About Crop Factors on lenses

Hello, I have a question about crop factors when comparing DX and non DX lenses.
I want to know when crop factors apply and if they apply on the lens f-stop. Hypothetical scenarios being:

A 100mm f/1.0 DX lens on a DX body means 100mm f/1.0, 125mm f/1.0 or 125 f/1.smth ? And what's the difference with a 100mm f/1.0 lens that does NOT indicate DX on it but used on a DX body?

A 100mm lens is the same 100mm lens on any camera body.

The FX sensor is just 1.5x larger than the DX sensor. So the DX sensor simply shows less of the lens image, crops it smaller, to show 1/1.5 as much of it. So then this VIEW of the subject scene is smaller overall in the smaller sensor. However, a quirk is that it can be compared as same as the scene VIEW of the larger sensor if the larger sensor used a lens 1.5x longer. That is ALL that "equivalent focal length" means.
It is a crop factor.

But the lens never changes, no matter what sensor you put behind it. The VIEW in that smaller sensor is simply smaller (cropped) as compared as the same scene VIEW of the larger sensor.

You can cause the same "equivalent" effect (comparing the two sensors as if with the same lens, standing in the same place) if you simply crop the larger FX sensors image to be smaller (2/3 the size = 1/1.5), later in the photo editor. You do of course lose pixels that way, but the scene VIEW is the same, in the cropped image in a DX sensor. It appears telephoto, but all it is is cropped smaller (and then enlarged to show same size).

Compatibility: A DX lens projects a circular image large enough to cover the smaller DX sensor, but the circle is not large enough to cover the corners of a FX sensor. FX needs a FX lens, which necessarily projects a larger image. But that larger glass is more expensive, so DX lenses don't.

So... DX appears telephoto, but FX is definitely more wide angle.

If you put a 16mm lens on FX, you get at 16mm view (as compared to 35mm film size).
But put the 16mm lens on DX, and you get a 24mm view (as compared to 35mm film size).


See Camera Sensor Crop Factor and Equivalent Lens Focal Length

If I'm understanding your question, it doesn't matter if the lens is DX or FX. A 100mm lens is a 100mm lens. Where you get the crop factor is solely from the size of the sensor that the lens is projecting the image onto.

So say you cut out a piece of paper roughly the size of a postcard (this represents your FX sensor) and tape it to the wall. Then cut another piece of paper roughly 2/3 the size of that first piece (this represents your DX sensor). Now tape that smaller piece directly in the center of the original piece of paper on the wall. Now take a flashlight and shine it on the two pieces of paper. The flash light represents the lens as it broadcasts its image on the sensor. As you move the flashlight away from the wall the image gets larger on the two sensors. If you look closely at what is on the actual sensor, this represents what the DX is cropping out in relation to the FX lens. So a DX 100mm lens is 100mm on FX or DX. The reason they designate DX on the lens is because the lens projection does not cover the edges of what you would consider the FX sensor. So if you put a DX 100mm lens on an FX camera then you'd have vignetting.

Focal length and aperture explained pretty well here...

https://youtu.be/YDbUIfB5YUc

I see, so basically whatever lens I use on my d3300 the view will always be x1.5 compared to what a full frame (let's say a D4) would see... What I don't understand though is the DX tag on the lenses I use. Let's say I use the Nikon 35mm DX 1.8g lens and the Nikon 50mm 1.8g (states no DX on it), apart from the more zoomed in perspective do those lenses have any other differences?

Imagine closing one eye and looking at something with your open eye through a toilet paper tube.

Now imagine closing one eye like you did before and looking at that same something with your open eye through a soda straw.

That, essentially, is the difference.
....

Aleks said:

I see, so basically whatever lens I use on my d3300 the view will always be x1.5 compared to what a full frame (let's say a D4) would see... What I don't understand though is the DX tag on the lenses I use. Let's say I use the Nikon 35mm DX 1.8g lens and the Nikon 50mm 1.8g (states no DX on it), apart from the more zoomed in perspective do those lenses have any other differences?

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The DX label is on them to indicate they are designed for DX cams but won't necessarily work on an FX. If there is no label on them they are designed for FX but can also be used on DX.

J-see said:

The DX label is on them to indicate they are designed for DX cams but won't necessarily work on an FX. If there is no label on them they are designed for FX but can also be used on DX.

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So for me who uses a DX machine I shouldn't mind at all if it has a DX tag or not right?
Also, after checking the video ScottinPollock posted I see that the f-stop number is also taken in the crop calculation, so does that mean that the Nikkor 50mm f1.8g on a D4 wide open and same iso/shutter speed with a D3300 will get a brighter image result on the D4?

