Crop factor confusion

[insomniac]

I thought I understood crop factors completely, but after reading up a lot on Nikon DX vs FX lenses (am thinking of buying or renting an FX lens for a DX body), I'm a bit confused again.

So my question is very simple. Assuming that the images in this link are taken with an FX lens on an FX body, if I put the same lens on a DX body (all other parameters unchanged), would the red house appear 1.5x larger?

Thank you.

 

[AJQS]

Yes

 

[HoppyUK]

I thought I understood crop factors completely, but after reading up a lot on Nikon DX vs FX lenses (am thinking of buying or renting an FX lens for a DX body), I'm a bit confused again.

So my question is very simple. Assuming that the images in this link are taken with an FX lens on an FX body, if I put the same lens on a DX body (all other parameters unchanged), would the red house appear 1.5x larger?
Thank you.

Yes, with a 24mm lens on DX, the red house would fill the viewfinder like the 35mm lens on FX, or a 200mm lens like the 300mm example.

So it 'effectively' appears larger, though the actual image on the sensor is the same size. Because the focal length has not changed, just that the DX sensor is smaller.

Think of it like this: take a full-frame FX camera, and put masking tape around the four sides, leaving an area in the middle that is crop-format DX size (roughly half the area of FF). You have now converted the full-frame FX camera to crop-format DX.

 

[insomniac]

Great, thanks for that clarification. So now my follow-up question, specifically the applications for wildlife photography.

I understand that for a full frame 70-300mm lens, the effective cropped focal length will be 450mm at the long end. But this is a bit misleading (right?), since physics dictates that the actual optical properties of the lens is unchanged.

How does this crop factor get applied in the field when zooming in on furry animals at a distance?

 

[HoppyUK]

Great, thanks for that clarification. So now my follow-up question, specifically the applications for wildlife photography.

I understand that for a full frame 70-300mm lens, the effective cropped focal length will be 450mm at the long end. But this is a bit misleading (right?), since physics dictates that the actual optical properties of the lens is unchanged.

How does this crop factor get applied in the field when zooming in on furry animals at a distance?

As explained above?

 

[drounding]

The way the change in effective lens fl is described when used on ff or crop just confuses things.

Just think of a bird in a tree in the middle of the vf. The light from the bird and the branches come back to the sensor. A FF sensor will show the bird and the branches, a crop sensor will show the bird and fewer branches. The rest of the branches of the tree will fall outside of the sensor and won't be captured because the sensor is smaller.

When you view the resultant digital images blown up to the same size the bird seems larger from the cropped sensor.

The zoom and lens used is irrelevant.

 

[insomniac]

Understood, thanks a lot - appreciate the input.

 

[Teflon-Mike]

Great, thanks for that clarification. So now my follow-up question, specifically the applications for wildlife photography.

I understand that for a full frame 70-300mm lens, the effective cropped focal length will be 450mm at the long end. But this is a bit misleading (right?), since physics dictates that the actual optical properties of the lens is unchanged.

How does this crop factor get applied in the field when zooming in on furry animals at a distance?

It doesn't. Its Still a 300mm lens. It behaves as a 300 mm lens. You get teh Dof you would get from a 300mm lens.

What you get in pixels, is the same as if you took the photo with 300mm as it IS a 300mm lens.

The only difference is that a crop-sensor camera chops 1/4 off the top, bottom and sides of the 'frame', compared to a 35mm 'full-frame' camera.

The clue is in 'CROP Factor'

What you get is the same 'frame' as if you had a full-frame sensor and 'Cropped' it in any other manner; masking the frame before shot; or taking printed photo and cutting top bottom and sides off it, or did similar in a digital photo-editor, or in days of dark room; putting 5x4 bit of printing paper under the enlarger, but instead of printing edge-to-edge the whole frame 'shot' on the neg, cranked the enlarger head up to enlarge the photo 1.5 times, which would require a 6x8 bit of paper, to get the whole frame in, BUT left smaller 5x4 printing paper under, so you only got the middle bit.

The 1.5x 'crop factor' equivilency, is a little abstract, and is reffering to the effective field of view you get, diagonally, corner to corner of the frame.

