Resolution: 35mm vs MF

[d_khong]

Hi

 

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My mind is going through some form of mathematics on resolving power.

 

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Presuming that the resolving power of an average MF lens is 50 lines/mm,

and that the negative area is about 3.5 times that compared to the 35mm

frame, then this is equivalent to a practical resolving power of 3.5 x 50 =

175 lines/mm. when equivalent focal lengths are compared.

 

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Put it in another way, let's say we are shooting the same scene with a 50mm

lens on a 35mm camera side by side with a 80mm lens on a 6x6 camera. They

would be equivalent in focal length and would capture the same scene.

Therefore the MF lens would be able to capture more detail than the lens on

the 35mm camera.

 

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Is this a logical deduction?

 

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Dan K.

[steve_rasmussen]

Dan: Given the state of optical perfection in today's designs, the difference between 35mm and MF is more a matter of film size than lines/mm that the lens can deliver. One example is that Pentax uses virtually identical designs on its 6x7 normal lens as it does on its 35mm. The 90 and 105mm for the 6x7 are both double gauss designs just like their 50mm lens on their 35mm cameras. The difference in lines/mm is negligable. So, it's all about film area.

Steve R.

[mel_brown1]

Dan, that's a good thought, and most all of us wish it were true. Your math is right, but your input data has two problems I'd like to explain.

 

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First, lens testing is based on lines/mm of resolution of the target, or subject, not lines/mm resolved at the film plane, which I think you had in mind. Bottom line is that if the image coming through the lens is blurry, making it bigger won't make it less blurry. So, lines/mm does not necessarily vary with focal length or film size.

 

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Second, a 6x6 or larger MF negative is only 2.3 times larger than a 35mm negative, comparing their shorter linear measurements. If all cameras made images with the same aspect ratio (length vs. width), area comparison would be as valid as linear comparison. Since aspect ratios vary widely, I prefer to use the linear method. To illustrate, let's compare 6x6 and 6x9 negatives. For a square print, each would be enlarged the same amount, while for a 6x9 or some proportionately equivalent print, the 6x6 negative must be enlarged 50% more (linearly) than the 6x9 negative. We can even the playing field by considering only square prints, regardless of the aspect ratio of the negatives.

 

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If our goal is to produce a fixed size print, whether from a 35mm or a MF or LF camera, the largest format typically wins hands down because less magnification (enlargement) of the negative is required to produce that print. That means smaller grain size on the print.

 

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Now let's change our goal so as to produce a print of fixed magnification, with each negative enlarged 20 times. A 35mm negative would produce a 48cm print (24mm x 20), and a 6x6 negative would produce a 112cm print (56mm x 20).

 

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Viewed from the same distance, both the smaller and the larger prints would have the same grain size because we enlarged each negative 20 times. That is a function of the film alone, not of the image recorded on the film.

 

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Now consider viewing those same prints from a proportional distance. From an arbitrary but reasonable 48cm for the smaller print and from 112cm for the larger one, the MF print will look better. That's because the film grains, which are physically the same size in both prints, will appear smaller to our eyes from 112cm than from 48cm.

 

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Lens resolution is but one factor that affects the quality of the final image. At least as important as resolution is contrast. It is easy to demonstrate that a lens with mediocre resolution and high contrast (low flare) will produce a subjectively "sharper" print than will a lens with high resolution and lower contrast. That is one reason many one-hour photo labs use high-contrast, glossy paper. To the untrained eyes of their target market, the extra contrast makes a "better" print than one printed on normal-contrast paper with a matte or luster finish. When you sell a vehicle, don't you polish it first?

 

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There is no perfect lens, and the larger the lens is, the more likely it is to deviate from perfection. The good news is that the average lens is much better than the average photographer. I think Ansel Adams said it well with, "I believe there is nothing more disturbing than a sharp image of a fuzzy concept!"

[steve_vancosin]

Both of your answers were true statements, however, They did not answer Dan's question - "Will the MF lens capture more detail?"

The answer is no. I think Dan needs to see that the film is the limiting factor here, not the lens.

[frank_sheeeran]

Hi Dan, <p>

 

You math is not quite correct. In terms of <em>square</em> millimeters, a

6x6 image is about 3.6 times bigger. Resolving power is in terms of

<em>linear</em> millimeters. It is also often stated in terms of line

<em>pairs</em> per millimeter.<p>

 

Also, lp/mm is measured on film. To another responder that hinted it was on

the subject plane: consider you take a picture of the moon, something like

15,000km across, and of a soccerball 3 meters away, the same angular

diameter. I don't think you think the lens provides millions of times higher

resolution for the moon! <p>

 

Since the 6x6 and 35mm frames are wildly different shapes, to take the same

photo you have to crop one or the other by 33% or so. But ignoring that, you'd

want to say sqrt(3.6) * 50mm (MF) = sharpness comperable to 95 lp/mm (on

35mm). <p>

 

To take ratio into account: if you want a 3:2 ratio print, you'd crop the square MF

to 37x56. If the MF lens was 50lp/mm, the 35mm lens would have to be

75lp/mm to render as sharply. If you want a square print, the 35mm lens would

have to be 50 * 56 / 24 = 117 lp/mm. <p>

 

However, for equal weight, complexity, cost, etc., lenses on different formats

will have about equal resolution accross the target. It that target is 8x10", the

lens will have much lower lp/mm resolution; if the target is APS, it could be

much higher. Or more to the point, expect a 50mm/35mm format lens to be

1.5-2x sharper than a comperable MF lens: in other words, if the standard

MF lens was 50 lp/mm, expect the standard 35mm lens to be 95 lp/mm. <p>

 

So why shoot with MF? Why not always APS? Three great reasons: <ul>

 

<li> More film. As pointed out, enlarging a 35mm, MF, and LF transparency of

the same scene will give the <em>same</em> sharpness more or less, but

the grain of the 35mm will be hideous while the LF is still butter-smooth. <p>

 

<li> More lens. a $100 50mm/1.8 would be vaguely as sharp as a MF's

$100 80mm/2. But if you spend $3000 on a Schneider 80/2, you can expect

some of that money went into better quality control or optical engineering and

will in fact render more lp/image than any 35mm lens. <p>

 

<li> More personal camera. The MF to suit you is probably already being

made. Only about 2-3 kinds of 35mm cameras, and they may not suit you.

If you want the benefits of a TLR design, or a Fuji 680, MF is your only option.

<p> </ul>

 

Last note: no lens has a simple resolution figure like this. Any lens in the world

will give 100% contrast (you hope) given a target yielding .05 lp/mm on the film,

and will give no scientifically detectable contrast for targets yielding 10000

lp/mm. Each lens, from a kids magnifying glass to the Hubble telescope, will

yield some kind of curve going from that 100% to 0% as lp/mm increases.

However, the curves aren't smooth, and vary incredibly from center to corner

of a lens. So, don't forget that this entire discussion is <em>very</em> "in

theory."

Frank