On: Thu, 29 Jan 2004 Giles Stokoe wrote:- > William Curwen wrote > >> For example, a 300mm standard lens on a 10x8in camera has the same depth of >> field as a 300mmm telephoto lens on a 35mm camera. > > But... > > To get the same field of view on a digicam as i do on 35mm, I have to use > wider angle lenses... (which have greater field of view)
With digicams, that is:- with small chips as light sensors, there are a whole myriad of complex technical problems with quantising an analog light signal. A main one being chromatic abberation in high contrast areas with smallish apertures. A wavelength of light is measured in micro-millimetres and how it hits a pixel on the sensor is critical. All digicams, whatever their sensor size and resolution - will, with sufficient magnification show some degree of chromatic abberation. >> Digicam lenses have a minimum aperture of around f8/11 due the small circular >> field of illumination necessary to cover a typically small sensor. Any >> smaller and all manner of lens abberations begin to creep in. > > But if I look at the middle portion of a 35mm tranny shot at f22 > (approximating to the area covered by the small sensor), I do not see all > manner of lens abberations? So I can't see why couldn't I use f22 on a > digicam? Because you do not need to use a smaller aperture, it is already small at f8/11ish due to the small sensor size, and with a short focal length to the optic, the depth of field will be more than adequate. > Or is it that compact digicams have crap lenses placed very close to the > 'film' plane, thus exacerbating abberations? Your sentence does not make any sense. You need to read up on lens design, construction and actual use. <G> Using digicams is a serious business, I know, I can wipe the floor with a 2meg Canon Powershot A40 - no probs (I digress.) Compact digicams have lenses that are miracles of design and engineering, and there is a definite limit as to what you can get out of a say, a 3meg digi-cam - a commonly agreed minimum for any kind of serious work, without the image falling apart in some way. It is all about the size of the chip, and how the signal is processed. > I guess that I should find out whether (say) a 17mm lens at f8 (field of > view x, min aperture) has more or less d.o.f than a 28mm lens at f22 (min > aperture for 35mm slr at same field of view) (I would imagine less). > Focal length dictates the depth of field that a lens can achieve. Field of View is a different but related subject. Hope this helps William Curwen http://www.william.ws =============================================================== GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
