Crop Factor... that monster!

BillGCH · Jan 14, 2019

(In this thread I am talking about Nikon bodies and lenses so I am using a crop factor of x1.5 in my examples. For Canon shooters a crop factor of x1.6 should be used. Just mentioning.)

Hello everyone. After 6 years in photography I am still fighting that monster we call crop factor. The reason why I got confused again is because of a youtube chanel called “Tony & Chelsea Northrup”. Watched a few videos about fx on dx and dx on fx and about multiplying the f and the iso and the noise and then found another channel saying that he is wrong etc and I almost smashed my computer.

Well for start let’s forget everything else and concentrate on one thing. Focal Length.

(Please ignore the fact that I am giving a prime DX as an example that does not exist. Let’s keep it for the example)

50mm FX lense on FX body = 50mm

50mm FX lense on DX body = 75mm

50mm DX lense on FX body = 75mm (crop mode)

50mm DX lense on DX body = 75mm (crop mode automatically since it’s a cut sensor)

Q1. Do we agree on the above?

Q2. Do you believe that when multiplying the mm on a lense the F also does get multiplied?

Real example for Question 2:

I used to shoot nightlife in clubs. Had a D5100 with a 18-55. After 3 years I got tired and stopped. Now they are asking me back in the nightlife but now I have a D610 with just a 50mm lense.

So my thoughts are. Just buy a 17-55 f2.8 and I will be 1mm wider than I used to be (big deal lol), have a big loss on MP (it’s going to use the center of the sensor so about 10mp) but I don’t mind since it’s only for facebook.

Will I lose that f2.8 so multiply it by x1.5 as the guy in the video said? He confused me!

Ysarex · Jan 14, 2019

BillGCH said:
(In this thread I am talking about Nikon bodies and lenses so I am using a crop factor of x1.5 in my examples. For Canon shooters a crop factor of x1.6 should be used. Just mentioning.)

Hello everyone. After 6 years in photography I am still fighting that monster we call crop factor. The reason why I got confused again is because of a youtube chanel called “Tony & Chelsea Northrup”. Watched a few videos about fx on dx and dx on fx and about multiplying the f and the iso and the noise and then found another channel saying that he is wrong etc and I almost smashed my computer.

Well for start let’s forget everything else and concentrate on one thing. Focal Length.

(Please ignore the fact that I am giving a prime DX as an example that does not exist. Let’s keep it for the example)

50mm FX lense on FX body = 50mm

Yes.

BillGCH said:
50mm FX lense on DX body = 75mm

No. 50mm is 50mm changing the sensor size does not change the lens focal length.

BillGCH said:
50mm DX lense on FX body = 75mm (crop mode)

No. 50mm is 50mm changing the sensor size does not change the lens focal length.

BillGCH said:
50mm DX lense on DX body = 75mm (crop mode automatically since it’s a cut sensor)

No. 50mm is 50mm changing the sensor size does not change the lens focal length.

BillGCH said:
Q1. Do we agree on the above?

No.

BillGCH said:
Q2. Do you believe that when multiplying the mm on a lense the F also does get multiplied?

No. f/xx is f/xx and changing the sensor size does not change the f/stop.

And away we go: One big bucket of troll chum in the water. The DPR troll feeding frenzy begins momentarily

BillGCH said:
Real example for Question 2:

I used to shoot nightlife in clubs. Had a D5100 with a 18-55. After 3 years I got tired and stopped. Now they are asking me back in the nightlife but now I have a D610 with just a 50mm lense.

So my thoughts are. Just buy a 17-55 f2.8 and I will be 1mm wider than I used to be (big deal lol), have a big loss on MP (it’s going to use the center of the sensor so about 10mp) but I don’t mind since it’s only for facebook.

Will I lose that f2.8 so multiply it by x1.5 as the guy in the video said? He confused me!

BillGCH · Jan 14, 2019

So you just throw the crop factor out of the window right now?

Are you sure?

My DX 18-105 @35mm on D5100 = my FX 50mm on my FX D610

= means I see the same "window".

Ysarex · Jan 14, 2019

BillGCH said:
So you just throw the crop factor out of the window right now?

Are you sure?

I'm sure. Crop factor doesn't change focal length it changes angle of view. It matters. A lens can't change it's focal length.

BillGCH said:
My DX 18-105 @35mm on D5100 = my FX 50mm on my FX D610

= means I see the same "window".

