75 lux on M8 vs. Film

[Kent10D]

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Question for M8 and film users.

 

I am going through infatuation phase with my new 75 'lux (absolutely lovin' it), and now that I have things adjusted properly can focus it pretty well wide open. Still the occasional whoops, of course. As many others have noted the DOF of this lens wide open and at close range is incredibly narrow. A cat's whisker, you might say (see proof below ... and please waive the unspoken rule about cats and kids in this case).

 

Now this is on the M8, with it's 1.3 crop factor. Correct me if I'm wrong, but I am under the impression that the DOF would be even narrower on film (!). The question is: how much narrower? Would it be an obvious difference?

 

Does anyone have any 75 'lux shots on film at f/1.4 they could post?

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[lars_bergquist]

D.o.f. (depth of field) as computed and measured does NOT change with the sensor size. If that were the case, you would be able to increase d.o.f. simply by cropping the image on the negative or sensor! Compact cameras with very small sensors do have more d.o.f., not because the sensors are small but because they use lenses with shorter focal lengths, just as a 50mm standard lens on a 35mm camera gives more d.o.f. (f-stop for f-stop) than a 135mm lens on a 9x12cm camera.

 

On the other hand, a small negative/sensor image will have to be enlarged more to produce a print of a given size, which increases the demand for sharpness. Manufacturers of large roll film or plate cameras could compute d.o.f. at a generous circle of confusion of 1/10th of a mm, because it was assumed that the negative would not be enlarged (1/10th is enough in the print itself) but Barnack & Berek & Ernst Leitz II reckoned that the Leica negative would be enlarged to 6x9 cm size, this being the most popular amateur format at the time. Hence the assumption of a circle of confusion of 1/30th of a mm, which is indeed enough for a 3x enlargement (i.e to 72x108mm!) This was hoplessly out of date already during the 1930's, but nobody dares change it because stupid customers would believe that smaller computed d.o.f. would mean that lens quality had deteriorated. People in the know reckon depth of field at two stops down, i.e. working aperture 8, read d.o.f. at the markings for f:4, and did that already with film. This is equivalent of a circle of confusion of 1/60th of a mm.

 

Irrespective of our assumptions, the d.o.f. of your 75 'lux at 1.4 is just about the same as that of a 90mm 'cron at f:2 however – shorter focal length does compensate for the larger aperture!

 

The old man from the Age of Roll Film Folders

[Kent10D]

Thanks very much for taking the time Lars,

 

Great explanation. My misunderstanding. So rather than comparing the same (full-frame-relative) 75-mm focal length on the M8's 1.33 crop sensor and full-frame film, my confused brain was effectively trying to compare the 75-mm focal length with an approximately 100-mm focal length (75 x 1.33). Comparing the same 75 'lux at the same aperture on the M8 and film is essentially a non-comparison. Now I get it.

 

You know, I've totally glossed over explanations of this in the past, but your explanation really hit the spot.

 

Thanks again!

[innerimager]

Thanks very much for taking the time Lars,

 

Great explanation. My misunderstanding. So rather than comparing the same (full-frame-relative) 75-mm focal length on the M8's 1.33 crop sensor and full-frame film, my confused brain was effectively trying to compare the 75-mm focal length with an approximately 100-mm focal length (75 x 1.33). Comparing the same 75 'lux at the same aperture on the M8 and film is essentially a non-comparison. Now I get it.

 

You know, I've totally glossed over explanations of this in the past, but your explanation really hit the spot.

 

Thanks again!

The reason that the exactly similarly framed image will have less DOF on a film camera than on the M8 is that one would be that much closer to the subject thus lessening DOF. The 1.3 crop M8 means we would be .3X farther away to yield the same image exactly as ay an M7, thus increasing DOF. There are only 3 factors that determine DOF, with a given circle of confusion-

1. Focal length by a squared factor

2. Aperture by a squared factor

3. Distance to subject by a linear factor.

best....Peter

[carstenw]

Depth of field is linear in aperture, inversely proportional to the square of the focal length, and proportional to the distance squared? Source: Ansel Adams, The Camera, pg. 49, ie.

 

DOF = K * aperture * distance^2 / Focal Length^2

 

K has to do with unit conversion and the circle of confusion.

[innerimager]

and here's a 75 lux @ 1.4 film shot as you asked for. (M7) best....Peter

[innerimager]

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Depth of field is linear in aperture, inversely proportional to the square of the focal length, and proportional to the distance squared? Source: Ansel Adams, The Camera, pg. 49, ie.

 

DOF = K * aperture * distance^2 / Focal Length^2

 

K has to do with unit conversion and the circle of confusion.

Correct Carsten, my typo. I learned it from the same page! So the influence of changing the distance to the subject to achieve the exact frame changes DOF as a squared factor of the distance. best...Peter

[jaapv]

The reason that the exactly similarly framed image will have less DOF on a film camera than on the M8 is that one would be that much closer to the subject thus lessening DOF. The 1.3 crop M8 means we would be .3X farther away to yield the same image exactly as ay an M7, thus increasing DOF. There are only 3 factors that determine DOF, with a given circle of confusion-

1. Focal length by a squared factor

2. Aperture by a squared factor

3. Distance to subject by a linear factor.

best....Peter

 

Missed one, Peter: Total enlargement. Which takes in: distance to subject, final print size and viewing distance, all of which influence DOF.

