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Digital Photography                    Back to Notes on Photography
by Makoto Honda               
Updated February 18, 2005
 

General

Anyone who has been exposed to the world of digital photography will surely appreciate the benefit of this digital technology.  You can view the image the moment after you shoot the picture.  With a proper setup, you can even evaluate the result critically.  The total number of pixels on the digital sensor -- that replaces the chemical film -- continues to increase.  Today, 10 mega-plus cameras are available, and 20 mega, 30 mega, .. are just around the corner (provided the market demand sustains the camera manufacturers' business model for high-mega-pixel cameras).

Digital-Camera/Film-Camera Comparison

How does digital photography stack up with film photography in terms of the system resolution of the final image?  Building on the knowledge of the previous sections, we are going to calculate how many pixels are needed to be in par with today's high-resolution films.  Looking at the film resolution chart presented earlier, we see some color slide films achieve 80 lp/mm resolution at 1.6:1 contrast, and twice as much at 1000:1 contrast.  Just speaking for myself, I would like to have at least the same resolution in digital photography as I get using a high resolution slide film.  This would mean a minimum of 80 lp/mm.  I would up an ante a bit and use a clean number, 100 lp/mm: If a digital camera gives me this resolution, I would feel comfortable converting myself to digital photography. 

The 35mm film size is 24mm x 36mm.  The 100 lp/mm film resolution means the film surface, or digital sensor surface in digital photography, must have 100 times 2 pixels per 1 mm.  (Remember our resolution unit is "line pairs", one black line and one white line, needing two pixels for each pair.)  This gives (24x100x2) x (36x100x2), or 34,560,000 pixels.  Now, I got my number.  I need a 35 mega pixel camera!

   

                   x 100 lp/mm                            x 2 pixels/lp
        36 mm ---------------->  3600 line pairs ---------------> 7200 pixels

                  x 100 lp/mm                            x 2 pixels/lp
        24 mm ---------------->  2400 line pairs ---------------> 4800 pixels

        Total pixel number that is required on the 24 mm x 36 mm digital sensor surface
        to achieve the same resolution as the 35mm film yielding 100 lp/mm is:

       Total Pixel Number  = 4800 x 7200 = 34,560,000 pixels  (35M)

Note:  If your film resolution requirement is 50 lp/mm, then the same calculation gives 8,329,000 pixels.  That is, an 8 mega pixel camera will give you a comparable resolution.

Soap Box - Nikon announced their newest digital SLR, Nikon D2x, featuring 12 Mega pixels. Canon immediately responded by their announcement of Canon EOS-1Ds Mark II having a 16.7 Mega pixel sensor. Canon EOS-1Ds Mark II is a full-frame 35 mm camera (24 x 36mm), while Nikon D2x still maintains a APS-C format. The APS-C format sensor size  is almost exactly half of that of the full 35 mm format.  It follows that if Nikon produces a full-frame digital, its pixel number would be twice that of D2x. That is, the digital sensor density needed to produce 24 Mega pixel, full-frame digital SLR is already achieved.


Full 35mm Format vs. APS-C Format
 (Click here for the table of total dot count on the image)

 (* somewhere around 16x24mm format, like Nikon's DX-format and assortment of Canon's small er sensors)

How does the film format affect the image quality of the photograph in terms of resolution?  To answer this question, we must first define how to measure the final outcome of the photograph.  The importance of resolution boils down to how many "dots" you have in the final picture you are looking at.  This may be a print you put in your family photo album, a large print you want to frame on the wall, a photograph in the book or in the magazine, or a projected image on the screen during a slide show of your last trip to Caribbean. Although the image captured on the film is generally not the final outcome of the whole photographic process, it is nonetheless a major milestone. In the following, we calculate the final total dot count captured on the film - be than chemical or digital - based on the system resolution. To be on a critical side of evaluation, we assume we are using a lens of a 600 lp/mm resolution.