PascalNJ · Jan 14, 2019

BillGCH said:
(In this thread I am talking about Nikon bodies and lenses so I am using a crop factor of x1.5 in my examples. For Canon shooters a crop factor of x1.6 should be used. Just mentioning.)

Hello everyone. After 6 years in photography I am still fighting that monster we call crop factor. The reason why I got confused again is because of a youtube chanel called “Tony & Chelsea Northrup”. Watched a few videos about fx on dx and dx on fx and about multiplying the f and the iso and the noise and then found another channel saying that he is wrong etc and I almost smashed my computer.

Well for start let’s forget everything else and concentrate on one thing. Focal Length.

(Please ignore the fact that I am giving a prime DX as an example that does not exist. Let’s keep it for the example)

50mm FX lense on FX body = 50mm

50mm FX lense on DX body = 75mm

50mm DX lense on FX body = 75mm (crop mode)

50mm DX lense on DX body = 75mm (crop mode automatically since it’s a cut sensor)

Q1. Do we agree on the above?

Actually, no, we don't. The focal length of a lens doesn't change because of the sensor size. 50 mm is 50 mm.

BillGCH said:
Q2. Do you believe that when multiplying the mm on a lense the F also does get multiplied?

No, you can't "multiply the mm" or "multiply the F". Aperture doesn't change when you use a lens on different cameras. That's why exposure (light-aperture-shutter speed) stays the same for a same scene with different cameras.

BillGCH said:
Real example for Question 2:

I used to shoot nightlife in clubs. Had a D5100 with a 18-55.

"a 18-55": which 18-55? Which max aperture? f/2.8 ? f/3.5 ?

BillGCH said:
After 3 years I got tired and stopped. Now they are asking me back in the nightlife but now I have a D610 with just a 50mm lense.

So my thoughts are. Just buy a 17-55 f2.8

I'm assuming this is a "crop" (DX) lens ?

BillGCH said:
and I will be 1mm wider than I used to be (big deal lol), have a big loss on MP (it’s going to use the center of the sensor so about 10mp)

And that's the problem you seem to have: you now have a 10 Mp crop sensor, that is no better in low light than that of you D5100. In fact, depending how large you're going to print, it could even be worse (10 Mp crop vs. 16 Mp crop).

BillGCH said:
but I don’t mind since it’s only for facebook.

BillGCH said:
Will I lose that f2.8 so multiply it by x1.5 as the guy in the video said? He confused me!

Use equivalence to compare field of view or depth of field, even comparing image quality in low light. But here, by using a crop (DX) lens on the D610, you're making your camera a crop sensor camera so there's no advantage compared with the D5100.

Now, maybe I have confused you even more? I hope not.

BillGCH · Jan 14, 2019

Ysarex said:
BillGCH said:

So you just throw the crop factor out of the window right now?

Are you sure?

Click to expand...

I'm sure. Crop factor doesn't change focal length it changes angle of view. It matters. A lens can't change it's focal length.

Ysarex said:

My DX 18-105 @35mm on D5100 = my FX 50mm on my FX D610

= means I see the same "window".

Click to expand...

Well that's what I mean. The angle of view...

50mm FX lense on FX body = 50mm -- Let's take that as a standard

50mm FX lense on DX body = 75mm -- So the above lense if could "zoom" it should go to 75mm to match the frame.

Same here:

50mm DX lense on FX body = 75mm (crop mode) -- I mean that what I see in that frame is the same as what I see in the above

50mm DX lense on DX body = 75mm (crop mode automatically since it’s a cut sensor) -- same here.

I am talking about frame...

Ysarex · Jan 14, 2019

BillGCH said:
Ysarex said:

BillGCH said:

So you just throw the crop factor out of the window right now?

Are you sure?

Click to expand...

I'm sure. Crop factor doesn't change focal length it changes angle of view. It matters. A lens can't change it's focal length.

BillGCH said:

My DX 18-105 @35mm on D5100 = my FX 50mm on my FX D610

= means I see the same "window".

Click to expand...

Click to expand...

Well that's what I mean. The angle of view...

50mm FX lense on FX body = 50mm -- Let's take that a standard

50mm FX lense on DX body = 75mm -- So the above lense if could "zoom" it should go to 75mm to match the frame.

No. 50mm lens on FX body has 40 degree angle of view. A 50mm lens which remain a 50mm lens has a 27 degree angle of view on a DX body. Make sure that what you mean and say are the same thing.

