of a 35mm lens on any format camera.

A crop-factor camera with a 35mm lens mounted on a tripod and shooting a scene will give the same (more or less) image as a full-frame camera carrying a 50mm lens right next to it on another tripod, looking at the same scene.

Experienced photographers know what field of view to expect from a certain focal length lens, and saying that a 35 lens behaves like on this camera as a 50 on that one shows them what to expect.

If you shot an image with the 35mm lens on a full frame, printed the image, then took scissors and cut out the part that wouldn't have been recorded by the crop-sensor camera, you'd have EXACTLY the picture that the lens would have produced on the crop-sensor camera.
 
There are two focal lengths:

1. The actual, real, true, whatever you want to call it focal length, usually just called focal length. The focal length is a property of the lens itself, and of the lens alone. It describes at which distance from the sensor the light passes the aperture of the lens (*) (**). Together with the knowledge about how large the sensor is, this defines the viewing angle of the lens.

2. The equivalent focal length. This is the actual focal length multiplied by the crop factor. It describes the focal length a lens would have on small format (also called full frame, 35mm or 135mm film, FX by Nikon, EF by Canon, etc - its a sensor of the size 36x24mm and an image circle of 43.3mm) to have the same viewing angle as the lens has with that sensor in question.

Thats why your 17-50mm will be a 25.5-75mm equivalent focal length on an half format (APS-C, Nikon DX, ~24x16mm etc) sensor.

If you would use the 17-50mm on a full frame sensor, it would indeed as 17-50mm, but the borders would be dark and/or of poor quality. It happends though that APS-C zooms might be able to fill the whole full frame image circle properly, then the 17-50mm would work fine in that part of the focal lenght range.


(*) This definition is assuming the most simple of lens construction. Optical construction ticks like telephoto and retrofocus allow the aperture to actually sit closer or more far away, respectively. Which is needed to have very long lenses which arent also physically absurdly long, and lenses which are wide angle but can handle a long distance between sensor and lens to make space for the mirror box of a SLR, or to allow less steep angles of light on a digital sensor, since many digital sensors cant handle too steep angles well.

(**) The aperture is specifically the place at which the image is turned around. This is often hidden through the camera construction; for example a SLR typically has a pentaprism which turns the image around again so the viewfinder shows it again in correct order. However, if you use a waistfinder with a SLR or if you use a large format camera, you can see that up and down as well as left and right are flipped by the optics.
 

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