What is the biggest, perfectly sharp format you can get from your sharpest negatives?

[RalphLambrecht]

That was last night. Prices have gone up since then.

 

[removed account4]

silly thread - about nothing

noooo
it is about something, just not the kool-aid i drink !

 

[removed account4]

ralph:

YOU ROCK !

john

 

[Edward_S]

I think it's an interesting thread because it relates to the way we perceive things, or maybe tells us about the differences between our perception of a subject and what a camera sees. For example, have you ever seen a painting that shows the effects of DoF? I've never seen any in the National Gallery that do, even though our eyes create DoF effects in the same way as a camera lens. DoF provides a tool (or a limitation) that is unique to the photographic way of picturing things.

There is a recently posted image in the gallery that shows a line of fence posts. It provides a good example of how the perceived depth of field changes with viewing distance. At 'normal' distance, the near post appears to be outside the DoF on the near side (i.e. too close to the camera to be sharp), but if I view the image from the other side of the room, the post appears completely sharp and the DoF seems larger. In some ways this image is not the best example because blurring occurs due to off-axis aberrations as well as DoF effects, but actually when viewed from a distance, the near post seems to float pleasingly in front of an out-of-focus background and looks quite three-dimensional. My perception of the picture changes as I move towards and away from it, although the information contained in the image is of course locked in at the point of exposure, as Ralph was saying in an earlier post.

 

[Diapositivo]

There are many, many threads on APUG. I am not interested in many of them, actually in most of them. I don't write in those forum saying that I am not interested in them. I just pass by. I find it useless and "deconstructing" to post a message in a thread just saying that the thread is not interesting. It just adds noise to a discussion.

Besides, a thread exists not just for the interest of those 10 persons who participate in it, but for the interest that many readers, now or in a distant future, might find in it.

So by all mean Ralph, as I am very interested in this discussion, do post the results of your test here, instead of by private mail.

The matter of DoF, enlargement, maximum sharpness obtainable etc. is IMO very interesting generally for photographers. Fora exist for the open and friendly discussion of such matters.

Fabrizio

 

[Rafal Lukawiecki]

Bill

Don't worry. Not posting them doesn't mean I won't email them to people who are interested.

Dear Ralph,

I hope you can publish the results here. If not, I'd be obliged if you email me. I am interested in these discussions very much.

Rafal

 

[RalphLambrecht]

I will have posted something by tomorrow evening.

 

[holmburgers]

Sorry to have corroborated with the neighsayer and giving my approval of his message with a thumbs up. I think he was just trying to provide some perspective on the issue. But please don't let 1 (or 2) man's opinion effect your passionate discussion. Address it if need be and continue.

 

[A49]

I'm willing to compile the following pictorial evidence if it is accepted as proof beforehand:

1. Two images, one taken with a 35mm the other with a 80mm lens from the same viewpoint and subject distance (u) and at the same aperture (N).
2. Both images enlarged to the same subject magnification.

Then we can visually investigate the images and compare their DoFs.

I think for these DoF examples a digital SLR cam is absolutely adequate. I very often use my DSLR for small format lens testings. They are much faster and more convinient to do with digital.

Ralph, if you tell me (us) the subject distance, the negative format, the aperture and how big the print for the 80 mm shot should be, then I do the little calculation of the DoF for both lenses and the needed 7 lpm resultion of both prints. Maybe we could then compare theory and real life.

Best regards,
Andreas

 

[RalphLambrecht]

... But enlarge both to the same size print, and you will have not only achieved equal magnification, but also equal DoF. ...

The assignment was to prove or disprove the above statement made in post #75.

To verify the claim, I used a camera on tripod and took two pictures, both shot from exactly the same distance, focused at the car's rear lamp, and with the same aperture. However, one was taken with an 85mm lens (approximately normal for medium-format) and the other with a 24mm lens (approximately normal for DX format). (see attachment 1)

Then, the 24mm image was enlarged to match the 85mm image in subject magnification and print size. (see attachment 2)

As we can clearly see, the DoF is not equal. The inherent DoF advantage of a shorter lens is not eliminated by the enlargement required to get to the same subject magnification. Smaller film formats have the (dis)advantage of providing more DoF, because typical focal lengths are getting smaller with film format.

If these two images would have been taken with a medium-format and a DX-format camera, and both would have been enlarged to the same print size, the DX-format camera would have given much more depth of field at the same f/stop. A mathematical DoF investigation came to the same conclusion.

The above statement, made in post #75, is incorrect.

 

[Diapositivo]

Ralph, that's impressive. Thanks for your effort and your positive contribution to the matter.

I am frankly puzzled. These results do not coincide with the theory as I understood it. I will not do, anyway, like those intellectuals in Brecht's Life of Galileo who, when invited to observe the Medicean "planets" (moons of Jupiter) in the telescope, said it was more interesting, before looking into the telescope, to dispute first if the planets can really exist, and if they are necessary.

Ralph might well be right on the point.

Some further investigations are nonetheless in order.

First, which is the point of focus, in both images? The picture with the 85mm seems to be not exactly on focus on the back of the car. Even if it was with the focus slightly behind it, that would not explain the difference in DoF that can be observed anyway.