BillGCH said:
Same here:

50mm DX lense on FX body = 75mm (crop mode) -- I mean that what I see in that frame is the same as what I see in the above

50mm DX lense on DX body = 75mm (crop mode automatically since it’s a cut sensor) -- same here.

I am talking about frame...

mamallama · Jan 14, 2019

Crop factor is a simple concept.

Richrf · Jan 14, 2019

I believe you are misunderstanding Northrup's video. The video is discussing equivalency, not physical change.

Here is a site that calculates equivalency of FOV and aperture.

https://mmcalc.com

Northrup demonstrated actual equivalencies using actual lenses, the proof of the pudding. No technical knowledge required.

Richrf · Jan 14, 2019

mamallama said:
Crop factor is a simple concept.

Unfortunately it is not. But actual, real life demonstrations such as the ones offered by Northrup help.

One thing is for sure, when it comes to light, there is no replacing it with less light. Less light is always going to be less light, no matter what.

BillGCH · Jan 14, 2019

Ysarex said:
BillGCH said:

Ysarex said:

BillGCH said:

So you just throw the crop factor out of the window right now?

Are you sure?

Click to expand...

I'm sure. Crop factor doesn't change focal length it changes angle of view. It matters. A lens can't change it's focal length.

Ysarex said:

My DX 18-105 @35mm on D5100 = my FX 50mm on my FX D610

= means I see the same "window".

Click to expand...

Click to expand...

Well that's what I mean. The angle of view...

50mm FX lense on FX body = 50mm -- Let's take that a standard

50mm FX lense on DX body = 75mm -- So the above lense if could "zoom" it should go to 75mm to match the frame.

Click to expand...

No. 50mm lens on FX body has 40 degree angle of view. A 50mm lens which remain a 50mm lens has a 27 degree angle of view on a DX body. Make sure that what you mean and say are the same thing.

Ysarex said:

Same here:

50mm DX lense on FX body = 75mm (crop mode) -- I mean that what I see in that frame is the same as what I see in the above

50mm DX lense on DX body = 75mm (crop mode automatically since it’s a cut sensor) -- same here.

I am talking about frame...

Click to expand...

I never said the lense physically changes. I am saying that depending on what body it goes changes the FRAME you see. The "how wide" or "how zoomed" you. see.

Agree.

50mm on FX 40 degree

50mm on DX 27 degree -> so on DX it x1.5 "zoomed / croped" the frame ("the degree"). (27x1.5 = 40.5)

Great Bustard · Jan 14, 2019

BillGCH said:
(In this thread I am talking about Nikon bodies and lenses so I am using a crop factor of x1.5 in my examples. For Canon shooters a crop factor of x1.6 should be used. Just mentioning.)

Hello everyone. After 6 years in photography I am still fighting that monster we call crop factor. The reason why I got confused again is because of a youtube chanel called “Tony & Chelsea Northrup”. Watched a few videos about fx on dx and dx on fx and about multiplying the f and the iso and the noise and then found another channel saying that he is wrong etc and I almost smashed my computer.

Well for start let’s forget everything else and concentrate on one thing. Focal Length.

(Please ignore the fact that I am giving a prime DX as an example that does not exist. Let’s keep it for the example)

50mm FX lens on FX body = 50mm

Yes.

BillGCH said:
50mm FX lens on DX body = 75mm

No. It's still a 50mm lens. However, its angle of view is *equivalent to* a 75mm lens on FX.

BillGCH said:
50mm DX lens on FX body = 75mm (crop mode)

Again, no. It's still a 50mm lens. However, the angle of view is *equivalent to* an uncropped 75mm lens on FX.

BillGCH said:
50mm DX lens on DX body = 75mm (crop mode automatically since it’s a cut sensor)

Q1. Do we agree on the above?

No -- as explained above.

BillGCH said:
Q2. Do you believe that when multiplying the mm on a lens the F also does get multiplied?

The *equivalent* f-number gets multiplied by the same factor. For example, 50mm f/2 on DX is *equivalent to* 75mm f/3 on FX because they yield the same angle of view and the same aperture diameter (where the same aperture diameter will result in the same DOF and same amount of light projected on the sensor for a given exposure time).

BillGCH said:
Real example for Question 2:

I used to shoot nightlife in clubs. Had a D5100 with a 18-55. After 3 years I got tired and stopped. Now they are asking me back in the nightlife but now I have a D610 with just a 50mm lens.

So my thoughts are. Just buy a 17-55 f2.8 and I will be 1mm wider than I used to be (big deal lol), have a big loss on MP (it’s going to use the center of the sensor so about 10mp) but I don’t mind since it’s only for facebook.