The 16x24mm 'crop-sensor' is half the area of a 24x36mm 'Full-Frame' Sensor; however, the liniar dimensions are aprox 1.5x longer on Full-Frame. So, if you do the geometry to work out the angles of view of different length lenses, and by 'similar triangles', you get the same angle of view from lenses also 1.5 times longer in focal length.... hence you get aprox 1.5 times 'effective' magnification from the same length lens on a crop sensor; however, back to the dark room 'cropped enlargement', you aren't actually getting that maginification from the LENS, but from magnifying smaller captured image by a greater degree in the enlargement, or on a computer screen to be the same viewing size.

Make more sense? Or completely confounded you?

Far away furry things? Would be bigger in the view-finder with 300mm lens on DX camera than 300mm lens on FX camera, about the same size as 450mm lens on FX Camera.

For same aperture; Depth of Focus around furry thing would be the same, on 300mm lens on either DX or FX camera. Same aperture on FX camera and 450mm lens, you would probably have less DoF around your furry thing, given same camera to subject distance.

 

[HoppyUK]

It doesn't. Its Still a 300mm lens. It behaves as a 300 mm lens. You get teh Dof you would get from a 300mm lens.

What you get in pixels, is the same as if you took the photo with 300mm as it IS a 300mm lens.

The only difference is that a crop-sensor camera chops 1/4 off the top, bottom and sides of the 'frame', compared to a 35mm 'full-frame' camera.

The clue is in 'CROP Factor'

What you get is the same 'frame' as if you had a full-frame sensor and 'Cropped' it in any other manner; masking the frame before shot; or taking printed photo and cutting top bottom and sides off it, or did similar in a digital photo-editor, or in days of dark room; putting 5x4 bit of printing paper under the enlarger, but instead of printing edge-to-edge the whole frame 'shot' on the neg, cranked the enlarger head up to enlarge the photo 1.5 times, which would require a 6x8 bit of paper, to get the whole frame in, BUT left smaller 5x4 printing paper under, so you only got the middle bit.

The 1.5x 'crop factor' equivilency, is a little abstract, and is reffering to the effective field of view you get, diagonally, corner to corner of the frame.

The 16x24mm 'crop-sensor' is half the area of a 24x36mm 'Full-Frame' Sensor; however, the liniar dimensions are aprox 1.5x longer on Full-Frame. So, if you do the geometry to work out the angles of view of different length lenses, and by 'similar triangles', you get the same angle of view from lenses also 1.5 times longer in focal length.... hence you get aprox 1.5 times 'effective' magnification from the same length lens on a crop sensor; however, back to the dark room 'cropped enlargement', you aren't actually getting that maginification from the LENS, but from magnifying smaller captured image by a greater degree in the enlargement, or on a computer screen to be the same viewing size.

Make more sense? Or completely confounded you?

Far away furry things? Would be bigger in the view-finder with 300mm lens on DX camera than 300mm lens on FX camera, about the same size as 450mm lens on FX Camera.

For same aperture; Depth of Focus around furry thing would be the same, on 300mm lens on either DX or FX camera. Same aperture on FX camera and 450mm lens, you would probably have less DoF around your furry thing, given same camera to subject distance.

Possibly the latter

And your comments re depth of field don't appear to read right.

 

[insomniac]

No, it's clear as mud.

Re DoF, I think the FX frame should have shallower DoF (all things being equal), right?

 

[HoppyUK]

No, it's clear as mud.

Re DoF, I think the FX frame should have shallower DoF (all things being equal), right?

Shallower than what? I think the answer is yes, but the wording with things like this has to be very precise to avoid confusion.

If you frame the subject the same, from the same distance, with full-frame FX (say 300mm lens) and crop-format DX (200mm), then at same f/number the full-frame FX image will have shallower depth of field. About one stop less, the conversion being f/number x crop factor.

 

[sk66]

DOF is affected by only 3 things:
The aperture, the subject distance, and the focal length.
Sensor size is not a direct factor. People say a FF has less DOF because they are assuming using a longer FL lens, or a shorter working distance, in order to keep the captured image "the same."