As a matter of fact , one does not change the distance to compensate for sensor size, as that would not produce exactly the same image. Perspective would be changed. If you want to get exactly the same image you would have to change the focal length.The only thing the sensor size does is change the crop.

[innerimager]

Missed one, Peter: Total enlargement. Which takes in: distance to subject, final print size and viewing distance, all of which influence DOF.

As a matter of fact , one does not change the distance to compensate for sensor size, as that would not produce exactly the same image. Perspective would be changed. If you want to get exactly the same image you would have to change the focal length.The only thing the sensor size does is change the crop.

Clearly, viewing distance to the print is an important but different issue than the physics involved with DOF as rendered on a digital or film sensor. If Ansel didn't feel the need to include this factor in his discussion, I can be excused as well. And yes, perspective changes with distance, but in real life shooting, people do either zoom their feet or their lens if they can to get a particular framed shot. We Leica types, prime lenses on camera, zoom with our feet, and thereby change DOF, (and perspective).

[t024484]

Missed one, Peter: Total enlargement. Which takes in: distance to subject, final print size and viewing distance, all of which influence DOF.

As a matter of fact , one does not change the distance to compensate for sensor size, as that would not produce exactly the same image. Perspective would be changed. If you want to get exactly the same image you would have to change the focal length.The only thing the sensor size does is change the crop.

 

I don't get your point.

Are you telling us that if you make two prints in different sizes from the same original that these prints have different a DOF ???

[Kent10D]

Source: Ansel Adams, The Camera, pg. 49.

 

You know what's really embarrassing ... I have that book ...

[Kent10D]

and here's a 75 lux @ 1.4 film shot as you asked for. (M7) best....Peter

 

Very cool! Thanks!

I'd be interested to know about how far the foreground flower is from the tree on the right. That flower is seriously pop-up isolated.

[jaapv]

I don't get your point.

Are you telling us that if you make two prints in different sizes from the same original that these prints have different a DOF ???

 

At the same viewing distance? Yes. DOF depends on the (lack of) resolution of the human eye. When you enlarge a print details that appeared in focus and within DOF in the smaller print will been seen as out-of-focus on the larger one, provided it is viewed from the same distance. Lars' post refers to this as well. DOF is not an absolute value.

[carstenw]

Correct Carsten, my typo. I learned it from the same page! So the influence of changing the distance to the subject to achieve the exact frame changes DOF as a squared factor of the distance. best...Peter

It's funny, I thought that it was also linear in the focal length, but decided to go to the source to verify. We just mis-remembered different parameters

[innerimager]

It's funny, I thought that it was also linear in the focal length, but decided to go to the source to verify. We just mis-remembered different parameters

Yep. It's why I think we really need friends and groups like this one. My equation goes something like this---- if the percentage decrease in functioning neurons of an individaual equals the percentage increase of other brains working on the same problem, the chance of success remains roughly equal. That is until our collective circle of confusion outweighs all factors. of course that's where younger members of the group come in! best....Peter

[carstenw]

That's roughly how Wikipedia works too. There is an Achilles heel though: if the common perception is distorted, and the expert is not present. The "facts" will then be wrong.

[ho_co]

DOF = K * aperture * distance^2 / Focal Length^2

 

K has to do with unit conversion and the circle of confusion.

 

Carsten--

I don't have the Adams book.

 

Could you be more specific about how K is defined?

 

Thanks!

 

--HC

[Kent10D]

OK, so I actually reached up above my desk and pulled down the book: "The Camera" by Ansel Adams. And here I will take the liberty of excerpting the paragraph on page 49 that I assume Carsten and Peter are referring to.

 

I quote:

"The factors affecting depth of field are governed by the following principles: (1) The depth of field doubles if the f-number is doubled (e.g. from f/8 to f/16). (2) If you double the subject distance, the depth of field increases four times; triple the distance, the depth of field increases nine times (depth of field is proportional to the square of the distance). (3) If you reduce the focal length by one half, the depth of field increases by four times (depth of field is inversely proportional to the square of the focal length)."

 

And just for the record, in the following paragraph he goes on to describe how acceptable sharpness is also affected by the degree of enlargement of the negative and the distance from which the final print is viewed.

 

So ... (note to myself), having the book is one thing, but READING it makes all the difference in the world!

I have actually learned something here.

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Hallo,

Look here: Leica M

[ho_co]

... So ... (note to myself), having the book is one thing, but READING it makes all the difference in the world!

I have actually learned something here.

 

Thanks, Kent!

 

So if I understand, Adams doesn't actually present a formula per se, just a series of relationships?

 

You say reading it makes a difference. I would submit that despite having read it, I still fall short of comprehension.

 

--HC

[Kent10D]

So if I understand, Adams doesn't actually present a formula per se, just a series of relationships?

 

That seems to be the case. At least in that particular section of the book.

 

You say reading it makes a difference. I would submit that despite having read it, I still fall short of comprehension.

 

I was pretty sure I read it too. But I must have just skimmed over anything that looked remotely like it might make my head hurt. Nice pictures though!

 

I'll try to remember this time.