Second: the theory states that although the total DoF is the same, their distribution will be different. The shorter focal lengths will have the DoF extending more behind than in front of the plane of perfect focus, while the longer focal length tend to have the DoF distributed more equally in front and behind the plane of perfect focus. This can be a partial explanation of the phenomenon, because we are not seeing the DoF of the tele lens in front of the plane of focus. Nonetheless the difference in DoF is so neat that I doubt this factor alone can explain it all.

I have "officially" lost my certitude and wait for some further explanation to this.

We could also make a seance and contact the soul of Ansel Adams (peace be upon him) and ask him why did he found the Group f/64 instead of the Group 35mm. :confused:

Fabrizio

 

[cowanw]

Now that I think of it, to say that Depth of Field depends on f stop is to say that Depth of Field is dependant on focal length and aperture size.
Fstop is the made up definition; Focal length and Aperture are the measurable facts.

 

[MattKing]

Ralph:

You deserve a beer!

I've seen a somewhat similar experiment done with digital capture, but assumed that the fact that the digital sensor used a Bayer array impacted the result in some way.

I wonder if the role that acutance plays in our perception of sharpness is a factor in this.

 

[RalphLambrecht]

... which is the point of focus, in both images? ...

Both lenses were focused on the rear lamp of the car.

... the theory states that although the total DoF is the same, their distribution will be different. The shorter focal lengths will have the DoF extending more behind than in front of the plane of perfect focus, while the longer focal length tend to have the DoF distributed more equally in front and behind the plane of perfect focus. ...

If the subject magnification is <1 (portrait, landscape, etc), front DoF (towards the camera) is smaller than rear DoF. If subject magnification is >1 (macro photography) front DoF is larger than front DoF. If the magnification is 1:1, front and rear DoF are identical. With a few meters of subject distance a 1/3 to 2/3 ratio between front and rear DoF is typical a rule of thumb. The ratio between front and rear DoF depends on subject magnification, not focal length.

 

[Diapositivo]

Bill, I don't know if I get you right, but when people say that DoF depends on aperture (among other factors) they usually mean f/value, not absolute aperture in mm. That f/value already takes into account that for different focal lengths, the absolute aperture in mm that results in a certain f/value is different.

I can't help thinking where is the "catch" here...

If you make this test using a different focal length, the same format, and a different point of capture, you obtain equal DoF, I think also Ralph concurs with this.

But he says, and shows, that if you use different focal lengths, same format, same point of capture (different field of view so different image) and then enlarge a portion corresponding to a higher focal lenght in magnification and field of view, so that you end up with the image that you wanted in the first place, you end up with a gain in DoF.

If I get it right that would mean that, in macrophotography, people should just use shorter focal lengths, and without need to get nearer to the subject, by subsequently enlarging the central part of the image they would get a better DoF (which is critical in macrophotography) than they would have obtained if using a larger focal length.

The common idea is that it is easier to take pictures of living insects while going in the field with a 100mm macro rather than a 50mm macro, because with the 100mm macro one can be at a higher distance from the insects, which normally fly away just a moment before finding the correct focus. By using the 100mm instead of the 50mm, one can remain further away and can hope the insects will not notice the pervert taking pictures of a sex scene. Well, the pervert might just use the 50mm, and take pictures from the same distance he would have used with the 100mm, and then enlarge the picture of the insects, and altough he would have lost something in resolution, he should have gained, if the test by Ralph is well conducted and if I derive the right conclusions from it, an advantage in DoF. Sounds like a "free meal" so to speak.

Fabrizio

 

[RalphLambrecht]

... I've seen a somewhat similar experiment done with digital capture, but assumed that the fact that the digital sensor used a Bayer array impacted the result in some way.

I wonder if the role that acutance plays in our perception of sharpness is a factor in this.

Matt

The pictorial examples just verify the equations provided in post #79, and those equation are older than you and me together. Human perception is the foundation of DoF. Without the resolution limitations of our eyes, there would be no DoF. True sharpness only exists at the plane of focus. Everything in front or behind that plane is only 'sharp enough' for us not to notice.

 

[Diapositivo]

If the subject magnification is <1 (portrait, landscape, etc), front DoF (towards the camera) is smaller than rear DoF. If subject magnification is >1 (macro photography) front DoF is larger than front DoF. If the magnification is 1:1, front and rear DoF are identical. With a few meters of subject distance a 1/3 to 2/3 ratio between front and rear DoF is typical a rule of thumb. The ratio between front and rear DoF depends on subject magnification, not focal length.

According to this many-times quoted text, which I begin to doubt :confused:, the ratio between front and rear should depend on focal length

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

It must be said, though, that in the asterisk note (marked as *Technical note) this author specifies that in particular circumstances, when the focus is near the hyperfocal distance, the wide angle shows a bigger DoF.

Fabrizio

 

[MattKing]

Matt

The pictorial examples just verify the equations provided in post #79, and those equation are older than you and me together. Human perception is the foundation of DoF. Without the resolution limitations of our eyes, there would be no DoF. True sharpness only exists at the plane of focus. Everything in front or behind that plane is only 'sharp enough' for us not to notice.