RE crop sensor and "furry things." It comes down to how many pixels are on the captured area (and sensor saturation/light). I can crop images from my 36mp D800 to DX size and have the "same image" as if I had shot with the 16MP D7000. (In fact, I often use my D800 in DX mode). I will have more pixels (detail) w/ a D7000 compared to cropping down my 16MP D4.

BUT, it's not quite that simple because we are forgetting about the lens. In order to have the benefit of the "crop factor" the lens must be able to resolve the image details at the pixel size. And there are other negatives to smaller pixels (crop factor)...

A longer lens on larger pixels is "the better answer." But it's also a lot more expensive.

 

[Keeper]

Is any of this affected by how furry the furry thing is?

 

[beyond the blue]

Is any of this affected by how furry the furry thing is?

Of course it is! That's obvious!

 

[HoppyUK]

DOF is affected by only 3 things:
The aperture, the subject distance, and the focal length.
Sensor size is not a direct factor. People say a FF has less DOF because they are assuming using a longer FL lens, or a shorter working distance, in order to keep the captured image "the same."

RE crop sensor and "furry things." It comes down to how many pixels are on the captured area (and sensor saturation/light). I can crop images from my 36mp D800 to DX size and have the "same image" as if I had shot with the 16MP D7000. (In fact, I often use my D800 in DX mode). I will have more pixels (detail) w/ a D7000 compared to cropping down my 16MP D4.

BUT, it's not quite that simple because we are forgetting about the lens. In order to have the benefit of the "crop factor" the lens must be able to resolve the image details at the pixel size. And there are other negatives to smaller pixels (crop factor)...

A longer lens on larger pixels is "the better answer." But it's also a lot more expensive.

Sensor size is a direct factor, because that dictates lens focal length. If focal length is not adjusted according to sensor size, then you end up with a completely different picture in terms of framing so no meaningful comparison can be made.

You will also notice on DoF Master.com that sensor size is part of the input data panel.

 

[woof woof]

This might help.

I have FF and MFT cameras, MFT being x2 crop so it makes the numbers easier

The first shot is a 5D shot of the dragon on my window ledge and what's beyond taken with a 5D fitted with a 50mm at f5.6. The next shot is from my MFT fitted with a 50mm lens set to f5.6.





and just for fun here's the 5D shot cropped to give about the same view as the MFT shot.

 

[Keeper]

This might help.

I have FF and MFT cameras, MFT being x2 crop so it makes the numbers easier

The first shot is a 5D shot of the dragon on my window ledge and what's beyond taken with a 5D fitted with a 50mm at f5.6. The next shot is from my MFT fitted with a 50mm lens set to f5.6.





and just for fun here's the 5D shot cropped to give about the same view as the MFT shot.

Yes that helps a lot.

Now, someone correct me if I've called this wrong. But if we assume the FF and MFT cameras here have the same pixel count then the latter has crammed then into a smaller area thus giving the extra 'reach' (or perceived reach). While the FF crop achieves the same image as the MFT with less pixels and so theoretically less detail. Simplification I know but is this right?

 

[sk66]

Sensor size is a direct factor, because that dictates lens focal length. If focal length is not adjusted according to sensor size, then you end up with a completely different picture in terms of framing so no meaningful comparison can be made.

Yes. But, when talking about action photography (sports, wildlife, etc) the choice for using DX (or a longer lens) is to CHANGE the image captured (i.e. larger subject).

Using a longer lens will change the DOF (reduce). Using a crop body will not (not at the pixel level).

You will also notice on DoF Master.com that sensor size is part of the input data panel.

Yes (and I believe you know this) but it's actually the "circle of confusion" which is essentially the pixel's relative size when displayed/viewed. The calculators "assume" that the captured images will be used "as is" and not that the FF image will be cropped first.

The COC for a D800 DX mode/crop image (~16MP) is roughly the same as for a D7000 image (16MP DX sensor)

 

[sk66]

Yes that helps a lot.

Now, someone correct me if I've called this wrong. But if we assume the FF and MFT cameras here have the same pixel count then the latter has crammed then into a smaller area thus giving the extra 'reach' (or perceived reach). While the FF crop achieves the same image as the MFT with less pixels and so theoretically less detail. Simplification I know but is this right?