1)  Full 35mm Format vs. APS-C Format - Using the Same Film (100 lp/mm)

     Full 35mm Format
   

            System resolution = 1 / ( 1/600 + 1/100) = 86 lp/mm
            Total dots = (24 x 86 x 2) x (36 x 86 x 2) = 25,372 = 25 Mega pixels
          

     APS-C Format  (around 16x24mm format, like Nikon's DX-format)

    

            System resolution = 1 / ( 1/600 + 1/100) = 86 lp/mm
            Total dots = (16 x 86 x 2) x (24 x 86 x 2) = 11,279 = 11 Mega pixels
            

This is a well-known, and predictable, result. Given the same resolution of the lens, the larger the film format, the greater the number of dots you get in your picture, provided the same film is available for larger formats (35mm --> 6x6 cm --> 4x5 inch --> 8x10 inch). This is true for such films as Fujichrome Velvia, but not Kodachrome-25 or 64. In a digital world, this "same film" requirement amounts to having the same sensor density, or the same lp/mm resolution. 
.

2)  Full 35mm Format vs. APS-C Format - Using Two Cameras with the Same 35 Mega Pixels

     Full 35mm Format

     As we calculated earlier, the 34,560,000 (35M) pixel, 
     full-frame, 35mm digital camera is equivalent to 
     using a film with 100 lp/mm resolution.
               

            System resolution = 1 / ( 1/600 + 1/100) = 86 lp/mm
            Total dots = (24mm x 86 lp/mm x 2 dots/lp) 
                                   x (36mm x 86 lp/mm x 2 dots/lp) = 25,323,356 = 25 Mega pixels


     APS-C Format

     The film resolution (Rf) of 150 lp/mm used here 
     was derived by solving the following equation.
     Total pixel number: 34,560,000 = (16 x Rf x 2) x (24 x Rf x 2)                                                                    = 1536 x Rf x Rf
     Rf = root of (34560000 / 1536) = root of 2250 = 150

  

            System resolution = 1 / ( 1/600 + 1/150) = 120 lp/mm
            Total dots = (16 x 120 x 2) x (24 x 120 x 2) = 22,118 = 22 Mega pixels


This is an important result.  This shows that even if the two cameras offer the same total mega pixel count, the camera with the small er size format is at a disadvantage. This comes from the simple fact that the lens must work harder in order to achieve the identical result on the small er image area. In the above example, of course, we are assuming the use of the same lens, so the final outcome is inferior for the small er format. The full-35mm format and the APS-C format do not create a level-plane field for manufacturers.
The total pixel count difference of 25 Mega vs. 22 Mega is reflective of that fact.

3)  Equivalent Mega Pixels for APS-C Format

How many mega pixels do we need to get the same final result (25 Mega total dot count in the picture) as the full-frame 35mm if we stick to the APS-C format sensor size? 

From the previous example, we need the system resolution (Rs) of 129 lp/mm, derived from the equation, 24 x Rs x 2 = 6192 --> Rs = 6192 / 48 = 129..   

In order to get this system resolution, we need the film resolution (Rf) of 164 lp/mm, as derived from the equation, 129 = 1 / ( 1 / 600 + 1 / Rf) -->  Rf = 1 / ( 1 / 129 - 1 / 600 ) = 164.

The total number of pixels to simulate this film resolution (164lp/mm) on the APS-C format digital surface is thus computed as 41 Mega pixels. 

To recap, the following mega pixels are needed to get the same total dot count of 25 mega on the image for the respective image sensor sizes:

     Full 35mm Format

               

    Film Resolution:  Rf = (7246 / 36) / 2  or  (4830 / 24) / 2 = 100 lp/mm
    System Resolution:  Rs = 1 / ((1 / 600) + (1 / 100)) = 85.7 lp/mm
    Total Pixels in the Picture:  (36 x 85.7 x 2) x (24 x 85.7 x 2) = 25.4 Mega pixels

    APS-C Format

  

    Film Resolution:  Rf = (7842 / 24) / 2  or  (5228 / 16) / 2 = 163 lp/mm
    System Resolution:  Rs = 1 / ((1 / 600) + (1 / 163)) = 128 lp/mm
    Total Pixels in the Picture:  (24 x 128 x 2) x (16 x 128 x 2) = 25.2 Mega pixels

 

