Fullframe lenses - softer on crop sensor bodies

[Ysarex]

f the SAME height, despite different sensor sizes and different lens lengths and maximum aperture values.

So if I took a lens, projected the image on the wall and then drew a rectangle out on the wall and labeled it "image area" the depth of field would change within the rectangle?

Not a correct usage of DOF. DOF and how it is calculated is well defined by the industry. I.e. DOF is not calculated at the film plane.

Calculations for DOF require a value for the circle of confusion: Circle of confusion - Wikipedia, the free encyclopedia

As I said, the CoC is calculated based on the size of the film or sensor. Show us the math that says otherwise, otherwise here is a simple proof:

View attachment 117769

Where H = hyper focal distance, f = focal length, N = f stop, and c = circle of confusion. The formula requires a value for c.

Reference: Hyperfocal distance - Wikipedia, the free encyclopedia

Joe

You're absolutely right. And come to think of it, it makes sense.

However, my math is still suggesting that magnification is a significantly greater factor until you get into higher magnifications.

Oh hey, I didn't say anything about how important a factor sensor size is -- just that it's a factor. Absolutely magnification is the dominant factor in controlling DOF -- you are correct. Reduced to most simple form DOF is a function of magnification and f/stop. Magnification expands to a fairly complex component of the equation that includes subject distance, focal length and ultimately enlargement. Sensor size plays it's role when you expand the factor enlargement.

Joe

 

[Alexr25]

DOF stays the same also. Working distance changes and this affects focus distance, which gives the impression that DOF changes.

As others have pointed out this assertion is wrong.

Cropping the image circle doesn't change the lens, only how the lens is used.

You got this right but your conclusion is wrong because you in common with many people assign depth of field to be a property of the lens. IT IS NOT. Depth of field is a property of human vision.
A lens has only one plane of focus and anything that is not in that plane will to some extent out of focus.
Depth of field is perceived by the viewer because human vision is not perfect and any object that subtends less than approx of 1 minute of arc is perceived as a point. This is why a photo may look sharp printed as a postcard sized image but look horrible when blown up to poster size.
Some people point to the depth of field markings you see on some lenses to argue that DoF is a lens property but those markings assume you will be viewing an 8" x 10" uncropped print of the full image at a distance of 1 foot, at any other print size or viewing distance the perceived depth of field will not match the lens markings.

 

[Ysarex]

DOF stays the same also. Working distance changes and this affects focus distance, which gives the impression that DOF changes.

As others have pointed out this assertion is wrong.

Cropping the image circle doesn't change the lens, only how the lens is used.

You got this right but your conclusion is wrong because you in common with many people assign depth of field to be a property of the lens. IT IS NOT.

Careful here. You don't want to entirely disassociate the lens (lens focal length) from DOF. Each lens focal length has a unique relationship with DOF and plays a determinant role in calculating DOF. In the math equations that are used to calculate DOF one of the required variables is lens focal length. If the lens doesn't have anything to do with it then it wouldn't be a variable in the math formulas.

Joe

Depth of field is a property of human vision.
A lens has only one plane of focus and anything that is not in that plane will to some extent out of focus.
Depth of field is perceived by the viewer because human vision is not perfect and any object that subtends less than approx of 1 minute of arc is perceived as a point. This is why a photo may look sharp printed as a postcard sized image but look horrible when blown up to poster size.
Some people point to the depth of field markings you see on some lenses to argue that DoF is a lens property but those markings assume you will be viewing an 8" x 10" uncropped print of the full image at a distance of 1 foot, at any other print size or viewing distance the perceived depth of field will not match the lens markings.

 

[Overread]

If you want a good practical example try macro. The working distances at 1:1 is identical if you use the same lens on crop and fullframe; however the depth of field should be more apparently different when you compare fullframe to crop sensor.

 

[astroNikon]

So ... just to throw some oil on the fire in regards to "magnification"

Does DOF change on an APS-C sensor if it is a 24 megapixel versus a 6 mpixel sensor of the same area, though highly different pixel density ?

 

[Derrel]

Depth of field is a fairly complex subject. The ***best*** writer on the net, as far as clearly and correctly explaining the issue is Bob Atkins. Period.

Something to keep in mind is that depth of field, the area of acceptably sharp focus, is NOT the same, exact thing as background blur. Background blur, the magnitude of the blur in the background, increases with the physical SIZE of the lens aperture in an ABSOLUTE sense. Take a look at the teaching illustration below at the link compare2.jpg. It has three photos, each which show the same depth of field, and each photos has the same object height and width within the picture frame area--but because the lenses used were of different focal lengths, the physical size, the actual width of the aperture opening inside the lens, was progressively larger and larger, and the longer the lens, the higher the degree/magnitude of background blurring.