Yes, assuming the same pixel count. But that is seldom the case.

 

[Keeper]

Yes, assuming the same pixel count. But that is seldom the case.

Yes abolsutely, and I think that's what causes a lot of confusion. A FF camera with a higher pixel count won't have less 'reach' than a lower pixel DX camera. With so many combinations of sensor size, pixel count and lens size any generalisations or comparisons need to be put firmly into context I think.

 

[PickleB]

I thought I understood crop factors completely, but after reading up a lot on Nikon DX vs FX lenses (am thinking of buying or renting an FX lens for a DX body), I'm a bit confused again.

So my question is very simple. Assuming that the images in this link are taken with an FX lens on an FX body, if I put the same lens on a DX body (all other parameters unchanged), would the red house appear 1.5x larger?

Thank you.

Try the Nikon Lens Simulator: link...choose a scene, your lens focal length and its format, then you can swap the body format to see the effect on fov and image selection.

 

[sk66]

The COC for a D800 DX mode/crop image (~16MP) is roughly the same as for a D7000 image (16MP DX sensor)

In reality the final "perceptual DOF" comes down to pixel size; and it has gotten to the point that there is so much variability w/in the FF/DX sensors that any "generalization" is erroneous.

 

[PickleB]

Try the Nikon Lens Simulator: link...choose a scene, your lens focal length and its format, then you can swap the body format to see the effect on fov and image selection.

And...a tutorial that seems to explain this and other related issues in a way that I can understand: link.

 

[HoppyUK]

In reality the final "perceptual DOF" comes down to pixel size; and it has gotten to the point that there is so much variability w/in the FF/DX sensors that any "generalization" is erroneous.

In practise, DoF has very little to do with pixels. The DoF standard is not very demanding on that score and every camera has way more than the minimum necessary to satisfy the primary CoC requirement - like under 1mp.

 

[HoppyUK]

Yes. But, when talking about action photography (sports, wildlife, etc) the choice for using DX (or a longer lens) is to CHANGE the image captured (i.e. larger subject).

Using a longer lens will change the DOF (reduce). Using a crop body will not (not at the pixel level).


Yes (and I believe you know this) but it's actually the "circle of confusion" which is essentially the pixel's relative size when displayed/viewed. The calculators "assume" that the captured images will be used "as is" and not that the FF image will be cropped first.

The COC for a D800 DX mode/crop image (~16MP) is roughly the same as for a D7000 image (16MP DX sensor)

Okay. And I know you know this but if any of the four parameters defining DoF are altered (focal length, shooting distance, lens aperture and sensor size), then DoF will change. The only fixed parameter assumed by DoF calculations is that the final print/image should be viewed from a distance equal to the diagonal length of the print.

Which leaves us with only one relevant and meaningful comparison, which is - a photo taken from the same position (perspective constant), with the same framing (focal length adjusted according to sensor size), and same lens aperture. And in that case, the larger format delivers less DoF.

 

[PickleB]

...a photo taken from the same position (perspective constant), with the same framing (focal length adjusted according to sensor size), and same lens aperture. And in that case, the larger format delivers less DoF.

Using a focal length of 50 mm and a DSLR with CF of 1.5 compared to a focal length of 75 mm and full frame on the calculator (link) confirmed this for me. (Have a play with the advanced mode.)

And there's a tutorial too: link.

 

[insomniac]

Thanks a lot for the information. That Nikon app is great.

 

[sk66]

And in that case, the larger format delivers less DoF.

Yes, anytime you use the two formats to accomplish "the same thing" (basically) then the larger format has less DOF. In the example you gave it is the change in focal length that caused the difference in DOF, not the sensor size. The sensor size "required" the change in focal length.... Tomaeto-Tomahto.

But most do not buy a longer lens or choose DX so that they can work from further away in order to capture "the same thing." At least not when we're talking about action/wildlife/sports photography. When we talk about using them to do "something different" then the rule fails. In fact, if you use a DX thru the same lens (only change sensor size) you wind up with less DOF because you have changed the lens' effective focal length (FOV).