Aleks said:

So for me who uses a DX machine I shouldn't mind at all if it has a DX tag or not right?
Also, after checking the video ScottinPollockposted I see that the f-stop number is also taken in the crop calculation, so does that mean that the Nikkor 50mm f1.8g on a D4 wide open and same iso/shutter speed with a D3300 will get a brighter image result on the D4?

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It's easier if you ignore the whole f/stop equivalence and such since it only indicates the performance of a DX vs an identical shot taken with an FX. That all matters little unless you need to replicate the exact same shot with both systems.

Aleks said:

So for me who uses a DX machine I shouldn't mind at all if it has a DX tag or not right?
Also, after checking the video ScottinPollockposted I see that the f-stop number is also taken in the crop calculation, so does that mean that the Nikkor 50mm f1.8g on a D4 wide open and same iso/shutter speed with a D3300 will get a brighter image result on the D4?

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No, the fstop is not a factor in FX/DX.

fstop is focal length / aperture diameter. Real lens focal length is not affected by putting it on a DX body. The view is just smaller.

A f/1.8 DX lens on DX will give the same exposure as a f/1.8 lens on FX, be they different lenses, or the same lens.


No, on a DX body, it will not matter if you use DX or FX lenses. It will matter on the day that you decide to switch to a FX body, when you need new lenses.
And FX lenses tend to be better (and larger and heavier and MORE EXPENSIVE) lenses.
Switching to FX is NOT an inexpensive plan.

Aleks said:

I see, so basically whatever lens I use on my d3300 the view will always be x1.5 compared to what a full frame (let's say a D4) would see... What I don't understand though is the DX tag on the lenses I use. Let's say I use the Nikon 35mm DX 1.8g lens and the Nikon 50mm 1.8g (states no DX on it), apart from the more zoomed in perspective do those lenses have any other differences?

Click to expand...

Correct. Focal length is an actual measurement of the lens, not impacted by sensor size (DX or FX). Your D3300 will always have a 1.5x crop over an FX image at the same focal length, because of the smaller sensor size.

Lenses that carry the DX brand, only project a DX sized image onto the sensor area, where an FX lens projects the larger FX image over the sensor area. If you put a DX lens onto an FX body, part of the FX sensor will sit in darkness, because the DX lens is not getting light to the outer portions of the sensor. The image seen by the DX sensor is the same either way, the FX lens just displays more detail outside of the DX sensor so that part of the image is lost.

The thinking is/was that since DX cameras use a smaller sensor size, lenses could be made at less cost since they didn't have to provide an FX sized image. Less glass on the front element (typically), and possibly a more compact design. My 55-300mm DX lens has a front element that is 58mm, while my 24-120mm FX lens is 77mm. Part of that relates to maximum aperture, but also to the larger image size for FX.


Sent from my iPhone using Tapatalk

The f/stop is only a factor when it comes to Depth of Field.

When you take an identical shot with a DX and FX, you need about a stop more aperture for the FX to have the same DoF as the DX. The reason for this has nothing to do with the lens. If you want to take a similar shot with the DX, you are 1.5 times the distance from the same subject as the FX before it projects the same image on the sensor.

Since DoF depends on focal length, aperture and the distance to the subject, the further you are, the more DoF that same aperture provides.

RocketCowboy said:

Lenses that carry the DX brand, only project a DX sized image onto the sensor area, where an FX lens projects the larger FX image over the sensor area. If you put a DX lens onto an FX body, part of the FX sensor will sit in darkness, because the DX lens is not getting light to the outer portions of the sensor. The image seen by the DX sensor is the same either way, the FX lens just displays more detail outside of the DX sensor so that part of the image is lost.

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Ah, that explains my confusion! Thanks!!

J-see said:

Since DoF depends on focal length, aperture and the distance to the subject, the further you are, the more DoF that same aperture provides.

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And sensor size! Sensor size is the first question any DOF calculator will ask you.

It is not just about the distance where you stand. DX has to be enlarged 50% more to compare at same standard print size assumed by DOF calculation.

Many don't realize DOF is affected by print size (and enlargement of it), but that is the first basic about DOF.
It is called Circle of Confusion.

The calculator and other DOF guides assume an 8x10 inch print, viewed at 10 inches. It is about the resolution of the eye, and what the eye sees there, and enlargement is a big factor. Enlargement makes fuzziness look worse, easier to see. So smaller images on the video screen may see better results.

WayneF said:

And sensor size! Sensor size is the first question any DOF calculator will ask you.

It is not just about the distance where you stand. DX has to be enlarged 50% more to compare at same standard print size assumed by DOF calculation.