The three photos were all made at f/2.8. The 50mm lens had a 17.8mm aperture width at its f/2.8 setting; the 85mm lens had an aperture width of 30.4mm at its f/2.8 setting; and the 135mm lens had an f/2.8 aperture that measured 48.2mm in width. So, a 50mm, an 85mm, and a 135mm lens, SAME MAGNIFICATION of the target subject, same depth of field region as seen in an actual photo--yet significantly more background blur the longer the lens length.

The degree of background blurring is something many people seem fixated on, enthralled by--this is why there are many fashion/portrait/lifestyle shooters who favor LONG focal length and very WIDE-aperture lenses, lenses which have physically very wide actual aperture openings: lenses like the 85mm f/1.2, 135 f/2 and 135mm f/1.4, 200mm f/2, and 300mm f/2.8 and 400mm f/2.8. The magnitude of background blurring is something many people really seem to be concerned with--in some types of photograpy, and Tony Northrup seems especially fixated on it as some type of measuring stick.

Again...background blur is not the same, exact "thing" as depth of field. When you want more and ore and more background blurring, the easiest way to get that is to use a longer focal length, one in which the massive lens diameter openings at each f/stop value are significantly larger than with short focal length lenses.

compare2.jpg


^^^^As Bob Atkins wrote in the caption for the above image: "As you would expect, at the same magnification, the faster the lens, the smaller the depth of field. This also means that the background close to subject will be blurred more by the faster lens. In this case the 50mm lens at f1.4 gives slightly greater blurring for objects up to about 1m behind the subject in focus. However as you go further back, the lens with the largest physical aperture starts to show the most blur, and by the time you're at infinity, the 135mm lens at f2 lens will give almost twice as much blurring (actually 1.9x as much). The following images show the effect quite clearly. All three were shot to produce the same magnification of the camera box and so have essentially the same depth of field (region of "acceptably sharp" focus), but the image shot with the larger physical aperture (longer focal length) lens shows the greatest degree of background blur."

Now....we're talking here about the magnitude of the background blurring--not the DOF, but something else. Bob's second article, and his software application bacgroundblur.exe can be downloaded from this page: Bokeh and Background Blur Calculator- Bob Atkins Photography

 

[DarkShadow]

All this technical crap gives me a migraine.I like to spend less time thinking about full frame vs crop or FX vs DX glass and spend more time thinking about shooting and Improving. I am happy as a clam with my D7200 and slow sigma 150-600mm and my slow 18-140 kit lens for general walk around things.

 

[Ysarex]

So ... just to throw some oil on the fire in regards to "magnification"

Does DOF change on an APS-C sensor if it is a 24 megapixel versus a 6 mpixel sensor of the same area, though highly different pixel density ?

No.

Joe

 

[unpopular]

So ... just to throw some oil on the fire in regards to "magnification"

Does DOF change on an APS-C sensor if it is a 24 megapixel versus a 6 mpixel sensor of the same area, though highly different pixel density ?

No.

Joe

Now wait!! If you have greater resolution shouldn't the minimum circle size be smaller? If so, wouldn't image resolution present a maximum DOF regardless of viewing resolution????

 

[unpopular]

(or does this only apply at 1:1 enlargements?)

 

[Ysarex]

So ... just to throw some oil on the fire in regards to "magnification"

Does DOF change on an APS-C sensor if it is a 24 megapixel versus a 6 mpixel sensor of the same area, though highly different pixel density ?

No.

Joe

Now wait!! If you have greater resolution shouldn't the minimum circle size be smaller? If so, wouldn't image resolution present a maximum DOF regardless of viewing resolution????

By minimum circle do you mean circle of confusion? No. Circle of confusion is not a function of the resolution of the recording medium. Think film. Back in the 80s there was a short fad to shoot 35mm recording film -- super high resolution but with a dynamic range of maybe 3 stops. DOF between recording film and Tri-X didn't change because of the res difference. Remember DOF is calculated to a standard output print size. You couldn't make a DOF comparison between Tri-x printed 8x10 with recording film printed 30x40. Same goes for digital today.

Joe

 

[unpopular]

What the what?

The circle of confusion has nothing to do with the recording medium nor the enlargement. From Wikipedia:

In optics, a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source.

This is how it has always been described to me. What you seem to be referring to is depth of field, or region of "acceptable sharpness". Again, wikipedia:

The depth of field is the region where the CoC is less than the resolution of the human eye (or of the display medium).