I still think it's the three factors and sensor size only causes/requires the change in (effective) focal length.

 

[sk66]

In practise, DoF has very little to do with pixels. The DoF standard is not very demanding on that score and every camera has way more than the minimum necessary to satisfy the primary CoC requirement - like under 1mp.

Doesn't the detail have to be recorded as sharp to start with? And doesn't pixel size/density/airy disk etc influence that?

Pixel count/relative to sensor size is not of much influence because if you have more pixels of smaller size, then they require less enlargement to create a print of a given size. It's basically a 1:1 ratio. That's why the D800 and D4 (36/16MP FF) have the same COC.

 

[woof woof]

In the example you gave it is the change in focal length that caused the difference in DOF, not the sensor size. The sensor size "required" the change in focal length.... Tomaeto-Tomahto.

Yes, and you can achieve DoF perception very similar to that of FF with a smaller format even if using the same aperture setting if you are willing to alter the FoV by using longer lenses or reducing the camera to subject distance.

 

[HoppyUK]

Doesn't the detail have to be recorded as sharp to start with? And doesn't pixel size/density/airy disk etc influence that?

Pixel count/relative to sensor size is not of much influence because if you have more pixels of smaller size, then they require less enlargement to create a print of a given size. It's basically a 1:1 ratio. That's why the D800 and D4 (36/16MP FF) have the same COC.

In order to perceive DoF changes, there obviously has to be a clear visible difference between the sharp and unsharp areas, but my point about pixels being largely unimportant is simply that even the most basic cameras have far more resolution/pixels than the DoF standard requires. In terms of basic resolution, the DoF standard needs only 0.96mp (CoC 0.03mm on full-frame).

Any on-screen image proves this, because the most that any VDU can show is somewhere between 1mp and 2mp max. That takes us into another area, where perceived sharpness is more about image contrast than resolution, but we've become so brainwashed with megapixels these days that this gets forgotten.

Yes, and you can achieve DoF perception very similar to that of FF with a smaller format even if using the same aperture setting if you are willing to alter the FoV by using longer lenses or reducing the camera to subject distance.

Changing the field of view, by using a longer lens, and then moving closer...

Depth of field may be similar in that case, but the picture will be completely different. Invalid comparison.

 

[woof woof]

Changing the field of view, by using a longer lens, and then moving closer...

That's not what I wrote. What I actually wrote was...

Yes, and you can achieve DoF perception very similar to that of FF with a smaller format even if using the same aperture setting if you are willing to alter the FoV by using longer lenses or reducing the camera to subject distance.

...but if you want to go with what you thought I wrote ... Yes. If you use a longer lens and move closer you'll get what looks like less DoF.

Depth of field may be similar in that case, but the picture will be completely different. Invalid comparison.

Only in your mind and as I've said before, you have a very rigid Taliban like view on this. There are choices and "Invalid comparison" isn't carved in stone anywhere and the police don't enforce this law and (I don't know why I'm wasting my time saying this to you again... except in an effort to be clear to the OP and anyone else who doesn't have easy access to different sensor sized cameras) if you are willing to change your FoV you can get shallow DoF from smaller systems by using longer lenses or reducing your camera to subject distance. That's not an invalid statement, an invalid comparison or an invalid choice. It's a choice which the photographer is free to make. Yes, it'll be a different picture but which is the better picture is down to personal choice.

 

[HoppyUK]

Thank you for the clarification, now that it's clear you're comparing apples and oranges.

 

[sk66]

In order to perceive DoF changes, there obviously has to be a clear visible difference between the sharp and unsharp areas, but my point about pixels being largely unimportant is simply that even the most basic cameras have far more resolution/pixels than the DoF standard requires. In terms of basic resolution, the DoF standard needs only 0.96mp (CoC 0.03mm on full-frame).

Any on-screen image proves this, because the most that any VDU can show is somewhere between 1mp and 2mp max. That takes us into another area, where perceived sharpness is more about image contrast than resolution, but we've become so brainwashed with megapixels these days that this gets forgotten.

