|
|
|
|
| • Home • What's new? • Gallery • Articles • Shop • About this site | ||
12. February 2004
Fast
A 420mm f/2.8 tele lens?
A demonstration Below are two pictures taken from the same point with the Olympus C-750 at it's widest setting (38mm) and it's longest setting (380mm). (The red rectangle shows the 380mm crop)   ©Børge Skov 2004 How is this possible? Making compact and fast tele-zooms has been made possible because of the small imaging chips (image sensors) of modern consumer digital cameras. A small "imaging area" requires lenses with smaller focal length to produce the same angle of view as longer focal length will on larger imaging areas. As you can see from the list above, the actual focal length is very small compared to the 35mm equivalent. Focal length  On the figure above we have a subject matter to the right (a bench) and we have the imaging areas (film or sensor) to the left. When we decrease the size of the imager, we have to move it closer to the lens to maintain the same angle of view, thus decreasing the focal length as the figure illustrates. For medium format film the normal lens is an 80mm, while for 35mm film the normal lens is a 50mm and for a smaller imaging chip the normal lens might be something like a 10mm. Aperture Aperture is a function of the focal length : aperture = focal length/f-stop-number If we take the Lumix with the very fast Carl Zeiss lens from the list above, we have an aperture of : 25,7mm = 72mm/2.8. That means that we need a lens with a diameter of 25,7 mm in order to get f/2.8 at 72mm. That's not a huge lens. If we compare to 35mm film (in which the 72mm focal length of the Lumix resembles a 420mm) that would then be : 150mm = 420mm/2.8. A 150mm front lens will not fit into just any pocket! So, this means that the lenses get very compact in size when we decrease the imaging area. (Please observe that exact measures of modern lenses depends on a lot of factors, like lens construction etc. and therefore the exacts sizes can be more difficult to calculate than the example above suggest. However the calculations above will give you a good hint of the difference in sizes of 35mm lenses and the size of digital consumer lenses.) Sensor sizes The 1/2,7" and 1/2.5" CCDs used by the cameras listed above are approx. 5.27 x 3.96mm and 5.50 x 4.10mm (something like that, not very big). The illustration below shows the relative sizes of 6x6 medium format, 35mm film and the two sensor sizes described here:  OK, what's the drawback? Well, there aren't that many, but one drawback of having a small imaging chip is, that selective focus (shallow depth of field) becomes almost impossible to create. This is because depth of field is a function of the focal length - not of the size of the imaging area. This means that you get impressive depth of field from the consumer grade digital cameras with their short focal lengths. This can be good depending on what kind of pictures you take, but it can also be somewhat annoying when taking portraits for example, in which you might like the background to get blurred. Please read more about calculating Depth Of Field on the Norman Koren website. Another drawback of the extremely small sensors is, that getting really wide angle lenses becomes difficult, but let's what the future will bring... Links • Sensor Sizes - Read more at DPReview.com • Depth Of Filed - Norman Koren |
|
|
________________________________________________________________
Do you like the content of this site?
Subscribe to the newsletter.
Find out how to support the PHOTOgraphical.NET site.
