FX lens on DX body - impact on aperture not just field of view

[DennisS]

Until I saw these videos, I believed that the only impact on using an FX lens on a DX body was the apparent reduction in the field of view (crop). This video suggests that the aperture is also impacted - meaning a 2.8 becomes a 4.2 on a Nikon DX body.

https://www.youtube.com/watch?v=f5zN6NVx-hY

https://www.youtube.com/watch?v=YDbU...WTY6O&index=81

I own a 60mm FX f/2.8 and a DX body and was about to drop some cash on the 85mm f/1.8 hoping to achieve that beautiful bokeh I see all over the web from people who used that lens on a full frame. Now am not sure I want to make that leap just yet - maybe just upgrade my body to full frame eventually....!!!

[Manfred M]

Dennis - the video is absolutely correct; the impact on depth of field (DoF) is impacted when you use are shooting the same lens on a crop frame sensor will impact both the apparent focal length as well as the depth of field. Regardless, you are quite correct; putting that 85mm f/1.8 lens on the DX body is not going to give you the same shallow DoF as it will on an FX body.

If you want to look at different scenarios, then use the CiC DoF calculator to work things out. The band of acceptable sharpness in front of and behind the focus plane does vary, based on a number of different parameters.

https://www.cambridgeincolour.com/tu...calculator.htm


This impact is one of the main reasons I went from a crop factor camera to a full frame one.

[William W]

. . . I believed that the only impact on using an FX lens on a DX body was the apparent reduction in the field of view (crop). This video suggests that the aperture is also impacted - meaning a 2.8 becomes a 4.2 on a Nikon DX body. . . I own . . . a DX body and was about to drop some cash on the 85mm f/1.8 hoping to achieve that beautiful bokeh . . . maybe just upgrade my body to full frame eventually....!!!

I think to understand exactly what happens, it is important to articulate precisely.

I think you understand the principle that the IMAGE CIRCLE or the FIELD of VIEW of the LENS does not change. But when an FX Lens is used on a DX Body the smaller sensor simply records a smaller area of the Lens’s Image Circle.

Similarly, the APERTURE of the Lens does not change: what does change is the computation for the Depth of Field and that is because the Circle of Confusion differs dependent upon the film or sensor size.

Also, firstly it is important to recognise that BOKEH is related to, but is NOT the same as Depth of Field. Secondly BOKEH has many elements that make it so – and those are not just limited to the lens. So you may still acquire the beautiful BOKEH using any particular lens on a DX format camera.

But yes, as a general comment, many Portrait Photographers will today specifically choose 135 Format DSLRs (aka ‘full frame cameras”) for their work, because of three reasons:
> Very VERY fast, very VERY high quality lenses
> Very VERY shallow DoF is possible
> Very affordable cost

On the other hand a Sports Photographer might choose an APS-C body with a very fast "Full Frame Lens" simply to leverage the smaller resultant Field of View - but because there is NOT any actual change in the Aperture, that Photographer will still have the (very close to)(*1) the same ability to choose Shutter Speeds to arrest Subject Motion.

***

When comparing the outcomes form different CAMERA FORMAT SIZES there are also other factors. This is the area of "Equivalence in Photography" and if you want to study that Subject in more detail, this is a good place to begin.


WW

Footnote (*1): "very close to" - when comparing two Final Images which are made by the same Lens and the same Shutter Speed of the same Moving Subject and both Full Frame Crop Images are viewed at the same Viewing Distance and the same Enlargement - because of the reduced Field of View on an APS-C Camera there will be situations where there is detectable SUBJECT MOTION BLUR in the APS-C image and not the other Image.

[DennisS]

Thank you Manfred and Bill for the simple and concise explanations...!!!

what does change is the computation for the Depth of Field and that is because the Circle of Confusion differs dependent upon the film or sensor size. WW

That explains a lot...!!!!

[William W]

Hi Dennis,

You're welcome.

Whilst you were replying I edited my previous and added another bit about Shutter Speeds.

If one wants to delve it can be quite a complex matter comparing all the digital Formats apropos the OUTCOMES that one can achieve with one an not as easily with another: there are many considerations when making comparisons.

*

Also often many comments about using smaller formats seem, on first glance, counter intuitive. For example, with a P&S or Bridge Camera with a Varying Maximum Aperture Lens, if one wants a the shallowest DoF for a Portrait - it is firstly best to make the Portrait a Tight Portrait and secondly it is best to use the Wide Angle end of the zoom Lens to make the lens work at the LARGEST APERTURE possible. Whilst many will jump to conclusion that the shallowest DoF will be attained by using the telephoto end of the lens, because they assume that there will be a shallower DoF, because using Telephoto Lens simply make it so - they forget about the Aperture changing.

