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Photo Forum / Digital Photography / DSLR Cameras / February 200635mm (film) lenses on digital camera, do I lose lens speed as well?
Okay, I have to multiply focal length by 1.5 when I put a lens for film on my digital camera. Does this mean I have to do the same with F-numbers like a teleconverter? Thanks, Steve > Okay, I have to multiply focal length by 1.5 when I put a lens for film >on my digital camera. Actually, it is more of a FOV divider. > Does this mean I have to do the same with F-numbers >like a teleconverter? No.  SignatureEd Ruf Lifetime AMA# 344007 (Usenet2@EdwardG.Ruf.com) http://edwardgruf.com/Digital_Photography/General/index.html >> Okay, I have to multiply focal length by 1.5 when I put a lens for film >>on my digital camera. [quoted text clipped - 5 lines] > >No. So, if I put a 200 mm f/4 lens with a 2x teleconverter (effectively a 400 mm f/8) on my Nikon digital SLR, I'm getting the equivalent of a 600 mm f/8? That's useful to know... >>> Okay, I have to multiply focal length by 1.5 when I put a lens for film >>>on my digital camera. [quoted text clipped - 9 lines] >400 mm f/8) on my Nikon digital SLR, I'm getting the equivalent of a >600 mm f/8? That's useful to know... Equivalent what? SignatureEd Ruf Lifetime AMA# 344007 (Usenet2@EdwardG.Ruf.com) http://edwardgruf.com/Digital_Photography/General/index.html Correct, in effect. >>> Okay, I have to multiply focal length by 1.5 when I put a lens for >>> film [quoted text clipped - 10 lines] > 400 mm f/8) on my Nikon digital SLR, I'm getting the equivalent of a > 600 mm f/8? That's useful to know... > Okay, I have to multiply focal length by 1.5 when I put a lens for film > on my digital camera. Does this mean I have to do the same with F-numbers > like a teleconverter? No. The multiplier is meant to mean "_effective_" focal length vs. the 35mm standard 36x24mm frame size. The sensor is smaller on your DSLR (~ 24x16mm). The image circle formed by the lens is still the same size, but the sensor is that much smaller than the std. 35mm frame. So it is like you're "zoomed in" by 1.5x the FL of the lens. Since the true FL has not really changed and the diameter of the aperture has not changed (for a given setting), the aperure number (ratio of FL over aperture diameter) has not changed either. So, exposure is not affected*. The DOF _has_ changed as that is purely related to print size. So since you would need to blow up the smaller sensor image to a greater factor than with the larger film image to get to a given print size, the DOF is shallower than the markings on the standard 35mm format lens suggest. Cheers, Alan. *As the different camera/sensor manufacturers do not seem to be perfectly lined up with respect to ISO on digital cameras, there is a variance between cameras in results. If you meter with a seperate meter then different cameras will give different saturation for the same scene. This difference is about 2/3 of a stop among the various brands/models. Which could fairly be said for the wide variety of film types in any case. Signature-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. > > Okay, I have to multiply focal length by 1.5 when I put a lens for film > > on my digital camera. Does this mean I have to do the same with F-numbers [quoted text clipped - 17 lines] > than with the larger film image to get to a given print size, the DOF is > shallower than the markings on the standard 35mm format lens suggest. Whoa, I followed you right up to the DOF changing. Does the DOF become shallower because the circle of confusion would be blown up along with the sensor image to get a given print size? > Cheers, > Alan. [quoted text clipped - 12 lines] > -- [SI] gallery & rulz: http://www.pbase.com/shootin > -- e-meil: Remove FreeLunch. >>> Okay, I have to multiply focal length by 1.5 when I put a lens for > [quoted text clipped - 27 lines] > shallower because the circle of confusion would be blown up along with the > sensor image to get a given print size? Yep. The CoC is constant in size v. the constant in size aperture, but now smaller sensor; DOF is related to intended print size. The lens manufacturer makes an assumption about print size (depending on what you read and when and where it was written the "standard" CoC print size is 8x10 (from sheet film cameras)) when putting the DOF scale on the lens. (and it could be that lenses for cropped sensor cameras (Nikon, Canon, Pentax, Sigma, etc.) take this into account, but I don't know for sure. To get an 8x12 from a 24x36mm negative, the blowup is 8.47 times. To get an 8x12 from a 16x24mm sensor, the blowup is 12.7 times. So the CoC's are "spread" out that much more so you get shallower "critical focus" DOF on the print. Of course to get the same (almost) image from a 1.5 cropped sensor at the same focal length you will have to step back 1.5 times further resulting in deeper DOF and somewhat restoring balance to the DOF universe. Or at the same distance use a shorter FL. But this will have a different outcome as the hf distance is related to the square of the FL so it's not linear like the focal plane distance from the film plane. I'm too lazy to bother with the math, I'll leave it for you... the main point to remember is that the DOF markings on a full frame lens are wrong when used with a cropped sensor. Cheers, Alan. Signature-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. > Whoa, I followed you right up to the DOF changing. Does the DOF > become shallower because the circle of confusion would be blown up > along with the sensor image to get a given print size? It becomes more shallow if you use the same actual focal length, but deeper if you adjust focal length to preserve field of view (FOV). The following web-pages demonstrates what happes to FOV, DOF and perspective when you use a cropped image circle: - http://hannemyr.com/photo/crop.html - http://hannemyr.com/photo/crop2.html Signature- gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ] ------------------------------------------------------------------------ SD10, Kodak DCS460, Canon Powershot G5, Olympus 2020Z ------------------------------------------------------------------------ |
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