Soap Box - I was rather surprised when Nikon introduced a few lenses (only four, so far) specifically designed for use in their APS-C format (DX-format) digital SLR cameras. Someone in a local photo store even told me the DX-format was going to be the main line for Nikon. A total and absolute impossibility!  The reason is simple. As long as we are dealing with sub-10 mega pixel digital sensors, the lens resolution has a mild effect on the final outcome. However, when we reach and pass the 10 mega threshold, approaching the today's high-res film resolution, the lens' optical resolution increasingly shifts its weight toward the final resolution. Nikon cannot compete with other major camera brands with this APS-C handicap. This leads me to believe these DX-format-specific lenses are temporary offerings, just a stop-gap measure on the part of Nikon, to provide a wide-angle coverage to the current users which was lost due to the 1.5x focal length factor for the DX-format size cameras. On the other hand, if Nikon's intent is to establish a new DX-format line, on the same lens mount, in addition to the full-frame 35mm offering, it certainly is an interesting business strategy. The small er image sensor size definitely have many advantages, including a small er camera body and a small er lens size. In fact, many point-and-shoot digital cameras achieved an amazingly compact size employing an image sensor far small er than the DX-format. This size reduction is unthinkable with the traditional film-based cameras.      

Can 35 mm Digital Cameras Overtake Large Format Cameras?   (Total dot count for various film format)

Proper Comparison 35mm 50mm 100mm 180mm 260mm standard lens

The resolution of the traditional chemical film will not improve drastically.

Megapixel number of the digital sensor could go up.

What mega pixel count is needed to produce a picture comparable to 4x5 and 8x10?

Manufacturers have more leeway for cost in this market.

35mm photography is about a million times superior compared with 4x5 and 8x10 format cameras in terms of its system versatility.

Image Area Comparison

APS-C Format        35mm            6 x 6 cm            4 x 5 inch

384                   864                3600                12900          (area in square mm)

0.45        :          1         :         4           :            15           (ratio)

A straight interpretation of this result is that, compared with the 35mm format, the 6 x 6 format and the 4 x 5 inch format are capable of capturing 4-times and 15-times the number of dots in the resultant image, respectively. This of course assumes the use of the same film. Also, we are assuming the same lens resolution. In order to get the same image (perspective included), we must use a lens having a correspondingly longer focal length for the larger format. For instance, a 50mm lens for the 35mm, a 100mm lens for the 6x6 cm, and a 200mm lens for the 4x5 inch, more or less, create the same "standard lens" field of view. Even so, the resolution of these lenses must be the same to capture 4-times and 15-times number of dots in the larger formats. This assumption may not be true, especially for 4x5 format optics. We have a reasonable expectation that, even for a large format camera optics, the resolution of the image center (corresponding to the 35mm frame area) is as good as that of 35mm optics. The question is whether the same resolution extends all the way to the corner of the image. The 4x5 optics requires a far larger image circle than the 35mm counterpart. Not only that, the "shift" and "tilt" operations make the lens' image circle requirement even greater. The average resolution of the entire image area for a 4x5 format camera is likely to be lower than that of the 35mm optics.

 


Heterogeneous Hybrid  Environment (Chemical/
Digital)

Taking pictures using a traditional film camera.

Making a photographic print using a traditional enlarger.

Scanning a film to make a digital image (file)

Making a photographic print from a digital image (file)

Taking pictures using a digital camera. Produces a digital image (file)


Digital Film Requirement (rolls of film)

Requirement:  500 - 1000 exposures a day --- > Hotel  download

Typically, a photographer carries many rolls of film for assignment.  What does this mean in the digital world?  Just to get some idea as to what preparation would be needed when a film photographer converts himself to a digital photographer, let us take a look at Nikon D70 digital SLR camera that has a six mega pixel sensor.

Using the RAW setting, each frame consists of 6 million pixels.  Nikon D70 uses 12-bit buffer depth for RAW setting. Nikon D70 uses Nikon's NEF format that compresses this image to 5M bytes.  This is a "lossless" compression:  No information is lost.  According to Nikon D70 brochure, you can store about 23 images (taken in RAW mode setting) in a 258 MB Compact Flush memory card. 

Nikon D70 - 6M pixels (RAW setting)

258 MB CF memory -------->   23 images
1 GB CF memory ---------->   92 images
2 GB CF memory ----------> 184 images
4 GB CF memory ----------> 360 images

I can shoot about 360 shots (RAW image) with one 4 GB CF memory card. To satisfy my 1000 shots per day requirement,  I have to carry 2 extra cards in my pocket.  This is in lieu of carrying 30 rolls of film.  

Nikon D2x - 12M pixels (RAW setting)

If my camera is Nikon D2x with 12M pixel sensor, I would expect the numbers to decrease accordingly....  I just have to carry twice as many memory cards!