Ralph:

I understand what you are saying, and agree. I was just musing on this - wondering if the enlargement step at the end of the process might have the effect of disproportionally affecting the perception of sharpness for those items that are 'sharp enough'.

 

[RalphLambrecht]

... If you make this test using a different focal length, the same format, and a different point of capture, you obtain equal DoF, I think also Ralph concurs with this. ...

Yes, if you use different focal lengths and adjust your viewpoint so that they both create the same subject magnification, DoF will be identical. The two websites, referenced earlier in this thread show that. By the way, it does not require the same format. The format is not relevant.

... But he says, and shows, that if you use different focal lengths, same format, same point of capture (different field of view so different image) and then enlarge a portion corresponding to a higher focal lenght in magnification and field of view, so that you end up with the image that you wanted in the first place, you end up with a gain in DoF. ...

Correct, but again, it does not require the same format.

... If I get it right that would mean that, in macrophotography, people should just use shorter focal lengths, and without need to get nearer to the subject, by subsequently enlarging the central part of the image they would get a better DoF (which is critical in macrophotography) than they would have obtained if using a larger focal length. ...

Yes up to a point, eventually they lose too much resolution when enlarging the smaller frame and the image is inferior despite an increased DoF on the negative.

 

[Diapositivo]

I have another doubt. Ralph, how did you enlarge the cut-out?

If you enlarged it with Photoshop, that might invalidate the results, at least partially, because the enlarging algorithm do tend to "stuff pixels between pixels" by taking into account (finding) edges, lines etc. Especially when working from a raw image, the "developing" of the raw image is always the result of an "intelligent" work by an algorithm which does not just reads what the cells captured, but makes an educated guess on what might have caused that capture.

The reason why digital images give, to the casual observer, a sensation of great detail is that they are "created" so as to "find" and maintain lines, edges etc.

Even if your images seem to tell a clear story, it might be that part of this is due to the "intelligent" enlarging rather than a purely optical one.

Not that I am very convinced myself that this might justify what we see, though.

Fabrizio

 

[RalphLambrecht]

Ralph:

I understand what you are saying, and agree. I was just musing on this - wondering if the enlargement step at the end of the process might have the effect of disproportionally affecting the perception of sharpness for those items that are 'sharp enough'.

Matt

The loss of resolution due to enlargement is linear (CoC), the increase of DoF due to focal length, on the other hand, is to the 2nd power.

 

[RalphLambrecht]

According to this many-times quoted text, which I begin to doubt :confused:, the ratio between front and rear should depend on focal length

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

It must be said, though, that in the asterisk note (marked as *Technical note) this author specifies that in particular circumstances, when the focus is near the hyperfocal distance, the wide angle shows a bigger DoF.

Fabrizio

All I can say is that I believe the ratio between rear and front DoF depends on subject distance, approaching infinity towards infinity focus. I may be wrong and better check, but that's another thread.

 

[Lee L]

Thanks for the examples Ralph.

I find this post useful.

http://theonlinephotographer.typepa...er/2009/06/depth-of-field-hellthe-sequel.html

Lee

 

[RalphLambrecht]

I have another doubt. Ralph, how did you enlarge the cut-out?

If you enlarged it with Photoshop, that might invalidate the results, at least partially, because the enlarging algorithm do tend to "stuff pixels between pixels" by taking into account (finding) edges, lines etc. Especially when working from a raw image, the "developing" of the raw image is always the result of an "intelligent" work by an algorithm which does not just reads what the cells captured, but makes an educated guess on what might have caused that capture.

The reason why digital images give, to the casual observer, a sensation of great detail is that they are "created" so as to "find" and maintain lines, edges etc.

Even if your images seem to tell a clear story, it might be that part of this is due to the "intelligent" enlarging rather than a purely optical one.

Not that I am very convinced myself that this might justify what we see, though.

Fabrizio

Fabrizio

Look at the detail of the post boxes in the 24mm image, and compare it to the lack of detail in the 85mm image. How would any software create this detail out of nothing in the 24mm image? Photoshop is good, but not that good. The raw untouched images in Abobe Bridge show the same difference in detail. The background in the 85mm image is just more blurry while out of focus.

 

[Diapositivo]

Fabrizio

Look at the detail of the post boxes in the 24mm image, and compare it to the lack of detail in the 85mm image. How would any software create this detail out of nothing in the 24mm image? Photoshop is good, but not that good. The raw untouched images in Abobe Bridge show the same difference in detail. The background in the 85mm image is just more blurry while out of focus.

Although I agree that as an explanation it can be just a very partial one, what I mean is not that Photoshop added details to the 24mm, but that with an "optically" made enlargement the CoCs would have grown "just like the enlargement", while with a digital enlargement the enlarged pixels are, in fact, "calculated" so the enlargement is less "confused". We don't have the "linear defocusing" of an optical enlargement so to speak.

But I repeat that this can be a factor but cannot tell the entire story, I recognise that the results are quite different from what I expected for such a test and I myself doubt that the enlarging algorithm can be so good as to justify the difference between the results and my expectations.

Fabrizio