WW

[DennisS]

another nice tidbit Bill..!!!! Good to know. The more I read this, the more am leaning towards a Full Frame body but am also thinking that buying an FX lens now would be an investment for the future anyway - $500 now for the 85mm FX (not too expensive considering my 60mm FX macro cost 50% more) and down the road put down some dough for the FF body..!!!

[William W]

. . . both Nikon and Canon make a very good, very affordable 85/1.8 Lens. (I have a Canon EF 85/1.8)

I am sure that you will be overjoyed with the Nikon 85/1.8 - it will give you heaps of pleasure on your current DX body.

WW

[george013]

I allways get confused with this kind of comparisons.

I understand you're talking of comparisons with the same distance. That's what the video does. But if you shoot portraits you're interested in the same image.

Dennis owns a DX body. Except that you lose the less image quality at the borders of the image circle using a FX lens, a 100mm DX lens and a 100mm FX lens will act the same on the same body.

George

[Mark von Kanel]

Hi Dinnis, Iwent full frame for exactly this reason and its worth it but then i shoot a lot of people stuff.... take your body into the camera store and try the lens.... Still waiting for a fas dx from nikon for sports though.....but thats another thread!

[William W]

I allways get confused with this kind of comparisons.

Maybe this will assist.

*

. . .I understand you're talking of comparisons with the same distance. That's what the video does. But if you shoot portraits you're interested in the same image.

Indeed. And if one compares 'the same image' (I assume you mean same FRAMING), using the same lens on each of the camera bodies, then the DoF will be GREATER when using the DX camera.

Example: Use 100mm lens on FX and DX and make the 'same image' (i.e. same FRAMING)

Lens = 100mm
Aperture = F/2.8
Body FX
Subject Distance = 5000mm
FoV = 1764mm x 1176mm
CoC = 0.025mm
DoF = 347mm smaller DoF


Lens = 100mm
Aperture = F/2.8
Body DX
Subject Distance = 7575mm
FoV = 1764mm x 1181mm
CoC = 0.016mm
DoF = 526mm bigger DoF

Note the two variables when we change the CAMERA BODY - the CoC changes, but also we have to step back farther away from the Subject to make the same FRAMING – the SUBJECT DISTANCE is much greater when we use a DX body and a 100mm lens, to make ‘the same image’.

***

Except that you lose the less image quality at the borders of the image circle using a FX lens, a 100mm DX lens and a 100mm FX lens will act the same on the same body.

No. That's not always absolutely correct, either. Many lenses are specifically made to best suit one camera format or another. It is not just as simple as saying that using an "FX" Lens on a "DX" body that we will get rid of the poorer Image Quality at the edges.

It might not be seen unless one is critically looking, but, just as one example, aberrations such as Chromatic Aberration and Spherical Aberration might be more obvious when an "FX" Lens (or EF Lens) is used on an APS-C Camera Body, than when the same lens is used on an FX Camera Body.

WW

[george013]

Maybe this will assist.

*



Indeed. And if one compares 'the same image' (I assume you mean same FRAMING), using the same lens on each of the camera bodies, then the DoF will be GREATER when using the DX camera.

Example: Use 100mm lens on FX and DX and make the 'same image' (i.e. same FRAMING)

Lens = 100mm
Aperture = F/2.8
Body FX
Subject Distance = 5000mm
FoV = 1764mm x 1176mm
CoC = 0.025mm
DoF = 347mm smaller DoF


Lens = 100mm
Aperture = F/2.8
Body DX
Subject Distance = 7575mm
FoV = 1764mm x 1181mm
CoC = 0.016mm
DoF = 526mm bigger DoF

Note the two variables when we change the CAMERA BODY - the CoC changes, but also we have to step back farther away from the Subject to make the same FRAMING – the SUBJECT DISTANCE is much greater when we use a DX body and a 100mm lens, to make ‘the same image’.

***



No. That's not always absolutely correct, either. Many lenses are specifically made to best suit one camera format or another. It is not just as simple as saying that using an "FX" Lens on a "DX" body that we will get rid of the poorer Image Quality at the edges.

It might not be seen unless one is critically looking, but, just as one example, aberrations such as Chromatic Aberration and Spherical Aberration might be more obvious when an "FX" Lens (or EF Lens) is used on an APS-C Camera Body, than when the same lens is used on an FX Camera Body.

WW

Thanks for your answer. A logical list. It's coming up again.

I presume that the different aberrations you ment are dependent on the pixelsize/pixeldensity, not that much on the sensorsize.


George

[William W]

You're welcome.

Does the comment, "A logical list. It's coming up again" - mean that you understand?

*

. . .I presume that the different aberrations you ment are dependent on the pixelsize/pixeldensity, not that much on the sensorsize.

Yes and No.