Will I lose that f2.8 so multiply it by x1.5 as the guy in the video said? He confused me!

A 17-55 / 2.8 on DX is *equivalent to* a 26-83 / 4.2 on FX. That is, they will have the same angle of view and the same aperture diameter, the effect of the latter resulting in the same DOF and same amount of light projected on the sensor for a given exposure time.

BillGCH · Jan 14, 2019

Great Bustard said:
No. It's still a 50mm lens. However, its angle of view is *equivalent to* a 75mm lens on FX.

Great Bustard said:

50mm DX lens on FX body = 75mm (crop mode)

Click to expand...

Again, no. It's still a 50mm lens. However, the angle of view is *equivalent to* an uncropped 75mm lens on FX.

You are saying no and then saying the same thing as I say. With the symbol = I mean equivalent...

Ysarex · Jan 14, 2019

BillGCH said:
Ysarex said:

BillGCH said:

Ysarex said:

BillGCH said:

So you just throw the crop factor out of the window right now?

Are you sure?

Click to expand...

I'm sure. Crop factor doesn't change focal length it changes angle of view. It matters. A lens can't change it's focal length.

BillGCH said:

My DX 18-105 @35mm on D5100 = my FX 50mm on my FX D610

= means I see the same "window".

Click to expand...

Click to expand...

Well that's what I mean. The angle of view...

50mm FX lense on FX body = 50mm -- Let's take that a standard

50mm FX lense on DX body = 75mm -- So the above lense if could "zoom" it should go to 75mm to match the frame.

Click to expand...

No. 50mm lens on FX body has 40 degree angle of view. A 50mm lens which remain a 50mm lens has a 27 degree angle of view on a DX body. Make sure that what you mean and say are the same thing.

BillGCH said:

Same here:

50mm DX lense on FX body = 75mm (crop mode) -- I mean that what I see in that frame is the same as what I see in the above

50mm DX lense on DX body = 75mm (crop mode automatically since it’s a cut sensor) -- same here.

I am talking about frame...

Click to expand...

Click to expand...

I never said the lense physically changes. I am saying that depending on what body it goes changes the FRAME you see. The "how wide" or "how zoomed" you. see.

Agree.

50mm on FX 40 degree

50mm on DX 27 degree -> so on DX it x1.5 "zoomed / croped" the frame ("the degree"). (27x1.5 = 40.5)

Sorry for the interruption -- had to take a break and watch Stephen Colbert heap ridicule upon Trump.

OK, so the reason this issue causes so much angst is because of the language used to describe what's happening. Sorry you watched Tony Northrup and got confused -- a lot of people get confused when they watch people who are confused.

You're struggling with the language and given your original post it's at the root of the problem.

A 50mm lens on a DX camera produces an angle of view of 27 degrees and a 75mm lens on an FX camera also produces an angle of view 27 degrees. So that both camera with respective lenses would take the same photograph from the same place.

What belongs then on either side of an "=" is 27 degree angle of view -- you could also put "same photograph" = "same photograph."

When you read about multiplying f/stops by crop factors what's being compared is DOF and it's DOF that belongs on the two sides of the =. F/2.8 is f/2.8 is always f/2.8. So the use of crop factor in this case would read: To produce equivalent DOF in a photo taken from the same place with a 75mm lens set to f/4.2 on an FX camera "=" a 50mm lens set to f/2.8 on a DX camera (same place).

BillGCH · Jan 14, 2019

Ysarex said:
A 50mm lens on a DX camera produces an angle of view of 27 degrees and a 75mm lens on an FX camera also produces an angle of view 27 degrees. So that both camera with respective lenses would take the same photograph from the same place.

What belongs then on either side of an "=" is 27 degree angle of view -- you could also put "same photograph" = "same photograph."

THANK YOU!!! That's what I am saying from the very beginning. I am making a small excel though to explain it better.

edit: here we go...

Richrf · Jan 14, 2019

BillGCH said:
BillGCH said:

A 50mm lens on a DX camera produces an angle of view of 27 degrees and a 75mm lens on an FX camera also produces an angle of view 27 degrees. So that both camera with respective lenses would take the same photograph from the same place.

What belongs then on either side of an "=" is 27 degree angle of view -- you could also put "same photograph" = "same photograph."

Click to expand...

THANK YOU!!! That's what I am saying from the very beginning. I am making a small excel though to explain it better.