2 GB CF memory ---------->  90 images
4 GB CF memory ----------> 180 images

Hotel Download

After exhausting several 4GB memory cards, I must download the image data to my laptop or some storage device, so that I am ready for the next day's shoot....  Maybe, the modus operandi for a future digital photographer is to upload the image data to a secure site wirelessly, using a transmitter attached to the camera.  This is no different from NASA receiving pictures from Mars Rover taken by an on-board Nikon......

Storage Requirement (Slide vs. Digital)

I can keep 40 or so slides (mounted on the standard paper mount) in a 1 x 2 x 4-inch box returned from Kodak or Fuji processing lab.  Where do I keep my digital images?

  Storage Cost Comparison - Cost per Gig byte: 

                     USB            External          Internal          CD-ROM           DVD
                          Drive           Hard Disk        Hard Disk        Disk                 Disk  

   Capacity        258MB      200GB        200GB        700MB         4.7GB

   Price              $50          $250          $150           $0.20          $0.50

   Cost/GB        $200/GB     $1/GB        $0.75/GB     $0.30/GB    $0.10/GB

The comparison shows the DVD disk is the cheapest storage medium, and probably the most stable.  I just have to make a copy every 5 years or so to avoid the disk deterioration, and/or to keep my eyes open for new software/hardware technology, including image format obsolescence.

Soap Box - I kept my slides (Kodachrome-25) for the past 30-some years very successfully.  The transparencies retained their original color quite well.  I am not so confident if I can manage to keep my digital image that I took yesterday for the next 10 years!      
 

Nikon D3 / Nikon S1

There are many sites in the web making interesting predictions about new digital cameras. Some sites I saw predict Nikon D3 to be a full-frame 35mm digital SLR camera. Here is my two cents. As I wrote somewhere in my Soap Box, Nikon DSLR will eventually go full-frame. Canon has EOS 1D series and there are other brands using a full-frame 35mm format. Nikon's decision to use a APS-C format for their new, top of the line DSLR camera, D2x, is probably based on their careful analysis (both technical and marketing) that they can still compete well with a full-frame 35mm format at the current sensor pixel count, which is around 12M (Nikon D2x) and 16M (Canon EOS 1Ds Mark II). As the market push continues to increase the sensor resolution beyond 20M pixels, however, the disadvantage of a small er sensor size becomes more difficult to overcome. Nikon has to move to the full-frame 35mm DSLR market, and, doing so is fairly easy. All Nikon has to do is to produce a "camera"; all the 35mm lens lines are available - they have been for many years. That having been said, my two cents is merely a naming of the new camera.

Whether it was intended from the very beginning or not, the fact of the matter is that the APS-C format market has been established. (I am just using the term "APS-C format" to mean Nikon's 23.7 x 15.7mm and Canon's 22.5 x 15mm and so on.) I think the APS-C format started out as just a stop-gap measure because of the huge cost of the large sensor in the past, but regardless, it now has a life of its own. There is no reason for Nikon, or any other manufacturers, to abandon this new APS-C format line. They should continue to foster this market. So coming back to Nikon D3 naming. I do not think Nikon will remove the APS-C format from the professional lineup. That is, my prediction is D3 will be another generation of a professional APS-C DSLR. Nikon will simply have to pick another letter from the alphabet soup, say, "S", for their new full-frame 35mm DSLR, so as not to disturb the APS-C digital line. Nikon is more conservative and more careful in their product planning than Canon (just my opinion). Nikon has maintained their lens mount unchanged from the days of Nikon F. Nikon seems to be keeping the tradition of careful thinking in their sensor size decision. When Nikon introduces a new full-format 35mm DSLR camera, it will be a monumental event. The Nikon S1 will be a 25M pixel full-frame 35mm DSLR camera, but don't hold your breath. Nikon will take their time before bringing this to the market. Nikon realizes very clearly that the sensor pixel count is just a small part of the overall equation in the professional arena that they serve. This is shown in their latest introduction of Nikon D2Hs. The D2Hs sports 8 frames per second firing, though the sensor is limited to 4.1 mega pixel.  When Nikon introduces S1, it will shoot just as fast at a 25 mega pixel sensor.

 

 

             

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Copyright 2001-2017 Makoto Honda. All Rights Reserved.                                       www.iCarnivorousPlants.com                                       since June 2001