The Pixel Size and Density have some relationship to the Sensor Size, to maximize the final Image Quality within the Image File.

WW

[jcuknz]

"...because the Circle of Confusion differs dependent upon the film or sensor size."

That does not ring true to me as the circle of confusion remains the same but only changes dependent on the degree of subsequent enlargement. CoC is a physical fact unchanged by what it is recorded on, but its size and acceptability depends of the degree of enlargement ... why what may look good in the LCD is horrible at A3

[William W]

"...because the Circle of Confusion differs dependent upon the film or sensor size."
That does not ring true to me as the circle of confusion remains the same but only changes dependent on the degree of subsequent enlargement. CoC is a physical fact unchanged by what it is recorded on, but its size and acceptability depends of the degree of enlargement ... why what may look good in the LCD is horrible at A3

For application to Depth of Field calculations, the CoC was worked our backwards (it seems) originally from mere assumptions of visual acuity and historically based on a 10" x 8" print made from a full frame crop of the negative and then held at about arm's length (about 2 1/2) times the diagonal of the print.

So, yes even from the beginning of thinking about "Depth of Field", there was a direct relationship between the CoC and the sensor/negative size; because the smaller the Sensor (or Negative) the greater the enlargement required to make that 10" x 8" print.

***

There have been many other and more scientific based calculations for the CoC and the Zeiss Formula is probably the most commonly known, and all, including the Zeiss Formula, do take into account the Camera Format/Negative size.

***

Note well that if using an (older) lens that was designed for a 135 Format Film SLR on a modern APS-C DSLR, the notations for calculating DoF on the Lens Barrel and the Focus Turret will be: wrong, by virtue of the fact that the lens's DoF markings are for 135 Format and NOT for APS-C Format.

The same applies when using Lenses made for 6x6 or 645 cameras, on DSLR's.

***

All DoF scales and the (now) downloadable DoF applications require the Sensor/negative size to be put in for the DoF calculation – that sensor size data simply changes the CoC in the DoF formula.

Example is here below – note the drop down menu selection and the CoC change:







***

Don't confuse the application of Circle of Confusion for purposes of Depth of Field calculations - with the definitive measure of the smallest blur spot achieved by a lens at the lens’s point of best focus.

Traditionally the term: Circle of Least Confusion is used for the latter, to avoid this confusion.

WW

[Glenn NK]

Very good thread with some very good information from Bill (William W). Thanks Bill.

[William W]

^ ta, you're most welcome.

[xpatUSA]

Note well that if using an (older) lens that was designed for a 135 Format Film SLR on a modern APS-C DSLR, the notations for calculating DoF on the Lens Barrel and the Focus Turret will be: wrong, by virtue of the fact that the lens's DoF markings are for 135 Format and NOT for APS-C Format.

Thank you, Bill, for reminding us that DX and FX are Nikon's trade descriptions which somehow get used here as if all lenses are so described

[William W]

<Pardon please Off Topic and at times terribly flippant, just for a moment>

ta, Ted -

haha! - that's my habit of: PATTERN

Pedantic Attention To Transcribing & Entering Reliable Nomenclature

Nikon's "FX" and "DX" is not the worst case . . . don't get me started on "EF" and "EF-S" Bayonet Mounts for Canon Lenses - (aka "Full Frame" and "Crop Lenses") . . . there's so much confusion and misunderstanding on that playground and it gets worse when the 'third party crop lenses' are referred to as 'EF-S Lenses'.

Next week we can discuss:
"Protection Filters and why every lens should have one"
"The must have one and only best Portrait Lens"
"P Mode - never use it because it is the same as Full Auto"
"The must have one and only Landscape Lens"
"Get the 50/1.8 because it is so cheap - even if you never use it, it's still great value for money"

< END Off Topic >

[george013]

"...because the Circle of Confusion differs dependent upon the film or sensor size."

That does not ring true to me as the circle of confusion remains the same but only changes dependent on the degree of subsequent enlargement. CoC is a physical fact unchanged by what it is recorded on, but its size and acceptability depends of the degree of enlargement ... why what may look good in the LCD is horrible at A3

CoC is not a physical fact, it's a variable factor for calculation.

Starting point is that a black point of 0.2mm is visible as a point at a certain distance. If you want a print of 35x24cm from a FF sensor,35x24mm,, then you have enlarged the sensor by a factor of 350/35=10. So that point must not be larger as 0.2/10=0.02mm on your sensor. With different sensor sizes you get different elargement factors.
When you multiply that viewing distance with x, than you can multiply that black point by x and so the CoC.
When you multiply the basic enlargement by y, you have to divide your acceptable black point with 10y for that basic distance.

George

[jcuknz]

You have not changed my mind, just tried to bamboozle me with a lot of totally irrelevant information. Par for the course IMO