I thought COC was based upon image resolution (5 lp/mm) and the "standard" was .01"/.25mm (often quoted as .2mm) and that this then gets divided by the magnification to generate a given print size. That works out to .03mm on FF for a standard print/viewing size. (because the resolution (lp/mm)requirement is presumed to go down due to longer viewing distances. It also ignores other variables such as individual visual acuity w/ 20-20 vision being more like 7 lp/mm, viewing conditions, etc etc)

I've never heard it converted into .96MP, but it makes sense given a D4 (16MP FF) has a pixel size of .0073mm (much smaller than the COC standard).


In the same discussion one could say that FL and distance are *not* factors in DOF. If we presume "capturing the same thing" they effectively negate each other. In which case the only factors we really have are the COC and aperture.
This is probably technically true, but more "confusing" than helpful IMO.

In a "practical application" I still think that (effective)FL/subject distance/and aperture are the factors (w/ sensor size causing/requiring the FL change) makes the most sense. Because these are the factors we have control over... and it even explains why (only) changing to a DX sensor (increasing the effective FL) results in less DOF. And if you understand these three factors all of the DOF calculators and other interactions make sense.

 

[HoppyUK]

I thought COC was based upon image resolution (5 lp/mm) and the "standard" was .01"/.25mm (often quoted as .2mm) and that this then gets divided by the magnification to generate a given print size. That works out to .03mm on FF for a standard print/viewing size. (because the resolution (lp/mm)requirement is presumed to go down due to longer viewing distances. It also ignores other variables such as individual visual acuity w/ 20-20 vision being more like 7 lp/mm, viewing conditions, etc etc)

I've never heard it converted into .96MP, but it makes sense given a D4 (16MP FF) has a pixel size of .0073mm (much smaller than the COC standard).


In the same discussion one could say that FL and distance are *not* factors in DOF. If we presume "capturing the same thing" they effectively negate each other. In which case the only factors we really have are the COC and aperture.
This is probably technically true, but more "confusing" than helpful IMO.

In a "practical application" I still think that (effective)FL/subject distance/and aperture are the factors (w/ sensor size causing/requiring the FL change) makes the most sense. Because these are the factors we have control over... and it even explains why (only) changing to a DX sensor (increasing the effective FL) results in less DOF. And if you understand these three factors all of the DOF calculators and other interactions make sense.

I only mention the 'conversion' to 0.96mp to make the point about how few pixels are needed to meet the DoF standard and to see visible changes, and therefore having loads more makes no difference in this debate (it's simply 36/0.03mm to get some kind of comparison). But on the other hand, it begins to ask more questions about the whole nature of what we call 'sharpness' and how little it has to do with simple resolution, but this is not the place to get into MTF.

And for sure we could take issue around things like visual acuity, but the key thing about the DoF standard is that it's universally recognised, has been for decades, so at least we know where we are with it.

I agree that focal length and distance are two halves of the same coin, and both are aspects of magnification that effectively cancel each-other out - but then distance changes perspective, so that throws things as far as direct comparisons go. And in the same vein, you could also say that aperture is also 'magnification' too, in that it's the physical diameter that matters. All getting a bit too deep and confusing.

I'll take issue with your last paragraph though, where you're discounting sensor size as a practical variable. It was never really considered when shooting film, if only because we never used the same lenses on different format cameras. But today we do, I do it regularly (having both a 5D2 and 7D) as do many others, and it always comes up the moment anyone mentions crop factor. And then you've thrown something else in at the end, that (although perfectly true) 'appears' to the uninitiated to contradict the understanding that smaller sensors deliver more DoF

 

[sk66]

I'll take issue with your last paragraph though, where you're discounting sensor size as a practical variable.

I'm not; I'm including it in "effective focal length." And to go just a little further one could (should) even consider cropping in post as a change to effective focal length, just as sensor size (crop factor) does.

I agree that focal length and distance are two halves of the same coin, and both are aspects of magnification that effectively cancel each-other out - but then distance changes perspective, so that throws things as far as direct comparisons go.

And distance also does not completely negate FL magnification because you haven't equally(proportionally) increased the distance to the BG. Nor does it negate the FOV change.
So I agree, (effective)FL/distance/aperture are the considerations in creating a picture (along w/ exposure).