Many don't realize DOF is affected by print size (and enlargement of it), but that is the first basic about DOF.
It is called Circle of Confusion.

The calculator and other DOF guides assume an 8x10 inch print, viewed at 10 inches. It is about the resolution of the eye, and what the eye sees there, and enlargement is a big factor. Enlargement makes fuzziness look worse, easier to see. So smaller images on the video screen may see better results.

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Wayne, if a DX camera sensor has the same number of pixels as an FX, wouldn't the picture be exactly the same size?

Woodyg3 said:

Wayne, if a DX camera sensor has the same number of pixels as an FX, wouldn't the picture be exactly the same size?

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Tricky question. My argument says No.
I am hoping to speak to all, not just to you, so please don't take offense when I state something you know.

On print paper, there is no debate, pixels are just a resolution factor of reproduction - Not an enlargement factor. Which is tricky to say, it's correct, but I feel my words are not the best possible choice, because of course printing 150 dpi is a 2x larger image than 300 dpi, but it is the same pixels, and the resolution is lower because it is more enlargement of the sensor size.
Which is actually true of the video screen too (more at bottom).

Bottom line, we are enlarging the lens image that might be DX 24x16 mm size, or it might be FX 36x24 mm size. Same as film in that respect - the sensor has size. This lens image is what it is all about. Pixels or pixel resolution or film grain do not create image detail, they just try to reproduce lens detail well.

For DOF calculation purposes, we are enlarging it to a standard 8x10 inch size on paper (we have to crop it from 8x12 inches for 3:2). The 8 inch dimension is left intact.
We are enlarging 16mm or 24 mm to be 8 inches on paper, which is roughly 12.7x or 8.5x enlargement without additional cropping, which enlarges any blur too. That is a 1.5x factor. We obviously see the enlarged DX lens blur larger and more clearly than from the same lens on FX.

Having more pixels will sample the lens image more accurately, and can increase printer resolution of printing dithered ink dots, but it does not affect enlargement of the original image size. The 8x10 inches determines enlargement of sensor size. It works exactly the same as at the darkroom enlarger. And FX and DX enlargements are like the 35mm and APS film sizes.

Circle of Confusion is the (arbitrary) hypothetical size of an out-of-focus point source dot in the image. We judge the maximum CoC size permitted to still call it "sharp" (if not visible). The CoC link above shows what this circle diameter is, in its first image at the top right. There is no abrupt cutoff of course, it is not sharp on one side and fuzzy across the line, it just gradually varies all along, but we state a numerical limit that can be calculated.

We necessarily must adjust the CoC limit used for degrees of enlargement. To be the same visibility criteria, it has to be larger at low enlargement, and smaller at high enlargement. That depends on sensor size.

History has had several estimates about the CoC limit we can see. Carl Zeiss is said to have defined it as film diagonal/1730, which is of course about sensor size and enlargement. Now CoC is commonly said to be 0.03 mm diameter for 35mm (FX), and 0.02 mm for DX (2/3 size, which is 1/1.5)... when enlarged to this standard 8x10 viewed at 10 inches. And only 0.004 or 0.005 mm is acceptable CoC for compact camera sizes, which have to be enlarged so much.

An online DOF calculator will show the CoC that they use for the sensor size you select. Sensor size is the first question they ask us regarding DOF.


On the video screen, which shows pixels (but only up to about 2 megapixels), I say it is obviously the same thing. We do use pixels to do it, but we are still enlarging the sensor size to show at some viewed size in inches. About like paper, different plan, but same result of visibility. We resample to use as many pixels are necessary to create that size. It is an enlargement of the original lens image size, and the pixels are a resolution factor of reproduction of that enlargement.

Hope that helps.

@WayneF I kind of figured this is what you were talking about. I understand the way you are looking at it.

Still, we see examples of FX pictures that are cropped to match DX sensor size and often find that the DX pictures can be sharper. So, it's a combination of factors, here.

Woodyg3 said:

Still, we see examples of FX pictures that are cropped to match DX sensor size and often find that the DX pictures can be sharper. So, it's a combination of factors, here.

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That's the advantage of their pixel size. If a DX would have pixels half the size of an FX, it would resolve the same surface area's detail using 4 pixels when the FX has to do with one. By cropping the FX image, we trade in the initial sharpness advantage for enlargement. Depending the resolutions of the cams used, at some point the one becomes "better" than the other.

J-see said:

That's the advantage of their pixel size. If a DX would have pixels half the size of an FX, it would resolve the same surface area's detail using 4 pixels when the FX has to do with one.

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I see, then the little 16 megapixel compact camera will beat all of them?

Pixels only attempt to reproduce the lens image, hopefully well. The way to bet is a large sharp lens image.