So essentially what you'd have in a theoretically perfect lens is a perfect cone of light being focused onto an infinitely small point on the film plane:


(courtesy Wikipedia, public domain: File:Cirles of confusion lens diagram.png - Wikimedia Commons)

If the circle of confusion as defined is smaller than the resolving power of the system in whole, including [reluctantly] viewing conditions, we can call that area "sharp" or "in focus" because we no longer can see the individual circles being projected.

If this is the case, then certainly the resolving power of the medium must have an impact to some extent. If the film or sensor cannot resolve a circle diameter difference between one working distance and another, provided fixed focal distance, then both distances will be recorded essentially the same when looking at diameter alone and thus nothing about depth.

I'm also having some issue with the concept of "enlargement" with digital photography in regard to sensor size, and I am questioning if you can make the same type of comparisons as you can with film ... but more on that later.

 

[Ysarex]

What the what?

The circle of confusion has nothing to do with the recording medium nor the enlargement. From Wikipedia:

In optics, a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source.

This is how it has always been described to me. What you seem to be referring to is depth of field, or region of "acceptable sharpness". Again, wikipedia:

The depth of field is the region where the CoC is less than the resolution of the human eye (or of the display medium).

So essentially what you'd have in a theoretically perfect lens is a perfect cone of light being focused onto an infinitely small point on the film plane:


(courtesy Wikipedia, public domain: File:Cirles of confusion lens diagram.png - Wikimedia Commons)

If the circle of confusion as defined is smaller than the resolving power of the system in whole, including [reluctantly] viewing conditions, we can call that area "sharp" or "in focus" because we no longer can see the individual circles being projected.

If this is the case, then certainly the resolving power of the medium must have an impact to some extent. If the film or sensor cannot resolve a circle diameter difference between one working distance and another, provided fixed focal distance, then both distances will be rendered essentially the same, or in the least with no additional information regarding CoF diameter, and thus about depth.

I'm also having some issue with the concept of "enlargement" with digital photography in regard to sensor size, and I am questioning if you can make the same type of comparisons as you can with film ... but more on that later.

"In photography, the circle of confusion diameter limit (“CoC”) for the final image is often defined as the largest blur spot that will still be perceived by the human eye as a point.

With this definition, the CoC in the original image (the image on the film or electronic sensor) depends on three factors:

1. Visual acuity. For most people, the closest comfortable viewing distance, termed the near distance for distinct vision (Ray 2000, 52), is approximately 25 cm. At this distance, a person with good vision can usually distinguish an image resolution of 5 line pairs per millimeter (lp/mm), equivalent to a CoC of 0.2 mm in the final image.
2. Viewing conditions. If the final image is viewed at approximately 25 cm, a final-image CoC of 0.2 mm often is appropriate. A comfortable viewing distance is also one at which the angle of view is approximately 60° (Ray 2000, 52); at a distance of 25 cm, this corresponds to about 30 cm, approximately the diagonal of an 8″×10″ image. It often may be reasonable to assume that, for whole-image viewing, a final image larger than 8″×10″ will be viewed at a distance correspondingly greater than 25 cm, and for which a larger CoC may be acceptable; the original-image CoC is then the same as that determined from the standard final-image size and viewing distance. But if the larger final image will be viewed at the normal distance of 25 cm, a smaller original-image CoC will be needed to provide acceptable sharpness.
3. Enlargement from the original image to the final image. If there is no enlargement (e.g., a contact print of an 8×10 original image), the CoC for the original image is the same as that in the final image. But if, for example, the long dimension of a 35 mm original image is enlarged to 25 cm (10 inches), the enlargement is approximately 7×, and the CoC for the original image is 0.2 mm / 7, or 0.029 mm."
   
   
   
   
   
   

The resolution of the recording medium could become a factor if it were to drop too low. As such it would be entirely unsuitable for photographic use.

Otherwise the size of the CoC is predicated on human visual acuity and the size of the recording medium (see chart above).

Joe

 

[Bebulamar]

All this technical crap gives me a migraine.I like to spend less time thinking about full frame vs crop or FX vs DX glass and spend more time thinking about shooting and Improving. I am happy as a clam with my D7200 and slow sigma 150-600mm and my slow 18-140 kit lens for general walk around things.

Sure the technical isn't necessary! However, it's interesting to some and it's enlightening to think about it.

 

[bribrius]

does anybody actually do photography and make money on it anymore? Seems everyone sells photography gear and talks about it for money instead.