 BetterPhoto Member |
focal length difference for film/digital Hi I am a bit confused between focal lengths of lenses used on film and digital respectively. EG If I have a 20-55mm lens, what is the true focal length on a digi camera? As I am still on film, I am more interested in the opposite. EG. Would the same lens for a digi. actually give me 20mm on a film camera? How do the two differ? Thanks for your help Gary
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Alan N. Marcus |
Hi Gary, Confused, don't read this! We always choose focal length based on need. It’s a good idea to think in terms of four groups 1. Normal 2. Wide Angle 3.Telephoto 4. Portrait. First you need to know that all camera lenses project an image of the outside world onto the surface of the media. That media, be it film or chip has dimensions. You will find your camera’s chip size in the specification pages of the manual. As an example 35mm film camera sports a frame that measures 24mm height by 36mm long. From this data we calculate the diagonal measure; it works out to be 43.3mm. We need to know the diagonal because this is the basis used to determine focal length requirements All lenses suffer from vignette (edges of frame receive less light than center). When a lens focal length is about equal to or longer than the diagonal, no special lens design countermeasures is needed to give even coverage power. If the lens is shorter than the diagonal, extra coverage power must designed in. For this reason, and because, if the lens is about equal to the diagonal measure the angle of view is consider normal i.e. the human experience which is 53°, this is the basis of normal for all cameras. For the 35mm camera, the diagonal measure of 43.3mm is a bit strange so lens makers like to round this value up to 50mm. Thus the 50mm is considered normal for a 35mm camera. Because the 35mm has been around for about 100 years its sizes and commonages are held up as examples by writers and teachers as if they the golden ruler. So in line with this thinking, a lens about 50mm in focal length is considered normal. Longer lenses are grouped as telephoto. Common thinking would have this group start at about 135mm which is about 270% longer than the normal 50mm normal. Shorter lenses are grouped as wide angle starting with the 35mm which is about 70% of 50mm normal. A modern kit zoom lens on a 35mm or fill frame digital would center up on 50mm. Portrait lenses start at about 2.5 times the diagonal which calculates rounded up to 105mm. All of the above can be applied to any format, all you need to know is the format dimensions. Some digitals are full frame thus they are so close to the 24mm x 36mm format, that being the case all above applies exactly. Most digitals however sport an imaging chip that is smaller than the 35mm standard frame they replace. While no chip size is yet standard, a typical digital is about 66% of the size of a 35mm film frame. Thus it likely measures 15.8mm x 23.7mm with a diagonal measuring 28.5mm. This means that a 30mm lens fills the bill for normal. So a kit zoom would be centered up on 30mm. A portrait lens would be 70mm or longer. Telephoto would we 80mm or longer. Wide angle would be 21mm or shorter. In common usage is a crop factor which is actually a conversion factor. In future the chip size will get smaller and the 35mm and its associated film will be in the museum next to Civil War Medical Instruments. It’s time to abandon the crop factor as a way of thinking and move on to the diagonal measure concept on which all this stuff is based. Alan Marcus (marginal technical gobbledygook or worse)
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Alan N. Marcus |
I know you don’t want to know! All camera lenses trace back to the thirteenth century to Althazen a scholor of Arabinan who published the properties of the biconvex lens. The first usage was in the camera obscure in the middle of the 16th century. No film available, however the camera obscure was in common usage by artist; they traced landscape and still-life on paper using pencil. The camera gets its name from a darkened chamber usually set-up in a tent. Newton described the lens error chromatic aberration in 1666. He observed that a lens can only image one color at a time in proper focus; the other colors are depicted out of focus. He pronounced this error incurable. The first to solve was John Dollton 1757, his was called an achromatic (without color error) design. The first landscape lens came in 1812 out of shop of William Wollaston , a meniscus design. Joseph Nicephore Niepce invented photography in 1826. In 1839 Louis-Jacques-Mande Daguerre specified an achromatic landscape lens; his plan for a camera was carried out by Chevalier from his shop in Paris. The focal length of a lens is fixed at manufacture, it does not change thus it remains the same if mounted on a film camera or a digital camera. We practice our trade because “we stand on the shoulders of giants” a quote from Newton, he in my option was the great one in this field of study we call light sciences. Alan Marcus
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- Gregory LaGrange Contact Gregory LaGrange Gregory LaGrange's Gallery |
The focal length stays the same, it's a narrower field of view that you get. If you took one of your film pictures and cut off maybe a 1/4 inch border all the way around it, that would be your digital field of view. Camera makers almost always say what the change is 1.6, 1.3 Just multiply the focal length by that.
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anonymous A. |
While most lenses designed for film SLR cameras will work on digital cameras, they may not perform as well as they did on film, partly because they were designed for a different sized medium, apartly because the digital chip surface is more reflective than film. The reverse is not true: most designed-for-digital lenses will not work on film cameras. Even if they will fit physically, they image they produce will not cover the full film frame. The exception to this is the digital lens designed for a full-frame digital like the Canon EOS1DS models, EOS5DS, Nikon D3, but I do not think these are compatible with your system. Sorry.
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Alan N. Marcus |
A crop factor of 1.6 means the chip is 1/1.6 = 0.625 or 62 1/2 % the size of a 35mm full frame. A crop factor of 1.3 means the chip is 1/1.3=0.77 or 77% smaller than a full frame 35mm. Mr. Grange is correct; the focal length remains the same however the angle of view is indeed reduced due to the smaller chip. My point is the use of a 1.6 or the 1.3 crop factor equates only to comparisons made full frame 35mm vs. the specific chip size of the camera in question. Use of a crop factor is good only if the photographer is are familiar with the 35mm camera and its associated lenses. The problem is, as time passes, the 35mm frame will fast become history. It’s clear we must fall back on the diagonal measure to relate angle of view and perspective. Keep in mind that the 24mm x 36mm frame size established by Oskar Barnack back in 1913 is likely at the end of its life cycle. If you think I am wrong consider how many 4x5 cameras remain or how many 3 ¼ x 4 ¼ cameras remain or how many 6x9 cameras remain in common usage. Format size is a function of media resolution. As media improves resolution gets finer, new smaller formats emerge. Surface reflection differences film vs. digital chip is not a factor. Alan Marcus
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- Gregory LaGrange Contact Gregory LaGrange Gregory LaGrange's Gallery |
I don't think just because larger formats are still here that that indicates 35mm size will go. The advantage of larger formats was the larger negative. Yet the camera bodies were always so much more expensive than a 35mm body. You get more portable size, more economical, more versatile in some ways. That's never out of style. Expensive electronics have always gotten cheaper with time and development. As sensors get more economical, full frame can become more prevalent.
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Alan N. Marcus |
Hi Gregory, Being a betting man I’ll stake my money on the 24mm x 36mm full frame format going bye-bye. While my crystal ball is always cloudy, I’ll put my money on the HDTV format which is 16:9. That means for every 9 units height there will be 16 units width. I for one hate it when formats are presented this way. I prefer 16/9 = 1.777 or 1:1.7777. That translates to 6mm x 9mm or 18mm x 32mm or 27mm x 48mm. I will bet on the 18mm x 32mm format. 18mm x 32mm has a digital measure of 36mm. That being the case: You need to know I have been wrong more that 10,000 times in my lifetime. Alan Marcus
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Samuel Smith |
gawd,my head hurts.
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