Unfortunately, that is not what you've been saying. Creating equivalent FOVs is not the same as two lenses being equal on different sensors. In your chart, you are introducing the concept of FOV or angle of view which clarifies.

What is mathematical true it's that equivalent focal length and fstops will create equal aperture areas which impacts the amount of light that is being collected.

BillGCH · Jan 14, 2019

Richrf said:
BillGCH said:

Richrf said:

A 50mm lens on a DX camera produces an angle of view of 27 degrees and a 75mm lens on an FX camera also produces an angle of view 27 degrees. So that both camera with respective lenses would take the same photograph from the same place.

What belongs then on either side of an "=" is 27 degree angle of view -- you could also put "same photograph" = "same photograph."

Click to expand...

THANK YOU!!! That's what I am saying from the very beginning. I am making a small excel though to explain it better.

Click to expand...

Unfortunately, that is not what you've been saying. Creating equivalent FOVs is not the same as two lenses being equal on different sensors. In your chart, you are introducing the concept of FOV or angle of view which clarifies.

What is mathematical true it's that equivalent focal length and fstops will create equal aperture areas which impacts the amount of light that is being collected.

Equal... well.. depends on what you mean by "equal".

If "they see" the same thing (equal angle and if I can say regarding to crop factor) yes they are equal. Not in quality or anything but in angle (what I call mm).

Richrf · Jan 14, 2019

Ysarex said:
Sorry for the interruption -- had to take a break and watch Stephen Colbert heap ridicule upon Trump.

OK, so the reason this issue causes so much angst is because of the language used to describe what's happening. Sorry you watched Tony Northrup and got confused -- a lot of people get confused when they watch people who are confused.

You're struggling with the language and given your original post it's at the root of the problem.

A 50mm lens on a DX camera produces an angle of view of 27 degrees and a 75mm lens on an FX camera also produces an angle of view 27 degrees. So that both camera with respective lenses would take the same photograph from the same place.

What belongs then on either side of an "=" is 27 degree angle of view -- you could also put "same photograph" = "same photograph."

When you read about multiplying f/stops by crop factors what's being compared is DOF and it's DOF that belongs on the two sides of the =. F/2.8 is f/2.8 is always f/2.8. So the use of crop factor in this case would read: To produce equivalent DOF in a photo taken from the same place with a 75mm lens set to f/4.2 on an FX camera "=" a 50mm lens set to f/2.8 on a DX camera (same place).

There was nothing wrong with Northrup's demonstration. He merely demonstrated in real practice the equivalency of DOF and FOV. I always enjoy practical demonstrations since not everyone had access to equivalent lenses in FF and crop sensor cameras.

Richrf · Jan 14, 2019

BillGCH said:
Richrf said:

BillGCH said:

BillGCH said:

A 50mm lens on a DX camera produces an angle of view of 27 degrees and a 75mm lens on an FX camera also produces an angle of view 27 degrees. So that both camera with respective lenses would take the same photograph from the same place.

What belongs then on either side of an "=" is 27 degree angle of view -- you could also put "same photograph" = "same photograph."

Click to expand...

THANK YOU!!! That's what I am saying from the very beginning. I am making a small excel though to explain it better.

Click to expand...

Unfortunately, that is not what you've been saying. Creating equivalent FOVs is not the same as two lenses being equal on different sensors. In your chart, you are introducing the concept of FOV or angle of view which clarifies.

What is mathematical true it's that equivalent focal length and fstops will create equal aperture areas which impacts the amount of light that is being collected.

Click to expand...

Equal... well.. depends on what you mean by "equal".

If "they see" the same thing (equal angle and if I can say regarding to crop factor) yes they are equal. Not in quality or anything but in angle (what I call mm).

Without introducing FOV or angle of view into the discussion, then there is no equivalency. The chart introduces this concept and therefore clarifies. Up until that point, the discussion is incomplete.

Great Bustard · Jan 14, 2019

BillGCH said:
BillGCH said:

No. It's still a 50mm lens. However, its angle of view is *equivalent to* a 75mm lens on FX.

BillGCH said:

50mm DX lens on FX body = 75mm (crop mode)

Click to expand...

Again, no. It's still a 50mm lens. However, the angle of view is *equivalent to* an uncropped 75mm lens on FX.

Click to expand...

You are saying no and then saying the same thing as I say. With the symbol = I mean equivalent...

The thing is, however, equivalent is not the same as equal. It is the failure to make this distinction that causes confusion for so many people.

Crop Factor... that monster!

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